Climate Change and...

Annotated Bibliography

Carbon Dynamics

Grasslands, Savannahs, Prairies, Steppes

Aires, L. M. I., Pio, C. A., Pereira, J. S. (2008). Carbon dioxide exchange above a Mediterranean C3/C4 grassland during two climatologically contrasting years. Global Change Biology 14 (3): 539-555

ABSTRACT: Eddy-covariance measurements of net ecosystem carbon exchange (NEE) were carried out above a grazed Mediterranean C3/C4 grassland in southern Portugal, during two hydrological years, 2004–2005 and 2005–2006, of contrasting rainfall. Here, we examine the seasonal and interannual variation in NEE and its major components, gross primary production (GPP) and ecosystem respiration (Reco ), in terms of the relevant biophysical controls. The first hydrological year was dry, with total precipitation 45% below the long-term mean (669 mm) and the second was normal, with total precipitation only 12% above the long-term mean. The drought conditions during the winter and early spring of the dry year limited grass production and the leaf area index (LAI) was very low. Hence, during the peak of the growth period, the maximum daily rate of NEE and the light-use and water-use efficiencies were approximately half of those observed in the normal year. In the summer of 2006, the warm-season C4 grass,Cynodon dactylon L., exerted an evident positive effect on NEE by converting the ecosystem into a carbon sink after strong rain events and extending the carbon sequestration for several days, after the end of senescence of the C3 grasses. On an annual basis, the GPP and NEE were 524 and 49 g C m−2 , respectively, for the dry year, and 1261 and −190 g C m−2 for the normal year. Therefore, the grassland was a moderate net source of carbon to the atmosphere, in the dry year, and a considerable net carbon sink, in the normal year. In these 2 years of experiment the total amount of precipitation was the main factor determining the interannual variation in NEE. In terms of relevant controls, GPP and NEE were strongly related to incident photosynthetic photon flux density on short-term time scales. Changes in LAI explained 84% and 77% of the variation found in GPP and NEE, respectively. Variations in Reco were mainly controlled by canopy photosynthesis. After each grazing event, the reduction in LAI affected negatively the NEE.

Ammann, C., Flechard, C.R., Leifeld, J., Neftel, A., Fuhrer, J. (2007). The carbon budget of newly established temperate grassland depends on management intensity. Agriculture, Ecosystems & Environment 121 (1-2): 5-20

ABSTRACT: The carbon exchange of managed temperate grassland, previously converted from arable rotation, was quantified for two levels of management intensities over a period of 3 years. The original field on the Swiss Central Plateau had been separated into two plots of equal size, one plot was subjected to intensive management with nitrogen inputs of 200 kg ha−1 year−1 and frequent cutting, and the other to extensive management with no fertilization and less frequent cutting. For both plots, net CO2 exchange (NEE) was monitored by the eddy covariance technique, and the flux data were submitted to extensive quality control and gap filling procedures. Cumulative NEE was combined with values for carbon export through biomass harvests and carbon import through application of liquid manure (intensive field only) to yield the annual net carbon balance of the grassland ecosystems. The intensive management was associated with an average net carbon sequestration of 147 (±130) g C m−2 year−1 , whereas the extensive management caused a non-significant net carbon loss of 57 (+130/−110) g C m−2 year−1 . Despite the large uncertainty ranges for the two individual systems, the special design of the paired experiment led to a reduced error of the differential effect, because very similar systematic errors for both parallel fields could be assumed. The mean difference in the carbon budget over the 3-year study period was determined to be significant with a value of 204 (±110) g C m−2 year−1 . The difference occurred in spite of similar aboveground productivities and root biomass. Additional measurements of soil respiration under standardized laboratory conditions indicated higher rates of soil organic carbon loss through mineralization under the extensive management. These data suggest that conversion of arable land to managed grassland has a positive effect on the carbon balance during the initial 3 years, but only if the system receives extra nitrogen inputs to avoid carbon losses through increased mineralization of soil organic matter.

Ansley, R. J., Boutton, T. W., Skjemstad, J. O. (2006). Soil organic carbon and black carbon storage and dynamics under different fire regimes in temperate mixed-grass savanna. Global Biogeochemical Cycles 20 (3): B3006

ABSTRACT: We quantified the effects of repeated, seasonal fires on soil organic carbon (SOC), black carbon (BC), and total N in controls and four fire treatments differing in frequency and season of occurrence in a temperate savanna. The SOC at 0–20 cm depth increased from 2044 g C m−2 in controls to 2393–2534 g C m−2 in the three treatments that included summer fire. Similarly, soil total N (0–20 cm) increased from 224 g N m−2 in the control to 251–255 g N m−2 in the treatments that included summer fire. However, winter fires had no effect on SOC or total N. Plant species composition coupled with lowerd13 C of SOC suggested that increased soil C in summer fire treatments was related to shifts in community composition toward greater relative productivity by C3 species. Lowerd15 N of soil total N in summer fire treatments was consistent with a scenario in which N inputs > N losses. The BC storage was not altered by fire, and comprised 13–17% of SOC in all treatments. Results indicated that fire and its season of occurrence can significantly alter ecosystem processes and the storage of C and N in savanna ecosystems.

Cannell, M. G. R., Thornley, J. H. M. (1998). N-poor ecosystems may respond more to elevated CO2 than N- rich ones in the long term. A model analysis of grassland. Global Change Biology 4 (4): 431-442

ABSTRACT: The Hurley Pasture Model was used to examine the short and long-term responses of grazed grasslands in the British uplands to a step increase from 350 to 700μmol mol–1 CO2 concentration ([CO2 ]) with inputs of 5 or 100 kg N ha–1 y–1 . In N-rich grassland, [CO2 ] doubling quickly increased net primary productivity (NPP), total carbon (Csys) and plant biomass by about 30%. By contrast, the N-poor grassland underwent a prolonged 'transient', when there was little response, but eventually NPP, Csys and plant biomass more than doubled. The 'transient' was due to N immobilization and severe depletion of the soil mineral N pool. The large long-term response was due to slow N accumulation, as a result of decreased leaching, decreased gaseous N losses and increased N2 -fixation, which amplified the CO2 response much more in the N-poor than in the N-rich grassland. It was concluded that (i) ecosystems use extra carbon fixed at high [CO2 ] to acquire and retain nutrients, supporting the contention of Gifford et al. (1996), (ii) in the long term, and perhaps on the real timescale of increasing [CO2 ], the response (in NPP, Csys and plant biomass) of nutrient-poor ecosystems may be proportionately greater than that of nutrient-rich ones, (iii) short-term experiments on nutrient-poor ecosystems may observe only the transient responses, (iv) the speed of ecosystem responses may be limited by the rate of nutrient accumulation rather than by internal rate constants, and (v) ecosystem models must represent processes affecting nutrient acquisition and retention to be able to simulate likely real-world CO2 responses.

Cleland, E. E., Chiariello, N. R., Loarie, S. R., Mooney, H. A., Field, C. B. (2006). Diverse responses of phenology to global changes in a grassland ecosystem. Proceedings of the National Academy of Sciences 103 (37): 13740-13744

ABSTRACT: Shifting plant phenology (i.e., timing of flowering and other developmental events) in recent decades establishes that species and ecosystems are already responding to global environmental change. Earlier flowering and an extended period of active plant growth across much of the northern hemisphere have been interpreted as responses to warming. However, several kinds of environmental change have the potential to influence the phenology of flowering and primary production. Here, we report shifts in phenology of flowering and canopy greenness (Normalized Difference Vegetation Index) in response to four experimentally simulated global changes: warming, elevated CO2 , nitrogen (N) deposition, and increased precipitation. Consistent with previous observations, warming accelerated both flowering and greening of the canopy, but phenological responses to the other global change treatments were diverse. Elevated CO2 and N addition delayed flowering in grasses, but slightly accelerated flowering in forbs. The opposing responses of these two important functional groups decreased their phenological complementarity and potentially increased competition for limiting soil resources. At the ecosystem level, timing of canopy greenness mirrored the flowering phenology of the grasses, which dominate primary production in this system. Elevated CO2 delayed greening, whereas N addition dampened the acceleration of greening caused by warming. Increased precipitation had no consistent impacts on phenology. This diversity of phenological changes, between plant functional groups and in response to multiple environmental changes, helps explain the diversity in large-scale observations and indicates that changing temperature is only one of several factors reshaping the seasonality of ecosystem processes.

J. D. Derner, G. E. Schuman (2007). Carbon sequestration and rangelands: A synthesis of land management and precipitation effects. Journal of Soil and Water Conservation 62 (2): 77-85

ABSTRACT: Management of rangelands can aid in the mitigation of rising atmospheric carbon dioxide concentrations via carbon storage in biomass and soil organic matter, a process termed carbon sequestration. Here we provide a review of current knowledge on the effects of land management practices (grazing, nitrogen inputs, and restoration) and precipitation on carbon sequestration in rangelands. Although there was no statistical relationship between change in soil carbon with longevity of the grazing management practice in native rangelands of the North American Great Plains, the general trend seems to suggest a decrease in carbon sequestration with longevity of the grazing management practice across stocking rates. The relationship of carbon sequestration to mean annual precipitation is negative for both the 0 to 10 cm (0 to 3.9 in) and 0 to 30 cm (0 to 11.8 in) soil depths across stocking rates. The threshold from positive to negative carbon change occurs at approximately 440 mm (17.3 in) of precipitation for the 0 to 10 cm soil depth and at 600 mm (23.6 in) for the 0 to 30 cm soil depth. We acknowledge that largely unexplored is the arena of management-environment interactions needed to increase our understanding of climate-plant-soil-microbial interactions as factors affecting nutrient cycling. Continued refinement of estimates of terrestrial carbon storage in rangelands will assist in the development of greenhouse gas emissions and carbon credit marketing policies, as well as potentially modifying government natural resource conservation programs to emphasize land management practices that increase carbon sequestration.

Ganjegunte, G. K., Vance, G. F., Preston, C. M., Schuman, G. E., Ingram, L. J., Stahl, P. D., Welker, J. M. (2005). Soil organic carbon composition in a northern mixed-grass prairie: effects of grazing. Soil Science Society of America JournalSoil Sci Soc Am J 69 (6): 1746-1756

ABSTRACT: Growing interest in the potential for soils to provide a sink for atmospheric C has prompted studies of effects of management on the amount and nature of soil organic C (SOC). In this study, we evaluated effects of different grazing management regimes (light grazing [LG], heavy grazing [HG], and non-grazed exclosures [EX]) on amount and composition of SOC at the USDA–ARS High Plains Grasslands Research Station (HPGRS), Cheyenne, WY. Soils (0–5 cm) from each treatment were analyzed for total C and N contents and lignin composition. Soil organic C and N contents were significantly greater in LG (SOC–13.8 Mg ha–1 ; total N–1.22 Mg ha–1 ) than HG (SOC–10.9 Mg ha–1 ; total N–0.94 Mg ha–1 ) or EX (SOC–10.8 Mg ha–1 ; total N–0.94 Mg ha–1 ). From CuO oxidation studies, significantly greater (P < 0.05) total lignin (Vanillyl [V] + Syringyl [S] + Cinnamyl [C] compounds) contents were noted in EX (21 g kg–1 SOC) than LG (12 g kg–1 SOC) and HG (15 g kg–1 SOC) soils. The lignin composition of humic (HA) and fulvic (FA) acids indicated that HA under LG contained significantly greater V and S than HG or EX. Fulvic acids contained S-depleted lignin compared with HAs and FAs from HG, which contained significantly greater V and C than FAs extracted from LG and EX. Nuclear magnetic resonance (NMR) spectra of HA and FA, however, did not vary significantly among the three grazing treatments. Results from CuO oxidation and NMR spectroscopy emphasized the familiar problem that determining the nature of soil organic matter (SOM) is a difficult task and sometimes different analytical techniques provide different information about the nature of SOM. Nonetheless, results of this study indicate that LG is the most sustainable grazing management system for northern mixed-grass prairies.

M. Huang, J. Ji., K. Li, Y. Liu, F. Yang, B. Tao (2007). The ecosystem carbon accumulation after conversion of grasslands to pine plantations in subtropical red soil of South China. Tellus B 59 (3): 439-448

ABSTRACT: Since 1980s, afforestation in China has led to the establishment of over 0.53 × 108 ha of new plantation forests. While this leads to rapid accumulation of carbon (C) in vegetation, the effects of afforestation on soil C are poorly understood. In this study, a new version of the Atmosphere-Vegetation Interaction Model (AVIM2) was used to examine how changes in plant C inputs following afforestation might lead to changes in soil C at one of the Chinaflux sites and to estimate the effect of afforestation on ex-grassland. The potential total C accumulation of tree plantation was also predicted. The model was calibrated by net ecosystem exchange (NEE), ecosystem respiration (RE) and gross primary production (GPP) based on eddy-covariance measurements. The simulated vegetation C and soil C stocks were compared with the filed observations.

The simulates indicate that after 22 yr of conversion of grassland to needle leaf forests (Pinus massoniana andPinus elliottii ), the net carbon accumulation in tree ecosystem was 1.96 times more than that in grassland. The soil C in the initial 7 yr of planting decreased at a rate of 0.1871 kg C m−2 yr−1 , and after that it increased at a rate of 0.090 kg C m−2 yr−1 . The C accumulation in the studied plantation ecosystem is estimated to be 76–81% of that value in equilibrium state (the net ecosystem productivity approaches to zero).

Sensitivity analyses show that conversion from grassland to plantation caused an initial (7 or 8 yr) periods of decrease in soil C stocks in wider red soil area of southern China. The soil C stocks were reduced between 19.2 and 20.4% in the initial decreasing period. After 7 or 8 yr C loss, the increased in soil C stocks was predicted to be between 0.073 and 0.074 kg C m−2 yr−1 .

A. F. G. Jacobs, B. G. Heusinkveld, A. A. M. Holtslag (2007). Seasonal and interannual variability of carbon dioxide and water balances of a grassland. Climatic Change 82 (1): 163-177

ABSTRACT: There is great international concern over the increase of atmospheric carbon dioxide and its effect on vegetation and climate, and vice versa. Many studies on this issue are based on climate model calculations or indirect satellite observations. In contrast we present a 12-year study (1994–2005) on the net ecosystem exchange of carbon dioxide (NEE) and precipitation surplus (i.e., precipitation–evaporation) of a grassland area in the centre of the Netherlands. On basis of direct flux observations and a process-based model we study and quantify the carbon uptake via assimilation and carbon release via soil and plant respiration. It appears that nearly year-round the assimilation term dominates, which indicates an accumulation of carbon dioxide. The mean net carbon uptake for the 12-year period is about 3 tonnes C per hectare, but with a strong seasonal and interannual variability depending on the weather and water budget. This variability may severely hamper the accurate quantification of carbon storage by vegetation in our present climates and its projection for future climates.

Jones, M. B., Donnelly, A. (2004). Carbon sequestration in temperate grassland ecosystems and the influence of management, climate and elevated CO2 . New Phytologist 164 (3): 423-439

ABSTRACT: The substantial stocks of carbon sequestered in temperate grassland ecosystems are located largely below ground in roots and soil. Organic C in the soil is located in discrete pools, but the characteristics of these pools are still uncertain. Carbon sequestration can be determined directly by measuring changes in C pools, indirectly by using13 C as a tracer, or by simulation modelling. All these methods have their limitations, but long-term estimates rely almost exclusively on modelling. Measured and modelled rates of C sequestration range from 0 to > 8 Mg C ha-1 yr-1 . Management practices, climate and elevated CO2 strongly influence C sequestration rates and their influence on future C stocks in grassland soils is considered. Currently there is significant potential to increase C sequestration in temperate grassland systems by changes in management, but climate change and increasing CO2 concentrations in future will also have significant impacts. Global warming may negate any storage stimulated by changed management and elevated CO2 , although there is increasing evidence that the reverse could be the case.

Jones, S. K., Rees, R. M., Kosmas, D., Ball, B. C., Skiba, U. M. (2006). Carbon sequestration in a temperate grassland; management and climatic controls. Soil Use and Management 22 (2): 132-142

ABSTRACT: Soil management practices that result in increased soil carbon (C) sequestration can make a valuable contribution to reducing the increase in atmospheric CO2 concentrations. We studied the effect of poultry manure, cattle slurry, sewage sludge, NH4 NO3 or urea on C cycling and sequestration in silage grass production. Soil respiration, net ecosystem exchange (NEE) and methane (CH4 ) fluxes were measured with chambers, and soil samples were analysed for total C and dissolved organic C (DOC). Treatments were applied over 2 years and measurements were carried out over 3 years to assess possible residual effects. Organic fertilizer applications increased CO2 loss through soil respiration but also enhanced soil C storage compared with mineral fertilizer. Cumulative soil respiration rates were highest in poultry manure treatments with 13.7 t C ha−1 in 2003, corresponding to 1.6 times the control value, but no residual effect was seen. Soil respiration showed an exponential increase with temperature, and a bimodal relationship with soil moisture. The greatest NEE was observed on urea treatments (with a CO2 uptake of −4.4 g CO2 m−1 h−1 ). Total C and DOC were significantly greater in manure treatments in the soil surface (0–10 cm). Of the C added in the manures, 27% of that in the sewage pellets, 32% of that in the cattle slurry and 39% of that in the poultry manure remained in the 0–10 cm soil layer at the end of the experiment. Mineral fertilizer treatments had only small C sequestration rates, although uncertainties were high. Expressed as global warming potentials, the benefits of increased C sequestration on poultry manure and sewage pellet treatments were outweighed by the additional losses of N2 O, particularly in the wet year 2002. Methane was emitted only for 2–3 days on cattle slurry treatments, but the magnitudes of fluxes were negligible compared with C losses by soil respiration.

Jones, S.K., Rees, R.M., Skiba, U.M., Ball, B.C. (2005). Greenhouse gas emissions from a managed grassland. Global and Planetary Change 47 (2-4): 201-211

ABSTRACT: Managed grasslands contribute to global warming by the exchange of the greenhouse gases carbon dioxide, nitrous oxide and methane. To reduce uncertainties of the global warming potential of European grasslands and to assess potential mitigation options, an integrated approach quantifying fluxes from all three gases is needed. Greenhouse gas emissions from a grassland site in the SE of Scotland were measured in 2002 and 2003. Closed static chambers were used for N2 O and CH4 flux measurements, and samples were analysed by gas chromatography. Closed dynamic chambers were used for soil respiration measurements, using infrared gas analysis. Three organic manures and two inorganic fertilizers were applied at a rate of 300 kg N ha−1 a−1 (available N) and compared with a zero-N control on grassland plots in a replicated experimental design. Soil respiration from plots receiving manure was up to 1.6 times larger than CO2 release from control plots and up to 1.7 times larger compared to inorganic treatments (p <0.05). A highly significant (p <0.001) effect of fertilizer and manure treatments on N2 O release was observed. Release of N2 O from plots receiving inorganic fertilizers resulted in short term peaks of up to 388 g N2 O–N ha−1 day−1 . However losses from plots receiving organic manures were both longer lasting and greater in magnitude, with an emission of up to 3488 g N2 O–N ha−1 day−1 from the sewage sludge treatments. During the 2002 growing season the cumulative total N2 O flux from manure treatments was 25 times larger than that from mineral fertilizers. CH4 emissions were only significantly increased (p <0.001) for a short period following applications of cattle slurry. Although soil respiration in manure plots was high, model predictions and micrometeorological flux measurements at an adjacent site suggest that all plots receiving fertilizer or manure acted as a sink for CO2 . Therefore in terms of global warming potentials the contribution of N2 O from manure treatments becomes particularly important. There were considerable variations in N2 O and CO2 fluxes between years, which was related to annual variations in soil temperature and rainfall.

Landi, A., Anderson, D., Mermut, A. (2003). Organic carbon storage and stable isotope composition of soils along a grassland to forest environmental gradient in Saskatchewan. Canadian Journal of Soil Science 83 (4): 405-414

ABSTRACT: Limited information is available about soil organic carbon accumulation rates and stable isotope composition in the boreal region of the Canadian prairies. The objectives of the study were to document soil development, measure carbon storage and accumulation rates, and determine the13 C/12 C ratio of organic matter in native prairie soils in the major soil-climatic zones of Saskatchewan. The mean thickness of the Ah horizon increases from 5 cm in the Brown Chernozems to 14 cm in Black Chernozems, and this horizon is absent in Gray Luvisols. The thickness and degree of development of B horizons increase from Brown to Gray soils. Total organic C storage to 1.2 m depth in Brown, Dark Brown, Black Chernozems, and Gray Luvisols is 9.08, 11.72, 14.88, 9.63 kg C m-2 , respectively. The long-term mean annual accumulation rates of organic C for Brown, Dark Brown, Black, and Gray soils are 0.57, 0.90, 1.18, and 0.84 g m-2 yr-1 , respectively. For a Rego Black Chernozem the rate is 1.83 g m-2 yr-1 . All these values are higher than those reported for temperate grasslands in the United States of America. Thed13 C values of organic C (an average of all profiles in each soil zone to 1.2-m depth) range from -22.9 ‰ for Dry Brown soils, -24.3‰ for Brown soils, -24.8‰ for Dark Brown soils, -25.3‰ for Black soils, and -26.8‰ for Gray soils. The relative contribution of C4 plants to soil organic C decreases from the warm semiarid grassland to the moist Boreal region, where C4 plants have not influenced organic C at all. Considering the net primary production (NPP) estimated for the soil zones, average aboveground carbon sequestration is estimated to be about 0.46% of NPP. These data provides a realistic assessment of C balances in native prairie soils of Saskatchewan.

Liebig, M.A., Gross, J.R., Kronberg, S.L., Hanson, J.D., Frank, A.B., Phillips, R.L. (2006). Soil response to long-term grazing in the northern Great Plains of North America. Agriculture, Ecosystems & Environment 115 (1-4): 270-276

ABSTRACT: Grazing management affects ecosystem function through impacts on soil condition. We investigated the effects of long-term (over 70 years) grazing on soil properties and nitrous oxide (N2 O) emission within a moderately grazed native vegetation pasture (MGP), heavily grazed native vegetation pasture (HGP), and a fertilized crested wheatgrass (Agropyron desertorum (Fisch. ex. Link) Schult.) pasture (FCWP) near Mandan, ND, USA. Grazing-induced changes in species composition and N fertilizer application contributed to differences in soil properties and N2 O emission between pastures. Soil organic C (SOC) was 5.7 Mg ha−1 greater in FCWP and HGP than MGP at 0–5 cm, whereas HGP had 2.4 Mg ha−1 more SOC than FCWP and MGP at 5–10 cm. At 30–60 cm, SOC in FCWP was 4.0 and 7.5 Mg ha−1 greater than in HGP and MGP, respectively. Particulate organic matter (POM) C and N in the surface 5 cm of FCWP were three- and five-fold greater, respectively, than in HGP and MGP. Acidification from N fertilization in FCWP decreased soil pH and cation exchange capacity compared to HGP and MGP in the surface 5 cm. Annual N2 O emission was over three-fold greater in FCWP compared to HGP and MGP, and was positively associated with POM-C across all pastures (P ≤ 0.0001; r2 = 0.85). Results from this study suggest fertilized crested wheatgrass enhances deep storage of SOC, but contributes to surface acidification and greater N2 O emission relative to native non-fertilized pastures in the northern Great Plains.

Luo, Y. Q., Wan, S. Q., Hui, D. F., Wallace, L. L. (2001). Acclimatization of soil respiration to warming in a tall grass prairie. Nature 413 (6856): 622-625

ABSTRACT: The latest report by the Intergovernmental Panel on Climate Change (IPCC) predicts a 1.4–5.8 °C average increase in the global surface temperature over the period 1990 to 2100 (ref. 1). These estimates of future warming are greater than earlier projections, which is partly due to incorporation of a positive feedback. This feedback results from further release of greenhouse gases from terrestrial ecosystems in response to climatic warming2, 3, 4 . The feedback mechanism is usually based on the assumption that observed sensitivity of soil respiration to temperature under current climate conditions would hold in a warmer climate5 . However, this assumption has not been carefully examined. We have therefore conducted an experiment in a tall grass prairie ecosystem in the US Great Plains to study the response of soil respiration (the sum of root and heterotrophic respiration) to artificial warming of about 2 °C. Our observations indicate that the temperature sensitivity of soil respiration decreases—or acclimatizes—under warming and that the acclimatization is greater at high temperatures. This acclimatization of soil respiration to warming may therefore weaken the positive feedback between the terrestrial carbon cycle and climate.

Ma, W. H., Yang, Y. H., He, J. S., Hui, Z., Fang, J. Y. (2008). Above- and belowground biomass in relation to environmental factors in temperate grasslands, Inner Mongolia. Science in China Series C-Life Sciences 51 (3): 263-270

ABSTRACT: Above- and belowground biomasses of grasslands are important parameters for characterizing regional and global carbon cycles in grassland ecosystems. Compared with the relatively detailed information for aboveground biomass (AGB), belowground biomass (BGB) is poorly reported at the regional scales. The present study, based on a total of 113 sampling sites in temperate grassland of the Inner Mongolia, investigated regional distribution patterns of AGB, BGB, vertical distribution of roots, and their relationships with environmental factors. AGB and BGB increased from the southwest to the northeast of the study region. The largest biomass occurred in meadow steppe, with mean AGB and BGB of 196.7 and 1385.2 g/m2 , respectively; while the lowest biomass occurred in desert steppe, with an AGB of 56.6 g/m2 and a BGB of 301.0 g/m2 . In addition, about 47% of root biomass was distributed in the top 10 cm soil. Further statistical analysis indicated that precipitation was the primary determinant factor in shaping these distribution patterns. Vertical distribution of roots was significantly affected by precipitation, while the effects of soil texture and grassland types were weak.

Manson, A.D., Jewitt, D., Short, A.D. (2007). Effects of season and frequency of burning on soils and landscape functioning in a moist montane grassland. African Journal of Range and Forage Science 24 (1): 9-18

ABSTRACT: The effects of burning on soil properties and landscape function were investigated in a long-term experiment comparing different burning strategies in a moist montane grassland. Total C, total N, total S, bulk density, plant-available nutrients, and soil acidity were determined in the top 200mm of soil, together with vegetation basal cover at the soil surface. The no-burn treatment had the lowest basal cover (14.8%). Basal cover for the burnt treatments ranged from 19.0% (five-year spring burn) to 25.4% (alternate autumn/spring, burnt every 18 months). The organic matter content of these soils was very high with total carbon ranging from 114g kg−1 in the 0-50mm layer to 77g kg−1 in the 150-200mm layer. Bulk density was very low, being 0.57g ml−1 in the 0-50mm layer. There were no significant effects of burning on the quantity of soil organic matter. The C:N ratio was significantly affected throughout the top 200mm by burning treatments; in the 0-50mm layer it ranged from 14.43 in the no-burn treatment to 16.14 in the treatment burnt every 18 months. Higher C:N ratios in frequently burnt treatments suggests that grassland productivity is N-limited in these treatments. In the top 50mm, soil pH is lower in treatments burnt infrequently (5 year and no burn) than in those burnt frequently, whereas concentrations of basic exchangeable cations (K, Ca and Mg) were lower in treatments burnt infrequently (five-year and no burn) than in those burnt frequently. The higher pH and concentrations of basic cations in frequently burnt treatments was probably due to greater cycling of nutrients to the soil surface as a result of higher productivity and deposition of nutrients in ash, together with reduced leaching of cations with nitrate. Landscape Function Analysis was used to measure the functioning of the landscape in terms of scarce resources and the processes that maintain these resources. All sites were highly functional, irrespective of the burning treatment applied. The infrequently burned sites had significantly higher nutrient cycling and infiltration indices than frequently burnt sites and these indices were correlated well with soil chemical properties (acidity, acid saturation, Ca, Cu, K, Mg, P and pH). No significant differences were found between treatments for the stability index.

Mariko, S., Urano, T., Asanuma, J. (2007). Effects of irrigation on CO2 and CH4 fluxes from Mongolian steppe soil. Journal of Hydrology 333 (1): 118-123

SUMMARY: Semi-arid temperate steppes comprise approximately 30% of the world’s temperate grassland, and consequently, are a significant component of the global carbon cycle. To better understand how precipitation affects soil carbon fluxes in semi-arid steppes, we examined the effects of irrigation (simulated rainfall) on CO2 and CH4 fluxes from Mongolian semi-arid steppe soil on 10–12 August 2002 and 19–22 August 2003. Meteorological data revealed that the soil was dry in 2002 and wet in 2003. Summer flux measurements in both years showed that the soil emitted CO2 at 75–250 mg m−2 h−1 and consumed atmospheric CH4 at 30–90μg m−2 h−1 . In 2002, the CO2 flux of the irrigated soil showed an increase of 50% over one day following irrigation compared to the non-irrigated soil, and thereafter, no increase. This enhancing effect of irrigation was found only immediately following irrigation in 2003. Soil CH4 fluxes showed little difference between the irrigated and non-irrigated soils in 2002 and 2003. There was also little difference in soil temperatures (at the surface and 5 cm depth) between the soils in 2002 and 2003. The water content of the irrigated soil increased following irrigation then rapidly decreased with time. These results demonstrate that rainfall events enhance carbon loss from semi-arid steppe soil at least within the day following irrigation. However, long-term meteorological observations of precipitation and soil water content in 2003 and 2004 suggest that usual rainfall pulses throughout the growing season (June–September) do not markedly enhance CO2 emission from such soils.

Morgan, J.A., Milchunas, D.G., LeCain, D.R., West, M., Mosier, A.R. (2007). Carbon dioxide enrichment alters plant community structure and accelerates shrub growth in the shortgrass steppe. Proceedings Of The National Academy Of Sciences Of The United States Of America 104 (37): 14724-14729

ABSTRACT: A hypothesis has been advanced that the incursion of woody plants into world grasslands over the past two centuries has been driven in part by increasing carbon dioxide concentration, [CO2 ], in Earth's atmosphere. Unlike the warm season forage grasses they are displacing, woody plants have a photosynthetic metabolism and carbon allocation patterns that are responsive to CO2 , and many have tap roots that are more effective than grasses for reaching deep soil water stores that can be enhanced under elevated CO2 . However, this commonly cited hypothesis has little direct support from manipulative experimentation and competes with more traditional theories of shrub encroachment involving climate change, management, and fire. Here, we show that, although doubling [CO2 ] over the Colorado shortgrass steppe had little impact on plant species diversity, it resulted in an increasingly dissimilar plant community over the 5-year experiment compared with plots maintained at present-day [CO2 ]. Growth at the doubled [CO2 ] resulted in an ≈40-fold increase in aboveground biomass and a 20-fold increase in plant cover ofArtemisia frigida Willd, a common subshrub of some North American and Asian grasslands. This CO2 -induced enhancement of plant growth, among the highest yet reported, provides evidence from a native grassland suggesting that rising atmospheric [CO2 ] may be contributing to the shrubland expansions of the past 200 years. Encroachment of shrubs into grasslands is an important problem facing rangeland managers and ranchers; this process replaces grasses, the preferred forage of domestic livestock, with species that are unsuitable for domestic livestock grazing.

Mosier, A. R., Milchunas, D. G., Morgan, J. A., King, J. Y., LeCain, D. (2002). Soil-atmosphere exchange of CH4 , CO2, NOx , and N2 O in the Colorado shortgrass steppe under elevated CO2 . Plant And SoilPlant Soil 240 (2): 201-211

ABSTRACT: In late March 1997, an open-top-chamber (OTC) CO2 enrichment study was begun in the Colorado shortgrass steppe. The main objectives of the study were to determine the effect of elevated CO2 (~720m mol mol–1 ) on plant production, photosynthesis, and water use of this mixed C3/C4 plant community, soil nitrogen (N) and carbon (C) cycling and the impact of changes induced by CO2 on trace gas exchange. From this study, we report here our weekly measurements of CO2 , CH4 , NOx and N2 O fluxes within control (unchambered), ambient CO2 and elevated CO2 OTCs. Soil water and temperature were measured at each flux measurement time from early April 1997, year round, through October 2000. Even though both C3 and C4 plant biomass increased under elevated CO2 and soil moisture content was typically higher than under ambient CO2 conditions, none of the trace gas fluxes were significantly altered by CO2 enrichment. Over the 43 month period of observation NOx and N2 O flux averaged 4.3 and 1.7 in ambient and 4.1 and 1.7m g N m–2 hr–1 in elevated CO2 OTCs, respectively. NOx flux was negatively correlated to plant biomass production. Methane oxidation rates averaged –31 and –34m g C m–2 hr–1 and ecosystem respiration averaged 43 and 44 mg C m–2 hr–1 under ambient and elevated CO2 , respectively, over the same time period.

Ni, J. (2004). Forage yield-based carbon storage in grasslands of China. Climatic Change 67 (2-3): 237-246

ABSTRACT: Forage yield-based carbon storage in 18 grasslands of China was estimated according to the detailed investigation of grassland area and forage yield (standing crop), which were derived from a 10-year national grassland survey. The total forage yield carbon in Chinese grasslands is 134.09 Tg C for ca. 299 × 106 ha of grassland area and 1232 kg/ha of mean forage yield. The carbon storage is different depending on grassland types and climatic regions. Meadow, steppe and tussock occupy 93.3% (125.14 Tg C), and desert and swamp only accounts for 6.7% (8.95 Tg C) of total forage yield carbon. Forage yield carbon is stored largely in temperate (38.4%, 51.54 Tg C) and alpine regions (30.4%, 40.78 Tg C), and to less extent in tropical regions (22.1%, 29.66 Tg C). These three regions take 91% of the forage yield carbon in grasslands of China. The warm-temperate region accounts for only 9% (12.1 Tg C) of forage yields carbon. The forage yield-based carbon in grasslands of China is more accurate than the site biomass-based carbon estimate and the carbon density-based estimate. Although, forage yield carbon storage is small compared with the total carbon storage in China, carbon budgets of grasslands are often a dominant component in many regions and provide an important management opportunity to enhance terrestrial carbon sinks in vast areas of China.

Nosetto, M.D., Jobbagy, E.G., Paruelo, J.M. (2006). Carbon sequestration in semi-arid rangelands: Comparison ofPinus ponderosa plantations and grazing exclusion in NW Patagonia. Journal of Arid Environments 67 (1): 142-156

ABSTRACT: The large global extension of arid and semi-arid regions together with their widespread degradation give these areas a high potential to sequester carbon. We explored the possibilities of semi-arid ecosystems to sequester carbon by means of rangeland exclusion and afforestation withPinus ponderosa in NW Patagonia (Argentina). We sampled all pools where organic carbon accumulates in a network of five trios of adjacent grazed, non-grazed and afforested stands (age: 12–25 years, density 605–1052 trees ha−1 ). After 15 years since trees were planted, afforestation added 50% more C to the initial ecosystem carbon pool, with annual sequestration rate ranging 0.5–3.3 Mg C ha−1 year−1 . Carbon gains in afforested stands were higher above than below-ground (150% vs. 32%). Root biomass differences (374% more in afforested vs. grazed stands,p =0.0011) explained below-ground carbon contrasts whereas soil organic carbon showed no differences with afforestation. By contrast, grazing exclosures did not result in significant changes in the total carbon storage in comparison with the adjacent grazed stands (p=0.42) suggesting a slow ecosystem recovery in the time frame of this study (15 years of exclusion). Nevertheless, higher litter amount was found in the former (+53%, p=0.07). Neither, soil organic carbon nor root carbon showed significant differences between grazed and non-grazed conditions. Considering that more than 1.1 millions of hectares of the studied ecosystems are highly degraded and suitable for tree planting, afforesting this area could result in a carbon sequestration rate of 1.7 Tg C year−1 , almost 6% of the current fossil fuel emissions of Argentina; however environmental consequences which could emerge from this deep land use shift must be taken into account when afforestation program are being designed.

Ojima, D. S., Schimel, D. S., Parton, W. J., Owensby, C. E. (1994). Long- and short-term effects of fire on nitrogen cycling in tallgrass prairie. Biogeochemistry 24 (2): 67-84

ABSTRACT: Fires in the tallgrass prairie are frequent and significantly alter nutrient cycling processes. We evaluated the short-term changes in plant production and microbial activity due to fire and the long-term consequences of annual burning on soil organic matter (SOM), plant production, and nutrient cycling using a combination of field, laboratory, and modeling studies. In the short-term, fire in the tallgrass prairie enhances microbial activity, increases both above-and belowground plant production, and increases nitrogen use efficiency (NUE). However, repeated annual burning results in greater inputs of lower quality plant residues causing a significant reduction in soil organic N, lower microbial biomass, lower N availability, and higher C:N ratios in SOM. Changes in amount and quality of below-ground inputs increased N immobilization and resulted in no net increases in N availability with burning. This response occurred rapidly (e.g., within two years) and persisted during 50 years of annual burning. Plant production at a long-term burned site was not adversely affected due to shifts in plant NUE and carbon allocation. Modeling results indicate that the tallgrass ecosystem responds to the combined changes in plant resource allocation and NUE. No single factor dominates the impact of fire on tallgrass plant production.

Omonode, R. A., Vyn, T. J. (2006). Vertical distribution of soil organic carbon and nitrogen under warm-season native grasses relative to croplands in west-central Indiana, USA. Agriculture, Ecosystems & Environment 117 (2-3): 159-170

ABSTRACT: Establishment of grasslands can be an effective means of sequestering soil organic carbon (SOC) and reducing atmospheric CO2 that is believed to contribute to global warming. This study evaluated the vertical distribution and overall sequestration of SOC and total nitrogen (N) under warm-season native grasses (WSNGs) planted 6–8 years earlier relative to a corn (Zea mays L.)–soybean (Glycine max L.) crop sequence, and switchgrass (Panicum virgatum ) relative to tall mixed grasses of big bluestem (Andropogon gerardi), indiangrass (Sorghastrum nutans ), and little bluestem (Andropogon scoparius ). Paired soil samples from 0–15, 15–30, 30–60 and 60–100 cm depth increments were taken from WSNGs and adjoining croplands at 10 locations, and from switchgrass and adjoining tall mixed grasses at four locations in three major soil types of alfisols, mollisols, and entisols in Montgomery County, Indiana. Significant differences in SOC and N concentrations of WSNGs and croplands were limited to the surface 30 cm. On average, SOC concentrations in the surface 15 cm depth were higher in WSNGs than croplands (average: 22.4 and 19.8 g kg−1 C, respectively) but significant differences were observed in just 4 of 10 locations. Similarly, surface soil SOC concentrations were not different for switchgrass (22.1 g kg−1 ) relative to tall mixed grasses (21.4 g kg−1 ). Soil N concentrations never differed significantly among land use treatments. On average, SOC mass calculated to 1.0 m depth was 9.4% higher under WSNGs than cropland (P < 0.058), and 8.1% higher in switchgrass relative to tall mixed grass (P < 0.054), but soil N mass was the same for both WSNGs and cropland. Vertical distribution under WSNGs of SOC mass was 26, 21, 28, and 25%, and of total N mass was 31, 25, 28 and 16%, in the 0–15, 15–30, 30–60, and 60–100 cm depth intervals, respectively. Even though we acknowledge the potential influence of soil variability or prior landscape processes on our results at some locations, we estimated that WSNGs sequestered an average 2.1 Mg C ha−1 yr−1 more than the corn–soybean sequence.

Parton, W.J., Morgan, J.A., Wang, G., Del Grosso, S. (2007). Projected ecosystem impact of the Prairie Heating and CO2 Enrichment experiment. New Phytologist 174 (4): 823-834

ABSTRACT: • The Prairie Heating and CO2 Enrichment (PHACE) experiment has been initiated at a site in southern Wyoming (USA) to simulate the impact of warming and elevated atmospheric CO2 on ecosystem dynamics for semiarid grassland ecosystems.
• The daycent ecosystem model was parametrized to simulate the impact of elevated CO2 at the open-top chamber (OTC) experiment in north-eastern Colorado (1996–2001), and was also used to simulate the projected ecosystem impact of the PHACE experiments during the next 10 yr.
• Model results suggest that soil water content, plant production, soil respiration, and nutrient mineralization will increase for the high-CO2 treatment. Soil water content will decrease for all years, while nitrogen mineralization, soil respiration, and plant production will both decrease and increase under warming depending on yearly differences in water stress. Net primary production (NPP) will be greatest under combined warming and elevated CO2 during wet years.
• Model results are consistent with empirical field data suggesting that water and nitrogen will be critical drivers of the semiarid grassland response to global change.

Parton, W. J., Ojima, D. S., Schimel, D. S. (1994). Environmental change in grasslands: Assessment using models. Climatic Change 28 (1-2): 111-141

ABSTRACT: Modeling studies and observed data suggest that plant production, species distribution, disturbance regimes, grassland biome boundaries and secondary production (i.e., animal productivity) could be affected by potential changes in climate and by changes in land use practices. There are many studies in which computer models have been used to assess the impact of climate changes on grassland ecosystems. A global assessment of climate change impacts suggest that some grassland ecosystems will have higher plant production (humid temperate grasslands) while the production of extreme continental steppes (e.g., more arid regions of the temperate grasslands of North America and Eurasia) could be reduced substantially. All of the grassland systems studied are projected to lose soil carbon, with the greatest losses in the extreme continental grassland systems. There are large differences in the projected changes in plant production for some regions, while alterations in soil C are relatively similar over a range of climate change projections drawn from various General Circulation Models (GCM's). The potential impact of climatic change on cattle weight gains is unclear. The results of modeling studies also suggest that the direct impact of increased atmospheric CO2 on photosynthesis and water use in grasslands must be considered since these direct impacts could be as large as those due to climatic changes. In addition to its direct effects on photosynthesis and water use, elevated CO2 concentrations lower N content and reduce digestibility of the forage.

Patrick, L., Cable, J., Potts, D., Ignace, D., Barron-Gafford, G., Griffith, A., Alpert, H., Gestel, N., Robertson, T., Huxman, T., Zak, J., Loik, M., Tissue, D. (2006). Effects of an increase in summer precipitation on leaf, soil, and ecosystem fluxes of CO2 and H2 O in a sotol grassland in Big Bend National Park, Texas. Oecologia 151 (4): 704-718

ABSTRACT: Global climate models predict that in the next century precipitation in desert regions of the USA will increase, which is anticipated to affect biosphere/atmosphere exchanges of both CO2 and H2 O. In a sotol grassland ecosystem in the Chihuahuan Desert at Big Bend National Park, we measured the response of leaf-level fluxes of CO2 and H2 O 1 day before and up to 7 days after three supplemental precipitation pulses in the summer (June, July, and August 2004). In addition, the responses of leaf, soil, and ecosystem fluxes of CO2 and H2 O to these precipitation pulses were also evaluated in September, 1 month after the final seasonal supplemental watering event. We found that plant carbon fixation responded positively to supplemental precipitation throughout the summer. Both shrubs and grasses in watered plots had increased rates of photosynthesis following pulses in June and July. In September, only grasses in watered plots had higher rates of photosynthesis than plants in the control plots. Soil respiration decreased in supplementally watered plots at the end of the summer. Due to these increased rates of photosynthesis in grasses and decreased rates of daytime soil respiration, watered ecosystems were a sink for carbon in September, assimilating on average 31 mmol CO2 m−2 s−1 ground area day−1 . As a result of a 25% increase in summer precipitation, watered plots fixed eightfold more CO2 during a 24-h period than control plots. In June and July, there were greater rates of transpiration for both grasses and shrubs in the watered plots. In September, similar rates of transpiration and soil water evaporation led to no observed treatment differences in ecosystem evapotranspiration, even though grasses transpired significantly more than shrubs. In summary, greater amounts of summer precipitation may lead to short-term increased carbon uptake by this sotol grassland ecosystem.

Pennock, D. J. (2005). Precision conservation for co-management of carbon and nitrogen on the Canadian Prairies. Journal of Soil and Water Conservation 60 (6): 396-401

ABSTRACT: Agriculture is targeted to make a substantial contribution to Canada's greenhouse gas reduction targets under the Kyoto Protocol. To achieve a net reduction in emissions any gains in soil organic carbon storage cannot come at the expense of enhanced nitrous oxide emissions from the soil. In nonlevel agricultural landscapes of the Canadian Prairies the potential for significant soil organic carbon gain due to adoption of soil conserving practices is greatest on convex upper slope positions, which have experienced major losses of soil organic carbon due to cultivation. The potential for soil organic carbon gain in lower slope positions is limited due to their high soil organic carbon contents, but targeted wetland and riparian vegetation restoration programs could lead to significant above ground carbon gains. Several studies have shown that emissions of nitrous oxide from lower slope positions are significantly higher than the convex slope positions, and that improvements in nitrogen fertilizer use efficiency through site-specific management has the potential to significantly reduce nitrous oxide (N2 O) emissions from these positions. Because of the close relationship between landform position and the key carbon and nitrogen processes, quantitative landform segmentation procedures can be used to delineate precision conservation management zones in these landscapes. Site-specific management practices such as reduced or no-till, seeding to grass, wetland restoration, and site-specific nitrogen (N) management can then be implemented to simultaneously increase soil organic carbon stores on eroded upper slope segments while preserving existing stores of soil organic carbon and reducing N2 O emissions from lower slope segments. Close cooperation between precision conservation professionals and agronomists is required to ensure that information required by producers is available to guide them in their decision making and implementation of precision conservation for co-management of carbon and nitrogen.

Pepper, D. A., Del Grosso, S. J., Mcmurtrie, R. E., Parton, W. J. (2005). Simulated carbon sink response of shortgrass steppe, tallgrass prairie and forest ecosystems to rising [CO2 ], temperature and nitrogen input. Global Biogeochemical Cycles 19 (GB1004): doi:10.1029/2004GB002226

ABSTRACT: The response of plant ecosystems to environmental change will determine whether the terrestrial biosphere will remain a substantial carbon sink or become a source during the next century. We use two ecosystem models, the Generic Decomposition And Yield model (G'DAY) and the daily time step version of the Century model (DAYCENT), to simulate net ecosystem productivity (NEP) for three contrasting ecosystems (shortgrass steppe in Colorado, tallgrass prairie in Kansas, and Norway spruce in Sweden) with varying degrees of water, temperature, and nutrient limitation, to determine responses to gradual increases in atmospheric CO2 concentration ([CO2 ]), temperature, and nitrogen input over 100 years. Using G'DAY, under rising [CO2 ], there is evidence of C sink “saturation,” defined here as positive NEP reaching an upper limit and then declining toward zero, at all three sites (due largely to increased N immobilization in soil organic matter) but a positive C sink is sustained throughout the 100 years. DAYCENT also predicts a sustained C sink at all three sites under rising [CO2 ], with evidence of C sink saturation for the Colorado grassland and the C sink levels off after 80 years for the Kansas grassland. Warming reduces soil C and the C sink in both grassland ecosystems but increases the C sink in the forest. Warming increases decomposition and soil N mineralization, which stimulates net primary productivity (NPP) at all sites except when inducing water limitation. At the forest site some of the enhanced N release is allocated to a woody biomass pool with a low N:C ratio so that warming enhances NEP without increased N input at the forest site, but not at the grassland sites. Responses to combinations of treatments are generally additive for DAYCENT but more interactive for G'DAY, especially under combined rising [CO2 ] and warming at the strongly water- and N-limited shortgrass steppe. Increasing N input alleviates C sink saturation and enhances NEP, NPP, and soil C at all sites. At the water-limited grassland sites the effect of rising [CO2 ] on growth is greatest during the drier seasons. Key sensitivities in the simulations of NEP are identified and include NPP sensitivity to gradual increase in [CO2], N immobilization as a long-term feedback, and the presence or not of plant biomass pools with low N:C ratio.

G. Piñiero, J.M. Paruelo, M. Oesterheld (2006). Potential long-term impacts of livestock introduction on carbon and nitrogen cycling in grasslands of Southern South America. Global Change Biology 12 (7): 1267-1284

ABSTRACT: Empirical evidence based on grazing exclusion at the scale of years to decades shows that grazing modifies carbon (C) and nitrogen (N) cycling. However, long-term effects at the scale of centuries are less known, yet highly relevant to understand local and global impacts of grazing. Additionally, most studies have focused on the isolated response of C and N, with little understanding of their interactions. Using CENTURY, a process-based biogeochemical model, we analyzed the impacts of 370 years of livestock grazing (i.e. long term, from early European colonization to present) in 11 sites across the Río de la Plata grasslands and compared them with those resulting from two decades of grazing (i.e. mid-term, typical exclosure experiment). In the long term, livestock grazing primarily altered the N cycle through faster N returns to the soil via urine and dung, which were offset by uninterrupted N outputs by volatilization and leaching. As a result, soil organic N decreased by −880 kg ha−1 or −19%. Higher N outputs (mainly as NH3) opened the N cycle, potentially decreasing N2 O and NOx emissions and increasing N depositions over the region. These greater outputs of N constrained C accumulation in soils, reducing soil organic C by −21 200 kg ha−1 (−22%, a reduction of −1.5 Pg of C for the whole region) and net primary production by −2192 kg ha−1 yr−1 (−24%). Mid-term simulations showed that the effects of livestock introduction in a decadal time scale were substantially different both in magnitude and direction from long-term responses. Long-term results were not substantially affected when atmospheric CO2 content, species composition and fire regime were changed within plausible ranges, but highlighted fire-grazing interactions as a major constraint of long-term C and N dynamics in these grasslands.

Poth, M., Anderson, I. C., Miranda, H. S., Miranda, A. C., Riggan, P. J. (1995). The magnitude and persistence of soil NO, N2 O, CH4 , and CO2 fluxes from burned tropical savanna in Brazil. Global Biogeochemical Cycles 9 (4): 503-513

ABSTRACT: Among all global ecosystems, tropical savannas are the most severely and extensively affected by anthropogenic burning. Frequency of fire in cerrado, a type of tropical savanna covering 25% of Brazil, is 2 to 4 years. In 1992 we measured soil fluxes of NO, N2 O, CH4 , and CO2 from cerrado sites that had been burned within the previous 2 days, 30 days, 1 year, and from a control site last burned in 1976. NO and N2 O fluxes responded dramatically to fire with the highest fluxes observed from newly burned soils after addition of water. Emissions of N-trace gases after burning were of similar magnitude to estimated emissions during combustion. NO fluxes immediately after burning are among the highest observed for any ecosystem studied to date. These rates declined with time after burning and had returned to control levels 1 year after the burn. An assessment of our data suggested that tropical savanna, burned or unburned, is a major source of NO to the troposphere. Cerrado appeared to be a minor source of N2 O and a sink for atmospheric CH4 . Burning also elevated CO2 fluxes, which remained detectably elevated 1 year later.

Potts, D. L., Huxman, T. E., Enquist, B. J., Weltzin, J. F., Williams, D. G. (2006). Resilience and resistance of ecosystem functional response to a precipitation pulse in a semi-arid grassland. Journal of Ecology 94 (1): 23-30

ABSTRACT: 1 In water-limited ecosystems, discrete precipitation events trigger brief but important episodes of biological activity. Differential responses of above- and below-ground biota to precipitation may constrain biogeochemical transformations at the ecosystem scale.

2 We quantified short-term dynamics of whole ecosystem response to 39 mm irrigation events (precipitation pulses) during June 2002 and 2003 using plant physiological and ecosystem gas-exchange measurements as state variables in a principal components analysis (PCA). Experimental plots consisted of either native (Heteropogon contortus L.) or non-native (Eragrostis lehmanniana Nees) bunchgrasses planted in monoculture on two distinct geomorphic surfaces in a semi-arid grassland.

3 For 15 days, treatments followed similar, non-linear trajectories through state variable space with measurement periods forming distinct clusters; PCA axes 1 and 2 combined to explain 80.7% of the variation during both 2002 and 2003.

4 During both years, bunchgrass species interacted with soil type such that there was a reduction in ecosystem functional resistance in plots planted with the non-native bunchgrass species on the fine-textured clay geomorphic surface.

5 System-level hysteresis, emerging as a result of independent responses of photosynthesis, respiration and evapotranspiration to precipitation, indicated the potential for alternative functional states.

6 Quantifying the frequency and duration of ecosystem alternative functional states in response to individual precipitation events within a season will provide insights into the controls of species, soils and climate on ecosystem carbon and water cycles.

Reich, P. B., Hobbie, S. E., Lee, T., Ellsworth, D. S., West, J. B., Tilman, D., Knops, J. M. H., Naeem, S., Trost, J. (2006). Nitrogen limitation constrains sustainability of ecosystem response to CO2 . Nature 440 (7086): 922-925

ABSTRACT: Enhanced plant biomass accumulation in response to elevated atmospheric CO2 concentration could dampen the future rate of increase in CO2 levels and associated climate warming. However, it is unknown whether CO2 -induced stimulation of plant growth and biomass accumulation will be sustained or whether limited nitrogen (N) availability constrains greater plant growth in a CO2-enriched world1, 2, 3, 4, 5, 6, 7, 8, 9 . Here we show, after a six-year field study of perennial grassland species grown under ambient and elevated levels of CO2 and N, that low availability of N progressively suppresses the positive response of plant biomass to elevated CO2 . Initially, the stimulation of total plant biomass by elevated CO2 was no greater at enriched than at ambient N supply. After four to six years, however, elevated CO2 stimulated plant biomass much less under ambient than enriched N supply. This response was consistent with the temporally divergent effects of elevated CO2 on soil and plant N dynamics at differing levels of N supply. Our results indicate that variability in availability of soil N and deposition of atmospheric N are both likely to influence the response of plant biomass accumulation to elevated atmospheric CO2 . Given that limitations to productivity resulting from the insufficient availability of N are widespread in both unmanaged and managed vegetation5, 7, 8, 9 , soil N supply is probably an important constraint on global terrestrial responses to elevated CO2 .

Schnitzer, M., Mcarthur, D. F. E., Schulten, H. R., Kozak, L. M., Huang, P. M. (2006). Long-term cultivation effects on the quantity and quality of organic matter in selected Canadian prairie soils. Geoderma 130 (1-2): 141-156

ABSTRACT: Sixteen Orthic Chernozemic surface soil samples, one half from virgin prairie and one half from adjacent cultivated prairie (cultivated for 31 to 94 years), were collected from eight sites throughout Southern Saskatchewan, Canada. Samples were analyzed for total organic C and a number of other chemical and physical properties. The virgin and cultivated soils at site No. 4 were selected for more detailed analysis by CP-MAS13 C NMR, Curie-point-pyrolysis-gas chromatography/mass spectrometry (Cp-Py-GC/MS), and by pyrolysis-field ionization mass spectrometry (Py-FIMS). Long-term cultivation resulted in large significant decreases in total SOM (soil organic matter), as represented by total soil organic C. There were significant increases in aromaticity of the SOM as a result of long-term cultivation as indicated by CP-MAS13 C NMR spectroscopy. This was mainly attributable to the result of cultivation-enhanced degradation of aliphatic C relative to aromatic C. Organic compounds identified in the Cp-Py-GC/MS spectra of the virgin and cultivated soils at site No. 4 consisted of n-alkanes (ranging from C11 to C22) and alkenes (ranging from C7:1 to C21:1), with the virgin soil being richer in alkenes than the cultivated soil. Other components identified were cyclic aromatics, carbocyclics, N-containing aromatics, N-heterocyclics, benzene and substituted benzenes, phenols and substituted phenols and substituted furans. The compounds identified appeared to originate from long-chain aliphatics, lignins, polyphenols, aromatics, polysaccharides, and N-containing compounds in the two soils. While qualitatively similar compounds were identified by Py-FIMS in the two soils, the total ion intensity (TII) of the virgin soil was almost 2.5 times as high as that of the cultivated soil. This suggests that cultivation made the organic matter less volatile, either by favouring the formation of higher molecular weight organic matter or by promoting the formation of non-volatile metal-organic matter complexes. The Py-FIMS spectra showed that the virgin soil contained relatively more lignin dimers, lipids, sterols, and n-C16 to n-C34 fatty acids than the cultivated soil. Thus, conversely, the cultivated soil was richer in carbohydrates, phenols and lignin monomers, alkyl aromatics and N-containing compounds, including peptides, than the virgin soil.

Seastedt, T. R., Coxwell, C. C., Ojima, D. S., Parton, W. J. (1994). Controls of plant and soil carbon in a semihumid temperate grassland. Ecological Applications 4 (2): 344-353

ABSTRACT: A modeling study evaluated the importance of photosynthetic pathways (C3, C4, or both) and management strategies to the foliage productivity and soil carbon characteristics of a semihumid temperate grassland subjected to various combinations of climate change. Model values for plant and soil characteristics were obtained at sites near Manhattan, Kansas, and the Manhattan climate record provided the nominal climatic drivers. Model runs used both actual monthly temperature and precipitation data for a 100—yr interval and average weather conditions generated from this record. Monthly temperatures were increased 2°C, left unchanged, or decreased 2°C; annual precipitation was increased 6 cm, left unchanged, or decreased 6 cm. All possible combinations of temperature and precipitation were then used in 100—yr simulations. Regardless of the specific climate scenario, plant production was lowest for C3 grasses and highest for the mixed C3—C4 community. The nominal seasonal pattern of precipitation favored an active C3 plant community in early to late spring, prior to the emergence of the C4 vegetation. However, the higher growth and water use efficiencies of C4 vegetation during summer contributed to the maximization response of the grasslands containing both C3 and C4 grasses. An analysis of variance of annual average values observed from 100—yr simulations was used to evaluate the relative importance of climate, photosynthetic pathways, and management activities (annually burned, burned every 4 yr, unburned, or lightly grazed) to plant production and soil carbon values. Photosynthetic pathway and precipitation were identified as the most significant single variables affecting foliage production; the interaction between photosynthetic and temperature was the most significant interaction term. Management treatments were by far the most important variables affecting soil carbon values, but 2°C warming did produce substantial soil carbon losses from C3 grasslands. Enhanced carbon fixation by the C4 and C3—C4 plant communities negated the losses of soil carbon caused by enhanced soil respiration at warmer temperatures.

Soussana, J.-F., Loiseau, P., Vuichard, N., Ceschia, E., Balesdent, J., Chevallier, T., Arrouays, D. (2004). Carbon cycling and sequestration opportunities in temperate grasslands. Soil Use and Management 20 (2): 219-230

ABSTRACT: Temperate grasslands account for c. 20% of the land area in Europe. Carbon accumulation in grassland ecosystems occurs mostly below ground and changes in soil organic carbon stocks may result from land use changes (e.g. conversion of arable land to grassland) and grassland management. Grasslands also contribute to the biosphere–atmosphere exchange of non-CO2 radiatively active trace gases, with fluxes intimately linked to management practices. In this article, we discuss the current knowledge on carbon cycling and carbon sequestration opportunities in temperate grasslands. First, from a simple two-parameter exponential model fitted to literature data, we assess soil organic carbon fluxes resulting from land use change (e.g. between arable and grassland) and from grassland management. Second, we discuss carbon fluxes within the context of farming systems, including crop–grass rotations and farm manure applications. Third, using a grassland ecosystem model (PaSim), we provide estimates of the greenhouse gas balance, in CO2 equivalents, of pastures for a range of stocking rates and of N fertilizer applications. Finally, we consider carbon sequestration opportunities for France resulting from the restoration of grasslands and from the de-intensification of intensive livestock breeding systems. We emphasize major uncertainties concerning the magnitude and non-linearity of soil carbon stock changes in agricultural grasslands as well as the emissions of N2 O from soil and of CH4 from grazing livestock.

Steinbeiss, S., Temperton, V.M., Gleixner, G. (2007). Mechanisms of soil carbon storage in experimental grasslands. Biogeosciences Discussions 4 (5): 3829-3862

ABSTRACT: We investigated the fate of root and litter derived carbon into soil organic matter and dissolved organic matter in soil profiles, in order to explain unexpected positive effects of plant diversity on carbon storage. A time series of soil and soil solution samples was investigated at the field site of The Jena Experiment. In addition to the main biodiversity experiment with C3 plants, a C4 species (Amaranthus retroflexus L.) naturally labeled with13 C was grown on an extra plot. Changes in organic carbon concentration in soil and soil solution were combined with stable isotope measurements to follow the fate of plant carbon into the soil and soil solution. A split plot design with plant litter removal versus double litter input simulated differences in biomass input. After 2 years, the no litter and double litter treatment, respectively, showed an increase of 381 g Cm-2 and 263g C m-2 to 20 cm depth, while 71 g C m-2 and 393 g C m-2 were lost between 20 and 30 cm depth. The isotopic label in the top 5 cm indicated that 11 and 15% of soil organic carbon were derived from plant material on the no litter and the double litter treatment, respectively. Without litter, this equals the total amount of carbon newly stored in soil, whereas with double litter this corresponds to twice the amount of stored carbon. Our results indicate that litter input resulted in lower carbon storage and larger carbon losses and consequently accelerated turnover of soil organic carbon. Isotopic evidence showed that inherited soil organic carbon was replaced by fresh plant carbon near the soil surface. Our results suggest that primarily carbon released from soil organic matter, not newly introduced plant organic matter, was transported in the soil solution and contributed to the observed carbon storage in deeper horizons.

Suyker, A. E., Verma, S. B. (2001). Year-round observations of the net ecosystem exchange of carbon dioxide in a native tallgrass prairie. Global Change Biology 7 (3): 279-289

ABSTRACT: An Ameriflux site was established in mid 1996 to study the exchange of CO2 in a native tallgrass prairie of north-central Oklahoma, USA. Approximately the first 20 months of measurements (using eddy covariance) are described here. This prairie, dominated by warm season C4 grasses, is typical of the central Kansas/northern Oklahoma region. During the first three weeks of the measurement period (mid-July–early August 1996), moisture-stress conditions prevailed. For the remainder of the period (until March 1998), however, soil moisture was nonlimiting. Mid-day net ecosystem CO2 exchange (NEE), under well-watered conditions, reached a maximum magnitude of 1.4 mg CO2 m−2 s−1 (flux toward the surface is positive) during peak growth (mid-July 1997), with green leaf area index of 2.8. In contrast, under moisture-stress conditions in the same growth stage in 1996, mid-day NEE was reduced to near-zero. Average night NEE ranged from near-zero, during winter dormancy, to − 0.50 mg CO2 m−2 s−1 , during peak growth. Most of the variance in average night NEE was explained by changes in soil temperature (0.1 m depth) and green leaf area. The daytime NEE measurements were examined in terms of a rectangular hyperbolic relationship with incident photosynthetically active radiation. The analysis showed that the quantum yield during peak growth was similar to those measured in other prairies and the y-intercept, so obtained, can be potentially used as an estimate of night-time CO2 emissions when eddy covariance data are unavailable. Daily integrated NEE reached its peak magnitude of 30.8 g CO2 m−2 d−1 (8.4 g C m−2 d−1 ) in mid-July when the green LAI was the largest (about 2.8). In general, the seasonal trend of daily NEE (on relatively clear days) followed that of green LAI. Annually integrated carbon exchange, between prescribed burns in 1997 and 1998, was 268 g C m−2 y−1 . After incorporating carbon loss during the prescribed burn , the net annual carbon exchange in this prairie was near-zero in 1998.

Tan, Z., Liu, S., Johnston, C. A., Liu, J., Tieszen, L. L. (2006). Analysis of ecosystem controls on soil carbon source-sink relationships in the northwest Great Plains. Global Biogeochemical Cyclies 20 (GB4012): doi:10.1029/2005GB002610

ABSTRACT: Our ability to forecast the role of ecosystem processes in mitigating global greenhouse effects relies on understanding the driving forces on terrestrial C dynamics. This study evaluated the controls on soil organic C (SOC) changes from 1973 to 2000 in the northwest Great Plains. SOC source-sink relationships were quantified using the General Ensemble Biogeochemical Modeling System (GEMS) based on 40 randomly located 10 × 10 km2 sample blocks. These sample blocks were aggregated into cropland, grassland, and forestland groups based on land cover composition within each sample block. Canonical correlation analysis indicated that SOC source-sink relationship from 1973 to 2000 was significantly related to the land cover type while the change rates mainly depended on the baseline SOC level and annual precipitation. Of all selected driving factors, the baseline SOC and nitrogen levels controlled the SOC change rates for the forestland and cropland groups, while annual precipitation determined the C source-sink relationship for the grassland group in which noticeable SOC sink strength was attributed to the conversion from cropped area to grass cover. Canonical correlation analysis also showed that grassland ecosystems are more complicated than others in the ecoregion, which may be difficult to identify on a field scale. Current model simulations need further adjustments to the model input variables for the grass cover-dominated ecosystems in the ecoregion.

Tan, Z. X., Liu, S. G., Johnston, C. A., Loveland, T. R., Tieszen, L. L., Liu, J. X., Kurtz, R. (2005). Soil organic carbon dynamics as related to land use history in the northwestern Great Plains. Global Biogeochemical Cycles 19 (GB3011): doi:10.1029/2005GB002536

ABSTRACT: Strategies for mitigating the global greenhouse effect must account for soil organic carbon (SOC) dynamics at both spatial and temporal scales, which is usually challenging owing to limitations in data and approach. This study was conducted to characterize the SOC dynamics associated with land use change history in the northwestern Great Plains ecoregion. A sampling framework (40 sample blocks of 10 × 10 km2 randomly located in the ecoregion) and the General Ensemble Biogeochemical Modeling System (GEMS) were used to quantify the spatial and temporal variability in the SOC stock from 1972 to 2001. Results indicate that C source and sink areas coexisted within the ecoregion, and the SOC stock in the upper 20-cm depth increased by 3.93 Mg ha−1 over the 29 years. About 17.5% of the area was evaluated as a C source at 122 kg C ha−1 yr−1 . The spatial variability of SOC stock was attributed to the dynamics of both slow and passive fractions, while the temporal variation depended on the slow fraction only. The SOC change at the block scale was positively related to either grassland proportion or negatively related to cropland proportion. We concluded that the slow C pool determined whether soils behaved as sources or sinks of atmospheric CO2 , but the strength depended on antecedent SOC contents, land cover type, and land use change history in the ecoregion.

Tang, J., Baldocchi, D. D., Xu, L. (2005). Tree photosynthesis modulates soil respiration on a diurnal time scale.. Global Change Biology 11 (8): 1298-1304

ABSTRACT: To estimate how tree photosynthesis modulates soil respiration, we simultaneously and continuously measured soil respiration and canopy photosynthesis over an oak-grass savanna during the summer, when the annual grass between trees was dead. Soil respiration measured under a tree crown reflected the sum of rhizosphere respiration and heterotrophic respiration; soil respiration measured in an open area represented heterotrophic respiration. Soil respiration was measured using solid-state CO2 sensors buried in soils and the flux-gradient method. Canopy photosynthesis was obtained from overstory and understory flux measurements using the eddy covariance method. We found that the diurnal pattern of soil respiration in the open was driven by soil temperature, while soil respiration under the tree was decoupled with soil temperature. Although soil moisture controlled the seasonal pattern of soil respiration, it did not influence the diurnal pattern of soil respiration. Soil respiration under the tree controlled by the root component was strongly correlated with tree photosynthesis, but with a time lag of 7–12 h. These results indicate that photosynthesis drives soil respiration in addition to soil temperature and moisture.

Tilman, D., Hill, J. K., Lehman, C. (2006). Carbon-negative biofuels from low-input high-diversity grassland biomass. Science 314 (5805): 1598-1600

ABSTRACT: Biofuels derived from low-input high-diversity (LIHD) mixtures of native grassland perennials can provide more usable energy, greater greenhouse gas reductions, and less agrichemical pollution per hectare than can corn grain ethanol or soybean biodiesel. High-diversity grasslands had increasingly higher bioenergy yields that were 238% greater than monoculture yields after a decade. LIHD biofuels are carbon negative because net ecosystem carbon dioxide sequestration (4.4 megagram hectare–1 year–1 of carbon dioxide in soil and roots) exceeds fossil carbon dioxide release during biofuel production (0.32 megagram hectare–1 year–1 ). Moreover, LIHD biofuels can be produced on agriculturally degraded lands and thus need to neither displace food production nor cause loss of biodiversity via habitat destruction.

Veenendaal, Elmar M., Kolle, Olaf, Lloyd, Jon (2004). Seasonal variation in energy fluxes and carbon dioxide exchange for a broad-leaved semi-arid savanna (Mopane woodland) in Southern Africa.. Global Change Biology 10 (3): 318-328

ABSTRACT: We studied the seasonal variation in carbon dioxide, water vapour and energy fluxes in a broad-leafed semi-arid savanna in Southern Africa using the eddy covariance technique. The open woodland studied consisted of an overstorey dominated by Colophospermum mopane with a sparse understorey of grasses and herbs. Measurements presented here cover a 19-month period from the end of the rainy season in March 1999 to the end of the dry season September 2000.

During the wet season, sensible and latent heat fluxes showed a linear dependence on incoming solar radiation (I) with a Bowen ratio (β) typically just below unity. Although β was typically around 1 at low incoming solar radiation (150 W m−2 ) during the dry season, it increased dramatically with I, typically being as high as 4 or 5 around solar noon. Thus, under these water-limited conditions, almost all available energy was dissipated as sensible, rather than latent heat.

Marked spikes of CO2 release occurred at the onset of the rainfall season after isolated rainfall events and respiration dominated the balance well into the rainfall season. During this time, the ecosystem was a constant source of CO2 with an average flux of 3–5μmol m−2 s−1 to the atmosphere during both day and night. But later in the wet season, for example, in March 2000 under optimal soil moisture conditions, with maximum leaf canopy development (leaf area index 0.9–1.3), the peak ecosystem CO2 influx was as much as 10μmol m−2 s−1 . The net ecosystem maximum photosynthesis at this time was estimated at 14μmol m−2 s−1 , with the woodland ecosystem a significant sink for CO2. During the dry season, just before leaf fall in August, maximum day- and night-time net ecosystem fluxes were typically −3μmol m−2 s−1 and 1–2μmol m−2 s−1 , respectively, with the ecosystem still being a marginal sink.

Over the course of 12 months (March 1999–March 2000), the woodland was more or less carbon neutral, with a net uptake estimated at only about 1 mol C m−2 yr−1 . The annual net photosynthesis (gross primary production) was estimated at 32.2 mol m−2 yr−1 .

Verburg, P. S. J., Larsen, J., Johnson, D. W., Schorran, D. E., Arnone, J. A., III (2005). Impacts of an anomalously warm year on soil CO2 efflux in experimentally manipulated tallgrass prairie ecosystems. Global Change Biology 11 (10): 1720-1732

ABSTRACT: Modeling analyses suggest that an increase in growth rate of atmospheric CO2 concentrations during an anomalously warm year may be caused by a decrease in net ecosystem production (NEP) in response to increased heterotrophic respiration (R h ). To test this hypothesis, 12 intact soil monoliths were excavated from a tallgrass prairie site near Purcell, Oklahoma, USA and divided among four large dynamic flux chambers (Ecologically Controlled Enclosed Lysimeter Laboratories (EcoCELLs)). During the first year, all four EcoCELLs were subjected to Oklahoma air temperatures. During the second year, air temperature in two EcoCELLs was increased by 4°C throughout the year to simulate anomalously warm conditions. This paper reports on the effect of warming on soil CO2 efflux, representing the sum of autotrophic respiration (R a ) andR h .

During the pretreatment year, weekly average soil CO2 efflux was similar in all EcoCELLs. During the late spring, summer and early fall of the treatment year, however, soil CO2 efflux was significantly lower in the warmed EcoCELLs. In general, soil CO2 efflux was correlated with soil temperature and to a lesser extent with moisture. A combined temperature and moisture regression explained 64% of the observed variation in soil CO2 efflux. Soil CO2 efflux correlated well with a net primary production (NPP) weighted greenness index derived from digital photographs. Although separate relationships for control and warmed EcoCELLs showed better correlations, one single relationship explained close to 70% of the variation in soil CO2 efflux across treatments and years. A strong correlation between soil CO2 efflux and canopy development and the lack of initial response to warming indicate that soil CO2 efflux is dominated byR a . This study showed that a decrease in soil CO2 efflux in response to a warm year was most likely dominated by a decrease in Ra instead of an increase inR h .

Vuichard, N., Soussana, J.-F., Ciais, P., Viovy, N., Ammann, C., Calanca, P., Clifton-Brown, J., Fuhrer, J., Jones, M., Martin, C. (2007). Estimating the greenhouse gas fluxes of European grasslands with a process-based model: 1. Model evaluation from in situ measurements. Global Biogeochemical Cycles 21 (GB1004): doi:10.1029/2005GB002611

ABSTRACT: We improved a process-oriented biogeochemical model of carbon and nitrogen cycling in grasslands and tested it against in situ measurements of biomass and CO2 and CH4 fluxes at five European grassland sites. The new version of the model (PASIM) calculates the growth and senescence of aboveground vegetation biomass accounting for sporadic removals when the grassland is cut and for continuous removals when it is grazed. Limitations induced by high leaf area index (LAI), soil water deficits and aging of leaves are also included. We added to this a simple empirical formulation to account for the detrimental impact on vegetation of trampling and excreta by grazing animals. Finally, a more realistic methane emission module than is currently used was introduced on the basis of the quality of the animals' diet. Evaluation of this improved version of PASIM is performed at (1) Laqueuille, France, on grassland continuously grazed by cattle with two plots of intensive and extensive grazing intensities, (2) Oensingen, Switzerland, on cut grassland with two fertilized and nonfertilized plots, and (3) Carlow, Ireland, on grassland that is both cut and grazed by cattle during the growing season. In addition, we compared the modeled animal CH4 emissions with in situ measurements on cattle for two grazing intensities at the grazed grassland site of Laqueuille. Altogether, when all improvements to the PASIM model are included, we found that the new parameterizations resulted into a better fit to the observed seasonal cycle of biomass and of measured CO2 and CH4 fluxes. However, the large uncertainties in measurements of biomass and LAI make simulation of biomass dynamics difficult to make. Also simulations for cut grassland are better than for grazed swards. This work paves the way for simulating greenhouse gas fluxes over grasslands in a spatially explicit manner, in order to quantify and understand the past, present and future role of grasslands in the greenhouse gas budget of the European continent.

Chen, S., G. Lin, J. Huang, G.D. Jenerette (2009). Dependence of carbon sequestration on the differential responses of ecosystem photosynthesis and respiration to rain pulses in a semiarid steppe. Global Change Biology 15 (10): 2450-2461

ABSTRACT: Precipitation pulses play an important role in regulating ecosystem carbon exchange and balance of semiarid steppe ecosystems. It has been predicted that the frequency of extreme rain events will increase in the future, especially in the arid and semiarid regions. We hypothesize that large rain pulses favor carbon sequestration, while small ones cause more carbon release in the semiarid steppes. To understand the potential response in carbon sequestration capacity of semiarid steppes to the changes in rain pulse size, we conducted a manipulative experiment with five simulated rain pulse sizes (0, 5, 10, 25, and 75 mm) in Inner Mongolia steppe. Our results showed that both gross ecosystem productivity (GEP) and ecosystem respiration (Re ) responded rapidly (within 24 h) to rain pulses and the initial response time was independent of pulse size. However, the time of peak GEP was 1–3 days later than that of Re , which depended on pulse size. Larger pulses caused greater magnitude and longer duration of variations in GEP and Re . Differences in the response time of microbes and plants to wetting events constrained the response pattern of heterotrophic (Rh ) and autotrophic (Ra ) components of Re following a rain event. Rh contributed more to the increase of Re in the early stage of rain pulse response, while Ra played an more important role later, and determined the duration of pulse response, especially for large rain events of >10 mm. The distinct responses of ecosystem photosynthesis and respiration to increasing pulse sizes led to a threshold in rain pulse size between 10 and 25 mm, above which post wetting responses favored carbon sequestration. The disproportionate increase of the primary productivity of higher plants, compared with those in the activities of microbial decomposers to larger pulse events suggests that the carbon sequestration capacity of Inner Mongolia steppes will be sensitive to changes in precipitation size distribution rather than just precipitation amount.

P. Millard, B. K. Singh (2009). Does grassland vegetation drive soil microbial diversity?. Nutrient Cycling in Agroecosystems Online First (September 2009)

ABSTRACT: Does plant diversity drive soil microbial diversity in temperate, upland grasslands? Plants influence microbial activity around their roots by release of carbon and pot studies have shown an impact of different grass species on soil microbial community structure. Therefore it is tempting to answer yes. However, evidence from field studies is more complex. This evidence is reviewed at three different scales. First, studies from the plant community scale are considered that have compared soil microbial community structure in pastures of different vegetation composition, as a consequence of pasture improvement. These show fungi dominating the biomass in unimproved pastures and bacteria when lime and fertilizers have been applied. Secondly, evidence for interactions between individual grass species and soil microbes is discussed at the level of the rhizosphere, by considering both pot experiments and field studies. These have produced contrasting and inconclusive results, often due to spatial heterogeneity of soil properties and microbial communities. In particular, increased soil pH and fertility in urine patches and other nutrient cycling processes interact to increase the spatially complexity of soil microbial communities. Finally three studies which have measured microbial community structure in the rhizoplane are considered. These show that bacterial diversity is not directly related to plant diversity, although fungal diversity is. In addition, the soil fungal community has been demonstrated to have an effect upon the composition of the bacterial community. We suggest that while current vegetation influences fungal communities (particularly mycorrhizae) and litter inputs fungal saprotrophs, bacterial community structure is influenced more by the quality or composition of soil organic matter, thereby reflecting carbon inputs to the soil over decades.

Bontti, E. E., Gathany, M. A., Przeszlowska, A., Haddix, M. L., Owens, S., Burke, I. C., Parton, W. J., Harmon, M. E., DeCant, J. P., Munson, S. M. (2009). Litter decomposition in grasslands of central North America (US Great Plains). Global Change Biology 15 (5): 1356-1363

ABSTRACT: One of the major concerns about global warming is the potential for an increase in decomposition and soil respiration rates, increasing CO2 emissions and creating a positive feedback between global warming and soil respiration. This is particularly important in ecosystems with large belowground biomass, such as grasslands where over 90% of the carbon is allocated belowground. A better understanding of the relative influence of climate and litter quality on litter decomposition is needed to predict these changes accurately in grasslands. The Long-Term Intersite Decomposition Experiment Team (LIDET) dataset was used to evaluate the influence of climatic variables (temperature, precipitation, actual evapotranspiration, and climate decomposition index), and litter quality (lignin content, carbon : nitrogen, and lignin : nitrogen ratios) on leaf and root decomposition in the US Great Plains. Wooden dowels were used to provide a homogeneous litter quality to evaluate the relative importance of above and belowground environments on decomposition. Contrary to expectations, temperature did not explain variation in root and leaf decomposition, whereas precipitation partially explained variation in root decomposition. Percent lignin was the best predictor of leaf and root decomposition. It also explained most variation in root decomposition in models which combined litter quality and climatic variables. Despite the lack of relationship between temperature and root decomposition, temperature could indirectly affect root decomposition through decreased litter quality and increased water deficits. These results suggest that carbon flux from root decomposition in grasslands would increase, as result of increasing temperature, only if precipitation is not limiting. However, where precipitation is limiting, increased temperature would decrease root decomposition, thus likely increasing carbon storage in grasslands. Under homogeneous litter quality, belowground decomposition was faster than aboveground and was best predicted by mean annual precipitation, which also suggests that the high moisture in soil accelerates decomposition belowground.

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