Climate Change and...

Annotated Bibliography

Carbon Dynamics

Soil Biota

Allen, M. F., Klironomos, J. N., Treseder, K. K., Oechel, W. C. (2005). Responses of soil biota to elevated CO2 in a chaparral ecosystem. Ecological Applications 15 (5): 1701-1711

ABSTRACT: Atmospheric CO2 is rapidly increasing without an integrative understanding of the responses of soil organisms. We sampled soils in a chaparral ecosystem at 18 intervals over a 3-yr period in replicated field chambers ranging from 250 to 750 ppm CO2 at 100 ppm increments. We assessed three distinct soil energy channels: mycorrhizal fungi, saprotrophic fungi-mite/collembola, and bacteria-protozoa/nematode. C allocation below-ground increased with elevated CO2 , Standing crops of fungi and bacteria rarely changed with CO2 . Mass of bacteria-feeding nematodes increased during wet periods, but the effects on soil bacteria were not,detectable. However, grazing of fungi by mites increased with increasing CO2 up to 550 PPM CO2 . Above this threshold, allocation of C to the fungal channel declined. Direct measures of mycorrhizal fungi (percentage infection, arbuscular mycorrhizal [AM] fungal hyphal length) showed no changes with CO2 enrichment, but indirect measures (macroaggregates with newly fixed Q increased suggesting increasing allocation of C through this channel. We postulate that the lack of change in standing crop in microbes to elevated CO2 is due to increasing turnover and to increasing N deficiency. Assessing C sequestration and other impacts of elevated CO2 on ecosystems requires a comprehensive, interactive, and dynamic evaluation of soil organismal responses.

Allison, S. D. (2004). Brown ground: a soil carbon analog for the green world. The American Naturalist 167: 619-627

ABSTRACT: For many decades, ecologists have asked what prevents herbivores from consuming most of the plant biomass in terrestrial ecosystems, or “Why is the world green?” Here I ask the analogous question for detritivores: what prevents them from degrading most of the organic material in soils, or “Why is the ground brown?” For fresh plant detritus, constraints on decomposition closely parallel constraints on herbivory: both herbivore and decomposer populations may be controlled by plant tissue chemistry from the bottom up and predators from the top down. However, the majority of soil carbon is not plant litter but carbon that has been consumed by detritivores and reprocessed into humic compounds with complex and random chemical structures. This carbon persists mainly because the chemical properties of humic compounds and interactions with soil minerals constrain decomposition by extracellular enzymes in soil. Other constraints on decomposers, such as nutrient limitation of enzyme production and competition with opportunistic microbes, also contribute to brown ground. Ultimately, the oldest soil carbon persists via transformation into complex molecules that are impervious to enzymatic attack and effectively decoupled from processing by the soil food web.

Bardgett, R. D., Kandeler, E., Tscherko, D., Hobbs, P. J., Bezemer, T. M., Jones, T. H., Thompson, L. J. (1999). Below-ground microbial community development in a high temperature world. Oikos 85 (2): 193-203.

ABSTRACT: The response of above-ground plant and ecosystem processes to climate change are likely to be influenced by both direct and indirect effects of elevated temperature on soil biota and their activities. This study examined the effects of elevated atmospheric temperature on the development of the soil microbial community in a model terrestrial ecosystem facility. The model system was characterized by a soil of low nutrient availability, a condition that simulates most native terrestrial plant communities. The experiment was run over three plant generations, broadly mimicking the early stages of a plant succession, and showed that microbial biomass, measured using phospholipid fatty acid (PLFA) analysis, increased significantly in response to elevated temperature during the first generation only. This increase was unrelated to changes in plant productivity or soil C-availability, and was largely due to a direct effect of elevated temperature on fast-growing Gram-positive bacteria. Slow growing soil microorganisms such as fungi and actinomycetes were unaffected by elevated temperature throughout the experimental period. Measures of microbial biomass, microbial respiration and N-mineralization were also unaffected by elevated atmospheric temperature over the three generations. The lack of effects on the soil microbial community is thought to be due to the fact that elevated temperature did not influence root biomass or soil C-availability. We suggest that the observed reductions in above-ground plant productivity, in response to elevated temperature, will become apparent in the longer term when litter decomposition pathways are more established. The temporal measures of PLFA and microbial biomass indicated that over the experimental period rapid initial changes occurred in most soil biological characteristics, followed by periods of stabilization during later plant succession. These changes were associated with increases in above-ground plant productivity and amounts of available C in the soil. In contrast, total microbial biomass declined during the last plant generation. Reductions in the diversity of PLFAs in later plant generations appeared to be associated with an increase in the proportion of fatty acids associated with fungi, relative to those from bacteria. These changes are likely to be related to increased competition for resources within the soil, and an associated reduction in N- and C-availability. These changes appear to be broadly consistent with those reported for other studies on the successional development of soil microbial and plant communities. Overall, our data suggest that elevated atmospheric temperature has little effect on the development of below-ground microbial communities and their activities in soils of low nutrient status.

Basiliko, N., Moore, T. R., Lafleur, P. M., Roulet, N. T. (2005). Seasonal and inter-annual decomposition, microbial biomass, and nitrogen dynamics in a Canadian bog. Soil Science 170 (11): 902-912

ABSTRACT: To establish the temporal and spatial variability of substrate contribution to ecosystem respiration (ER), we measured the seasonal and inter-annual microbial carbon dioxide (CO2 ) production potential, microbial biomass, and nitrogen dynamics over a period of 2 years in the upper 30 cm of a peat bog in southern Ontario. Samples collected during a warmer year with lower average summer water table position had larger inorganic and organic nitrogen (N) concentrations and microbial CO2 production potentials. Across all sampling dates, the distance of the water table beneath the surface was significantly and positively correlated with N availability, and in turn N availability was significantly and positively correlated with CO2 production, although direct correlation between water table position and CO2 production was only significant at P = 0.1. Inter-seasonal variability in CO2 production, microbial biomass, or N did not follow consistent patterns between years, and inorganic N species, particularly nitrate, concentrations varied relatively the most between sampling dates, although concentrations were always small relative to microbial biomass N and potassium sulfate- extractable organic N. Microbial CO2 production from the surface peat profile was calculated to be between 2.5 and 5.7 g CO2 m-2 day-1 . Data extrapolation showed that microbial production of CO2 can be between 41 and 67% of the CO2 emitted as ER with the larger value falling in a warmer, drier year and that inter-annual changes in production potentials may partially explain increased ER in warmer, drier years. These results suggest that changes in microbial CO2 production and microbial community and nutrient characteristics may play an important role in controlling the emission of CO2 from terrestrial ecosystems such as peatlands.

Briones, M. J. I., Ostle, N. J., Garnett, M. H. (2007). Invertebrates increase the sensitivity of non-labile soil carbon to climate change. Soil Biology and Biochemistry 39 (3): 816-818

ABSTRACT: The fate of global soil carbon stores in response to predicted climate change is a ‘hotly’ debated topic. Considerable uncertainties remain as to the temperature sensitivity of non-labile soil organic matter (SOM) to decomposition. Currently, models assume that organic matter decomposition is solely controlled by the interaction between climatic conditions and soil mineral characteristics. Consequently, little attention has been paid to adaptive responses of soil decomposer organisms to climate change and their impacts on the turnover of long-standing terrestrial carbon reservoirs. Using a radiocarbon approach we found that warming increased soil invertebrate populations (Enchytraeid worms) leading to a greater turnover of older soil carbon pools. The implication of this finding is that until soil physiology and biology are meaningfully represented in ecosystem carbon models, predictions will underestimate soil carbon turnover.

Carney, K. M., Matson, P. (2005). Plant communities, soil microorganisms, and soil carbon cycling: does altering the world belowground matter to ecosystem functioning?. Ecosystems 8 (8): 928-940

ABSTRACT: Soil microorganisms mediate many critical ecosystem processes. Little is known, however, about the factors that determine soil microbial community composition, and whether microbial community composition influences process rates. Here, we investigated whether aboveground plant diversity affects soil microbial community composition, and whether differences in microbial communities in turn affect ecosystem process rates. Using an experimental system at La Selva Biological Station, Costa Rica, we found that plant diversity (plots contained 1, 3, 5, or > 25 plant species) had a significant effect on microbial community composition (as determined by phospholipid fatty acid analysis). The different microbial communities had significantly different respiration responses to 24 labile carbon compounds. We then tested whether these differences in microbial composition and catabolic capabilities were indicative of the ability of distinct microbial communities to decompose different types of litter in a fully factorial laboratory litter transplant experiment. Both microbial biomass and microbial community composition appeared to play a role in litter decomposition rates. Our work suggests, however, that the more important mechanism through which changes in plant diversity affect soil microbial communities and their carbon cycling activities may be through alterations in their abundance rather than their community composition.

Carney, K. M., Hungate, B. A., Drake, B. G., Megonigal, J. P. (2007). Altered soil microbial community at elevated CO2 leads to loss of soil carbon. Proceedings of the National Academy of Sciences: PNAS 104 (12): 4990-4995

ABSTRACT: Increased carbon storage in ecosystems due to elevated CO2 may help stabilize atmospheric CO2 concentrations and slow global warming. Many field studies have found that elevated CO2 leads to higher carbon assimilation by plants, and others suggest that this can lead to higher carbon storage in soils, the largest and most stable terrestrial carbon pool. Here we show that 6 years of experimental CO2 doubling reduced soil carbon in a scrub-oak ecosystem despite higher plant growth, offsetting ≈52% of the additional carbon that had accumulated at elevated CO2 in aboveground and coarse root biomass. The decline in soil carbon was driven by changes in soil microbial composition and activity. Soils exposed to elevated CO2 had higher relative abundances of fungi and higher activities of a soil carbon-degrading enzyme, which led to more rapid rates of soil organic matter degradation than soils exposed to ambient CO2 . The isotopic composition of microbial fatty acids confirmed that elevated CO2 increased microbial utilization of soil organic matter. These results show how elevated CO2 , by altering soil microbial communities, can cause a potential carbon sink to become a carbon source.

Cheng, W. (1999). Rhizosphere feedbacks in elevated CO2 . Tree Physiology 19 (4-5): 313-320.

ABSTRACT: Understanding rhizosphere processes in relation to increasing atmospheric CO2 concentrations is important for predicting the response of forest ecosystems to environmental changes, because rhizosphere processes are intimately linked with nutrient cycling and soil organic matter decomposition, both of which feedback to tree growth and soil carbon storage. Plants grown in elevated CO2 substantially increase C input to the rhizosphere. Although it is known that elevated CO2 enhances rhizosphere respiration more than it enhances root biomass, the fate and function of this extra carbon input to the rhizosphere in response to elevated CO2 are not clear. Depending on specific plant and soil conditions, the increased carbon input to the rhizosphere can result in an increase, a decrease, or no effect on soil organic matter decomposition and nutrient mineralization. Three mechanisms may account for these inconsistent results: (1) the "preferential substrate utilization" hypothesis; (2) the "priming effect" hypothesis; and (3) the "competition" hypothesis, i.e., competition for mineral nutrients between plants and soil microorganisms. A microbial growth model is developed that quantitatively links the increased rhizosphere input in response to elevated CO2 with soil organic matter decomposition. The model incorporates the three proposed mechanisms, and simulates the complexity of the rhizosphere processes. The model also illustrates mechanistically the interactions among nitrogen availability, substrate quality, and microbial dynamics when the system is exposed to elevated CO2 .

D'Angelo, E. M., Karathanasis, A. D., Sparks, E. J., Ritchey, S. A., Wehr-McChesney, S. A. (2005). Soil carbon and microbial communities at mitigated and late successional bottomland forest wetlands. Wetlands 25 (1): 162-175

ABSTRACT: The practice of wetland mitigation has come into question during the past decade because the relative capacity of the mitigated wetlands to perform normal wetland functions is mostly unknown. In this study, we wanted to determine whether soil microbial communities were significantly different in early successional mitigated wetlands (<10 years) (ES) compared to late successional bottomland hardwood forest wetlands (LS) due to differences in soil properties, such as carbon quality and storage and water-holding capacity. Carbon storage in litter and soil was 1.5 times greater in LS wetlands than ES wetlands. Soil water-holding capacity was significantly greater in LS wetlands and was related to soil organic C content (r2 =0.87, p-value=0.0007). Gravimetric water content was a moderately strong predictor of microbial respiration (r2 =0.55–0.61, p-value=0.001–0.0004) and microbial biomass (r2 =0.70, p-value=0.0019). Anaerobic microbial groups were enriched in soils from LS wetlands in both the dry and wet seasons, which suggested that LS soils were wetter for longer periods of the year than ES soils. The capacity of these wetlands to support anaerobic microbial processes depends on soil water retention characteristics, which were dictated by organic matter content. As an integrator of microbial growth conditions in soils, determination of microbial community composition by phospholipid fatty acid (PLFA) analysis may be an important new tool for monitoring successional development of compensatory mitigation wetlands.

Dilustro, J. J., Collins, B., Duncan, L., Crawford, C. (2005). Moisture and soil texture effects on soil CO2 efflux components in southeastern mixed pine forests. Forest Ecology and Management 204 (1): 87-97

ABSTRACT: Monitoring soil CO2 efflux rates and identifying controlling factors, such as forest composition or soil texture, can help guide forest management and will likely gain relevance as atmospheric CO2 continues to increase. We examined soil CO2 efflux and potential controlling factors in managed mixed pine forests in southwestern Georgia. Soil CO2 efflux was monitored periodically in two stands that differed in soil texture in 2001 and 2002, and in six additional stands in 2003. We also monitored controlling factors: soil temperature, moisture, organic layer mass, and A layer depth. Soil moisture and CO2 efflux varied with soil texture differences among the stands. As expected, soil temperature had a strong influence on soil CO2 efflux. Soil moisture, organic layer mass, and A layer depth also were correlated with soil CO2 efflux during periods of water stress, but these relationships differed with soil texture. Forest management activities can alter components of soil CO2 efflux, including soil carbon pools, temperature, and moisture; understanding the underlying variation of these components and resultant CO2 efflux over soil types can help guide management toward desired forest carbon balance trends in southeastern mixed pine forests.

Hamman, S. T., Burke, I. C., Stromberger, M. E. (2007). Relationships between microbial community structure and soil environmental conditions in a recently burned system. Soil Biology and Biochemistry 39 (7): 1703-1711

ABSTRACT: Most wildfires, even the most severe, burn at mixed intensities across a landscape, depending on local fuel loads, fuel moistures, and wind strength and direction. This heterogeneous patchwork of fire effects can influence the patterns of above- and belowground biotic recovery through altered environmental conditions, nutrient availability, and biotic sources for microbial and vegetative re-colonization. We quantified the effects of low- and high-severity fire 14 months post-burn on key environmental variables typically limiting to microbial activity. We characterized the soil microbial community structure through ester-linked fatty acid analysis (EL-FAME) and identified the soil environmental factors that best explain the pattern of microbial community profiles through canonical correspondence analysis (CCA). Low-severity burning caused no change in soil moisture, pH or temperature while high-severity burning caused an increase in soil moisture, temperature, and a decrease in pH levels, relative to the unburned sites. Soil respiration rates were significantly lower in both the low- and high-severity burn sites, relative to unburned sites, likely due to initial root and microbial death. Overall microbial biomass did not change with either low- or high-severity burning, but the microbial community ordination biplots showed separation of communities by fire, and slight separation by fire severity along three axes. This community separation was driven primarily by a decrease in fungal biomarkers (18:2ω6c, 18:3ω6c) with both low- and high-severity fire. Only 23% of the variation in the microbial community distribution could be explained by three environmental variables: soil pH, temperature, and carbon. These results suggest that the microbial communities in both the low- and high-severity burn sites are structurally different from the populations in the unburned sites.

Hassall, M., Adl, S., Berg, M., Griffiths, B., Scheu, S. (2006). Soil fauna-microbe interactions: towards a conceptual framework for research: ICSZ - Soil Animals and Ecosystems Services, Proceedings of the XIVth International Colloquium on Soil Biology. European Journal of Soil Biology 42 (Supplement 1): S54-S60

ABSTRACT: We explore the potential for applying broad ecological theories to interactions between soil animals and micro-organisms to generate a predictive framework within which more hypothesis led research can be undertaken. The paper stems from discussions during a workshop at the XIVth International Symposium on Soil Zoology. The possible linkage between biodiversity and ecosystem functions forms a good example of how soil zoology research can be productively stimulated by addressing a broader ecological concept but also how the concept can be tested below ground at fundamentally different scales to those commonly used above ground. Other areas of theory rapidly developing above ground, which are yet to be fully tested below ground, include: spatial variability in food webs; indirect interactions mediated through changes in plant secondary chemistry; signalling, including tritrophic interactions; optimal foraging theory, including depletion theory when patches differ in quality as well as quantity; adaptive plasticity in life history traits in relation to temporal variability in resources; trade-offs and facultative non-symbiotic and symbiotic mutualism. We identify modelling of effects of climate change on the soil compartment of the global carbon cycle as an area in which understanding of soil fauna–microbe interaction may outstrip current ecological theory and as a major challenge facing soil biologists in the future.

Heinemeyer, A., Hartley, I.P., Evans, S.P., Carreira De La Fuente, J.A., Ineson, P. (2007). Forest soil CO2 flux: uncovering the contribution and environmental responses of ectomycorrhizas. Global Change Biology 13 (8): 1786-1797

ABSTRACT: Forests play a critical role in the global carbon cycle, being considered an important and continuing carbon sink. However, the response of carbon sequestration in forests to global climate change remains a major uncertainty, with a particularly poor understanding of the origins and environmental responses of soil CO2 efflux. For example, despite their large biomass, the contribution of ectomycorrhizal (EM) fungi to forest soil CO2 efflux and responses to changes in environmental drivers has, to date, not been quantified in the field. Their activity is often simplistically included in the 'autotrophic' root respiration term. We set up a multiplexed continuous soil respiration measurement system in a young Lodgepole pine forest, using a mycorrhizal mesh collar design, to monitor the three main soil CO2 efflux components: root, extraradical mycorrhizal hyphal, and soil heterotrophic respiration.

Mycorrhizal hyphal respiration increased during the first month after collar insertion and thereafter remained remarkably stable. During autumn the soil CO2 flux components could be divided into ~60% soil heterotrophic, ~25% EM hyphal, and ~15% root fluxes. Thus the extraradical EM mycelium can contribute substantially more to soil CO2 flux than do roots. While EM hyphal respiration responded strongly to reductions in soil moisture and appeared to be highly dependent on assimilate supply, it did not responded directly to changes in soil temperature. It was mainly the soil heterotrophic flux component that caused the commonly observed exponential relationship with temperature. Our results strongly suggest that accurate modelling of soil respiration, particularly in forest ecosystems, needs to explicitly consider the mycorrhizal mycelium and its dynamic response to specific environmental factors. Moreover, we propose that in forest ecosystems the mycorrhizal CO2 flux component represents an overflow 'CO2 tap' through which surplus plant carbon may be returned directly to the atmosphere, thus limiting expected carbon sequestration from trees under elevated CO2 .

Johnson, D., Campbell, C. D., Lee, J. A., Callaghan, T. V., Gwynn-Jones, D. (2002). Arctic microorganisms respond more to elevated UV-B radiation than CO2 . Nature 416 (6876): 82-83

ABSTRACT: Surface ultraviolet-B radiation and atmospheric CO2 concentrations have increased as a result of ozone depletion and burning of fossil fuels1, 2 . The effects are likely to be most apparent in polar regions3 where ozone holes have developed and ecosystems are particularly sensitive to disturbance4 . Polar plant communities are dependent on nutrient cycling by soil microorganisms, which represent a significant and highly labile portion of soil carbon (C) and nitrogen (N). It was thought5 that the soil microbial biomass was unlikely to be affected by exposure of their associated plant communities to increased UV-B. In contrast, increasing atmospheric CO2 concentrations were thought to have a strong effect as a result of greater below-ground C allocation6 . In addition, there is a growing belief that ozone depletion is of only minor environmental concern because the impacts of UV-B radiation on plant communities are often very subtle7 . Here we show that 5 years of exposure of a subarctic heath to enhanced UV-B radiation both alone and in combination with elevated CO2 resulted in significant changes in the C:N ratio and in the bacterial community structure of the soil microbial biomass.

Kandeler, E., Mosier, A. R., Morgan, J. A., Milchunas, D. G., King, J. Y., Rudolph, S., Tscherko, D. (2006). Response of soil microbial biomass and enzyme activities to the transient elevation of carbon dioxide in a semi-arid grassland. Soil Biology and Biochemistry 38 (8): 2448-2460

ABSTRACT: Although elevation of CO2 has been reported to impact soil microbial functions, little information is available on the spatial and temporal variation of this effect. The objective of this study was to determine the microbial response in a northern Colorado shortgrass steppe to a 5-year elevation of atmospheric CO2 as well as the reversibility of the microbial response during a period of several months after shutting off the CO2 amendment. The experiment was comprised of nine experimental plots: three chambered plots maintained at ambient CO2 levels of 360μmol mol−1 (ambient treatment), three chambered plots maintained at 720μmol mol−1 CO2 (elevated treatment) and three unchambered plots of equal ground area used as controls to monitor the chamber effect.

Elevated CO2 induced mainly an increase of enzyme activities (protease, xylanase, invertase, alkaline phosphatase, arylsulfatase) in the upper 5 cm of the soil and did not change microbial biomass in the soil profile. Since rhizodeposition and newly formed roots enlarged the pool of easily available substrates mainly in the upper soil layers, enzyme regulation (production and activity) rather than shifts in microbial abundance was the driving factor for higher enzyme activities in the upper soil. Repeated soil sampling during the third to fifth year of the experiment revealed an enhancement of enzyme activities which varied in the range of 20–80%. Discriminant analysis including all microbiological properties revealed that the enzyme pattern in 1999 and 2000 was dominated by the CO2 and chamber effect, while in 2001 the influence of elevated CO2 increased and the chamber effect decreased.

Although microbial biomass did not show any response to elevated CO2 during the main experiment, a significant increase of soil microbial N was detected as a post-treatment effect probably due to lower nutrient (nitrogen) competition between microorganisms and plants in this N-limited ecosystem. Whereas most enzyme activities showed a significant post-CO2 effect in spring 2002 (following the conclusion of CO2 enrichment the previous autumn, 2001), selective depletion of substrates is speculated to be the cause for non-significant treatment effects of most enzyme activities later in summer and autumn, 2002. Therefore, additional belowground carbon input mainly entered the fast cycling carbon pool and contributed little to long-term carbon storage in the semi-arid grassland.

Kandeler, E., Mosier, A. R., Morgan, J. A., Milchunas, D. G., King, J. Y., Rudolph, S., Tscherko, D. (2008). Transient elevation of carbon dioxide modifies the microbial community composition in a semi-arid grassland. Soil Biology and Biochemistry 40 (1): 162-171

ABSTRACT: Using open-top chambers (OTC) on the shortgrass steppe in northern Colorado, changes of microbial community composition were followed over the latter 3 years of a 5-year study of elevated atmospheric CO2 as well as during 12 months after CO2 amendment ended. The experiment was composed of nine experimental plots: three chambered plots maintained at ambient CO2 levels of 360±20μmol mol−1 (ambient treatment), three chambered plots maintained at 720±20μmol mol−1 CO2 (elevated treatment) and three unchambered plots. The abundance of fungal phospholipid fatty acids (PLFAs) shifted in the shortgrass steppe under the influence of elevation of CO2 over the period of 3 years. Whereas the content of the fungal signature molecule (18:2ω6) was similar in soils of the ambient and elevated treatments in the third year of the experiment, CO2 treatment increased the content of 18:2ω6 by around 60% during the two subsequent years. The shift of microbial community composition towards a more fungal dominated community was likely due to slowly changing substrate quality; plant community forage quality declined under elevated CO2 because of a decline of N in all tested species as well as shift in species composition towards greater abundance of the low forage quality species (Stipa comata ). In the year after which CO2 enrichment had ceased, abundances of fungal and bacterial PLFAs in the post-CO2 treatment plots shifted slowly back towards the control plots. Therefore, quantity and quality of available substrates had not changed sufficiently to shift the microbial community permanently to a fungal dominated community. We conclude from PLFA composition of soil microorganisms during the CO2 elevation experiment and during the subsequent year after cessation of CO2 treatment that a shift towards a fungal dominated system under higher CO2 concentrations may slow down C cycling in soils and therefore enhance C sequestration in the shortgrass steppe in future CO2 -enriched atmospheres.

Kandeler, E., Tscherko, D., Bardgett, R. D., Hobbs, P. J., Kampichler, C., Jones, T. H. (1998). The response of soil microorganisms and roots to elevated CO2 and temperature in a terrestrial model ecosystem. Plant And SoilPlant Soil 202 (2): 251-262

ABSTRACT: We investigate the response of soil microorganisms to atmospheric CO2 and temperature change within model terrestrial ecosystems in the Ecotron. The model communities consisted of four plant species (Cardamine hirsuta ,Poa annua ,Senecio vulgaris ,Spergula arvensis ), four herbivorous insect species (two aphids, a leaf-miner, and a whitefly) and their parasitoids, snails, earthworms, woodlice, soil-dwelling Collembola (springtails), nematodes and soil microorganisms (bacteria, fungi, mycorrhizae and Protista). In two successive experiments, the effects of elevated temperature (ambient plus 2 °C) at both ambient and elevated CO2 conditions (ambient plus 200 ppm) were investigated. A 40:60 sand:Surrey loam mixture with relatively low nutrient levels was used. Each experiment ran for 9 months and soil microbial biomass (Cmic and Nmic), soil microbial community (fungal and bacterial phospholipid fatty acids), basal respiration, and enzymes involved in the carbon cycling (xylanase, trehalase) were measured at depths of 0–2, 0–10 and 10–20 cm. In addition, root biomass and tissue C:N ratio were determined to provide information on the amount and quality of substrates for microbial growth.

Elevated temperature under both ambient and elevated CO2 did not show consistent treatment effects. Elevation of air temperature at ambient CO2 induced an increase in Cmic of the 0–10 cm layer, while at elevated CO2 total phospholipid fatty acids (PLFA) increased after the third generation. The metabolic quotient qCO2 decreased at elevated temperature in the ambient CO2 run. Xylanase and trehalase showed no changes in both runs. Root biomass and C:N ratio were not influenced by elevated temperature in ambient CO2 . In elevated CO2 , however, elevated temperature reduced root biomass in the 0–10 cm and 30–40 cm layers and increased N content of roots in the deeper layers. The different response of root biomass and C:N ratio to elevated temperature may be caused by differences in the dynamics of root decomposition and/or in allocation patterns to coarse or fine roots (i.e. storage vs. resource capture functions). Overall, our data suggests that in soils of low nutrient availability, the effects of climate change on the soil microbial community and processes are likely to be minimal and largely unpredicatable.

Maljanen, M., Nykanen, H., Moilanen, M., Martikainen, P. J. (2006). Greenhouse gas fluxes of coniferous forest floors as affected by wood ash addition. Forest Ecology and Management 237 (1-3): 143-149

ABSTRACT: Wood ash has been used to alleviate nutrient deficiencies of peat forests and to combat acidification of forest soils. Ash may change the activities of soil microbes, including those producing or consuming greenhouse gases, such as methane (CH4 ), nitrous oxide (N2 O) and carbon dioxide (CO2 ). We studied the effects of wood ash (loose wood ash originating from pulp mill or power plants) on the fluxes of CH4 , N2 O and CO2 in forests with mineral or peat soils in northern Finland. The ash doses were from 3 to 8 t ha−1 . Gas fluxes were measured with a closed chamber method from five recently fertilized experiments for 1 year after application of ash and from five long-term trials 14–50 years after application. Wood ash did not affect N2 O gas fluxes. In the long-term experiments, wood ash increased the soil CO2 production and the CH4 uptake and lowered the CH4 emissions.

Meyer, H., Kaiser, C., Biasi, C., Haemmerle, R., Rusalimova, O., Lashchinsky, N., Baranyi, C., Daims, H., Barsukov, P., Richter, A. (2006). Soil carbon and nitrogen dynamics along a latitudinal transect in western Siberia, Russia. Biogeochemistry 81 (2): 239-252

ABSTRACT: An 1800-km South to North transect (N 53°43′ to 69°43′) through Western Siberia was established to study the interaction of nitrogen and carbon cycles. The transect comprised all major vegetation zones from steppe, through taiga to tundra and corresponded to a natural temperature gradient of 9.5°C mean annual temperature (MAT). In order to elucidate changes in the control of C and N cycling along this transect, we analyzed physical and chemical properties of soils and microbial structure and activity in the organic and in the mineral horizons, respectively. The impact of vegetation and climate exerted major controls on soil C and N pools (e.g., soil organic matter, total C and dissolved inorganic nitrogen) and process rates (gross N mineralization and heterotrophic respiration) in the organic horizons. In the mineral horizons, however, the impact of climate and vegetation was less pronounced. Gross N mineralization rates decreased in the organic horizons from south to north, while remaining nearly constant in the mineral horizons. Especially, in the northern taiga and southern tundra gross nitrogen mineralization rates were higher in the mineral compared to organic horizons, pointing to strong N limitation in these biomes. Heterotrophic respiration rates did not exhibit a clear trend along the transect, but were generally higher in the organic horizon compared to mineral horizons. Therefore, C and N mineralization were spatially decoupled at the northern taiga and tundra. The climate change implications of these findings (specifically for the Arctic) are discussed.

Park, J. H., Kalbitz, K., Matzner, E. (2002). Resource control on the production of dissolved organic carbon and nitrogen in a deciduous forest floor. Soil Biology and Biochemistry 34 (6): 813-822

ABSTRACT: The forest floor in temperate forests has become recognized for its importance in the retention of elevated inputs of dissolved inorganic nitrogen (DIN) and as a source of dissolved organic matter (DOM). A laboratory leaching experiment was conducted over the period of 98 d to examine the origin of dissolved organic carbon (DOC) and nitrogen (DON) in a deciduous forest floor, and the effect of resource availability and microbial activity on the production mechanisms involved. The experiment was composed of different types of treatments: exclusion of specific forest floor layers (no Oi, no Oe) and addition of carbon sources (glucose, cellulose, leaf, wood) and NH4 NO3 (nitrogen). The cumulative amount of CO2 evolution was positively related to the availability of C sources at each treatment: glucose>leaf=wood=cellulose>control=no Oe=nitrogen>no Oi. DOC release was related to the amount of C sources but showed no clear correlation with CO2 evolution. An increase in C availability generally led to a reduction in the release of DON as well as DIN. In contrast, the amendment of NH4 NO3 reduced the cumulative DOC release but enhanced the release of both DON and DIN. Fresh leaf litter was a more important DOC source than labile substrates (glucose and cellulose) as well as more stable substrates (forest floor materials and wood). Among forest floor layers, more humified horizons (Oe and Oa) were the primary source of DIN and made a similar contribution to DOM release as the Oi layer. The changes in DOM composition detected by a humification index of the leachates, in combination with a shift in the final microbial biomass C, suggested that DOM released from the soluble pools of added litter or the Oi layer contained a substantial amount of microbially processed organic matter. Our study demonstrated the importance of C availability in regulating microbial activity and immobilization of dissolved N in an N-enriched forest floor. However, the discrepancy between substrate lability and DOC production, in combination with a rapid microbial processing of DOC released from labile C pools, illustrated the complicated nature of microbial production and consumption of DOC in the forest floor.

Powlson, D. S., Hirsch, P. R., Brookes, P. C. (2001). The role of soil microorganisms in soil organic matter conservation in the tropics. Nutrient Cycling in Agroecosystems 61 (1-2): 41-51

ABSTRACT: Soil is a large sink for organic carbon within the terrestrial biosphere. Practices which cause a decline in soil organic matter cause CO2 release, in addition to damaging soil resilience and, often, agricultural productivity. The soil micro-organisms (collectively the soil microbial biomass) are the agents of transformation of soil organic matter, nutrients and of most key soil processes. Their activities are much influenced by soil physico-chemical and ecological interactions. This paper addresses two key issues. Firstly, ways of managing, and the extent to which it is possible to manage, soil biological functions. Secondly, the methodologies currently available for studying soil micro-organisms, and the functions they mediate, are discussed. It is concluded that, as the world population develops in this new millennium, there will be an increased dependence upon biological processes in soil to provide adequate crop nutrition for the majority of the world's farmers. Although a major increase in the use of artificial fertilisers will be necessary on a global scale, this will not be an option for large numbers of farmers due to their poverty. Instead they will rely on recycling of nutrients from animal and vegetable composts and urban wastes, and biological cycling from nitrogen fixation and mycorrhizae. The challenge is to select the most appropriate topics for further research. Not all aspects are likely to lead to significantly improved agricultural productivity, or sustainability within the foreseeable future.

Schimel, J. P., Gulledge, J. M., Clein-Curley, J. S., Lindstrom, J. E., Braddock, J. F. (1999). Moisture effects on microbial activity and community structure in decomposing birch litter in the Alaskan taiga. Soil Biology and Biochemistry 31 (6): 831-838

ABSTRACT: We carried out a field experiment to evaluate the effect of moisture regime on microbial biomass and activity in birch litter in the Alaskan taiga. Litter bags were placed in one of three treatments: continuously moist (0.5 cm water d−1 ), cycling (0.5 cm water weekly), and ‘natural', which experienced two natural dry–wet cycles of 2 weeks dry followed by rain. The experiment lasted for 1 month. Each week we collected litter bags and analyzed microbial respiration and biomass C and N. In the last two cycles we analyzed bacterial substrate use on Biolog GN plates. There were strong overall correlations between biomass, respiration and litter moisture content. However, the different treatments had significantly different rates of respiration, biomass and respiratory quotient (qCO2 ) that could not be explained by moisture content directly. The natural treatment had lower respiration rates and biomass than the wet or cycling samples, indicating that the 2-week droughts in the natural treatment reduced microbial populations and activity to a greater degree than did shorter droughts. Episodic drying and rewetting considerably decreased the number of Biolog substrates used. This experiment showed that the size and functioning of the litter microbial community was strongly affected by its stress history.

Thormann, M. N. (2006). Diversity and function of fungi in peatlands: A carbon cycling perspective. Canadian Journal of Soil Science 86 (2): 281-293

ABSTRACT: Peatlands are a dominant landform in the northern hemisphere, accumulating carbon in the form of peat due to an imbalance between decomposition and plant production rates. Decomposer (saprobes) and mycorrhizal fungi significantly influence carbon dynamics by degrading organic matter via the synthesis of extracellular enzymes. As organic matter decomposes, litter quality variables figure most prominently in the succession of fungi. Hence, litters composed primarily of complex polymers decompose very slowly. Surprisingly, recalcitrant polymer degraders (mostly basidiomycetes) are rarely isolated from peat, which may explain the accumulation of complex polymers in peat profiles. While enzymatic profiles of mycorrhizal fungi and other root endophytes may be more limited compared with saprobes, many of these fungi can degrade polymers of varying complexity as well and hence may also be significant decomposers of organic matter. To date, anamorphic ascomycetes and zygomycetes are the most frequently isolated fungi from peatlands (63 and 10% of all taxa, respectively), and chytridiomycetes, teleomorphic ascomycetes, and basidiomycetes appear to be less common (11% of all taxa). The remaining 16% of taxa remain unidentified or are sterile taxa. How disturbances affect peatland microbial communities and their roles is virtually unknown. This aspect of peatland microbial ecology requires immediate attention. The objective of this paper is to review the current state of knowledge of the diversity of fungi and their roles in carbon cycling dynamics in peatlands.

Waldrop, M., Firestone, M. (2006). Response of microbial community composition and function to soil climate change. Microbial Ecology 52 (4): 716-724

ABSTRACT: Soil microbial communities mediate critical ecosystem carbon and nutrient cycles. How microbial communities will respond to changes in vegetation and climate, however, are not well understood. We reciprocally transplanted soil cores from under oak canopies and adjacent open grasslands in a California oak–grassland ecosystem to determine how microbial communities respond to changes in the soil environment and the potential consequences for the cycling of carbon. Every 3 months for up to 2 years, we monitored microbial community composition using phospholipid fatty acid analysis (PLFA), microbial biomass, respiration rates, microbial enzyme activities, and the activity of microbial groups by quantifying13 C uptake from a universal substrate (pyruvate) into PLFA biomarkers. Soil in the open grassland experienced higher maximum temperatures and lower soil water content than soil under the oak canopies. Soil microbial communities in soil under oak canopies were more sensitive to environmental change than those in adjacent soil from the open grassland. Oak canopy soil communities changed rapidly when cores were transplanted into the open grassland soil environment, but grassland soil communities did not change when transplanted into the oak canopy environment. Similarly, microbial biomass, enzyme activities, and microbial respiration decreased when microbial communities were transplanted from the oak canopy soils to the grassland environment, but not when the grassland communities were transplanted to the oak canopy environment. These data support the hypothesis that microbial community composition and function is altered when microbes are exposed to new extremes in environmental conditions; that is, environmental conditions outside of their “life history” envelopes.

Hanson, C. A., Allison, S. D., Bradford, M. A., Wallenstein, M. D., Treseder, K. K. (2008). Fungal taxa target different carbon sources in forest soil. Ecosystems 11 (7): 1157-1167

ABSTRACT: Soil microbes are among the most abundant and diverse organisms on Earth. Although microbial decomposers, particularly fungi, are important mediators of global carbon and nutrient cycling, the functional roles of specific taxa within natural environments remain unclear. We used a nucleotide-analog technique in soils from the Harvard Forest to characterize the fungal taxa that responded to the addition of five different carbon substrates—glycine, sucrose, cellulose, lignin, and tannin-protein. We show that fungal community structure and richness shift in response to different carbon sources, and we demonstrate that particular fungal taxa target different organic compounds within soil microcosms. Specifically, we identified eleven taxa that exhibited changes in relative abundances across substrate treatments. These results imply that niche partitioning through specialized resource use may be an important mechanism by which soil microbial diversity is maintained in ecosystems. Consequently, high microbial diversity may be necessary to sustain ecosystem processes and stability under global change.

D. R. Zak, K. S. Pregitzer, P. S. Curtis, W. E. Holmes (2000). Atmospheric CO2 and the composition of soil microbial communities. Ecologcial Applications 10 (1): 47-59

ABSTRACT: Elevated atmospheric CO2 has the potential to increase the production and alter the chemistry of organic substrates entering soil from plant production, the magnitude of which is constrained by soil-N availability. Because microbial growth in soil is limited by substrate inputs from plant production, we reasoned that changes in the amount and chemistry of these organic substrates could affect the composition of soil microbial communities and the cycling of N in soil. We studied microbial community composition and soil-N transformations beneathPopulus tremuloides Michx. growing under experimental atmospheric CO2 (35.7 and 70.7 Pa) and soil-N-availability (low N = 61 ng N·g−1 ·d−1 and high N = 319 ng N·g−1 ·d−1 ) treatments. Atmospheric CO2 concentration was modified in large, open-top chambers, and we altered soil-N availability in open-bottom root boxes by mixing different proportions of A and C horizon material. We used phospholipid fatty-acid analysis to gain insight into microbial community composition and coupled this analysis to measurements of soil-N transformations using 15N-pool dilution techniques. The information presented here is part of an integrated experiment designed to elucidate the physiological mechanisms controlling the flow of C and N in the plant–soil system. Our objectives were (1) to determine whether changes in plant growth and tissue chemistry alter microbial community composition and soil-N cycling in response to increasing atmospheric CO2 and soil-N availability and (2) to integrate the results of our experiment into a synthesis of elevated atmospheric CO2 and the cycling of C and N in terrestrial ecosystems.

After 2.5 growing seasons, microbial biomass, gross N mineralization, microbial immobilization, and nitrification (gross and net) were equivalent at ambient and elevated CO2 , suggesting that increases in fine-root production and declines in fine-root N concentration were insufficient to alter the influence of native soil organic matter on microbial physiology; this was the case in both low- and high-N soil. Similarly, elevated CO2 did not alter the proportion of bacterial, actinomycetal, or fungal phospholipid fatty acids in low-N or high-N soil, indicating that changes in substrate input from greater plant growth under elevated CO2 did not alter microbial community composition. Our results differ from a substantial number of studies reporting increases and decreases in soil-N cycling under elevated CO2 . From our analysis, it appears that soil-N cycling responds to elevated atmospheric CO2 in experimental situations where plant roots have fully colonized the soil and root-associated C inputs are sufficient to modify the influence of native soil organic matter on microbial physiology. In young developing ecosystems where plant roots have not fully exploited the soil, microbial metabolism appears to be regulated by relatively large pools of soil organic matter, rather than by the additional input of organic substrates under elevated CO2 .

D. R. Zak, W. E. Holmes, D. C. White, A. D. Peacock, D. Tilman (2003). Plant diversity, soil microbial communities, and ecosystem function: are there any links?. Ecology 84 (8): 2042-2050

ABSTRACT: A current debate in ecology centers on the extent to which ecosystem function depends on biodiversity. Here, we provide evidence from a long-term field manipulation of plant diversity that soil microbial communities, and the key ecosystem processes that they mediate, are significantly altered by plant species richness. After seven years of plant growth, we determined the composition and function of soil microbial communities beneath experimental plant diversity treatments containing 1–16 species. Microbial community biomass, respiration, and fungal abundance significantly increased with greater plant diversity, as did N mineralization rates. However, changes in microbial community biomass, activity, and composition largely resulted from the higher levels of plant production associated with greater diversity, rather than from plant diversity per se. Nonetheless, greater plant production could not explain more rapid N mineralization, indicating that plant diversity affected this microbial process, which controls rates of ecosystem N cycling. Greater N availability probably contributed to the positive relationship between plant diversity and productivity in the N-limited soils of our experiment, suggesting that plant–microbe interactions in soil are an integral component of plant diversity's influence on ecosystem function.

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.

S. C. Hart, T. H. DeLuca, G. S. Newman, M. D. MacKenzie, S. I. Boyle (2005). Post-fire vegetative dynamics as drivers of microbial community structure and function in forest soils. Forest Ecology and Management 220 (1-3): 166-184

ABSTRACT: Soil microorganisms have numerous functional roles in forest ecosystems, including: serving as sources and sinks of key nutrients and catalysts of nutrient transformations; acting as engineers and maintainers of soil structure; and forming mutualistic relationships with roots that improve plant fitness. Although both prescribed and wildland fires are common in temperate forests of North America, few studies have addressed the long-term influence of such disturbances on the soil microflora in these ecosystems. Fire alters the soil microbial community structure in the short-term primarily through heat-induced microbial mortality. Over the long-term, fire may modify soil communities by altering plant community composition via plant-induced changes in the soil environment. In this review, we summarize and synthesize the various studies that have assessed the effects of fire on forest soil microorganisms, emphasizing the mechanisms by which fire impacts these vital ecosystem engineers. The examples used in this paper are derived primarily from studies of ponderosa pine-dominated forests of the Inland West of the USA; these forests have some of the shortest historical fire-return intervals of any forest type, and thus the evolutionary role of fire in shaping these forests is likely the strongest. We argue that the short-term effects of fire on soil microflora and the processes they catalyze are transient, and suggest that more research be devoted to linking long-term plant community responses with those of the mutually dependent soil microflora.

Waldrop, M. P., Harden, J. W. (2008). Interactive effects of wildfire and permafrost on microbial communities and soil processes in an Alaskan black spruce forest.. Global Change Biology 14 (11): 2591-2602

ABSTRACT: Boreal forests contain significant quantities of soil carbon that may be oxidized to CO2 given future increases in climate warming and wildfire behavior. At the ecosystem scale, decomposition and heterotrophic respiration are strongly controlled by temperature and moisture, but we questioned whether changes in microbial biomass, activity, or community structure induced by fire might also affect these processes. We particularly wanted to understand whether postfire reductions in microbial biomass could affect rates of decomposition. Additionally, we compared the short-term effects of wildfire to the long-term effects of climate warming and permafrost decline. We compared soil microbial communities between control and recently burned soils that were located in areas with and without permafrost near Delta Junction, AK. In addition to soil physical variables, we quantified changes in microbial biomass, fungal biomass, fungal community composition, and C cycling processes (phenol oxidase enzyme activity, lignin decomposition, and microbial respiration). Five years following fire, organic surface horizons had lower microbial biomass, fungal biomass, and dissolved organic carbon (DOC) concentrations compared with control soils. Reductions in soil fungi were associated with reductions in phenol oxidase activity and lignin decomposition. Effects of wildfire on microbial biomass and activity in the mineral soil were minor. Microbial community composition was affected by wildfire, but the effect was greater in nonpermafrost soils. Although the presence of permafrost increased soil moisture contents, effects on microbial biomass and activity were limited to mineral soils that showed lower fungal biomass but higher activity compared with soils without permafrost. Fungal abundance and moisture were strong predictors of phenol oxidase enzyme activity in soil. Phenol oxidase enzyme activity, in turn, was linearly related to both13 C lignin decomposition and microbial respiration in incubation studies. Taken together, these results indicate that reductions in fungal biomass in postfire soils and lower soil moisture in nonpermafrost soils reduced the potential of soil heterotrophs to decompose soil carbon. Although in the field increased rates of microbial respiration can be observed in postfire soils due to warmer soil conditions, reductions in fungal biomass and activity may limit rates of decomposition.

Antoninka, A., Wolf, J. E., Bowker, M., Classen, A. T., Johnson, N. C. (2009). Linking above- and belowground responses to global change at community and ecosystem scales. Global Change Biology 15 (4): 914-929

ABSTRACT: Cryptic belowground organisms are difficult to observe and their responses to global changes are not well understood. Nevertheless, there is reason to believe that interactions among above- and belowground communities may mediate ecosystem responses to global change. We used grassland mesocosms to manipulate the abundance of one important group of soil organisms, arbuscular mycorrhizal (AM) fungi, and to study community and ecosystem responses to CO2 and N enrichment. Responses of plants, AM fungi, phospholipid fatty acids and community-level physiological profiles were measured after two growing seasons. Ecosystem responses were examined by measuring net primary production (NPP), evapotranspiration, total soil organic matter (SOM), and extractable mineral N. Structural equation modeling was used to examine the causal relationships among treatments and response variables. We found that while CO2 and N tended to directly impact ecosystem functions (evapotranspiration and NPP, respectively), AM fungi indirectly impacted ecosystem functions by influencing the community composition of plants and other root fungi, soil fungi and soil bacteria. We found that the mycotrophic status of the dominant plant species in the mesocosms determined whether the presence of AM fungi increased or decreased NPP. Mycotrophic grasses dominated the mesocosm communities during the first growing season, and the mycorrhizal treatments had the highest NPP. In contrast, nonmycotrophic forbs were dominant during the second growing season and the mycorrhizal treatments had the lowest NPP. The composition of the plant community strongly influenced soil N, and the community composition of soil organisms strongly influenced SOM accumulation in the mesocosms. These results show how linkages between above- and belowground communities can determine ecosystem responses to global change.

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