Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
N-Deposition, Saturation, Dynamics
J. Aber, W. McDowell, K. Nadelhoffer, A. Magill, G. Berntson, M. Kamakea, S. McNulty, W. Currie, L. Rustad, I. Fernandez (1998). Nitrogen saturation in temperate forest ecosystems. BioScience 48 (11): 921-934
FIRST PARAGRAPH: Nitrogen emissions to the atmosphere due to human activity remain elevated in industrialized regions of the world and are accelerating in many developing regions (Galloway 1995). Although the deposition of sulfur has been reduced over much of the United States and Europe by aggressive environmental protection policies, current nitrogen deposition reduction targets in the US are modest. Nitrogen deposition remains relatively constant in the northeastern United States and is increasing in the Southeast and the West (Fenn et al. in press)
John D. Aber, Christine L. Goodale, Scott V. Ollinger, Marie-Louise Smith, Alison H. Magill, Mary E. Martin, Richard A. Hallett, John L. Stoddard (2003). Is Nitrogen Deposition Altering the Nitrogen Status of Northeastern Forests?. BioScience 53 (4): 375-389
ABSTRACT: Concern is resurfacing in the United States over the long-term effects of excess nitrogen (N) deposition and mobility in the environment. We present here a new synthesis of existing data sets for the northeastern United States, intended to answer a single question: Is N deposition altering the N status of forest ecosystems in this region? Surface water data suggest a significant increase in nitrate losses with N deposition. Soil data show an increase in nitrification with decreasing ratio of soil carbon to nitrogen (C:N) but weaker relationships between N deposition and soil C:N ratio or nitrification. Relationships between foliar chemistry and N deposition are no stronger than with gradients of climate and elevation. The differences in patterns for these three groups of indicators are explained by the degree of spatial and temporal integration represented by each sample type. The surface water data integrate more effectively over space than the foliar or soil data and therefore allow a more comprehensive view of N saturation. We conclude from these data that N deposition is altering N status in northeastern forests.
Aber, J. D., Ollinger, S. V., Federer, C. A., Reich, P. B., Goulden, M. L., Kicklighter, D. W., Melillo, J. M., Lathrop, R. G., Jr. (1995). Predicting the effects of climate change on water yield and forest production in the northeastern United States. Climate Research 5 (3): 207-222
ABSTRACT: Rapid and simultaneous changes in temperature, precipitation and the atmospheric concentration of CO2 are predicted to occur over the next century. Simple, well-validated models of ecosystem function are required to predict the effects of these changes This paper describes an improved version of a forest carbon and water balance model (PnET-II) and the application of the model to predict stand- and regional-level effects of changes in temperature, precipitation and atmospheric CO2 (gross and net) driven by nitrogen availability as expressed through foliar N concentration. Improvements was parameterized and run for 4 forest/site combinations and validated against available data for water yield, gross and net carbon exchange and biomass production. The validation exercise suggests that the determination of actual water availability to stands and the occurrence or non-occurrence of soil-based water stress are critical to accurate modeling of forest net primary production (NPP) and net ecosystem production (NEP). The model was then run for the entire New England/New York (USA) region using a 1 km resolution geographic information system. Predicted long-term NEP ranged from -85 to +275 g C m-2 yr-1 for the 4 forest/site combinations, and from -150 to 350 g cm-2 yr-1 for the region, with a regional average of 76 g C m-2 yr-1 . A combination of increased temperature (+6 degree C), decreased precipitation (-15%) and increased Water use efficiency (2x, due to doubling of CO2 ) resulted generally in increases in NPP and decreases in water yield over the region
Adams, A. B., Harrison, R. B., Sletten, R. S., Strahm, B. D., Turnblom, E. C., Jensen, C. M. (2005). Nitrogen-fertilization impacts on carbon sequestration and flux in managed coastal Douglas-fir Stands of the Pacific Northwest. Forest Ecology and Management 220 (1-3): 313-325
ABSTRACT: We examined whether N-fertilization and soil origin of Douglas-fir [Psuedotsuga menziesii (Mirb.) Franco] stands in western Washington state could affect C sequestration in both the tree biomass and in soils, as well as the flux of dissolved organic carbon (DOC) through the soil profile. This study utilized four forest sites that were initially established between 1972 and 1980 as part of Regional Forest Nutrition Research Project (RFNRP). Two of the soils were derived from coarse-textured glacial outwash and two from finer-textured volcanic-source material, primarily tephra, both common soil types for forestry in the region. Between 1972 and 1996 fertilized sites received either three or four additions of 224 kg N ha-1 as urea (672-896 kg N ha-1 total). Due to enhanced tree growth, the N-fertilized sites (161 Mg C ha-1 ) had an average of 20% more C in the tree biomass compared to unfertilized sites (135 Mg C ha-1 ). Overall, N-fertilized soils (260 Mg C ha-1 ) had 48% more soil C compared to unfertilized soils (175 Mg C ha-1 ). The finer-textured volcanic-origin soils (348 Mg C ha-1 ) had 299% more C than glacial outwash soils (87.2 Mg C ha-1 ), independent of N-fertilization. Soil-solution DOC collected by lysimeters also appeared to be higher in N-fertilized, upper soil horizons compared to unfertilized controls but it was unclear what fraction of the difference was lost from decomposition or contributed to deep-profile soil C by leaching and adsorption. When soil, understory vegetation and live-tree C compartments are pooled and compared by treatment, N-fertilized plots had an average of 110 Mg C ha-1 more than unfertilized controls. These results indicate these sites generally responded to N-fertilization with increased C sequestration, but differences in stand and soil response to N-ferfilization might be partially explained by soil origin and texture.
Adviento-Borbe, M. A. A., Haddix, M. L., Binder, D. L., Walters, D. T., Dobermann, A. (2007). Soil greenhouse gas fluxes and global warming potential in four high-yielding maize systems. Global Change Biology 13 (9): 1972-1988
ABSTRACT: Crop intensification is often thought to increase greenhouse gas (GHG) emissions, but studies in which crop management is optimized to exploit crop yield potential are rare. We conducted a field study in eastern Nebraska, USA to quantify GHG emissions, changes in soil organic carbon (SOC) and the net global warming potential (GWP) in four irrigated systems: continuous maize with recommended best management practices (CC-rec) or intensive management (CC-int) and maize–soybean rotation with recommended (CS-rec) or intensive management (CS-int). Grain yields of maize and soybean were generally within 80–100% of the estimated site yield potential. Large soil surface carbon dioxide (CO2 ) fluxes were mostly associated with rapid crop growth, high temperature and high soil water content. Within each crop rotation, soil CO2 efflux under intensive management was not consistently higher than with recommended management. Owing to differences in residue inputs, SOC increased in the two continuous maize systems, but decreased in CS-rec or remained unchanged in CS-int. N2 O emission peaks were mainly associated with high temperature and high soil water content resulting from rainfall or irrigation events, but less clearly related to soil NO3 -N levels. N2 O fluxes in intensively managed systems were only occasionally greater than those measured in the CC-rec and CS-rec systems. Fertilizer-induced N2 O emissions ranged from 1.9% to 3.5% in 2003, from 0.8% to 1.5% in 2004 and from 0.4% to 0.5% in 2005, with no consistent differences among the four systems. All four cropping systems where net sources of GHG. However, due to increased soil C sequestration continuous maize systems had lower GWP than maize–soybean systems and intensive management did not cause a significant increase in GWP. Converting maize grain to ethanol in the two continuous maize systems resulted in a net reduction in life cycle GHG emissions of maize ethanol relative to petrol-based gasoline by 33–38%. Our study provided evidence that net GHG emissions from agricultural systems can be kept low when management is optimized toward better exploitation of the yield potential. Major components for this included (i) choosing the right combination of adopted varieties, planting date and plant population to maximize crop biomass productivity, (ii) tactical water and nitrogen (N) management decisions that contributed to high N use efficiency and avoided extreme N2 O emissions, and (iii) a deep tillage and residue management approach that favored the build-up of soil organic matter from large amounts of crop residues returned.
FIRST PARAGRAPH: Global cycles of carbon (C) and nitrogen (N) are coupled through processes of terrestrial and marine biomass accumulation, decomposition, and storage. Alfred Redfield (1958) proposed that nearly constant carbon-to-nutrient ratios in marine phytoplankton and bacteria required that changes in one biogeochemical element be matched by changes in other essential elements. The "Redfield ratio" approach has proven valuable in understanding not only marine biogeochemistry (Broecker et al. 1979, Howarth 1988), but also carbon and nutrient cycles on land (Bolin and Cook 1983, Melillo and Gosz 1983, Reiners 1986, Rosswall 1981, Vitousek 1982). Both plant species diversity and their ability to produce varying amounts of structural material cause greater variation in carbon to nutrient ratios within terrestrial biomass than is found in the ocean (Vitousek et al. 1988). Yet nutrient limitation of net terrestrial primary production is still commonplace: In particular, the photosynthetic requirement for nitrogen, coupled with relatively low levels of available nitrogen in many terrestrial ecosystems, causes carbon uptake and storage on land to be tightly regulated by the nitrogen cycle (Vitousek and Howarth 1991).
Ball, T., Smith, K.A., Moncrieff, J.B. (2007). Effect of stand age on greenhouse gas fluxes from a Sitka spruce [Picea sitchensis (Bong.) Carr.] chronosequence on a peaty gley soil. Global Change Biology 13 (10): 2128-2142
ABSTRACT: The influence of forest stand age in aPicea sitchensis plantation on (1) soil fluxes of three greenhouse gases (GHGs – CO2 , CH4 and N2 O) and (2) overall net ecosystem global warming potential (GWP), was investigated in a 2-year study. The objective was to isolate the effect of forest stand age on soil edaphic characteristics (temperature, water table and volumetric moisture) and the consequent influence of these characteristics on the GHG fluxes. Fluxes were measured in a chronosequence in Harwood, England, with sites comprising 30- and 20-year-old second rotation forest and a site clearfelled (CF) some 18 months before measurement. Adjoining unforested grassland (UN) acted as a control. Comparisons were made between flux data, soil temperature and moisture data and, at the 30-year-old and CF sites, eddy covariance data for net ecosystem carbon (C) exchange (NEE). The main findings were: firstly, integrated CO2 efflux was the dominant influence on the GHG budget, contributing 93–94% of the total GHG flux across the chronosequence compared with 6–7% from CH4 and N2 O combined. Secondly, there were clear links between the trends in edaphic factors as the forest matured, or after clearfelling, and the emission of GHGs. In the chronosequence sites, annual fluxes of CO2 were lower at the 20-year-old (20y) site than at the 30-year-old (30y) and CF sites, with soil temperature the dominant control. CH4 efflux was highest at the CF site, with peak flux 491±54.5μg m−2 h−1 and maximum annual flux 18.0±1.1 kg CH4 ha−1 yr−1 . No consistent uptake of CH4 was noted at any site. A linear relationship was found between log CH4 flux and the closeness of the water table to the soil surface across all sites. N2 O efflux was highest in the 30y site, reaching 108±38.3μgN2 O-N m−2 h−1 (171μgN2 Om−2 h−1 ) in midsummer and a maximum annual flux of 4.7±1.2 kg N2 O ha−1 yr−1 in 2001. Automatic chamber data showed a positive exponential relationship between N2 O flux and soil temperature at this site. The relationship between N2 O emission and soil volumetric moisture indicated an optimum moisture content for N2 O flux of 40–50% by volume. The relationship between C:N ratio data and integrated N2 O flux was consistent with a pattern previously noted across temperate and boreal forest soils.
ABSTRACT: Mountain terrestrial and aquatic ecosystems are responsive to external drivers of change, especially climate change and atmospheric deposition of nitrogen (N). We explored the consequences of a temperature-warming trend on stream nitrate in an alpine and subalpine watershed in the Colorado Front Range that has long been the recipient of elevated atmospheric N deposition. Mean annual stream nitrate concentrations since 2000 are higher by 50% than an earlier monitoring period of 1991–1999. Mean annual N export increased by 28% from 2.03 kg N ha−1 yr−1 before 2000 to 2.84 kg N ha−1 yr−1 in Loch Vale watershed since 2000. The substantial increase in N export comes as a surprise, since mean wet atmospheric N deposition from 1991 to 2006 (3.06 kg N ha−1 yr−1 ) did not increase. There has been a period of below average precipitation from 2000 to 2006 and a steady increase in summer and fall temperatures of 0.12 °C yr−1 in both seasons since 1991. Nitrate concentrations, as well as the weathering products calcium and sulfate, were higher for the period 2000–2006 in rock glacier meltwater at the top of the watershed above the influence of alpine and subalpine vegetation and soils. We conclude the observed recent N increases in Loch Vale are the result of warmer summer and fall mean temperatures that are melting ice in glaciers and rock glaciers. This, in turn, has exposed sediments from which N produced by nitrification can be flushed. We suggest a water quality threshold may have been crossed around 2000. The phenomenon observed in Loch Vale may be indicative of N release from ice features such as rock glaciers worldwide as mountain glaciers retreat.
Bowden, R. D., Castro, M. S., Melillo, J. M., Steudler, P. A., Aber, J. D. (1993). Fluxes of greenhouse gases between soils and the atmosphere in a temperate forest following a simulated hurricane blowdown. Biogeochemistry 21 (2): 67-71
ABSTRACT: Fluxes of nitrous oxide (N2 O), carbon dioxide (CO2 ), and methane (CH4 ) between soils and the atmosphere were measured monthly for one year in a 77-year-old temperate hardwood forest following a simulated hurricane blowdown. Emissions of CO2 and uptake of CH4 for the control plot were 4.92 MT C ha−1 y−1 and 3.87 kg C ha−1 y−1 , respectively, and were not significantly different from the blowdown plot. Annual N2 O emissions in the control plot (0.23 kg N ha−1 y−1 ) were low and were reduced 78% by the blowdown. Net N mineralization was not affected by the blowdown. Net nitrification was greater in the blowdown than in the control, however, the absolute rate of net nitrification, as well as the proportion of mineralized N that was nitrified, remained low. Fluxes of CO2 and CH4 were correlated positively to soil temperature, and CH, uptake showed a negative relationship to soil moisture. Substantial resprouting and leafing out of downed or damaged trees, and increased growth of understory vegetation following the blowdown, were probably responsible for the relatively small differences in soil temperature, moisture, N availability, and net N mineralization and net nitrification between the control and blowdown plots, thus resulting in no change in CO2 or CH4 fluxes, and no increase in N2 O emissions.
ABSTRACT: An estimate of net carbon (C) pool changes and long-term C sequestration in trees and soils was made at more than 100 intensively monitored forest plots (level II plots) and scaled up to Europe based on data for more than 6000 forested plots in a systematic 16 km × 16 km grid (level I plots). C pool changes in trees at the level II plots were based on repeated forest growth surveys At the level I plots, an estimate of the mean annual C pool changes was derived from stand age and available site quality characteristics. C sequestration, being equal to the long-term C pool changes accounting for CO2 emissions because of harvest and forest fires, was assumed 33% of the overall C pool changes by growth. C sequestration in the soil were based on calculated nitrogen (N) retention (N deposition minus net N uptake minus N leaching) rates in soils, multiplied by the C/N ratio of the forest soils, using measured data only (level II plots) or a combination of measurements and model calculations (level I plots). Net C sequestration by forests in Europe (both trees and soil) was estimated at 0.117 Gton yr−1 , with the C sequestration in stem wood being approximately four times as high (0.094 Gton yr−1 ) as the C sequestration in the soil (0.023 Gton yr−1 ). The European average impact of an additional N input on the net C sequestration was estimated at approximately 25 kg C kg−1 N for both tree wood and soil. The contribution of an average additional N deposition on European forests of 2.8 kg ha−1 yr−1 in the period 1960–2000 was estimated at 0.0118 Gton yr−1 , being equal to 10% of the net C sequestration in both trees and soil in that period (0.117 Gton yr−1 ). The C sequestration in trees increased from Northern to Central Europe, whereas the C sequestration in soil was high in Central Europe and low in Northern and Southern Europe. The result of this study implies that the impact of forest management on tree growth is most important in explaining the C pool changes in European forests.
Fenn, M. E., Haeuber, R., Tonnesen, G. S., Baron, J. S., Grossman-Clarke, S., Hope, D., Jaffe, D. A., Copeland, S., Geiser, L., Rueth, H. M., Sickman, J. O. (2003). Nitrogen emissions, deposition, and monitoring in the western United States. BioScience 53 (4): 391-403
ABSTRACT: Nitrogen (N) deposition in the western United States ranges from 1 to 4 kilograms (kg) per hectare (ha) per year over much of the region to as high as 30 to 90 kg per ha per year downwind of major urban and agricultural areas. Primary N emissions sources are transportation, agriculture, and industry. Emissions of N as ammonia are about 50% as great as emissions of N as nitrogen oxides. An unknown amount of N deposition to the West Coast originates from Asia. Nitrogen deposition has increased in the West because of rapid increases in urbanization, population, distance driven, and large concentrated animal feeding operations. Studies of ecological effects suggest that emissions reductions are needed to protect sensitive ecosystem components. Deposition rates are unknown for most areas in the West, although reasonable estimates are available for sites in California, the Colorado Front Range, and central Arizona. National monitoring networks provide long-term wet deposition data and, more recently, estimated dry deposition data at remote sites. However, there is little information for many areas near emissions sources.
Gallo, M. E., Lauber, C. L., Cabaniss, S. E., Waldrop, M. P., Sinsabaugh, R. L., Zak, D. R. (2005). Soil organic matter and litter chemistry response to experimental N deposition in northern temperate deciduous forest ecosystems. Global Change Biology 11 (9): 1514-1521
ABSTRACT: The effects of atmospheric nitrogen (N) deposition on organic matter decomposition vary with the biochemical characteristics of plant litter. At the ecosystem-scale, net effects are difficult to predict because various soil organic matter (SOM) fractions may respond differentially. We investigated the relationship between SOM chemistry and microbial activity in three northern deciduous forest ecosystems that have been subjected to experimental N addition for 2 years. Extractable dissolved organic carbon (DOC), DOC aromaticity, C : N ratio, and functional group distribution, measured by Fourier transform infrared spectra (FTIR), were analyzed for litter and SOM. The largest biochemical changes were found in the sugar maple–basswood (SMBW) and black oak–white oak (BOWO) ecosystems. SMBW litter from the N addition treatment had less aromaticity, higher C : N ratios, and lower saturated carbon, lower carbonyl carbon, and higher carboxylates than controls; BOWO litter showed opposite trends, except for carbonyl and carboxylate contents. Litter from the sugar maple–red oak (SMRO) ecosystem had a lower C : N ratio, but no change in DOC aromaticity. For SOM, the C : N ratio increased with N addition in SMBW and SMRO ecosystems, but decreased in BOWO; N addition did not affect the aromaticity of DOC extracted from mineral soil. All ecosystems showed increases in extractable DOC from both litter and soil in response to N treatment. The biochemical changes are consistent with the divergent microbial responses observed in these systems. Extracellular oxidative enzyme activity has declined in the BOWO and SMRO ecosystems while activity in the SMBW ecosystem, particularly in the litter horizon, has increased. In all systems, enzyme activities associated with the hydrolysis and oxidation of polysaccharides have increased. At the ecosystem scale, the biochemical characteristics of the dominant litter appear to modulate the effects of N deposition on organic matter dynamics.
K. van Groenigen, J. Six, B. A. Hungate, M. de Graaff, N. van Breemen, C. van Kessel (2006). Element interactions limit soil carbon storage. Proceedings Of The National Academy Of Sciences Of The United States Of America 103 (17): 6571-6574
ABSTRACT: Rising levels of atmospheric CO2 are thought to increase C sinks in terrestrial ecosystems. The potential of these sinks to mitigate CO2 emissions, however, may be constrained by nutrients. By using metaanalysis, we found that elevated CO2 only causes accumulation of soil C when N is added at rates well above typical atmospheric N inputs. Similarly, elevated CO2 only enhances N2 fixation, the major natural process providing soil N input, when other nutrients (e.g., phosphorus, molybdenum, and potassium) are added. Hence, soil C sequestration under elevated CO2 is constrained both directly by N availability and indirectly by nutrients needed to support N2 fixation.
Hirsch, A. I., Michalak, A. M., Bruhwiler, L. M., Peters, W., Dlugokencky, E. J., Tans, P. P. (2006). Inverse modeling estimates of the global nitrous oxide surface flux from 1998–2001. Global Biogeochemical Cycles 20 (GB1008): doi:10.1029/2004GB002443
ABSTRACT: Measurements of nitrous oxide in air samples from 48 sites in the Cooperative Global Air Sampling Network made by NOAA/ESRL GMD CCGG (the Carbon Cycle Greenhouse Gases group in the Global Monitoring Division at the NOAA Earth System Research Laboratory in Boulder, Colorado) and the three-dimensional chemical transport model TM3 were used to infer global nitrous oxide fluxes and their uncertainties from 1998–2001. Results are presented for four semihemispherical regions (90°S–30°S, 30°S to equator, equator to 30°N, 30°N–90°N) and six broad “super regions” (Southern Land, Southern Oceans, Tropical Land, Tropical Oceans, Northern Land, and Northern Oceans). We found that compared to our a priori estimate (from the International Geosphere-Biosphere Programme's Global Emissions Inventory Activity), the a posteriori flux was much lower from 90°S–30°S and substantially higher from equator to 30°N. Consistent with these results, the a posteriori flux from the Southern Oceans region was lower than the a priori estimate, while Tropical Land and Tropical Ocean estimates were higher. The ratio of Northern Hemisphere to Southern Hemisphere fluxes was found to range from 1.9 to 5.2 (depending on the model setup), which is higher than the a priori ratio (1.5) and at the high end of previous estimates. Globally, ocean emissions contributed 26–36% of the total flux (again depending on the model setup), consistent with the a priori estimate (29%), though somewhat higher than some other previous estimates.
ABSTRACT: Field studies have shown that elevated CO2 can cause increased forest growth over the short term (<6 years) even in the face of N limitation. This is facilitated to some degree by greater biomass production per unit N uptake (lower tissue N concentrations), but more often than not, N uptake is increased with elevated CO2 as well. Some studies also show that N sequestration in the forest floor is increased with elevated CO2 . These findings raise the questions of where the “extra” N comes from and how long such growth increases can continue without being truncated by progressive N limitation (PNL). This paper reviews some of the early nutrient cycling literature that describes PNL during forest stand development and attempts to use this information, along with recent developments in soil N research, to put the issue of PNL with elevated CO2 into perspective. Some of the early studies indicated that trees can effectively “mine” N from soils over the long term, and more recent developments in soil N cycling research suggest mechanisms by which this might have occurred. However, both the early nutrient cycling literature and more recent simulation modeling suggest that PNL will at some point truncate the observed increases in growth and nutrient uptake with elevated CO2 , unless external inputs of N are increased by either N fixation or atmospheric deposition.
T. G. F. Kittel, N. A. Rosenbloom, T. H. Painter, D. S. Schimel (1995). The VEMAP integrated database for modelling United States ecosystem/vegetation sensitivity to climate change. Journal of Biogeography 22 (4/5): 857-862
ABSTRACT: For the Vegetation/Ecosystem Modelling and Analysis Project (VEMAP), we developed a model database of climate, soils and vegetation that was compatible with the requirements of three ecosystem physiology models and three vegetation life-form distribution models. A key constraint was temporal, spatial and physical consistency among data layers to provide these daily or monthly time step models with suitable common inputs for the purpose of model inter-comparison. The database is on a 0.5° latitude/longitude grid for the conterminous United States. The set has both daily and monthly representations of the same long-term climate. Daily temperature and precipitation were stochastically simulated with WGEN and daily solar radiation and humidity empirically estimated with CLIMSIM. We used orographically adjusted precipitation, surface temperature and surface windspeed monthly means to maintain consistency among these fields and with vegetation distribution. Vegetation classes were based on physiognomic and physiological properties that influence biogeochemical dynamics. Soils data include characteristics of the 1-4 dominant soils per cell to account for subgrid variability.
ABSTRACT: The Catskill Mountains of southeastern New York receive relatively high rates of atmospheric N deposition, and NO3 − concentrations in some streams have increased dramatically since the late 1960s. We measured the chemistry of 39 first- and second-order streams with forested watersheds to determine the variability of nitrogen concentrations within the Catskill Mountain area. We found that some streams have low NO3 − concentrations throughout the year, some have seasonal cycles of varying amplitude, and some have relatively high concentrations year round. If the concentration and seasonality of NO3 − in stream water are used as indices of nitrogen saturation, then most stages of nitrogen saturation are evident in our survey of Catskill watersheds. Organic nitrogen was a small portion of the total nitrogen for streams with high NO3 − concentration, but organic N was the dominant form of N (up to 73% of the total) in the streams with lowest nitrate. Estimated retention of N in these watersheds (based on total N in stream water) ranged from 49% to 90% of the atmospheric input. The variation in stream water NO3 − concentration and the amplitude of the seasonal fluctuations did not appear to be attributable to differences among watersheds in atmospheric deposition, watershed topography, or groundwater influx to the stream. We hypothesize that differences among watersheds in forest species composition and forest history, which are interrelated, produce most of the variation in NO3 − concentration that we observed.
ABSTRACT: The capability of terrestrial ecosystems to sequester carbon (C) plays a critical role in regulating future climatic change yet depends on nitrogen (N) availability. To predict long-term ecosystem C storage, it is essential to examine whether soil N becomes progressively limiting as C and N are sequestered in long-lived plant biomass and soil organic matter. A critical parameter to indicate the long-term progressive N limitation (PNL) is net change in ecosystem N content in association with C accumulation in plant and soil pools under elevated CO2 . We compiled data from 104 published papers that study C and N dynamics at ambient and elevated CO2 . The compiled database contains C contents, N contents, and C:N ratio in various plant and soil pools, and root:shoot ratio. Averaged C and N pool sizes in plant and soil all significantly increase at elevated CO2 in comparison to those at ambient CO2 , ranging from a 5% increase in shoot N content to a 32% increase in root C content. The C and N contents in litter pools are consistently higher in elevated than ambient CO2 among all the surveyed studies whereas C and N contents in the other pools increase in some studies and decrease in other studies. The high variability in CO2 -induced changes in C and N pool sizes results from diverse responses of various C and N processes to elevated CO2 . Averaged C:N ratios are higher by 3% in litter and soil pools and 11% in root and shoot pools at elevated relative to ambient CO2 . Elevated CO2 slightly increases root:shoot ratio. The net N accumulation in plant and soil pools at least helps prevent complete down-regulation of, and likely supports, long-term CO2 stimulation of C sequestration. The concomitant C and N accumulations in response to rising atmospheric CO2 may reflect intrinsic nature of ecosystem development as revealed before by studies of succession over hundreds to millions of years.
Luo, Y., Su, B., Currie, W.S., Dukes, J,S., Finzi, A., Hartwig, U., Hungate, B. A., McMurtrie, R.E., Oren, R., Parton, W.J., Pataki, D.E., Shaw, M.R., Zak, D. R., Field, C. B. (2004). Progressive nitrogen limitation of ecosystem responses to rising atmospheric carbon dioxide. BioScience 54 (8): 731-739
ABSTRACT: A highly controversial issue in global biogeochemistry is the regulation of terrestrial carbon (C) sequestration by soil nitrogen (N) availability. This controversy translates into great uncertainty in predicting future global terrestrial C sequestration. We propose a new framework that centers on the concept of progressive N limitation (PNL) for studying the interactions between C and N in terrestrial ecosystems. In PNL, available soil N becomes increasingly limiting as C and N are sequestered in long-lived plant biomass and soil organic matter. Our analysis focuses on the role of PNL in regulating ecosystem responses to rising atmospheric carbon dioxide concentration, but the concept applies to any perturbation that initially causes C and N to accumulate in organic forms. This article examines conditions under which PNL may or may not constrain net primary production and C sequestration in terrestrial ecosystems. While the PNL-centered framework has the potential to explain diverse experimental results and to help researchers integrate models and data, direct tests of the PNL hypothesis remain a great challenge to the research community.
ABSTRACT: The concept that nitrogen (N) availability can limit plant productivity is well established based on (1) N fertilization that stimulates productivity and (2) increases in productivity along gradients of soil fertility. Nitrogen limitations to plant productivity are regulated by processes such as mineralization, immobilization, and plant physiological adjustments. However, this production-centric perspective might not fully explain patterns in carbon (C) sequestration in terrestrial ecosystems. Carbon sequestration involves both plant and soil pools. The plant pool, which is the main concern of production research, can be much smaller than the soil pool. To quantify terrestrial C sequestration, therefore, we have to develop an ecosystem perspective to examine how C and N interact in both plant and soil pools.
Due to fossil fuel burning and deforestation, atmospheric CO2 concentration has increased by approximately 35% since the Industrial Revolution. In general, elevated CO2 enhances photosynthesis and stimulates initial C sequestration in terrestrial ecosystems. How sustainable the CO2 -induced C sequestration can be depends, in part, on ecosystem N availability and supply. Thus, the interdependence of C and N cycles is an issue that is not only interesting to ecologists, but also has important implications for global change policy.
Increased C influx into an ecosystem under elevated CO2 generally requires more N to support plant growth than is required at ambient CO2 and, in turn, sequesters N into long-lived plant biomass and soil organic matter pools. This N sequestration can decrease soil N availability for plant uptake and lead to progressive N limitation (PNL) over time. The PNL hypothesis states that N sequestration in long-term organic matter pools will, without new N input and/or decreases in N losses, lead to a decline in mineral N availability over time at elevated CO2 compared to ambient CO2 . On the other hand, increased plant N demand and/or sequestration could induce changes in N supply. When elevated CO2 increases N use efficiency (NUE) and stimulates N transfer from the soil organic pools with narrow C:N ratios to plants with broad C:N ratios, PNL may be delayed. If additional C input at elevated CO2 stimulates capital gain of N through fixation, decreased losses, increased forage for soil N, or any combinations of them, PNL may not occur. If it does, CO2 -induced C sequestration in ecosystems declines over time. In short, N will constrain C sequestration over time unless additional C input at elevated CO2 stimulates N gain in ecosystems.
This Special Feature consists of six papers that examine various aspects of PNL against field data collected from ecosystems that have been exposed to elevated CO2 treatments. The first two papers show sustained CO2 stimulation of net primary production (NPP) in forest ecosystems. Norby and Iversen present data from a sweetgum forest stand in Oak Ridge, Tennessee, that has been exposed to free-air CO2 enrichment (FACE) for six years. The sustained CO2 stimulation of NPP was associated primarily with increased N uptake, since NUE did not change significantly under elevated CO2 . Sufficient N supply from soil at Oak Ridge may help delay or even avoid PNL as elevated CO2 substantially stimulated root growth to explore N sources in deeper soil layers. At the Duke Forest FACE site, Finzi and colleagues demonstrate that the CO2 stimulation of NPP was sustained at 18–24% during the first six years of the experiment. Sustained NPP stimulation occurred together with significantly more N uptake by trees and higher NUE at elevated than at ambient CO2 . Their mass balance analysis shows that significantly more N accumulated at elevated CO2 in plants and in forest floor litter. The forest ecosystem accrued N capital at an average rate of 12 g N·m−2·yr−1, perhaps due to N uptake from deeper in the soil profile.
However, PNL of plant growth and C sequestration appears to occur in a scrub-oak ecosystem, Florida (Hungate et al.), and a C3/C4 grassland, Texas (Gill et al.) in response to elevated CO2 . Initial CO2 stimulation of plant biomass growth was supported by more N uptake from soil in the scrub-oak ecosystem. As N was accumulated in plant biomass and litter layers in the O horizon in years 4–7, soil N availability progressively declined, as did the CO2 stimulation of plant growth. Initially, PNL of plant growth was avoided by increased N uptake from the soil and alleviated later through increased NUE. Elevated CO2 did not change total ecosystem N content but caused a redistribution of N from the mineral soil to plants and litter. In the Texas grassland, increased CO2 along a gradient from 200 to 560μmol/mol also caused reallocation of N from soil to plant and from more recalcitrant to more labile fractions within the soil. The N reallocation alleviates PNL and allows plant production to increase with increasingCO2 . However, it does not support much long-term C sequestration in the soil at elevatedCO2 , since the C gained from increased plant production can be rapidly lost through decomposition.Results at experimental sites are often highly variable. To synthesize results from multiple sites, Luo et al. conducted a meta-analysis of data from 104 published papers and found significant increases in C and N contents on average in all the plant and soil pools under elevatedCO2 . The net N accumulation in plant and soil pools at least helps prevent complete down-regulation of, and likely supports, long-termCO2 stimulation of C sequestration. The net C and N accumulations under elevatedCO2 are consistent with C and N dynamics during succession over hundreds to millions of years, suggesting that ecosystems may have intrinsic capabilities to stimulate N accumulation by C input. Johnson reviews the early nutrient cycling literature related to PNL during forest stand development and more recent studies on C and N interactions under elevatedCO2 . In general, trees can“mine” N from soils over the long term, but PNL will constrain CO2 stimulation of plant growth unless external inputs of N are increased by N fixation or atmospheric deposition.
The six papers in this Special Feature provide experimental evidence on ecosystem C and N interactions but do not fully resolve the issue of whether N constrains the C cycle or additional C input stimulates the N cycle in response to elevated CO2 . Against the backdrop of diverse responses in nature, the challenge is how we can incorporate the diverse mechanisms of C and N interactions into models to predict future C sequestration. In the end, we hope this Special Feature will stimulate research to test the PNL hypothesis further and advance our understanding of the biogeochemical coupling of C and N cycles.
A. H. Magill, J. D. Aber, J. J. Hendricks, R. . Bowden, J. M. Melillo, P. A. Steudler (1997). Biogeochemical response of forest ecosystems to simulated chronic nitrogen deposition. Ecologcial Applications 7 (2): 402-415
ABSTRACT: Reported in this paper are foliar chemistry, tree growth (above- and belowground), soil chemistry, nitrogen cycling (net mineralization and nitrification) and soil N2 O flux responses to the first 6 yr of chronic nitrogen amendments at the Harvard Forest (Massachusetts, USA). A 70-yr-old red pine (Pinus resinosa Ait.) stand and a 50-yr-old mixed hardwood stand received control, low nitrogen (50 kg·ha−1 ·yr−1 ), high nitrogen (150 kg·ha−1 ·yr−1 ), and low nitrogen plus sulfur treatments, with additions occurring in six equal doses over the growing season as NH4 NO3 and Na2 SO4 . Foliar N concentrations increased up to 25% in the hardwood stand and 67% in the pines, and there was no apparent decrease of N retranslocation due to fertilization. Wood production increased in the hardwood stand in response to fertilization but decreased in the pine stand. Fine-root nitrogen concentrations increased with N additions, and fine roots were a significant sink for added nitrogen. Nitrate leaching losses increased continuously over the 6-yr period in the treated pine stands but remained insignificant in the hardwoods. Annual net N mineralization increased substantially in response to treatments in both stands but declined in the pine high-N plot by the end of year six. Net nitrification increased from 17% of net mineralization in 1988 to 51% in 1993 for the pine high-N plot. Only a slight increase in net nitrification was measured in the hardwood stand, and only in 1993. Extractable NH4 was consistently higher in treated plots than in controls in both stands, where extractable NO3 was higher than controls only in the treated pine plots. Soil extracts yielded <1.5 kg/ha of NO3 -N for all plots in the hardwood stand throughout the experiment. Effluxes of N2 O were consistently greater in the pine high-N plot than in the pine control plot, but there were no observed large-scale increases in N2 O emissions immediately following fertilizer application. Calculated nitrogen budgets for the first 6 yr showed extremely high N retention (85–99%). Of the retained N, 50–83% appears to be in the long-term, recalcitrant soil pool. The relative importance of biotic and abiotic mechanisms of N incorporation into soils remains uncertain. Size, kinetics, and uptake capacity of this soil pool are critical and largely unknown factors determining ecosystem response to increased N loading and may be related to land-use history.
Manning, P., Newington, J. E., Robson, H. R., Saunders, M., Eggers, T., Bradford, M. A., Bardgett, R. D., Bonkowski, M., Ellis, R. J., Gange, A. C., Grayston, S. J., Kandeler, E., Marhan, S., Reid, E., Tscherko, D., Godfray, H. C. J., Rees, M. (2006). Decoupling the direct and indirect effects of nitrogen deposition on ecosystem function. Ecology Letters 9 (9): 1015-1024
ABSTRACT: Elevated nitrogen (N) inputs into terrestrial ecosystems are causing major changes to the composition and functioning of ecosystems. Understanding these changes is challenging because there are complex interactions between 'direct' effects of N on plant physiology and soil biogeochemistry, and 'indirect' effects caused by changes in plant species composition. By planting high N and low N plant community compositions into high and low N deposition model terrestrial ecosystems we experimentally decoupled direct and indirect effects and quantified their contribution to changes in carbon, N and water cycling. Our results show that direct effects on plant growth dominate ecosystem response to N deposition, although long-term carbon storage is reduced under high N plant-species composition. These findings suggest that direct effects of N deposition on ecosystem function could be relatively strong in comparison with the indirect effects of plant community change.
Mansson, K. F., Falkengren-Grerup, U. (2003). The effect of nitrogen deposition on nitrification, carbon and nitrogen mineralisation and litter C : N ratios in oak (Quercus robur L.) forests. Forest Ecology and Management 179 (1-3): 455-467
ABSTRACT: The present study addresses the question why there is a positive relationship between nitrogen deposition and potential net nitrogen mineralisation and nitrification in oak (Quercus robur L.) forest soils in south Sweden (Falkengren-Grerup et al., 1998), and how this is related to the carbon mineralisation. We tested three hypotheses based on European studies ( Persson et al., 2000a and Persson et al., 2000b) that postulate lower availability of carbon due to chemical binding of nitrogen to lignin remains and phenolic compounds or a more decomposed, recalcitrant organic matter due to faster initial decomposition rates of fresh litter. This in turn leads to increased net nitrogen mineralisation, and nitrifiers that may adapt to acid soils when ammonium availability increases. We used soils from two regions exposed to a total deposition of 17 and 10 kg N ha−1 per year and incubated the soils in the laboratory separately as well as in mixtures between the regions. To be able to evaluate how the microbial communities and organic matter interacted in the soil mixtures, we divided the observed values of the net carbon and nitrogen mineralisation and nitrification for the soil mixtures by the calculated expected values. C:N ratios of litter, fresh leafs ofDeschampsia flexuosa and microbial biomass were also measured. Contrary to the assumptions in the hypothesis, the soil respiration was somewhat higher in soils subjected to high nitrogen deposition. Furthermore, the observed rate of nitrogen mineralisation was higher than expected in the majority of soil mixtures, while observed rates of carbon mineralisation only showed a weak tendency to be higher than expected. All the results taken together indicate that there has been a positive change in litter quality that leads to increased carbon and nitrogen mineralisation. This conclusion is supported by the C:N ratio of oak litter and fresh leaves ofDeschampsia flexuosa that was lower in the most nitrogen-exposed sites and which might indicate an increase in decomposability. The observed values of nitrification were significantly higher than the calculated expected values. Thus, the increased net nitrogen mineralisation in the region with high nitrogen deposition seems to allow nitrifiers to adapt to these acid soils when they are no longer limited by ammonium.
McGuire, A.D., Melillo, J.M., Kicklighter, D.W., Pan, Y., Xiao, X., Helfrich, J., Moore, B., III, Vorosmarty, C.J., Schloss, A.L. (1997). Equilibrium responses of global net primary production and carbon storage to doubled atmospheric carbon dioxide: Sensitivity to changes in vegetation nitrogen concentration. Global Biogeochemical Cycles 11 (2): 173-189
ABSTRACT: We ran the terrestrial ecosystem model (TEM) for the globe at 0.5° resolution for atmospheric CO2 concentrations of 340 and 680 parts per million by volume (ppmv) to evaluate global and regional responses of net primary production (NPP) and carbon storage to elevated CO2 for their sensitivity to changes in vegetation nitrogen concentration. At 340 ppmv, TEM estimated global NPP of 49.0 1015 g (Pg) C yr−1 and global total carbon storage of 1701.8 Pg C; the estimate of total carbon storage does not include the carbon content of inert soil organic matter. For the reference simulation in which doubled atmosphericCO2 was accompanied with no change in vegetation nitrogen concentration, global NPP increased 4.1 Pg C yr−1 (8.3%), and global total carbon storage increased 114.2 Pg C. To examine sensitivity in the global responses of NPP and carbon storage to decreases in the nitrogen concentration of vegetation, we compared doubled CO2 responses of the reference TEM to simulations in which the vegetation nitrogen concentration was reduced without influencing decomposition dynamics ("lower N" simulations) and to simulations in which reductions in vegetation nitrogen concentration influence decomposition dynamics ("lower N+D" simulations). We conducted three lower N simulations and three lower N+D simulations in which we reduced the nitrogen concentration of vegetation by 7.5, 15.0, and 22.5%. In the lower N simulations, the response of global NPP to doubled atmospheric CO2 increased approximately 2 Pg C yr−1 for each incremental 7.5% reduction in vegetation nitrogen concentration, and vegetation carbon increased approximately an additional 40 Pg C, and soil carbon increased an additional 30 Pg C, for a total carbon storage increase of approximately 70 Pg C. In the lower N+D simulations, the responses of NPP and vegetation carbon storage were relatively insensitive to differences in the reduction of nitrogen concentration, but soil carbon storage showed a large change. The insensitivity of NPP in the N+D simulations occurred because potential enhancements in NPP associated with reduced vegetation nitrogen concentration were approximately offset by lower nitrogen availability associated with the decomposition dynamics of reduced litter nitrogen concentration. For each 7.5% reduction in vegetation nitrogen concentration, soil carbon increased approximately an additional 60 Pg C, while vegetation carbon storage increased by only approximately 5 Pg C. As the reduction in vegetation nitrogen concentration gets greater in the lower N+D simulations, more of the additional carbon storage tends to become concentrated in the north temperate-boreal region in comparison to the tropics. Other studies with TEM show that elevated CO2 more than offsets the effects of climate change to cause increased carbon storage. The results of this study indicate that carbon storage would be enhanced by the influence of changes in plant nitrogen concentration on carbon assimilation and decomposition rates. Thus changes in vegetation nitrogen concentration may have important implications for the ability of the terrestrial biosphere to mitigate increases in the atmospheric concentration of CO2 and climate changes associated with the increases.
ABSTRACT: As a consequence of chronically high N depositions in forest ecosystems, the C-to-N ratio of forest floors has narrowed in many forest ecosystems. This might affect the sequestration of soil C and the partitioning of C during decomposition. We investigated samples from Oa layers of 15 different forest floors under Norway spruce (Picea abies Karst. L.) with varying C-to-N ratios in respect to soil respiration, N mineralization and dissolved organic carbon (DOC) release under standardized laboratory conditions. Samples were incubated aerobically at 15 °C and water holding capacity over a period of 10 months. Soil respiration decreased significantly with decreasing C-to-N and increasing N content. The release of DOC increased with increasing C-to-N ratio, while N-mineralization was not affected by C-to-N ratio and N content. Our results support the hypothesis that low C-to-N ratios in later stages of decomposition stabilize soil organic matter and that chronically high N deposition will lead to increased accumulation of C in forest floors.
ABSTRACT: The interactions between the biotic processes of reproduction, growth, and death and the abiotic processes which regulate temperature and water availability, and the interplay between the biotic and abiotic processes regulating N and light availabilities are important in the dynamics of forest ecosystems. We have developed a computer simulation that assembles a model ecosystem which links these biotic and abiotic interactions through equations that predict decomposition processes, actual evapo-transpiration, soil water balance, nutrient uptake, growth of trees, and light penetration through the canopy. The equations and parameters are derived directly from field studies and observations of forests in eastern North America, resulting in a model that can make accurate quantitative predictions of biomass accumulation, N availability, soil humus development and net primary production.
Pregitzer, K. S., Zak, D. R., Burton, A. J., Ashby, J. A., MacDonald, N. W. (2004). Chronic nitrate additions dramatically increase the export of carbon and nitrogen from northern hardwood ecosystems. Biogeochemistry 68 (2): 179-197
ABSTRACT: A long-term field experiment was initiated to simulate chronic atmospheric N deposition, a widespread phenomenon in industrial regions of the world. Eight years of experimental nitrate (NO3 - ) additions (3 g NO3 -- N m-2 per year) to four different northern hardwood forests located along a 500 km geographic gradient dramatically increased leaching losses of NO3 -- N, dissolved organic carbon (DOC), and dissolved organic nitrogen (DON). During the last two water years, the average increase in solution NO3 -- N and DON leaching from the NO3 - amended plots was 2.2 g N m-2 , equivalent to 72% of the annual experimental N addition. Results indicate that atmospheric N deposition may rapidly saturate some northern hardwood ecosystems across an entire biome in the upper Great Lakes Region of the USA. Changes in soil C and N cycling induced by chronic N deposition have the potential in this landscape to significantly alter the flux of DOC and DON from upland to aquatic ecosystems. Michigan Gradient study site characteristics are similar to those of European forests most susceptible to N saturation.
ABSTRACT: We have investigated the storage and spatial distribution of soil nitrogen (N) in China based on a data set of 2480 soil profiles and a map of Chinese soil types at a spatial resolution of 1:1,000,000. Our estimate indicates that the total N storage in China is 8.29x1015 g, representing 5.9-8.7% of the total global N storage. The total N storage in China is on average or slightly above the average of its share in the global N storage, even though low nitrogen content soils cover a large area in China. N density varies substantially with soil types and regions. Peat soils in the southeast of Tibet, southwest China, show the highest averaged N density with a value of 7314.9 g/m3 among all soil types. This is more than 30 times of the lowest N density of brown desert soils in the western desert and arid region. The highest N storages among all the soil types are the felty soil in southeast of Tibet, dark-brown earths in northeast China, and red earths in southeast China with values of 921.1, 611.4, and 569.6 Tg, respectively. N density also varies with land cover types in China. Wetlands in southwest China exhibit the highest N density at 6775.9 g/m3 and deserts in northwest China have the least at 447.5 g/m3 . Our analysis also indicates that land cover types are poor predictors of N content. Further research is needed to examine how transformation from organic agriculture to increased use of fertilizers and pesticides has influenced N storage in China.