Spatial and temporal patterns in terrestrial carbon storage due to deposition of anthropogenic nitrogen
Fertilization of the biosphere by nitrogen deposition represents an important connection between atmospheric chemistry and the global carbon cycle. We describe a modeled estimate of terrestrial carbon storage arising from deposition of nitrogen derived from fossil fuels that accounts for spatial distributions in deposition and vegetation types, turnover of plant and soil carbon pools, and the cumulative effects of deposition, Vegetation type has a pronounced effect on C uptake; the combination of high C:N ratios and long lifetimes in wood may create a significant sink in forests, but much of the nitrogen falls on cultivated areas and grasslands, where there is limited capacity for long-term carbon storage. We estimate 1990 net carbon uptake due to deposition of fossil-fuel N to be between 0.3 and 1.3 Pg C/yr [1 Pg = 10(15) g], depending on the fraction of C allocated to wood, with a best estimate of 0.44-0.74 Pg/yr. Cumulative C storage since 1845 is estimated to be about 25% of the proposed terrestrial sink for anthropogenic CO2. Continued exposure to high N deposition, however, will decrease the extent of N limitation in terrestrial ecosystems, thereby limiting the persistence of any N-derived carbon sink.
Mechanisms of shrubland expansion: Land use, climate, or CO2
Encroachment of trees and shrubs into grasslands and the ‘thicketization’ of savannas has occurred worldwide over the past century. These changes in vegetation structure are potentially relevant to climatic change as they may be indicative of historical shifts in climate and as they may influence biophysical aspects of land surface-atmosphere interactions and alter carbon and nitrogen cycles. Traditional explanations offered to account for the historic displacement of grasses by woody plants in many arid and semi-arid ecosystems have centered around changes in climatic, livestock grazing and fire regimes. More recently, it has been suggested that the increase in atmospheric CO2 since the industrial revolution has been the driving force. In this paper we evaluate the CO2 enrichment hypotheses and argue that historic, positive correlations between woody plant expansion and atmospheric CO2 are not cause and effect.
On the contribution of biosphere CO2 fertilization to the missing sink
A gridded biospheric carbon model is used to investigate the impact of the atmospheric CO2 increase on terrestrial carbon storage. The analysis shows that the calculated CO2 fertilization sink is dependent not just on the mathematical formulation of the “β factor” but also on the relative controls of net primary productivity (NPP), carbon residence times, and resource availability. The modeled evolution of the biosphere for the period 1850–1990 shows an increasing lag between NPP and the heterotrophic respiration. The time evolution of the modeled biospheric sink (i.e., difference between enhanced NPP and enhanced respiration) does not match that obtained by deconvolution of the ice core CO2 time series. Agreement between the two is reasonable for the first half of the period, but during the recent decades the deconvoluted CO2 increase is much too fast to be explained by the CO2 fertilization effect only. Therefore other mechanisms than CO2 fertilization should also contribute to the missing sink. Our results suggest that about two thirds to three fourths of the 1850–1990 integrated missing sink is due to the CO2 greening of the biosphere. The remainder may be due to the increased level of nitrogen deposition starting around 1950.
Variability in temperature regulation of CO2 fluxes and N mineralization from five Hawaiian soils: Implications for a changing climate
We examined the possibility that microbial adaptation to temperature could affect rates of CO2, N2O and CH4 release from soils. Laboratory incubations were used to determine the functional relationship between temperature and CO2, N2O and CH4 fluxes for five soils collected across an elevational range in Hawaii. Initial rates of CO2 production and net N mineralization increased exponentially from 15 °C to 55 °C; initial rates of CH4 and N2O release were more complex. No optimum temperature (in which rates decline at higher and lower temperatures) was apparent for any of the gases, but respiration declined with time at higher temperatures, suggesting rapid depletion of readily available substrate. Mean Q10S for respiration varied from 1.4 to 2.0, a typical range for tropical soils. The functional relationship between CO2 production and temperature was consistent among all five soils, despite the substantial differences in mean annual temperature, soils, and land-use among the sites. Temperature responses of N2O and CH4 fluxes did not follow simple Q10 relationships suggesting that temperature functions developed for CO2 release from heterotrophic respiration cannot be simply extrapolated. Expanding this study to tropical heterotrophic respiration, the flux is more sensitive to changes in Q10 than to changes in temperature on a per unit basis: the partial derivative with respect to temperature is 2.4 Gt C ·° C−1 with respect to Q10, it is 3.5 Gt C · Q10 unit−1. Therefore, what appears to be minor variability might still produce substantial uncertainty in regional estimates of gas exchange.
Analysis of nitrogen saturation potential in Rocky Mountain tundra and forest: Implications for aquatic systems
We employed grass and forest versions of the CENTURY model under a range of N deposition values (0.02–1.60 g N m–2 y–1) to explore the possibility that high observed lake and stream N was due to terrestrial N saturation of alpine tundra and subalpine forest in Loch Vale Watershed, Rocky Mountain National Park, Colorado. Model results suggest that N is limiting to subalpine forest productivity, but that excess leachate from alpine tundra is sufficient to account for the current observed stream N. Tundra leachate, combined with N leached from exposed rock surfaces, produce high N loads in aquatic ecosystems above treeline in the Colorado Front Range. A combination of terrestrial leaching, large N inputs from snowmelt, high watershed gradients, rapid hydrologic flushing and lake turnover times, and possibly other nutrient limitations of aquatic organisms constrain high elevation lakes and streams from assimilating even small increases in atmospheric N. CENTURY model simulations further suggest that, while increased N deposition will worsen the situation, nitrogen saturation is an ongoing phenomenon.
Fluxes of nitrous oxide and methane from nitrogen-amended soils in a Colorado alpine ecosystem
In order to determine the effect of increased nitrogen inputs on fluxed of N2O and CH4 from alpine soils, we measured fluxes of these gases from fertilized and unfertilized soils in wet and dry alpine meadows. In the dry meadow, the addition of nitrogen resulted in a 22-fold increase in N2O emissions, while in the wet meadow, we observed a 45-fold increase in N2O emission rates. CH4 uptake in the dry meadow was reduced 52% by fertilization; however, net CH4 production occurred in all the wet meadow plots and emission rates were not significantly affected by fertilization. Net nitrification rates in the dry meadow were higher in fertilized plots than in non-fertilized plots throughout the growing season; net mineralization rates in fertilized dry meadow pots were higher than those in non-fertilized plots during the latter half of the growing season.
Climatic, edaphic and biotic controls over storage and turnover of carbon in soils
Soil carbon, a major component of the global carbon inventory, has significant potential for change with changing climate and human land use. We applied the Century ecosystem model to a series of forest and grassland sites distributed globally to examine large-scale controls over soil carbon. Key site-specific parameters influencing soil carbon dynamics are soil texture and foliar lignin content; accordingly, we perturbed these variables at each site to establish a range of carbon concentrations and turnover times. We examined the simulated soil carbon stores, turnover times, and C:N ratios for correlations with patterns of independent variables. Results showed that soil carbon is related linearly to soil texture, increasing as clay content increases, that soil carbon stores and turnover time are related to mean annual temperature by negative exponential functions, and that heterotrophic respiration originates from recent detritus (~50%), microbial turnover (~30%), and soil organic matter (~20%) with modest variations between forest and grassland ecosystems. The effect of changing temperature on soil organic carbon (SOC) estimated by Century is dSOC/dT=183e-0.034T.Global extrapolation of this relationship leads to an estimated sensitivity of soil C storage to a temperature of -11.1 Pg°C-1, excluding extreme arid and organic soils.In Century, net primary production (NPP) and soil carbon are closely coupled through the N cycle, so that as temperatures increase, accelerated N release first results in fertilization response, increasing C inputs. The Century-predicted effect of temperature on carbon storage is modified by as much as 100% by the N cycle feedback. Century-estimated soil C sensitivity (-11.1 Pg°C-1) is similar to losses predicted with a simple data-based calculation (-14.1 Pg°C-1). Inclusion of the N cycle is important for even first-order predictions of terrestrial carbon balance. If the NPP-SOC feedback is disrupted by land use or other disturbances, then SOC sensitivity can greatly exceed that estimated in our simulations. Century results further suggest that if climate change results in drying of organic soils (peats), soil carbon loss rates can be high.
Controls on methane flux from terrestrial ecosystems in Agricultural Ecosystem Effects on Trace Gases and Global Climate Change. L. Harper, A. Mosier, J. M. Duxbury, and D. Rolston, (eds.)
This chapter identifies some of the key controls on Methane (CH4) flux between terrestrial ecosystems and the atmosphere, and discusses new research that begins to address them. It focuses on CH4 production, CH4 consumption, their interactions and the role of plant and soil-mediated transport to the atmosphere. CH4 consumption occurs in all aerobic soils and can serve two distinct ecological roles. First, CH4 consumption in the aerobic surface soil and rhizosphere of wetlands can provide a barrier to efflux of CH4 produced at depth, and if the water table drops far enough it can become a sink. Second, CH4 consumption in upland soils acts as a net sink for atmospheric CH4. Predicting changes in CH4 over time requires incorporating the major controls on CH4 consumption into large-scale models. Nitrogen availability appears to be an important control on CH4 consumption in upland systems, while O2 may be important in wetlands.
Tropical soils could dominate the short-term carbon cycle feedbacks to increased global temperatures
Results of a simple model of the effects of temperature on net ecosystem production call into question the argument that the large stocks of soil carbon and greater projected warming in the boreal and tu ndra regions of the world will
lead to rapid efflux of carbon from these biomes to the atmosphere. We show that low rates of carbon turnover in these regions and a relatively greater response of net primary production to changes in temperature may lead to carbon storage over some limited range of warming. In contrast, the high rates of soil respiration found in tropical ecosystems are highly sensitive to small changes in temperature, so that despite the less pronounced warming expected in equatorial regions, tropical soils are likely to release relatively large amounts of carbon to the atmosphere. Results for high-latitude biomes are highly sensitive to parameter values used, while the net efflux of carbon from the tropics appears robust.