B33F-01
Consequences of Carbon-Nitrogen Interactions on the Feedbacks Between Climate and the Terrestrial Carbon Cycle
Most earth system models used in climate-change assessments do not consider the influence of nitrogen availability on terrestrial carbon sequestration. We explore how carbon-nitrogen interactions in terrestrial ecosystems affect feedbacks to the climate system using the MIT Integrated Global Systems Model (IGSM) with two different versions of its terrestrial ecosystems sub-model, the Terrestrial Ecosystems Model (TEM): one that considers carbon-nitrogen interactions (CN-TEM) and one that considers only carbon dynamics (C-TEM). Nitrogen constraints on CO2 fertilization cause the terrestrial biosphere simulated by the CN-TEM to take up less atmospheric carbon than that simulated by C-TEM, resulting in a larger increase in atmospheric CO2 concentration and warmer temperatures for a given amount of anthropogenic carbon emitted. Furthermore, consideration of carbon-nitrogen interactions also changes the sign of the carbon feedback with climate change. In the simulations with C-TEM, surface warming significantly reduces carbon sequestration in both vegetation and soil, leading to a positive carbon-cycle feedback to the climate system similar to that found by other earth system models. However, in simulations with CN-TEM, the increased decomposition of soil organic matter with higher temperatures releases soil nitrogen to stimulate plant growth and carbon storage in the vegetation that is greater than the carbon lost from soil. As a result, sequestration of carbon in terrestrial ecosystems increases, in comparison to the fixed climate case, and the carbon cycle feedback to the climate system becomes negative for much of the next three centuries. Consideration of carbon-nitrogen interactions should be included in future assessments of climate-change impacts.
B33F-02 INVITED
A synergistic effect of anthropogenic N and CO2 on land carbon uptake and its implications for global warming
Increased carbon uptake of land in response to elevated atmospheric CO2 concentration and nitrogen deposition could offset the future rate of increase in CO2 levels and mitigate climate warming. None of the coupled climate- carbon cycle models used for analysis of climate-biogeochemistry feedbacks in the C4MIP intercomparison and the Fourth IPCC Assessment Report explicitly treats nitrogen cycle. Using a coupled model of climate, ocean, and land biogeochemistry driven by SRES scenarios of CO2 emissions and anthropogenic N deposition, we show that neither increasing nitrogen deposition nor physiological effect of CO2 alone can enhance global carbon uptake considerably to dampen the rising atmospheric CO2 concentration. Atmospheric nitrogen deposition and atmospheric CO2 have however a strong synergetic effect on the carbon uptake of land (~30 percent). We show that this synergetic effect increases strongly carbon uptake of land and has a potential to reduce atmospheric CO2 concentration by 30-40 ppmv by 2030. This potential depends on prevailing ecosystem types in areas with high nitrogen deposition and their responses to simultaneous increases in atmospheric CO2 and nitrogen deposition.
B33F-03
Examining the Role of Nitrogen Cycling in the Terrestrial Response to CO2, Climate, and Nitrogen Deposition
Assessment of simulations to date with coupled carbon cycle-climate models show that carbon cycle feedbacks to climate change could significantly alter the rate of atmospheric CO2 concentration increase and climate change over the century. However, the terrestrial carbon cycle is not only directly altered by increasing atmospheric CO2 and climate change; it is also indirectly altered by feedbacks from nitrogen(N) cycle perturbations induced by changes in CO2 concentration, climate and N deposition. A process-based terrestrial nitrogen cycle model has been developed and coupled with the terrestrial carbon cycle component of Integrated Science Assessment Model (ISAM) to study terrestrial carbon cycle and nitrogen cycle in an integrated way. The coupled carbon-nitrogen model has been applied to a series of modeling experiments examining the influence of nitrogen cycling on the response of the terrestrial biosphere to elevated CO2, climate change, and nitrogen deposition. The results show that the interactions between carbon and nitrogen cycles greatly influence the sensitivity of terrestrial biosphere to the increase of CO2, temperature, precipitation and N deposition leads to an important carbon sink in the coming decades. This model accounts for all the major nitrogen processes such as immobilization, mineralization, nitrification, denitrification, leaching and can be used to estimate nitrogen gas emissions. This talk will focus on describing the results of a series of modeling experiments examining the influence of nitrogen cycling on the response of the terrestrial biosphere to elevated CO2, climate change, and nitrogen deposition.
B33F-04 INVITED
The Role of Nitrogen Dynamics in the Responses of Terrestrial Carbon Dynamics to Changes in Atmospheric Carbon Dioxide, Climate, and Land Use
While it has long been appreciated that alterations of the nitrogen cycle can substantially affect the carbon dynamics of terrestrial ecosystems, most large-scale models of terrestrial carbon dynamics have ignored carbon-nitrogen interactions in making projections of how carbon dynamics will respond to changes in atmospheric carbon dioxide, climate, and land use. Numerous experimental studies have documented that the uptake of carbon by terrestrial ecosystems is enhanced by nitrogen fertilization under baseline and elevated atmospheric carbon dioxide concentrations. Ecosystem warming studies often identify that the uptake of carbon is enhanced when mineralization of soil organic nitrogen increases in response to warming, but the response often depends on how warming affects soil moisture. Nitrogen amendments are a standard practice in heavily managed agro-forestry ecosystems because of the enhanced response of plant growth to nitrogen fertilization. We have used the Terrestrial Ecosystem Model (TEM) as a tool to explore the regional and global implications of how carbon-nitrogen interactions may influence the responses of terrestrial carbon dynamics to environmental change and land use. Comparisons of the model with and without nitrogen dynamics indicate that the response of carbon uptake to increases in atmospheric carbon dioxide are clearly constrained by nitrogen dynamics. In contrast, carbon uptake is enhanced in situations in which warming enhances the mineralization of soil organic nitrogen, and this response can lead to increases in vegetation carbon storage that are greater than losses of carbon from increases in decomposition of soil organic matter. Land use can result in substantial depletion of nitrogen from terrestrial ecosystems in the harvest of agricultural products. As substantial sink activity is associated with forest re-growth after agricultural land abandonment, we conducted simulations with TEM in the eastern United State to evaluate to role of nitrogen replacement in the carbon dynamics of these ecosystems. The comparison of TEM simulations with inventory-based analyses of carbon storage were most consistent for simulations in which soil nitrogen was minimally depleted and in which there was no photosynthetic response to increased atmospheric carbon dioxide. In summary, experimental studies and our analyses with TEM indicate that carbon-nitrogen interactions are important in the regional and global response of carbon dynamics to environmental change and land use. These interactions should be considered by carbon cycle models that are used to assess the responses of terrestrial carbon storage to future environmental change and land use.
B33F-05
The Effects of Disturbance and Nitrogen Deposition on Carbon Uptake by Boreal and Temperate Forests
Fourteen chronosequences (83 sites) in temperate and boreal forests were used to examine the effects of disturbance and nitrogen deposition on net carbon uptake. Because effects of disturbance, temperature, and nitrogen deposition are difficult to disentangle, process-based and empirical models were used to interpolate measurements between ages to estimate C fluxes at every age in the rotation and obtain averages without risks associated with a limited sample size. The temporal dynamics following stand-replacing disturbances account for a very large fraction of the overall variability in forest carbon sequestration. Data from the 14 chronosequences of individual stands showed that annual NEP was only poorly correlated with wet N deposition at the stands (R2 = 0.16) as a result of the predominant effect of age on C fluxes. This was in contrast with the very good correlation observed with rotation-averaged NEP (NEPav) for the same chronosequences (R2 = 0.92). Time since disturbance explained 70% of the total variability in the dataset. After the confounding effects of disturbance were factored out, NEPav appeared to be driven by wet nitrogen deposition. No signs of N saturation were apparent in our data set, which explored a broad range of wet deposition up to 9.8 kgN/ha/yr (15 kgN/ha/yr of total N deposition), representing more than 90% of Western Europe and the conterminous United States. Preliminary analysis of data from over 100 sites was conducted to understand the mechanisms responsible for the increase in NEP. We found that N deposition stimulated GPP and woody biomass production proportionally, while a large amount of carbon was used for production of short-lived tissues. Yet, N deposition appeared to reduce heterotrophic respiration and associated carbon losses from soil. Modeling considerations include C:N coupling and changes in processes with time since disturbance.
B33F-06 INVITED
Historical Land Use Change Impacts in the Great Plains
Human activity and climate change have had a big impact on the ecosystem dynamics of managed and unmanaged terrestrial systems. The impact of human activities and climatic changes have been observed using numerous field experiments and computer models are capable of simulating the impact of climatic and land use changes on terrestrial systems. This talk will demonstrate how human land use changes during the last 150 years in the US Great Plains have altered carbon balance and trace gas fluxes for the region. DayCent model results suggest the 30 to 50% of the soil carbon has been lost from the soils and N2O fluxes have been increased 10 to 50% for different parts of the region. Plant production has also been greatly changes with decreases occurring in the dryland cropping systems and increases associated with irrigated cropping systems. The talk will also project the potential future impact of biofuel production cropping systems on US Agricultural systems. Biofuel cropping systems have the potential to alter ecosystem dynamics of agricultural systems in both positive and negative ways. Conversion of existing agricultural land into biofuel production systems generally has a positive impact of reducing net greenhouse gas fluxes, however, conversion of conservation reserve program land and undisturbed natural systems into biofuel production systems can greatly reduce the potential positive impact of biofuel systems on reducing greenhouse gas fluxes.
B33F-07
Fundamental influence of carbon-nitrogen cycle coupling on climate-carbon cycle feedbacks
A long history of ecological and biogeochemical research demonstrates the critical role of nutrients in general, and nitrogen in particular, in the dynamics of the terrestrial carbon cycle. The current generation of global coupled climate-carbon cycle models has not included an explicit (prognostic) representation of the nitrogen cycle over land. Recent development of the NCAR Community Climate System Model (CCSM) includes the introduction of coupled carbon and nitrogen cycles in the Community Land Model component (CLM-CN). The most important new mechanism captured in the model is the two-way coupling between net primary production (NPP) and heterotrophic respiration (HR) through carbon and nitrogen pathways. Previous models have represented the dependence of HR on NPP as the source of carbon to the decomposition pathways, but CLM-CN introduces the dependence of NPP on HR as the primary source of mineral (plant-available) nitrogen for new growth. Introduction of carbon-nitrogen cycle dynamics in a fully-coupled global simulation fundamentally alters the nature of the predicted climate-carbon cycle feedbacks: the land biosphere response to CO2 fertilization is reduced by about a factor of three, and the sign of the climate-carbon cycle gain is switched from positive to negative feedback, compared to previous carbon-only model predictions. So, with prognostic nitrogen cycle included, the land biosphere takes up substantially less of the fossil fuel emissions than previously predicted, but the global warming (and wetting) associated with greenhouse gas radiative forcing results in a modest additional uptake of carbon, as opposed to the release of carbon due to climate change predicted by all previous coupled climate- carbon cycle models.
B33F-08 INVITED
Global N2 fixation and its response to global climate change and increasing CO2 level
Biological nitrogen fixation is the largest nitrogen input to many natural terrestrial ecosystems, particularly tropical ecosystems, thereby influencing primary production, CO2 uptake, and responses to climate change. However, our understanding of biological nitrogen fixation is still very limited, and the dominant plant family capable of fixing N2 symbiotically, the Leguminasae, exhibits considerable geographic variation in the terrestrial biosphere. Based on the principles of resource optimization, we developed a new model to constrain our understanding of the geographic distribution of N fixation globally. Our model treats N fixation according to the C cost of fixing N, coupled with the N cost associated with acquiring P from the soil for plant growth. The model was used to estimate the rate of global symbiotic N2 fixation and the response of symbiotic N2 fixers to changes in climate and rising atmospheric CO2. We shall discuss global N limitation of terrestrial carbon uptake and its implications for climate-carbon cycle feedbacks from present to year 2100.