B31I-01 INVITED
Consequences of considering carbon/nitrogen interactions on the feedbacks between climate and the terrestrial carbon cycle.
The impact of carbon/nitrogen dynamics in terrestrial ecosystems on the interaction between carbon cycle and climate is studied using an Earth system model of intermediate complexity, the MIT Integrated Global Systems Model (IGSM). Numerical simulations were carried out with two versions of the IGSM's Terrestrial Ecosystems Model, one with and one without carbon/nitrogen dynamics. Our simulations show that consideration of carbon/nitrogen interactions not only limits the effect of CO2 fertilization, but also changes the sign of the feedback between climate and terrestrial carbon cycle. In the absence of carbon/nitrogen interactions, surface warming significantly reduces carbon sequestration in both vegetation and soil by increasing respiration and decomposition (a positive feedback). If plant carbon uptake, however, is assumed to be nitrogen limited, an increase in decomposition leads to an increase in nitrogen availability stimulating plant growth. The resulting increase in carbon uptake by vegetation exceeds carbon loss from soil, leading to enhanced carbon sequestration (a negative feedback). Under very strong surface warming, however, terrestrial ecosystems become a carbon source whether or not carbon/nitrogen interactions are considered. Overall, for small or moderate increases in surface temperatures, consideration of carbon/nitrogen interactions result in a larger increase in atmospheric CO2 concentration in the simulations with prescribed carbon emissions. This suggests that models which ignore terrestrial carbon/nitrogen dynamics will underestimate reductions in carbon emissions required to achieve atmospheric CO2 stabilization at a given level. At the same time, compensation between climate-related changes in the terrestrial and oceanic carbon uptakes significantly reduces uncertainty in projected CO2 concentration.
B31I-02 INVITED
Particle Nucleation Over and in a Forest: How Important is the Canopy?
Year-long measurements of particle size distributions (6 to 400 nm) at three levels over and in a Midwestern deciduous forest from two Scanning Mobility Particle Sizers (SMPS) and one Fast Mobility Particle Sizer (FMPS) indicate a high frequency with which ultra-fine particles are observed. Approximately 1 in 5 days exhibit class A nucleation events according to the University of Helsinki classification. Highest frequency of nucleation events are observed in spring (subsequent to leaf-on) but nucleation is observed in all seasons. Lowest ultra-fine particle concentrations are uniformly observed in the trunk space, and during leaf-on highest concentrations always occur at the upper-most measurement height (46 m). However, during leaf-off frequently the highest concentrations of ultra-fines (6-30 nm) are observed close to the canopy level (at 34 m above the canopy which extends to about 28-30 m). This situation, of highest concentrations being observed closest to the canopy, is nearly uniformly the case for event days from mid-February through March (i.e. prior to bud-break in mid-April). It may be due to a reduction in canopy dry deposition of ultra-fines during the leaf-off period, but may also reflect the role of an apparently senescent canopy or under-storey in stabilizing recently nucleated particles or simulating the growth of sub-10 nm particles. We will present comparisons of the characteristics of these events, postulates for their source and analyses concerning the growth rates and removal of the resulting particles.
B31I-03 INVITED
Isotopic constraints on the global terrestrial N cycle
Nitrogen (N) limitation to terrestrial primary productivity is widespread; the amount of N that enters and leaves terrestrial ecosystems holds considerable leverage over how much CO2 they can store. We use variations in natural N isotope abundance of soil to estimate the dominant pathways by which N is lost from the global terrestrial environment. In this analysis we assume that denitrification is the major isotope fractionating loss term, the mean δ15N of inputs is approximately 0 per mil, and total soil N pools are close to steady state with respect to N inputs and losses. Formulating these assumptions into a simple analytical model, we estimate that 40 % of the N that enters the terrestrial biosphere is lost back to the atmosphere via gaseous pathways; the remaining 60 % escapes via rivers and streams. This partitioning falls within the range of values determined by global simulation modeling and data synthetic approaches, which suggest that anywhere from ~32 % to 59 % of N is lost via denitrification worldwide. Our analysis also points to a strong continental pattern in N loss pathways. In particular, those land masses that fall largely within tropical/sub tropical climate zones--Africa, S. America, Australia--seem to lose N mainly via gaseous compounds (52 % to 62 %), whereas N. America, Asia and Europe export N predominantly via leaching (66 % to 71 %). With the exception of N. America, where isotope abundance suggests lower gaseous N losses than models, simulations of N loss pathways correlate strongly with isotope-based estimates across the continents (r2 = 0.75).
B31I-04 INVITED
The role of anthropogenic disturbance in the carbon balance of terrestrial ecosystems
The terrestrial carbon cycle is directly affected by increasing atmospheric CO2, climate change, and disturbance, and indirectly affected by feedbacks from changes in nutrient cycling. Disturbance effects on carbon uptake by terrestrial ecosystems are often ignored in regional to continental assessments, yet this information is needed to inform policy decisions. Predictions of increased fire frequency have driven attempts to reduce fire risk by thinning forests, particularly in areas with shorter intervals between fires. Field data in a thinning study showed shifts in NPP from trees to understory shrubs, and shifts in C allocation from foliage and wood to fine roots, as well as C emissions from harvest activities, which should be considered when evaluating the effects of thinning on regional C budgets. Other considerations include assumptions about area that will potentially burn at high severity in the future relative to certain losses from harvest removals to reduce fire risk. On the other end of the spectrum, it is commonly assumed that old forests are carbon neutral, but a new dataset on boreal and temperate forests suggests this is not necessarily the case (i.e. a majority of the forest sites > 150 y old were net sinks, and RH:NPP was on average, < 1). Observations suggest modeling of integrated effects of CO2, N deposition, and climate should elevate the importance of disturbance in regional and continental assessments, and in the evaluation of potential mitigation actions.
B31I-05 INVITED
Uncertainty in carbon, climate and nutrient interactions following human disturbance in simulating terrestrial greenhouse-gas exchange with the atmosphere
Human change in vegetation, especially deforestation for agriculture, is second only to the combustion of fossil fuels as an anthropogenic source of CO2 in the Earth's atmosphere. At the same time, the recovery of forests from historical land use, both agriculture and timber harvest, is the single largest contributor to the North American carbon sink, partially mitigating fossil-fuel contributions to increased atmospheric CO2 concentration. These trends are likely to continue in the future, but in a future of changing climate with increasing demand for land to produce food and fuel for growing, more affluent human populations. Understanding, and accurately simulating, terrestrial-ecosystem response to human disturbance and how these disturbed systems respond to management, climate and other environmental change are critical to understanding historical anthropogenic climate forcing and for projecting future trends and uncertainties. These responses remain, however, one of the most uncertain elements of earth system modeling. A great deal is known about individual components: carbon and nutrient cycling, physiological and ecological response to climate and nutrient variability and perturbation; but much less is known about interactions and feedbacks among these components especially in response to disturbance and how they change over time after disturbance, with and without human management. What is known is certainly not well integrated into global earth system models. Here, I use three types of models, a phenomenological model of ecosystem response to land-use change, a mechanistic model of carbon-cycle response to climate and atmospheric CO2, and an individual-based model of forest succession to illustrate how interactions among climate, carbon, nutrients, and human activities are, or are not, represented in terrestrial ecosystem modeling and by extension in global climate models. I also use these models to investigate model sensitivities and their expression as uncertainties in simulation results. I argue that understanding these sensitivities and uncertainties can provide a triage for determining the critical elements of this exceedingly complex system that must be included in integrated terrestrial models for global earth system modeling.
B31I-06
Modeling Nitrogen Leaching With A Biogeochemical Model Coupled With Soil Hydrology Model
Land use changes for cropland, excessive application of fertilizers in agriculture, and increase in anthropogenic activities such as fossil fuel burning have lead to widespread increases in anthropogenic production of reactive N and NH3 emissions, and N deposition rates. An important consequence of these processes is intensification of soil nutrient leaching activities, leading to serious ground water contamination problems. The current study focuses on the issue of nitrogen (nitrate and ammonium) leaching due to land cover changes for cropland, excess N fertilizer application, and atmospheric nitrogen deposition on nitrogen leaching at a global scale. Simulations of nitrogen leaching require integration of processes involving soil hydrology and biogeochemical cycles. An existing terrestrial coupled carbon-nitrogen cycle model, Integrated Science Assessment Model (ISAM), was used to estimate nitrogen leaching. The N-cycle in ISAM includes the major processes associated with nitrogen (immobilization, mineralization, nitrification, denitrification, leaching, nitrogen fixation, and vegetation nitrogen uptake). ISAM also considers how carbon and nitrogen dynamics are influenced by the effects of human perturbations to the N cycle including atmospheric deposition and fertilizer application, and the fate of N in land use activities, i.e., deforestation and agricultural harvest. In this study, the ISAM soil hydrology was extended and improved with CLM 3.5 hydrology processes and algorithms, which extended the modeling capabilities to consider the prediction of nitrogen leaching. The model performance was evaluated with flow and nutrient data at several locations within the Upper Sangamon River Basin in Illinois, and flow data in contrasting watersheds in Oklahoma. This talk will focus on describing the results of a series of modeling experiments examining the influence of land management changes for cropland and nitrogen deposition on nitrogen leaching at a global scale. These experiments were conducted based on the measured activities of land use and nitrogen deposition over the last century.
B31I-07
An Integrated Modeling Framework for Assessment of Impacts of Multiple Global Changes on Terrestrial Productivity
Independent changes in atmospheric carbon dioxide, tropospheric ozone, nitrogen deposition and climate change directly impact terrestrial productivity. Less well understood are the interactive effects of these globally changing factors on terrestrial productivity and the resultant impact on rising atmospheric carbon dioxide concentrations. This study uses the Integrated Science Assessment Model (ISAM) to quantify the impacts of these multiple global changes on terrestrial productivity and further, to project how these changes feedback on atmospheric carbon dioxide concentrations via respiratory carbon fluxes. The ISAM is modified to include a mechanistic model of leaf photosynthesis including the sensitivity of leaf photosynthesis to tropospheric ozone. Leaf-level photosynthetic carbon gain is scaled to the canopy with a sun-shade microclimate model to estimate the gross primary productivity of major biomes comprised of representative plant functional types. The modified carbon cycle in ISAM is coupled to a detailed model of the terrestrial nitrogen cycle therefore providing the integrated modeling framework required to assess the interactive effects of rising carbon dioxide, tropospheric ozone, nitrogen deposition and climate change on global productivity.
B31I-08
Nitrogen Attenuation of Terrestrial Carbon Cycle Response to Global Environmental Change
The magnitude of worldwide terrestrial carbon sinks driven by CO2 fertilization are found to be attenuated by nitrogen dynamics. However, the terrestrial nitrogen cycle also has the potential to interact with carbon cycle responses to changes in climate, nitrogen inputs, and land use. In this study, a terrestrial carbon and nitrogen cycle model was used to evaluate how the nitrogen cycle influences the terrestrial carbon sinks in the 20th Century in response to changes in atmospheric CO2, climate, nitrogen inputs, and land use. Two series of simulations were performed. First, the model of the nitrogen cycle was fixed at the 1765 levels. Next, nitrogen availability was allowed to vary dynamically according to plant nitrogen supply and demand. These simulations were driven by a single driving variable. Comparisons of these applications of the model with a fully dynamic nitrogen cycle to applications in which nitrogen availability was fixed at 1765 levels revealed that in 1990s there was (1) a decreased sink associated with increasing atmospheric CO2, (2) a decreased source associated with changes in climate, (3) an increased sink associated with nitrogen inputs, and (4) an increased source associated with changes in land use. While the analysis for individual driving variables indicates that during the 1990s the role of the nitrogen cycle in changing atmospheric CO2, climate, nitrogen and land use counterbalance each other to some extent, model applications that simultaneously considered all of these effects indicate that the nitrogen and carbon cycles are in fact currently playing an important role in changing the terrestrial CO2 sinks at the global scale. Results indicate the importance of including the nitrogen cycle in coupled carbon-climate system models.