The overarching goals of this project are: 1) to constrain the global budget of reactive nitrogen (Nr) through a combination of numerical simulations, data synthesis and analysis; and 2) to produce initial estimates of the overall interaction between reactive nitrogen cycling and climate. The nitrogen cycle is a key regulator of the Earth's climate system, linking terrestrial, marine, photochemical, and industrial processes, and modulating the carbon cycle. Over the last century and a half, expansion and intensification of agriculture and fossil fuel combustion have led to a more than doubling of Nr emissions to the atmosphere with profound impacts on the earth system. A closed global Nr cycle will be simulated within the CCSM (Community Climate System Model) by tracking Nr across three model domains: (1) atmosphere, (2) land, including native and agroecosystems, and (3) fresh and oceanic waters. While the basis for much of this work has already been developed within the CCSM, the nitrogen cycle has not been coupled across the different model domains. In the fully coupled system, each model domain will simulate the transport and production of Nr within its domain and the chemistry and loss of Nr from its domain, with the requirement that the nitrogen fluxes between the domains be self-consistent. This mass-balanced approach will avoid the untracked losses of Nr that occur when the nitrogen cycle is modeled in isolation within a single domain. Moreover, it will consider the diverse and opposing impacts of Nr on terrestrial carbon sinks and on the radiatively important species nitrous oxide, ozone, methane and aerosol ammonium sulfate. It is hypothesized that the overall effect of a changing Nr cycle on these four atmospheric species will lead to a warming sufficient to offset the cooling associated with increased Nr availability and increased terrestrial carbon uptake.
The synthesized datasets for evaluating the nitrogen cycle in Earth System Models, the coupled nitrogen-carbon-climate model developed here, and the simulations with the CCSM will be made broadly available to university and national laboratory communities and will contribute to the upcoming IPCC 5th Assessment. Most of the principal investigators in this project are actively engaged in graduate student education and training, and most participate in undergraduate activities as well. The research will support four additional graduate students and one postdoctoral associate as well as a number of undergraduates. All will become involved in the research activities both at their respective local institutions, as well as across all institutions and disciplines involved in the project.
Nitrogen is a key regulator of Earth’s climate, linking terrestrial, marine, photochemical, and industrial processes, and modulating the carbon cycle. Over the last 150 years, expansion and intensification of agriculture and fossil fuel combustion have led to a more than doubling of reactive nitrogen emissions to the atmosphere with profound impacts on the Earth system. The goals of this project were to: evaluate the current representation of the terrestrial carbon and nitrogen cycles in the Community Land Model (CLM), the terrestrial component of the Community Earth System Model (CESM); and improve the carbon-nitrogen biogeochemistry. Model simulations identified deficiencies in the current version of the model with respect to: (1) how nitrogen availability in the soil constrains plant productivity; (2) litter decomposition and nitrogen mineralization; (3) representation of microbial processes; (4) representation of soil biogeochemistry in croplands; and (5) nitrogen and phosphorus constraints on the terrestrial carbon cycle. 1. The Community Land Model overestimates gross primary production compared with observational estimates. Strong down-regulation of productivity by nitrogen is required to better match the observations. Model simulations showed that the imposed nitrogen reduction compensates for deficiency in the plant canopy parameterization that produces high productivity. A revised biophysically- and physiologically-based canopy parameterization solved this problem without the required nitrogen down-regulation. 2. Comparisons of the Community Land Model and the Daycent biogeochemical model with observed 10-year litter decomposition studies showed that the CLM underestimates carbon mass remaining and overestimates nitrogen mass remaining during litter decomposition. Daycent provided a better simulation compared with the observations. Those parameterizations were incorporated into the CLM. 3. The soil carbon response to climate change is uncertain, and Earth system models omit key biogeochemical mechanisms such as direct microbial control over soil carbon dynamics. We developed and tested a new model that explicitly represents microbial mechanisms of soil carbon cycling on the global scale. Compared with traditional models, the microbial model simulated soil carbon pools that more closely matched observations. It also projected a wider range of soil carbon responses to climate change over the twenty-first century. These results indicate that Earth system models should simulate microbial physiology to more accurately project climate change feedbacks. 4. Croplands have unique soil biogeochemistry related to soil cultivation and nitrogen fertilization. We developed a soil cultivation parameterization for the Community Land Model, based on the Daycent model. Model simulations showed that increased soil decomposition as a result of cultivation reduces the simulated soil carbon accumulation over the 20th century. Current generation Earth system models may improve their simulations of carbon cycle-climate feedbacks by accounting for enhanced decomposition from cultivation. Croplands also differ from natural systems because of addition of nitrogen fertilizers. We developed a nitrogen fertilizer parameterization for the CLM. 5. Carbon and nitrogen cycles are coupled in terrestrial ecosystems through multiple processes including photosynthesis, tissue allocation, respiration, nitrogen fixation, nitrogen uptake, and decomposition of litter and soil organic matter. Capturing the constraints of nitrogen on terrestrial carbon uptake and storage has been a focus of the Earth system modeling community. However, there is little understanding of the trade-offs and sensitivities of allocating carbon and nitrogen to different plant tissues in order to optimize the productivity of plants. We developed and tested a framework to represent these interactions and tradeoffs in Earth system models. This included: (i) In many forest ecosystems, nitrogen deposition enhances plant carbon uptake, thus reducing climate warming from fossil fuel emissions. Accurately modeling how forest carbon sequestration responds to nitrogen addition is critical for understanding how future changes in nitrogen availability influence climate. We used observations of forest carbon response to nitrogen inputs along deposition gradients and with fertilization experiments to test and improve the Community Land Model. The modeled plant growth response to nitrogen deposition was smaller than observed and the modeled response to nitrogen fertilization was larger than observed. A set of model modifications improved the correspondence between model predictions and observational data. (ii) We developed a new conceptual framework to represent multiple constraints of nitrogen on photosynthesis, tissue allocation, respiration, and plant nitrogen uptake. (iii) Phosphorus is an additional nutrient that controls the carbon cycle. We showed that omission of phosphorus constraints on terrestrial carbon cycle projections from Earth system models is as important as omission of nitrogen constraints. This project trained one Ph.D. graduate student and two post-doctoral researchers in the science and software engineering of Earth system models and the representation of biogeochemical processes in those models. The project contributed to the Community Earth System Model (CESM) and its terrestrial component the Community Land Model (CLM). These are major model facilities for the university research community.
