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 an essential element needed by all life on Earth. However, one of the largest reservoirs, the abundant nitrogen gas that makes up most of the atmosphere, is quite unreactive and is not usable directly by most organisms. Some microbes called nitrogen-fixers have the unique ability to transform nitrogen gas into more biologically and chemically reactive forms. The resulting reactive nitrogen from these microbes and other abiotic sources fuels biological productivity in both land and ocean ecosystems. As part of the global nitrogen cycle, reactive nitrogen is used, recycled, and chemically modified by other microbes, plants, and animals. It is also transported by winds, rivers, and ocean currents as well as by sinking particles in the sea. Human activities are perturbing the global nitrogen cycle, adding large amounts of additional reactive nitrogen through the production of fertilizers and fossil-fuel combustion. With NSF support from this grant, the research team from the Woods Hole Oceanographic Institution (WHOI) is collaboratively building and evaluating a new dynamic computer model of the global nitrogen cycle and the links to climate, the carbon cycle, ecosystems, agriculture, atmospheric chemistry, and fossil fuel use. A key element of the collaboration involves the compilation and analysis of environmental datasets on reactive nitrogen distributions and transformation rates needed to evaluate model simulations. The modeling and data analysis contributes directly to the Community Earth System Model, a publically available model that is widely used to study the mechanisms connecting physical climate to the biogeochemistry and ecology of the atmosphere, ocean and land biosphere. The WHOI team’s researched focused on modeling and improving understand of three specific aspects of the marine nitrogen cycle. In the first project, the WHOI team worked with collaborators at Cornell University to characterize how the growing inputs of reactive nitrogen and phosphorus to estuarine and coastal waters leads to nutrient eutrophication, widespread low-oxygen levels (hypoxia), and other ecological damage. Several factors can influence whether the input of excess nutrient will generate hypoxia in coastal waters. For example, the degree and spatial extent of hypoxia can be modulated by ocean physics (e.g. extent of stratification, residence time, and so forth). Recent changes in ocean physics such as climate warming have made some ecosystems more sensitive to hypoxic events. Nutrient eutrophication can also set up biogeochemical feedbacks that decrease the availability of silica – conditions that can favor the formation and persistence of harmful algal blooms. Coastal hypoxia also contributes to ocean acidification, which harms calcifying organisms such as mollusks and some crustaceans. In the second project, the WHOI group analyzed the spatial patterns of marine nitrogen fixation based on a recently constructed, comprehensive database. Using about 500 depth-integrated field measurements covering the Pacific and Atlantic Oceans, the team explored how nitrogen fixation is related to sunlight, turbulence, temperature, nutrients and trace metals such as iron. The study confirmed the hypothesis that the large-scale spatial patterns of marine nitrogen fixation are coupled to regional loss of fixed nitrogen induced by subsurface low oxygen concentrations. The group also produced new global spatial maps of nitrogen fixation that can be used to evaluate model simulations. In the third project, the WHOI group collaborated with investigators at Harvard and NOAA/GFDL to analyze field data and model estimates of air-sea ammonia flux. Ammonia is released into seawater as a by-product when small planktonic predators feed on prey. As a result, the ocean is a source of reactive ammonia gas to the atmosphere. Preliminary results from two global ocean biogeochemistry models indicate large discrepancies among model predicted ammonia fluxes. These differences may reflect differences in how the models mathematically incorporate different biological processes including: nutrient limitations on phytoplankton growth and phytoplankton loss terms such as natural mortality and zooplankton grazing.
