Using response categories to classify the impacts of reactive nitrogen

Jill Baron and Timothy Weinmann, US Geological Survey, Fort Collins, CO, Jill Baron, Natural Resource Ecology Laboratory, Colorado State University, Hideaki Shibata, Field Science Center for Northern Biosphere, Hokkaido University, Sapporo, Japan, Hideaki Shibata, International Long-Term Ecological Research Network, Daniel Liptzin, Soil Health Institute, Hans van Grinsven, PBL Netherlands Environment Agency, The Hague, Netherlands, Wim de Vries, Wageningen University and Research (WUR), Wageningen, Netherlands, Jana Compton, Center for Public Health and Environmental Assessment, US EPA, Office of Research and Development, Corvallis, OR, Azusa Oita, Institute for Agro-Environmental Sciences, NARO, Tsukuba, Japan, Allison M. Leach, Natural Resource and Earth Systems Science and The Sustainability Institute, University of New Hampshire, Durham, NH

Background/Question/Methods
As part of the program “Towards an International Nitrogen Management System,” we have developed a guide to threat and benefit methodologies for determining the impacts of reactive nitrogen (Nr). Nr is beneficial for the production of food and manufactured products such as fibers and munitions but can harm human health and terrestrial and aquatic ecosystems. Nr contributes to climate change and impacts cultural services such as recreation and visibility. Nr causes these beneficial and detrimental impacts through many pathways: industrial production of ammonia, atmospheric deposition of N, contributing to surface level ozone and particulate matter, destruction of stratospheric ozone by the greenhouse gas, nitrous oxide, and leaching of nitrate from soils to surface and groundwater, and to coasts and the open ocean. Here, we synthesize our understanding of Nr pressure-state-impact relations to create categories of simple response functions to inform nitrogen management and policy. Several dozen beneficial and detrimental impacts from Nr were examined to determine their responses to increasing concentrations of different Nr compounds.
Results/Conclusions
A small number of patterns describe pressure to state responses, and state to impact responses. Tree biomass for nearly all species declines linearly, for instance, in response to NOx emissions that contribute to phytotoxic ozone doses, in an example of a pressure to state to impact response. Crop yields and biomass of natural vegetation (the impact) increase and then level off with increasing Nr (the pressure) that alters soil pH and soil base cation availability, (the state). Nitrogen saturation (the state), which defines the tipping point where availability of Nr (the pressure) is in excess of biotic demand (the impact), is a similar example of a linear increase to a plateau. Terrestrial and aquatic species richness (a pressure to impact response) can be characterized either by a negative curvilinear response, as shown in many papers, but also by a parabola, showing the increase in richness at the very onset of Nr enrichment, followed by the decline from species competition for resources.
Pressure-state-impact functions using empirical or modeled critical loads to the environment have already been used to develop pollution abatement agreements at many political levels. We are working to identify how other functions described may be useful for developing Nr management policies related to health, well-being, and climate.