The California Nitrogen Assessment: Prioritizing Practices and Policies For Mitigating Environmental and Human Health Impacts.

Nitrogen (N) plays a vital role in sustaining the agricultural economy of California, as well as the global food supply. However, nitrogen used in agriculture also converts to nitrous oxide (N₂O) and nitrate (NO₃^-) which respectively contribute to climate change and water pollution.  Despite the important benefits and tradeoffs of N to human and ecological systems, there remains a paucity of quantitative knowledge on the interrelationships between California’s N flows and the various ecosystem services that contribute to the public good. To fill this knowledge gap the Agricultural Sustainability Institute at UC Davis conducted a comprehensive “California Nitrogen Assessment” informed by an intensive stakeholder engagement process to establish a baseline of credible information on the sources, sinks and flows of N into, out of, and within the state.  We present results of a statewide N Mass Balance, focusing particular attention on quantifying the fluxes of N₂O to the atmosphere and losses of nitrate (NO₃^-) to groundwater from the major source categories. We also evaluate the relative contribution and impact of N losses on climate change versus groundwater pollution. Whereas about 2.3% of N inputs in California are eventually emitted as N₂O, leaching of NO₃^-  to groundwater accounts for roughly 21% of the state’s nitrogen flows, with approximately 90% of the losses originating from crop and livestock production. We conclude by considering several technical and policy-based strategies for balancing the important productive uses of N while minimizing the environmental tradeoffs.