We study the biogeochemistry of elements that impact agricultural productivity and environmental quality. Much of our work focuses on nitrogen (N) which is applied to agricultural soils as chemical fertilizers or animal manures to increase crop productivity. The efficiency of crop utilization of this applied N is often well below 100%. The applied N which is not used by the crop can become transported from the agricultural field to the greater environment (land, atmosphere, or water) in different chemical forms. We study the reactive nitrogen compounds - including nitrous oxide, nitric oxide, nitrate, nitrite and ammonia - each of which can have ecological impacts at local, regional and/or global scales. Our overall objectives are to (1) evaluate land management practices that maintain or enhance agricultural production capacity while minimizing N loss to the environment, and (2) to better quantify and predict the fluxes of reactive N species using lab, field and modeling techniques.