One of the most important challenges for global food security in the near future is increasing crop production to meet food and energy demands in a sustainable manner. The agronomic challenge is well documented, and will necessitate overcoming plateauing yield potentials in major cereals, continued development of new varieties, and decreasing the difference between yield potentials and yields achieved on farms, among other hurdles. Less attention has been given to improving nutrient management, but it is clear that global fertilizer use has already caused profound alterations in the nitrogen (N) cycle with unsustainable consequences for the environment. Meeting future food production demand in a sustainable manner will require improving the efficiency of fertilizer use on farms, and the purpose of this thesis was to identify the major opportunities and constraints to such a feat.

The need for increased nitrogen use efficiency (NUE) is established in this thesis by synthesizing more than a decade of research in the Yaqui Valley in Sonora, Mexico for a regional perspective on the major N transfers and transformations in the Valley. Large reductions in gaseous emissions and leached N were shown to be possible with changes in management. Simulation modeling is used to determine the potential for improved NUE in the Valley, the variables most responsible for the increase, and the biogeochemical implications of improved N management. The greatest gains in NUE were facilitated by technologies that allowed for site-specific N management by allowing farmers to manage for differences in residual N and crop demand in individual fields. Simulations suggested that agronomic efficiency could be doubled in the Yaqui Valley with no loss in profit or yield. A synthesis of field trials in wheat systems throughout the world is used to place the Yaqui Valley in a global perspective. All field trials demonstrated that large gains in agronomic efficiency were possible with no loss in profit or yield.

There are also many constraints to managing for high NUE. The two primary reasons for the success of site-specific N management are better synchronization of fertilizer supply with plant demand and the ability to change rates in-season depending on differences in demand on a field-by-field basis. The ability to measure differences in N supply and N demand from field-to-field—and the causes of those differences—is one of the major hurdles for increasing NUE. Controlled experiments on soils in the laboratory are used with simulation modeling to demonstrate the importance of accounting for regional differences in soil N retention, climate, and management in regional scale studies of N cycling and crop growth.

Large improvements in NUE and N pollution are possible, but implementation of site-specific N management depends on a thorough understanding of the importance of spatial variability in N flows and transformations in the soil, air, groundwater, and surface water.