Nitrogen (N) leaching to surface waters from grain farms in the Mississippi River Basin (MRB) is the primary cause of hypoxia in the Gulf of Mexico. Regional-scale N mass balances indicate that a small area of the upper MRB, which is intensively cropped and tile-drained, contributes disproportionately to nitrate loading. These aggregate balances miss small-scale variability, especially that caused by differences in farm management. A better understanding of the flows and fates of N at the field level is needed as an indicator of directional change towards deficit or surplus (i.e., potential for N loss) and to compare the relative efficiency and sustainability of diverse farming systems in the MRB. We constructed N mass balances for a gradient of farm types from intensive corn-soybean monocultures to diversified grain farms that rely on biological N fixation (BNF) as a primary N source to explore the relationship between agroecosystem management and the potential for N losses. Mass balances were constructed using data on major field-scale N fluxes collected from interviews, legume biomass and corn grain samples. Interviews with 95 grain farmers were conducted between 2007-2009 on grain farms in Iowa, Ohio, Minnesota and Wisconsin.
Results/Conclusions
Nitrogen balances ranged from high average annual surpluses (145 kg N ha-1) to large deficits (-45 kg N ha-1), and differed based on N source and management practice. Fields with greater than 70% of total N additions from legume N sources were in balance compared to fertilizer-based practices with average annual surpluses near 30 kg N ha-1. Results also indicate that N surplus is inversely related to the proportion of total N input from biological nitrogen fixation for medium (80-160 kg N ha-1 yr-1) to high (>160 kg N ha-1 yr-1) N rates. The N concentration of corn grain samples averaged 1.23% across fields in 2008, which is lower than values reported in the literature, suggesting that recent corn varieties have reduced N contents. Spring legume biomass inputs varied three-fold, demonstrating the importance of using sampling data to improve estimates of uncertain N fluxes, such as legume nitrogen fixation, for working farms. Preliminary research results indicate that management practices which most effectively reduce the potential for N loss from farm fields are currently rare on agricultural landscapes in the Midwestern United States.
