95th ESA Annual Meeting (August 1 -- 6, 2010)

COS 19-7 - Comparing nitrogen cycling on grain farms in the Mississippi River Basin using a mass balance approach

Tuesday, August 3, 2010: 10:10 AM
333, David L Lawrence Convention Center
Jennifer B. Gardner, Dept of Crop and Soil Sciences, Cornell University, Ithaca, NY and Laurie E. Drinkwater, Horticulture, Cornell University, Ithaca, NY

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.  


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.