Agroecosystem Management and Nitrogen Balance On Grain Farms in the Midwestern United States.

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. This research is part of an interdisciplinary project exploring links between social and ecological processes in the MRB using Gulf hypoxia as a model system.  Regional-scale N mass balances show 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 sustainability of diverse management systems in the MRB. We constructed N 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 N source to understand how agroecosystem management affects field-scale N mass balance. We collected data on the largest and most important N flows regulated by farm managers. Interviews were conducted from 2007-2009 with over 80 grain farmers in four study sites: the Des Moines lobe, Iowa; the Darby Creek Watershed, Ohio; the Driftless Region, Wisconsin; and in central Minnesota. Field samples of corn grain and legume biomass were collected from a subset of farms because N inputs from BNF and N exports in corn grain represent two large areas of uncertainty in N balance estimates. Nitrogen balances ranged from high surpluses to large deficits, and differed based on N source and management practice. Results also indicate that N surplus is inversely related to the proportion of total N input from BNF.