How Does Agroecosystem Management Affect Potential Nitrogen Loss from Grain Farms in the Mississippi River Basin?.

Intensively managed grain farms are saturated with large inputs of nitrogen (N) fertilizer, which leads to N losses and environmental degradation.  This research is part of an interdisciplinary project exploring linkages between human and biogeochemical processes in agricultural landscapes in the Mississippi River Basin (MRB) to address the problem of N-loss and hypoxia in the Gulf of Mexico.  Fieldwork on farms in the Midwest was informed by a meta-analysis of 217 field-scale experiments following the stable isotope, ¹⁵N, in temperate grain crops and soil to compare management practices that alter inorganic fertilizer additions, such as application timing or rate, with practices that manage carbon (C) and N inputs together, such as organic N sources or diverse crop rotations.  Practices that aimed to increase crop fertilizer use efficiency had a lower impact on total ¹⁵N recovery in crops and soil (3 – 21% increase) than practices that re-coupled C and N cycling (30 – 42% increase). We expanded on these results by calculating N balances for a gradient of agroecosystem management practices on working farms in the MRB to assess whether practices that manage C and N inputs simultaneously result in reduced N surplus and N loss potential.  The management gradient spans continuous corn systems, conventional farms with winter annuals or cover crops, and diversified organic grain farms. We constructed 5-year N balances for approximately 100 farms from 4 study sites in the MRB using data gathered with farmer interviews.  Here we present preliminary results from the interviews and discuss key N cycle fluxes that are the most uncertain terms in the balances—biological N fixation and N exported in corn grain.  We will measure these fluxes on a subset of farms to improve the accuracy of the N balances.