Nitrate Dynamics in Organic Cropping Systems.

Managing nitrogen is a central challenge in agriculture, requiring growers to strike a balance between N provisioning to support crop growth and N retention to limit pollution.  The use of biological nitrogen inputs complicates this balancing act due to greater uncertainty in inorganic N availability.  The purpose of this study was to investigate the impacts of management on inorganic nitrogen pool dynamics in organic production systems.  Four organic cropping systems were initiated in 2007 and 2008 at Rock Springs, PA.  The systems varied in crop sequence, tillage, and organic N inputs, and our goal was to use this variation to identify dominant controls on N availability and leaching loss.  Surface soil nitrate concentrations were measured bi-weekly, and nitrate leaching was estimated from tension lysimeters buried at the soil-bedrock interface.  All systems demonstrated variability in soil NO₃-N concentration, measured as the coefficient of variation (CV) from the mean concentration across all sampling dates.  Coefficients of variation range from 0.83 to 1.54.  Using partial least squares regression, soil temperature, soil moisture, days since moldboard tillage, and days since manure application account for 32% of the variability in measured soil NO₃-N concentrations.  The timing of elevated NO₃-N concentrations (above 10 ppm) in deeper soil water corresponds with fallow periods.  These findings identify a trade-off between weed management strategies common in organic systems (tillage and tilled fallow) and N losses. This trade-off, however, may be minimized by rotation with perennial crops, cover cropping, and adapted timing of management operations.