Nitrate-N Loads to Subsurface Drains as Affected by Drainage Intensity and Agronomic Management Practices.

Few field studies have documented the impact of subsurface drainage intensity on nitrate loads in drainage waters.  A long-term (23-yr) study has been conducted on a silt loam soil in southeastern Indiana, USA, to determine the impacts of drainage intensity (5-, 10-, and 20-m drain spacings) and changes in crop production system on nitrate loads to drainage water.  Annual rainfall, drainflow, and drainage efficiency were higher in the 2000-2007 period compared to the 1985-99 time period.  Average drainflow per unit area was 72% greater for the 5-m spacing compared with the 20-m spacing during the 1985-99 time period and 24% greater during the 2000-2007 period.  The greater drainflow per unit area with the narrow spacing compared to wider spacings led to proportionately greater N loads to surface waters.  The relative differences in drainflow among spacings has become smaller with time, possibly due to improved soil structure and permeability with time after drain installation or resulting from the no-till cropping system.  No consistent differences in nitrate-N concentrations between corn and soybean years were evident except during the month or two following sidedress urea ammonium nitrate fertilizer application.  Nitrate-N concentrations have generally remained below 10 mg/liter for most of the year during the past 12 years of the study.  Nitrate loads were higher in 2000-2007 than in 1997-99, however, due to higher drainflow.  Addition of a winter cover crop along with lower fertilizer N rates, have significantly reduced the nitrate concentrations and loads in drainflow over the experimental period.