Nitrate Leaching to Subsurface Drains As Affected By Cover Crops, Drainage Intensity, and Rainfall Distribution.

Nitrate leaching through the vadose zone to shallow subsurface waters or deeper groundwater continues to be a challenge in highly productive agricultural regions.  In areas where agricultural drainage is a common water management practice, the use of subsurface drains to evaluate practices to reduce nitrate leaching is valuable.  A long-term (30-yr) drainage-water quality study has been conducted in southeastern Indiana, and highlights of the findings will be presented.  Some of the key findings include the effects of drainage intensity, cover crops, N fertilizer rate and form, and rainfall amounts and distribution on nitrate loads in the drainage waters.  The N-loads are smaller when winter cover crops are grown, when drainage intensity is less (more widely spaced drain lines), when fertilizer N rates are close to the optimum vs. higher rates, when less of the applied N is in the nitrate form at application, and when rainfall during the main leaching season is lower.  Seasonal variations in nitrate concentrations often do not exist unless the fertilizer N contains some nitrate.  Using subsurface drains as a research methodology for nitrate leaching must consider the differential travel time of water and nitrate from different distances from the drain, meaning that drain outflow is an integration of leaching losses over time periods of perhaps a year, depending on site-specific soil, restricting layers, and precipitation.  This integration across time and space can also be an advantage, however, as a way to scale up from a point-scale to a field-scale assessment of nitrate leaching.