Nitrate Losses to Subsurface Drains as Affected by Winter Cover Crop, Fertilizer N Rates, and Drain Spacing

Abstract

Subsurface tile drainage is a common water management practice in agricultural regions withseasonal high water tables. This paper reports the results of a 16-yr subsurface tile drainageresearch study on a site located in southeastern Indiana, USA. The site includes three tile drainspacings (5, 10, and 20 m) managed for 10 years with chisel tillage in monoculture corn (Zeamays L.) and currently managed under a no-till corn-soybean (Glycine max L.) rotation.Drainflow volumes are continuously recorded and the water is sampled for nitrate-N on a flowproportionalbasis. Additional field measurements have included soil water content andpotential, water table, and crop yield. In general, drainflow and nitrate-N losses per unit areaincrease as drain spacing becomes narrower. Drainflow removed between 8 and 26% of annualrainfall, depending on the year and drain spacing. Nitrate-N concentrations in the drainflow didnot vary with spacing, but concentrations have significantly decreased from the beginning to theend of the experiment. Concentrations dropped from a mean of 28 mg L-1 in the 1986-1988period, to a mean of 8 mg L-1 in the 1997-1999 period. The reduction in concentration was dueto both a reduction in fertilizer N rates over the course of the experiment and to the addition of awinter cover crop as a trap crop after corn in the corn-soybean rotation. Average annualnitrate-N mass losses dropped from 38 kg ha-1 in the 1986-1988 period, to 15 kg ha-1 in the 1997-1999 period, and losses were greatest for the narrowest spacing. Most of the nitrate-N lossesoccurred during the dormant season, when most of the drainage occurred. Results of this studyunderscore the necessity of long-term research on different soil types and in different climaticzones, in order to develop appropriate management strategies for both economic crop productionand protection of environmental quality.