Reducing Nitrate Loss in Tile Drainage Water with Cover Crops and Watertable Management.

Nitrate leached from agricultural soils can be both an environmental concern when it enters rivers and lakes, and a human health concern when it enters aquifers used for potable water. Management practices such as cover crops and controlled tile drainage – subirrigation (CDS) have been identified as possible ways to reduce nitrate losses.  A 6-yr study (1999-2005) was conducted in Southwestern Ontario in which the effectiveness of a winter wheat cover crop and CDS were assessed for a corn-soybean rotation. The four treatments included: cover crop with and without traditional tile drainage (DR), and cover crop with and without CDS.   Corn received 15 kg N ha^-1 at planting and 150 kg N ha^-1 as a sidedress application at the 6 leaf stage.  No fertilizer N was applied to soybean.  Surface runoff and tile drainage water were collected using autosamplers and analyzed for nitrate-N.  Relative to no cover crop, winter wheat reduced surface runoff by 10.9% and increased tile drainage by 8.2% for DR; and it reduced surface runoff by 29.4% and increased tile drainage by 35% for CDS. The flow weighted mean (FWM) nitrate concentration in tile drainage from the DR treatment was 9.29 mg N L^-1 and this was reduced by 19.5% when the cover crop was planted (7.48 mg N L^-1).  The CDS system reduced FWM nitrate concentrations in tile drainage by 15.5% (7.85 mg N L^-1) compared to DR.  However when both the cover crop and CDS were used, the FWM nitrate concentrations (5.01 mg N L^-1) were 46% lower than those obtained using no cover crop and DR. The 6-yr cumulative nitrate loss with the DR treatment (101 kg N ha^-1) was reduced by 12% with cover crops (89.1 kg N ha^-1) and by 37.5% with the CDS system (63.3 kg N ha^-1) whereas when both cover crops and CDS were used, cumulative nitrate loss was reduced by 46.7% to 53.9 kg N ha^-1.  Although the cover crop assimilated residual N in the soil profile, this benefit is partially offset by increased tile flow. Hence both a cover crop and CDS can effectively reduce nitrate losses from agricultural soils, and they are particularly effective when used in combination.