Effect of tile drain depth and spacing on nitrogen losses under corn and soybean

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<abstract> <bold>Abstract</bold> Nutrient losses from agricultural drainage lands may differ depending on tile drain depth and spacing. Studies were conducted using large field plots equipped with automatic flow volume measurement and sampling systems over a 4-year period to compare two different tile drain depths (0.65 m vs. 0.85 m) and two tile drain spacing (4.2 m vs. 7.5 m) on nitrate-N losses under regular free drainage (RFD) and controlled drainage and sub-irrigation (CDS) systems in a corn and soybean rotation. Over a 4-year period, under RFD system, 0.85 m tile depth with average tile spacing of 4.2 m produced 11.4 kg/ha more surface nitrate-N loss and 45.3 kg/ha more sub-surface tile nitrate-N loss, compared with the 0.65 m tile depth. Deep tile depth of 0.85 m produced 57 kg/ha more combined total surface and sub-surface tile nitrate-N losses relative to shallow tile depth of 0.65 m. However, under CDS system, 0.85 m tile depth with average tile spacing of 4.2 m, produced 44.3 kg/ha more surface nitrate-N loss and 41.0 kg/ha less tile nitrate-N loss, compared with 0.65 m tile depth. Deep tile depth of 0.85 m with CDS system only produced 3.3 kg/ha more combined total surface and sub-surface tile nitrate-N losses relative to shallow tile depth of 0.65 m. Under RFD system, tile spacing of 4.2 m with the average tile depth of 0.65 m, produced 10.5 kg/ha less surface nitrate-N loss and 30.3 kg/ha more sub-surface tile nitrate-N loss compared with the 7.5 m tile spacing. Close tile spacing of 4.2 m produced 20 kg/ha more combined total surface and sub-surface tile nitrate-N losses relative to 7.5 m tile spacing. Under CDS system, tile spacing of 4.2 m with average tile depth of 0.65 m, produced 4.8 kg/ha less surface nitrate-N loss and 41.8 kg/ha more tile nitrate-N loss compared with the 7.5 m tile spacing. Close tile spacing of 4.2 m with CDS system produced 37 kg/ha more combined total surface and sub-surface tile nitrate-N losses relative to tile spacing of 7.5 m. Under RFD system, for a tile drain spacing of 4.2 m, increasing the tile drain depth from 0.65 m to 0.85 m increased total combined surface and sub-surface drain volume and nitrate-N loss by 140% and 64%, respectively. However, for a tile drain depth of 0.65 m, decreasing the tile spacing from 7.5 m to 4.2 m reduced total combined surface and sub-surface drain volume by 10% but increased total nitrate-N loss by 28%. Under CDS system, for a tile drain spacing of 4.2 m, increasing the tile drain depth from 0.65 m to 0.85 m increased total combined surface and sub-surface drain volume and nitrate-N loss by 146% and 5%, respectively. However, for a tile drain depth of 0.65 m, decreasing the tile spacing from 7.5 m to 4.2 m reduced total combined surface and sub-surface drain volume by 20% but increased total nitrate-N loss by 109%.