Drainage Discharge and Nitrogen Losses Influenced by Tile Depth and Spacing under Corn and Soybean Rotation

Abstract. 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 tile drain spacing (4.2 m vs. 7.5 m) on nitrate-N losses under regular free drainage (RFD) and controlled drainage/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. 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. 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 in drainage water compared with the 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 in drainage water compared with the 7.5 m tile spacing. 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 in drainage water 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%.