Influence of subsurface drain depth and spacing on nitrate losses from paddy fields during canola growth season

A subsurface drainage system, in addition to controlling the water table and leaching out dissolved salts from the root zone, may cause losses of nitrogen via drainage effluent. In this research, the effect of different subsurface drainage systems on the losses of nitrate, was investigated at the consolidated paddy fields of Sari Agricultural Sciences and Natural Resources University, during a canola growing season of 2011-2012. Drainage treatments included: three conventional subsurface drainage systems with mineral envelope including drainage system with drain depth of 0.9 m and drain spacing of 30 m (D0.9L30S), drain depth of 0.65 m and drain spacing of 30 m (D0.65L30S), and drain depth of 0.65 m and drain spacing of 15 m (D0.65L15S) and a conventional subsurface drainage system with artificial envelope with drain depth of 0.65 m and drain spacing of 15 m (D0.65L15F). The nitrate concentration of effluent of subsurface drains was measured every 15 days. During the study period, the average of nitrate concentration of drainage effluent and total nitrate loss in D0.9L30S, D0.65L30S, D0.65L15S, and D0.65L15F treatments were 4.65, 3.47, 4.5, and 2.97 mgL-1 and 7.5, 4.57, 9.12, and 15.46 kg.ha-1, respectively. Based on the results, increase in drain spacing resulted in reduction of nitrate concentration of drainage effluent. Also, by increasing in drain depth, the nitrate concentration of drainage effluent was increased. Therefore, water management through a subsurface drainage system with proper drain depth and spacing, could diminish the negative effects of nitrate losses from paddy fields on water resources.