Nitrate, Phosphorus, and Salt Export through Subsurface Drainage from Corn Fields in the Canadian Prairies

Abstract. High fertilizer inputs and uncontrolled subsurface drainage can result in the export of nutrients from farmland, leading to negative environmental impacts. Controlled drainage can be an effective best management practice to minimize the export of nutrients. However, information about controlled drainage in the Canadian Prairies is scarce. The objective of this study was to compare free drainage with overhead irrigation (FDIR) to controlled drainage with sub-irrigation (CDSI) in terms of nitrogen, phosphorus, and salt exports from corn fields in the Canadian Prairies. The irrigation water had low concentrations of nitrate (≤0.2 mg L^-1) and orthophosphate (<0.1 mg L^-1) and low electrical conductivity (0.55 dS m^-1), which were assumed to have low influence on the export of nutrients and salts. In 2010, the exports of NO₃-N (138 kg ha^-1), PO₄-P (0.6 kg ha^-1), and salts (2.34 Mg ha^-1) from the FDIR treatment during the growing season were significantly larger (p < 0.05) than exports from the CDSI treatment, which were 0.07 kg ha^-1, 0.08 kg ha^-1, and 0.41 Mg ha^-1, respectively. This comparison did not include the drainage outflow arising from spring snowmelt due to delays in the installation of monitoring equipment. In 2011, which included the drainage outflow due to spring snowmelt events, the exports of NO₃-N (36 kg ha^-1), PO₄-P (0.27 kg ha^-1), and salts (1.1 Mg ha^-1) from FDIR were significantly larger (p < 0.05) than the exports from CDSI, which were 10 kg ha^-1, 0.08 kg ha^-1, and 0.39 Mg ha^-1, respectively. Therefore, controlled drainage with subirrigation (CDSI) showed significant reductions in export of NO₃-N (71%), PO₄-P (69%), and salts (64%) compared to free drainage with overhead irrigation (FDIR). This study shows the advantage of controlled drainage over free drainage in reducing the export of nutrients and salts.