Water Discharge, Nitrogen Loss and Crop Production in Response to Phosphorus Draw Down in High P Soils Under Free Drainage and Water Table Control Systems.

Agricultural production practices could have a substantial impact on water quality by altering the timing and the amount of surface and sub-surface water and nutrient movement. Over 50 % of farm land in eastern US and Canada is tile drained, and pollution of water resources by inorganic fertilizer nutrients from tile runoff has been evident. The objectives of this paper were to study the impacts of phosphorus drawn down on nutrient losses and crop production in high P soil under free drainage and water table control systems in corn and soybean rotation. The treatments included a P drawn down (inorganic fertilizer N and K only), an inorganic fertilizer control (inorganic fertilizer N, P, and K), and two water management strategies (traditional free drainage vs. water table control). The field site is located at the AAFC research farm, Woodslee, Ontario, and consists of 8 field plots (each plot 15 m wide by 67 m long) linked to a central automated monitoring station. Surface and sub-surface flow volumes from individual plots were recorded by transmitters and samples of water collected automatically with auto-samplers. Two separate reservoirs and water recycling system were used to maintain desired water table control. The P drawn down treatment had lower cumulative total (surface + tile) water discharge than inorganic fertilizer control treatment. However, the flow- weighed mean nitrate concentration in both surface runoff and sub-surface tile drainage under the P draw down had much higher values than the inorganic fertilizer control treatment. Therefore, the P drawn down treatment had higher cumulative total (surface + tile) nitrate loss than inorganic fertilizer control treatment. In general, cumulative sub-surface tile nitrate loss under free drainage had higher values than water table control system. However, cumulative surface nitrate loss under free drainage had lower values than water table control system. The P drawn down treatment had higher corn yields than the inorganic fertilizer control treatment. The P drawn down treatment had low soybean yields relative to the inorganic fertilizer control treatment.