Crop Yield and Water Quality Assessment with Continuous Soil P Draw-Down, a Seven-Year Study.

Legacy phosphorus (P) of past agricultural management is one of the major causes for the failure of many watershed-based conservation programs on delivering improvement of surface water quality. Soil P draw-down (PDD) is a potential BMP for high P soils to overcome legacy P effect and mitigate soil P loss. A field experiment was conducted to assess the effects of PDD on crop yield, soil test P, and soil P losses in both surface runoff and tile drainage under a corn-soybean rotation in a Brookston clay loam soil in a 7-year period from 2008 to 2014. Corn grain yields with PDD were largely identical to those with continuous P addition (CPA) treatment. However, soybean yield decreased by 0.3 Mg ha^-1 in PDD plots in two of the three years, causing the net income loss of CAD $74.3 ha^-1. Over the early 4-yr period, soil Olsen P concentration remained unchanged, regardless of the P treatment. In the PDD plots, dissolved reactive P (DRP), particulate P (PP), and total P (TP) losses in both surface runoff and tile drainage did not decrease with time after halting P application. Relative to CPA, however, PDD significantly decreased DRP, PP, and TP losses. Our results indicated that legacy P of past management and its effect on surface water quality impairment are unlikely to be remedied by short-term PDD. Also, this study emphasized a need to adopt PDD to manage high P soils, which would prevent further increases in soil P losses, though supplemental P application may be required for soybeans to ensure the maximized production.