Phosphorus Loss Through Subsurface Tile Drainage During Eleven Years of Manure or Fertilizer Application for Corn and Soybean.

Phosphorus loss through subsurface drainage can be an important mechanism of P delivery to water resources. A study was conducted during 11 years in a typical Northeast Iowa soil to determine impacts of liquid swine manure or fertilizer P application for corn and soybean on soil P and P loss with tile drainage. Three nutrient management systems that involved chisel-plow tillage were manure applied every year for both crops of a corn-soybean rotation or continuous corn based on N need or N removal with grain harvest (MN2Y), N-based manure only for corn of a corn-soybean rotation (MNC), and N-P fertilizer for both crops of a corn soybean rotation according to Iowa recommendations (FNP). A fourth system that involved no-till management for a corn-soybean rotation was N-based manure only for corn (MNCNT). Swine manure always was injected, and fertilizer P was broadcast and incorporated into the soil. Profile soil-test P (STP) was measured to 120 cm. The STP for all systems was greatest in the 0-15 cm soil layer, and decreased abruptly to background initial levels below 15 cm for all systems. Topsoil STP (15-cm depth) by the last year of the study was 96 and 19 mg P kg^-1 for MN2Y and FNP, and was intermediate but highly variable for MNC and MNCNT. A soil P saturation index (15-cm depth) was well correlated with STP across manure and fertilizer management systems. The mean annual P loss was highest for MN2Y (17 g P ha^-1), intermediate for MNC and MNCNT (12 g P ha^-1), and lowest for FNP (4 g P ha^-1). Annual application of N-based manure greatly increased STP and P loss with tile drainage compared with fertilizer P-based application. Effects of N-based manure application for corn of corn-soybean rotation with or without tillage resulted in intermediate STP and P loss.