Characterizing phosphorus impacts on crop yields, soil nutrient profiles, and water quality in soils at risk of flooding for several phosphorus-based fertilizers of varying source and rate

Abstract

In the Red River Valley of northwestern Minnesota, phosphorus (P) is the main nutrient of concern that threatens the Lake Winnipeg watershed. Climate change has altered crop production in the Northern Great Plains, as increased summer temperatures have brought corn and soybean rotations to northern regions. Consequentially, farmers in the Red River Valley are implementing P rates for two years at once on corn-soybean rotations which do not have long-established fertility documentation in the region. Additionally, soluble orthophosphate (ortho-P) release from calcareous, alkaline soils to spring floodwater over agricultural fields is understudied. To characterize the impact of phosphorus-based fertilizers by source and rate on crop yields, soil nutrient concentrations, biomass nutrients, and nitrogen mineralization, field trials with corn and soybean rotations were conducted over two years at the University of Minnesota’s Northwest Research & Outreach Center, followed by a laboratory incubation study to characterize dissolved P release from the treatments over eight weeks of a cool, anaerobic, saturated environment designed to replicate spring flooding. Treatments included the following commonly used fertilizers: monoammonium phosphate (MAP, NPK: 11-52-0), MicroEssentials SZ (MESZ, NPK-Zn: 12-40-0-10S-1), and 25% Struvite/Crystal Green (NPK: 5-28-0). While increased fertilizer rates resulted in slightly higher yields for some datasets, two-year rates of P-based fertilizer did not make differences in crop yields. These field trials indicate that applying excess phosphorus to save time does not impact yields, but simply increases phosphorus available in soil water to run off under flooded conditions. Laboratory incubation of mesocosms from field samples by fertilizer source and rate at two depths (0-15 cm and 15-30 cm) showed distinct release of soluble P from the soil to water between treatments. Ortho-P release from soil to surface water and pH increased over time, with ortho-P being greater in concentration in the surface layer. The 3-week mark was a significant point in spikes of ortho-P release in both surface and subsurface soil layers, which characterizes a timeline for water quality concerns in spring flooding of agricultural fields, after iron reduction occurs and previously sorbed phosphorus is released. MAP + Struvite and MESZ at double rates had consistently higher dissolved ortho-P release, possibly due to the addition of magnesium and zinc. While double rates were not noticeable by crop yield in a second-year soybean rotation, double rates showed distinct risk of runoff to floodwaters in simulated conditions.