Blended soil amendment effects on phosphorus loss from manured soils under simulated snowmelt flooding

Phosphorus (P) loss from soils via snowmelt runoff is a major contributor to eutrophication in water bodies across the Canadian Prairies. Reductions in P losses are often achieved using single-component soil amendments. Blended amendments have been shown to stabilize P more effectively than single amendments; however, their effectiveness in reducing P loss with snowmelt flooding is not well understood. This laboratory incubation study, conducted under simulated snowmelt flooding, compared the effectiveness of blended soil amendments with single amendments in reducing P release from six agricultural soils from southern Manitoba. The treatments were unamended (control), single amendment of alum KAl(SO4)2·12H2O, or ferric chloride (FeCl3) at 2.5 Mg ha-1, single amendment of gypsum (CaSO4·2H2O) or magnesium sulfate (MgSO4) at 2.5 Mg ha-1 or 5 Mg ha-1, and eight amendment blends of gypsum/magnesium sulfate with alum/ferric chloride at different combinations. Treated soils were packed in vessels, flooded, and incubated at 4°C for 56 days. Floodwater samples were collected bi-weekly and analyzed for dissolved reactive P (DRP) concentrations. Blended amendments typically led to greater DRP reductions in floodwater, achieving maximum decreases of 51-89%, compared to 38-64% reductions observed with individual amendments. The gypsum and ferric chloride blend (1:1 ratio at 2.5 Mg ha-1) demonstrated consistent effectiveness across all soil types, whereas ferric chloride was the most effective when applied individually. Single amendment of ferric chloride was only slightly inferior to blended amendments, suggesting it would be a viable option to reduce floodwater DRP in most soils.