Phosphorus fractions and speciation in an alkaline, manured soil amended with alum, gypsum, and Epsom salt

Abstract

AbstractSnowmelt runoff is a dominant pathway of phosphorus (P) losses from agricultural lands in cold climatic regions. Soil amendments effectively reduce P losses from soils by converting P to less soluble forms; however, changes in P speciation in cold climatic regions with fall‐applied amendments have not been investigated. This study evaluated P composition in soils from a manured field with fall‐amended alum (Al2(SO4)3·18H2O), gypsum (CaSO4·2H2O), or Epsom salt (MgSO4·7H2O) using three complementary methods: sequential P fractionation, scanning electron microscopy with energy‐dispersive X‐rays (SEM‐EDX) spectroscopy, and P K‐edge X‐ray absorption near‐edge structure spectroscopy (XANES). Plots were established in an annual crop field in southern Manitoba, Canada, with unamended and amended (2.5 Mg ha−1) treatments having four replicates in 2020 fall. Soil samples (0–10 cm) taken from each plot soon after spring snowmelt in 2021 were subjected to P fractionation. A composite soil sample for each treatment was analyzed using SEM‐EDX and XANES. Alum‐ and Epsom salt‐treated soils had significantly greater residual P fraction with a higher proportion of apatite‐like P and a correspondingly lower proportion of P sorbed to calcite (CaCO3) than unamended and gypsum‐amended soils. Backscattered electron imaging of SEM‐EDX revealed that alum‐ and Epsom salt‐amended treatments had P‐enriched microsites frequently associated with aluminum (Al), iron (Fe), magnesium (Mg), and calcium (Ca), which was not observed in other treatments. Induced precipitation of apatite‐like species may have been responsible for reduced P loss to snowmelt previously reported with fall application of amendments.