Phosphorus Speciation in Soils Under Different Land Use in Southwestern Saskatchewan.

Understanding phosphorus (P) speciation and cycling under different land uses is vital to improve P use efficiency in agriculture while mitigating P loss to water bodies. The complexity of P cycling in soils calls for analytical methods to structurally and functionally speciate soil P. We used multiple techniques, including sequential extraction, K-edge X-ray absorption near-edge structure (XANES) and solution ³¹P nuclear magnetic resonance (P-NMR) spectroscopy, to investigate soil P concentration and speciation under different land use. We collected soil samples from five locations in southwestern Saskatchewan (Auvergne Wise Creek [AWC] 1, AWC 2, Val Marie [VM], Masefield [MF] 1, MF 2), where each contained adjacent sites for all four land use types (native grassland, crested wheatgrass pasture, annual cropland and roadside). The soil classification was the same at all sites. These soils contained 289-595 mg kg^-1 total P (TP), of which organic P (P_o) was dominant (51-91% by digestion) in all except three samples (36-49%). According to sequential fractionation, most P_o was extracted by NaOH (Fe-associated P_o) in all except one sample. This was confirmed by P-NMR results which indicated the predominance of orthophosphate monoesters in NaOH-EDTA extracts of all samples. The dominant P_o form was consistently inositol hexakisphosphate (IHP) by both P-NMR and P K-edge XANES spectroscopies. In all the five locations, it is consistent that both tame and native pastures were dominated by IHP as indicated by P-NMR spectroscopy and the featureless XANES spectra. For roadside soils from four of the five locations (not AWC2), XANES spectra showed evident features of calcium-associated P (Ca-P), of which tricalcium phosphate (TCP) accounted for about half of TP. The results suggest that soil P speciation on the same soil type under varied land usage differs across locations through different management practice, including differences in plant species, fertilization and soil disturbance.