A P-NMR spectroscopic study of phosphorus forms in two phosphorus-fertilized grassland soils in eastern Canada

Phosphorus (P) fertilization can increase grassland production, but will also alter P forms, changing their cycling and potential for loss in runoff. We assessed the effects of mineral P fertilization on soil P forms in timothy swards at two sites in Quebec, Canada. Soil samples (10 cm depth) were collected in autumn 2013 from replicate plots at Levis on a Kamouraska clay and at Normandin on a Labarre clay loam, each having received three rates of triple superphosphate (0, 20, and 40 kg P ha(-1)) for 4 yr. These were analyzed for pH, total carbon (TC), total nitrogen (TN), and total phosphorus (TP); Mehlich-3-extractable aluminium (AlM3), iron (FeM3), calcium (CaM3), and P (PM3); and P-31 nuclear magnetic resonance spectroscopy (P-31-NMR) following sodium hydroxide-ethylenediaminetetraacetic acid (NaOH-EDTA) extraction. Phosphorus fertilization had no significant effects on soil TC, TN, AlM3, FeM3, CaM3, and pH, but significantly increased TP, NaOH-EDTA-extractable total P and total inorganic P, PM3, orthophosphate, and glucose 6-phosphate at both sites. In contrast, NaOH-EDTA-extractable total organic P, total orthophosphate diesters, and scyllo-inositol hexaphosphate decreased with P fertilization. Phosphorus fertilization over 4 yr increased soluble inorganic P and decreased organic P at both grassland sites.