Long-Term Impact of Tillage Practices and Phosphorus Fertilization on Soil Phosphorus Forms as Determined by 31P Nuclear Magnetic Resonance Spectroscopy

Conservation tillage practices have become increasingly common in recent years to reduce soil erosion, improve water conservation, and increase soil organic matter. Research suggests that conservation tillage can stratify soil test phosphorus (P), but little is known about the effects on soil organic P. This study was conducted to assess the long-term effects of tillage practices (no-till [ NT] and mouldboard plowing) and P fertilization (0 and 35 kg P ha(-1)) on the distribution of P species in the soil profile. Soil samples from a long-term corn-soybean rotation experiment in Quebec, Canada, were collected from three depths (0-5, 5-10, and 10-20 cm). These samples were analyzed for total P (TP), total C (TC), total N (TN), pH, and Mehlich-3 P (PM3); P forms were characterized with solution phosphorus-31 nuclear magnetic resonance spectroscopy (P-31-NMR). Results showed a stratification of TP, TC, TN, pH, PM3, and Mehlich-3-extractable aluminum and magnesium under NT management. The PM3 and orthophosphate concentrations were greater at the soil surface (0-5 cm) of the NT-P-35 (soil treatment with 35 kg P ha(-1)) treatment. Organic P forms (orthophosphate monoesters, especially scyllo-IP6, and nucleotides) had accumulated in the deep layer of NT treatment possibly due to preferential movement. We found evidence that the NT system and P fertilization changed the distribution of P forms along the soil profile, potentially increasing soluble inorganic P loss in surface runoff and organic P in drainage and decreasing bioavailability of inorganic and organic P in deeper soil layers.