The chemical nature of organic phosphorus that accumulates in fertilized soils of a temperate pasture as determined by solution 31P NMR spectroscopy

Abstract

Inefficiency of fertilizer phosphorus (P) use in grazing systems is often associated with the accumulation of inorganic and organic P in fertilized soil. However, the chemical nature of the accumulated organic P remains poorly understood. The aim of this study was to use solution 31P nuclear magnetic resonance (NMR) spectroscopy on sodium hydroxide-ethylenediaminetetraacetic acid (NaOH-EDTA) extracts to identify the chemical nature of organic P in soils from a medium-term (13 years) permanent pasture field experiment. This included an unfertilized pasture (P0), and treatments designed to maintain soil P fertility at near 'optimum' (P1) and 'supraoptimum' (P2) levels for pasture growth; pastures at all levels of soil P fertility were continuously grazed with either a moderate or high stocking rate (SR09 and SR18). Approximately 20% of the fertilizer P added to pastures was recovered as organic P in NaOH-EDTA extracts at the P1 level of soil P fertility in the 0-10 cm soil layer, and the majority (≈ 65%) of this was detected as the broad phosphomonoester signal. In addition, several specific forms of phosphomonoesters (myo- and scyllo-inositol hexakisphosphate, α- and β-glycerophosphate, and RNA mononucleotides) and phosphodiesters were detected across all soils but at low concentrations. This study shows that phosphate fertilization of pastures primarily results in the accumulation of complex forms of phosphomonoesters rather than that of specific forms of recognizable biomolecules (e.g., myo-inositol hexakisphosphate).