The phosphorus composition of temperate pasture soils determined by NaOH–EDTA extraction and solution 31P NMR spectroscopy

Abstract

Information on the composition and dynamics of soil phosphorus (P) remains limited, but is integral to understanding soil biogeochemical cycles. We used solution 31P nuclear magnetic resonance (NMR) spectroscopy to characterise NaOH–EDTA extractable P in 29 permanent pasture soils from England and Wales (total carbon 29–80 g kg−1 soil, clay 219–681 g kg−1 soil, pH 4.4–6.8). Total P ranged between 376 and 1981 mg P kg−1 soil, of which between 45 and 88% was extracted with NaOH–EDTA. The extracts were dominated by orthophosphate monoesters (29–60% extracted P) and inorganic orthophosphate (21–55% extracted P), with smaller concentrations of orthophosphate diesters (2–10% extracted P), pyrophosphate (1–7% extracted P), phosphonates (0–3% extracted P), and traces of polyphosphates. Orthophosphate diesters were subclassified into phospholipids (1–7% extracted P) and DNA (1–6% extracted P). Signals slightly downfield of inorganic orthophosphate were tentatively assigned to aromatic orthophosphate diesters similar in structure to R-(−)-1,1′-binaphthyl-2,2′-diyl hydrogen phosphate. Such signals are rarely detected in soil extracts, but were present in relatively large concentrations in the samples analysed here (2–5% extracted P). Relationships between functional P groups and soil properties suggested that the various functional groups are involved in the soil P cycle to different extents. In particular, concentrations of orthophosphate monoesters appeared to be controlled by the potential for chemical stabilisation in soil, whereas DNA and pyrophosphate were strongly correlated with the microbial biomass, suggesting an active involvement in biological nutrient turnover.