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 ‘supra‐optimum' (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).

The structural composition of soil phosphomonoesters as determined by solution 31P NMR spectroscopy and transverse relaxation (T2) experiments

“…Whilst there is much research on the mechanisms associated with specific biomolecules of organic P, particularly that of myo-IHP (Turner et al, 2002), there is limited insight on the mechanisms governing pools of organic P as the broad signal in a NMR spectrum. There is supporting evidence to suggest their sequestration is related to factors affecting plant growth, soil organic matter and climate, which involve slow and soil-based mechanisms (Doolette et al, 2017;McLaren et al, 2017;Schefe et al, 2015).…”

A31P-NMR spectroscopic study of phosphorus forms in two phosphorus-fertilized grassland soils in eastern Canada

“…Other unidentified peaks in the monoester region were grouped into the Monoester 1 (Mono1, 7.0-6.0 ppm), Monoester 2 (Mono2; peaks not specifically identified between 5.9 and 3.8 ppm), and Monoester 3 (Mono3, 3.2-2.5 ppm). The broad peak detected by others in the monoester region would be included in the Mono2 category (e.g., McLaren et al 2016). Others unknown peaks were detected in the diester region and divided into other diester 1 (Di1, 2.5 Fig.…”

“…They reported an accumulation of P i and inositol phosphate in the top 5 cm soil depth. Results from studies carried out in P-fertilized grasslands in Netherlands over 11 yr (Koopmans et al 2003), and in Australia for 13 yr (McLaren et al 2016) revealed increased P i concentrations in the top 5 cm and increased orthophosphate monoesters in the top 10 cm soil depths, respectively. In Northern Ireland, high rates of P fertilization increased the concentrations of orthophosphate and total P i , and decreased the concentrations of total inositol hexaphosphate (IHP).…”

Innovations of phosphorus sustainability: implications for the whole chain

“…In addition to environmental risk zones, controlled-release P fertilizers offer the potential to reduce runoff losses [93] . In terms of chemical form, many studies have focused on organic P fertilizers [94] , chelates and polymers. One study has also shown that based on temperature and pH controlled-release rates, targeted P fertilizers are a promising new direction [95] .…”