Soil phosphomonoesters in large molecular weight material comprise multiple components

Abstract

The majority of soil organic P is typically that of “unresolved” phosphomonoesters, which are associated with organic P (Po) in large molecular weight (MW) fractions. However, the composition of this pool of Po remains unclear and could contain more than one component. Our aim was to identify the number and nature of components in soil extracts of different MW using solution 31P nuclear magnetic resonance (NMR) spectroscopy and spectral deconvolution fitting. Three surface soils were collected from native sites, which were extracted for Po using 0.25 M NaOH + 0.05 M Na2–EDTA (unfractionated), followed by size separation using ultrafiltration, resulting in four MW fractions (>10 kDa, <10 kDa, <10 kDa and >3 kDa, and <3 kDa). The NMR analyses revealed a diversity of Po species in all MW fractions across the three soils. In particular, the phosphomonoester region of the >10-kDa fraction comprised up to 10 components of varying linewidth (0.5–18.6 Hz) and a broad signal (228 Hz). This broad signal was also present in the <3-kDa fraction, albeit at a lower concentration (average was 29% of that in the unfractionated extracts). Furthermore, solution 31P NMR spectra on the <3-kDa fraction revealed a diversity of sharp signals (up to 28) in the phosphomonoester region. Overall, our findings show that soil phosphomonoesters in large MW fractions comprise multiple components of varying linewidth.