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

Abstract

In terrestrial ecosystems, a large proportion of the phosphorus (P) in soil is often found within soil organic matter. However, the majority of organic P in soil remains ‘unresolved’ and is largely observed as a ‘broad’ signal within the phosphomonoester region of a solution 31P nuclear magnetic resonance (NMR) spectrum on soil extracts. Our aim was to gain insight into the composition of four soils using the transverse relaxation (T2) time of the magnetisation in solution 31P NMR spectroscopy as a probe of their structure. We found the broad signal within the phosphomonoester region rapidly decayed compared to the sharp signals (i.e. myo- and scyllo-inositol hexakisphosphate) across all soils, which corresponded to the former having a shorter T2 time than the latter, and supports the existence of a broad signal due to supra-/macro-molecular structures. Furthermore, measures of the broad signal's line-width at half peak intensity based on T2 times were found to be less than that obtained from spectral deconvolution fitting. Therefore, our results strongly suggest that the broad signal is itself comprised of more than one component. The significance of this is that the chemical nature of a large proportion of soil organic P appears to be structurally complex.