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 31 P 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 31 P 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.

Quantitative measures of myo-IP6 in soil using solution 31P NMR spectroscopy and spectral deconvolution fitting including a broad signal

“…9 The two main SDF approaches applied to soil extracts are that of 10 or modications thereof, 11 and Bünemann et al, (2008) 12 or modications thereof. 13 Turner et al, (2003) 10 were the rst to propose a SDF procedure that could partition the NMR signal within the phosphomonoester region and quantify myo-IP 6 in soil extracts. The SDF procedure was carried out using the Bruker WinNMR program, and involved tting a series of sharp signals from the peak maxima of myo-IP 6 to the baseline of the spectra.…”

“…The authors hypothesized that the broad signal was caused by phosphomonoesters in large and complex molecules, which was later conrmed by McLaren et al, (2015) 14 and McLaren et al, (2019). 13 Bünemann et al, (2008) 12 did not describe the SDF procedure in detail or test its efficacy at the time. However, Doolette et al, (2010) 15 compared the two SDF procedures (with and without a broad signal) to quantitatively recover myo-IP 6 , which was added to a soil extract.…”

“…myo-IP 6 ) with very similar linewidths, which are presumably from small organic molecules. 17,18 In contrast, the implication of the SDF procedure with a broad signal is that it interprets the phosphomonoester region as comprised of a broad signal, which is considered to be P org in the form of large molecular structures and associated with the soil organic matter (SOM), 13,14 and in addition an array of sharp peaks from small organic molecules containing P. It is also important to reconcile these views with previous studies using non-NMR techniques, which oen reported large concentrations of P org in large molecular weight fractions. 14,19 The aim of this study was to assess the efficacy of the two SDF procedures to quantitatively determine myo-IP 6 in soil extracts.…”

Options for Improved Phosphorus Cycling and Use in Agriculture at the Field and Regional Scales

“…• Study soil organic P accumulation under different cropping systems. • Improve identification of organic P compounds (i.e., those in the monoester category and those associated with the soil humic fraction [Simpson et al, 2011;McLaren et al, 2019]).…”

Is the enzymatic hydrolysis of soil organic phosphorus compounds limited by enzyme or substrate availability?

“…The soils studied here were previously characterized using 31 P nuclear magnetic resonance (NMR) spectroscopy and adding enzymes to alkaline soil extracts (Jarosch et al, 2015). This identified inositol phosphate P as the largest identifiable organic P class in most soils, followed by non-phytate phosphomonoesters ("monoester-like P", excluding yet unidentified monoesters (McLaren et al, 2019)). We hypothesized that the hydrolysis of phytate as a typically dominant soil organic P class is enzyme-limited, whereas the hydrolysis of less abundant organic P classes (non-phytate phosphomonoesters and phosphodiesters) is substrate-limited.…”