Speciation of soil organic phosphorus: Steps from NMR spectra to bioavailability

Phosphorus (P) is an essential element for all life on Earth. Understanding P cycling is in the context of global change crucial both for modelling of global biogeochemical cycles and for agricultural productivity. Recently, concerns about the future of P fertilizer supply have prompted much research on soil P and method development. ³¹P Nuclear Magnetic Resonance (NMR) Spectroscopy has been used to analyse speciation of inorganic and of organic P species (Po), using in alkaline soil extracts¹. The region containing signals from phosphomonoesters is particularly important because these compounds are considered biologically active, but there are still significant problems to be resolved particularly for this region of P NMR spectra, including: 1. Poor signal resolution often makes quantification of Po species in this region very challenging. 2. It is unclear if observed signals are due to free P species, or originate from P compounds bound to high-molecular weight soil matter. 3. The question needs to be addressed how signals observed in alkaline extracts relate to P species that were originally present in the soil. Here we present two approaches to address these problems:

In a study of a 5000-year soil chronosequence in Northern Sweden², we found that humus P composition barely changed, although time since fire varied up to 5000 years. We will present a new method to back-calculate original Po speciation from the observed composition. Results of this method indicate absence of “recalcitrant” Po species, and instead indicate that most Po was originally present as biologically active P metabolites, probably present in live soil organisms. We will discuss implication of these findings for P biogeochemistry.

Second, we studied a diverse group of soils to address how the poorly resolved phosphomonoester region should best be analysed. Deconvolution techniques are required to handle the overlap, but a better understanding of the nature of the signals is required for reliable quantification. Based on combined analysis of 1D ³¹P NMR, 2D ¹H-³¹P NMR and ³¹P linewidth measurements, we present a strategy for quantification of phosphomonoester species, as next step in linking observed Po speciation to P bioavailability.

 

(1) Cade-Menun BJ, Preston CM (1996) A comparison of soil extraction procedures for 31P NMR spectroscopy. Soil Sci 161:770–785

(2) Andrea G. Vincent, Jürgen Schleucher, Reiner Giesler, David A. Wardle (2022) Soil phosphorus forms show only minor changes across a 5000‑year‑old boreal wildfire chronosequence. Biogeochemistry (2022) 159:15–32  https://doi.org/10.1007/s10533-022-00910-2

(3) Vestergren J, Vincent AG, Jansson M et al (2012) High-resolution characterization of organic phosphorus in soil extracts using 2D 1H–31P NMR correlation spectroscopy. Environ Sci Technol 46:3950–3956. https:// doi. org/ 10. 1021/ es204016h