Unveiling Soil Organic Phosphorus Dynamics Using one- and two-dimensional 31P NMR

Organic phosphorus (P) is a large fraction of soil P and is essential for sustaining soil health,supporting microbial activityand contributing significantly to soil biological functionality. A thorough understanding of organic P composition in soils is vital across fields, from agriculture to ecology. In recent decades, substantial efforts have been made to characterize and quantify soil organic P compounds and to understand their turnover rates.

Among analytical techniques, ³¹P NMR spectroscopy stands out as a robust tool for exploring P speciation and dynamics. This method enables the identification of diverse P compounds in complex soil extracts. For instance, orthophosphate monoesters dominate ³¹P NMR spectra of NaOH-EDTA extracts. However, the monoester region often contains sharp signals overlaid on a broad background, challenging both quantification and interpretation of the spectra. Analyzing soils from seven ecosystems, we discovered that this background comprises numerous sharp signals representing small organic P molecules (1). Ongoing research aims to identify these P molecules, their origins and ecological significance.

A recent study of a 5000-year boreal wildfire chronosequence demonstrated that most observed organic P compounds in soil originate from biomass, including biologically active molecules such as RNA, phospholipids, and DNA (2). This research, which used 1D ³¹P NMR to quantify overall P speciation and 2D ¹H- ³¹P NMR to resolve overlapping signals, revealed that P diesters constitute 93% of extractable organic P (3). These findings highlight the critical role of microbial processes and biomass turnover in shaping soil organic P composition over long timescales, even in ecosystems impacted by diverse fire histories.

Building on this knowledge—that the NaOH/EDTA extract contains small molecules (1) and diester degradation products (3)—we hypothesize that the background observed in the 1D ³¹P NMR spectra could be composed of diester degradation products. To test this hypothesis, we conducted a detailed analysis of several reference P compounds using 1D ³¹P NMR and 2D ¹H-³¹P NMR. This approach aimed to replicate the spectral patterns observed in soil extracts and to further unravel the composition of organic P compounds in soil. By recreating these spectra, we aim to gain a deeper understanding of the nature and origins of diester degradation products and their ecological significance within soil.

(1) Haddad, L.; Vincent, A. G.; Giesler, R.; Schleucher, J. Small Molecules Dominate Organic Phosphorus in NaOH-EDTA Extracts of Soils as Determined by 31P NMR. Sci. Total Environ. 2024, 931, 172496.

 (2) Vestergren, J.; Vincent, A. G.; Jansson, M.; Persson, P.; Ilstedt, U.; Gröbner, G.; Giesler, R.; Schleucher, J. High-Resolution Characterization of Organic Phosphorus in Soil Extracts Using 2D 1H–31P NMR Correlation Spectroscopy. Environmental Science & Technology 2012, 46 (7),

(3) Vincent, A.G., Schleucher, J., Giesler, R. et al. Soil phosphorus forms show only minor changes across a 5000-year-old boreal wildfire chronosequence. Biogeochemistry 159, 15–32 (2022)