Dual isotopic (33P and 18O) tracing and solution 31P NMR spectroscopy to reveal organic phosphorus synthesis in organic soil horizons

Soil microorganisms can do both, mineralize and synthesize organic and condensed phosphate (P) species. Whereas P mineralization has been extensively studied, few studies have assessed the biological synthesis of organic P species, which can potentially accumulate in soil. The goal of this study was to investigate biotic and abiotic P transformations, particularly the synthesis of organic P species, upon water-soluble P addition in the organic (O) horizons of two beech forest sites with contrasting P availability.

The two O horizons (low-P and high-P) were subjected to four different nutrient addition treatments (Control without addition, CN, P, and CNP additions) in an incubation experiment of up to 104 days. We combined isotopic tracing (³³P-labelled P addition and ¹⁸O-enriched soil water) into sequentially extracted P pools with the characterization of organic P species (solution ³¹P nuclear magnetic resonance (NMR) spectroscopy) and soil respiration measurements.

The P availability of the two O horizons shaped the microbial response to the nutrient additions. In the low-P O horizon, P addition stimulated microbial activity together with the increase of organic (phosphodiesters and phosphonates) and condensed (polyphosphates) P species, most likely from microbial origin. In the high-P O horizon, microbes were unaffected by the added P and abiotic processes controlled its fate. CN addition had no effect on P fate in the high-P O horizon but reduced the transformation of added P into organic P and increased soil-derived P in the resin P pool in the low-P O horizon. The ¹⁸O isotopic values in phosphate of the resin P pool suggest that the released P was biologically cycled.

Our study confirms with a unique multi-analytical approach the microbial synthesis of phosphodiesters, phosphonates, and polyphosphates upon inorganic P addition under low P availability.