Phosphorus speciation and C:N:P stoichiometry of functional organic matter fractions in temperate forest soils

Background and aims: Phosphorus (P) availability is crucial for forest ecosystem productivity and soil organic matter (SOM) is an important source for P. This study was conducted to reveal carbon (C), nitrogen (N) and P distributions in functional SOM fractions. We hypothesised that (1) most of the organic P (P_org) is part of the particulate SOM, (2) particulate SOM stores increasing share of P with decreasing soil P content and (3) the C:P_org ratio of mineral-associated SOM is smaller than that of particulate SOM. Methods: We analysed soil samples from five temperate forest sites (Fagus sylvatica) under different geological parent material with a wide range of total P concentrations. Density fractionation was used to separate free light fraction (fLF), particulate SOM occluded within soil aggregates (occluded light fraction; oLF), and mineral associated SOM (heavy fraction; HF). We determined the mass balance of P in these fractions, in addition to the C and N concentrations. Additionally, the P speciation of the topsoil was analysed by X-ray absorption near edge structure (XANES) spectroscopy at the P K-edge. Results: The fLF contained 18–54% and the oLF 1–15% of total P (P_tot). High percentage of P in these light fractions was associated to soil minerals. Phosphorous in particulate SOM within aggregates tend to increase with decreasing soil P. The HF containing mineral-associated OM, comprised 38–71% of P_tot and their C:P_org ratios were consistently lower than those of the fLF irrespective of the P status of the soil. Conclusions: We show that all three functional SOM fractions contain variable amount of both organic and inorganic P species. The free light fraction shows no response to changing P stocks of soils. Despite physically protected particulate SOM, oLF, becomes increasingly relevant as P cache in soils with declining P status.