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 (Porg) is part
of the particulate SOM, (2) particulate SOM stores increasing share of P with decreasing soil P content
and (3) the C:Porg 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 (Ptot). 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 mineralassociated OM, comprised 38–71% of Ptot and their C:Porg 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. Keywords: Ecosystemnutrition . Density fractions . Soil organic matter . C:N:P ratio . Phosphorus . P K-edge XANES
