Speciation of phosphorus in temperate zone forest soils as assessed by combined wet-chemical fractionation and xanes spectroscopy

Phosphorus availability in terrestrial ecosystems is strongly dependent on soil P speciation. Here we present information on the P speciation of 10 forest soils in Germany developed from different parent materials as assessed by combined wet-chemical P fractionation and synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy. Soil P speciation showed clear differences among different parent materials and changed systematically with soil depth. In soils formed from silicate bedrock or loess, Fe-bound P species (FePO₄, organic and inorganic phosphate adsorbed to Fe oxyhydroxides) and Al-bound P species (AlPO₄, organic and inorganic phosphate adsorbed to Al oxyhydroxides, Al-saturated clay minerals and Al-saturated soil organic matter) were most dominant. In contrast, the P speciation of soils formed from calcareous bedrock was dominated (40–70% of total P) by Ca-bound organic P, which most likely primarily is inositol hexakisphosphate (IHP) precipitated as Ca₃-IHP. The second largest portion of total P in all calcareous soils was organic P not bound to Ca, Al, or Fe. The relevance of this P form decreased with soil depth. Additionally, apatite (relevance increasing with depth) and Al-bound P were present. The most relevant soil properties governing the P speciation of the investigated soils were soil stocks of Fe oxyhydroxides, organic matter, and carbonate. Different types of P speciation in soils on silicate and calcareous parent material suggest different ecosystem P nutrition strategies and biogeochemical P cycling patterns in the respective ecosystems. Our study demonstrates that combined wet-chemical soil P fractionation and synchrotron-based XANES spectroscopy provides substantial novel information on the P speciation of forest soils.