Phosphorus chemistry in managed forest soils : effects of weathering and wood ash fertilisation

Sammanfattning: Weathering and Podzolisation are key mechanisms that transform primary mineral apatite into a phosphorus (P) pool with low solubility. In addition, intensive forest harvesting removes nutrients from the soil, reducing P availability. In this thesis, a combination of wet chemical extractions, bulk X-ray absorption near-edge structure (bulk-XANES) spectroscopy, microscopic X-ray fluorescence (μ-XRF) imaging, and μ-XANES was applied to seven Podzolised soils (down to 1 m depth) across Sweden, to study molecular P speciation in the bulk soil and in microsites. Moreover, this thesis examined the fate of wood ash-bound P, when added alone or with repeated nitrogen (N) fertilisation, to the organic layer to return P removed after harvest. Total P (TP) in the upper 80 cm was 69−379 g m−2, with 94% of all P residing between 20−80 cm. More than 50% of all P in B and C horizons was Al-bound P, bound mainly to imogolite-type nanoparticles (ITN), while apatite comprised about 26%. Wood ash increased TP in the organic layer by 6−28 kg P ha−1, equivalent to 17−39% of the initial ash-P content. More bioavailable P (Olsen-P) and aboveground biomass P were observed in the ash treatment than in the control, probably due to the dissolution of Ca-bound P from the ash. Wood ash application, especially at a high dose, also increased Al-bound P (p < 0.001) by up to 15.6 kg P ha−1. P speciation across soil profiles was found to be strongly influenced by weathering and Podzolisation, with TP and apatite depletion in the E horizon and mainly Al-bound P and Fe-bound P in the B horizon. Dissolution of Ca-bound P from the ash was almost complete 13−24 years after ash application, which contributed to increased P availability and P uptake. These novel findings can be useful in improving forest management practices to ensure the long-term sufficiency of P supply to trees. Further work is required to understand the importance of the subsoil for the P supply from the subsoil and to study the fate of wood ash-bound P in the mineral soil.