Significance of phosphorus inclusions and discrete micron‐sized grains of apatite in postglacial forest soils

Recent advances in soil phosphorus (P) studies have revealed unique P hot spots and discrete micron-sized grains at soil microsites, but the significance of these so-called ‘hot spots’ and grains in P cycling and long-term supply is yet to be determined. We examined soil particles and pore space distribution at a micro-scale in two postglacial forest soils by laser ablation ICP-MS imaging. This allowed us to semi-quantitatively reveal both axial and lateral abundance, distribution, and co-localization of P with elements known to influence its chemical speciation (e.g., Si, Al, Mn, Ca, and Fe). The results show topsoil P to be co-localised predominantly with Si, Al, and Fe. However, in the subsoils, P was co-localised mainly with Ca, Si, Al, and Mg in spots within Si and Al-bearing minerals and with only Ca in discrete micron-sized grains. While the spots of P-Ca inclusions were ~ 1000 μm apart and present at 40–100 cm depth in Tärnsjö, the discrete grains of P-Ca were ~ 700–1200 μm apart and present at 90–100 cm depth in Tönnersjöheden. The P concentrations in these ‘hot spots’ and grains were 7 to 600 times greater than the average soil P concentrations, with the highest values (3434–8716 mmol P kg−1) occurring in the C horizons of the two soils. When combined with previous P speciation results obtained by synchrotron P K-edge XANES in the same soils, our work confirms geogenic apatite to have been dissolved in the topsoil and its P transformed to P adsorbed by Al-Si and Fe phases, and to organic P. Most importantly, our work shows subsoil spots of P-Ca inclusions and micron-sized grains to be a long-term source of P and Ca.