Significance of phosphorus inclusions and discrete micron‐sized grains of apatite in postglacial forest soils. (18th October 2022)

• Record Type: Journal Article
• Authors: Adediran, Gbotemi A.; Kielman‐Schmitt, Melanie; Kooijman, Ellen; Gustafsson, Jon‐Petter
• Is Part Of: European journal of soil science. Volume 73:Number 5(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-10-18
• Subjects: apatite inclusions -- chemical speciation -- discrete particles -- elemental co‐localizations -- LA‐ICP‐MS imaging -- phosphorus cycling; Soil science -- Periodicals; 631.4
• DOI: 10.1111/ejss.13310
• Languages: English
• ISSNs: 1351-0754

Abstract

Abstract: 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. Highlights: The significance of high‐P spots and discrete grains to long‐term P supply is largely unknown. For the first time, P concentration and speciation was resolved by LA‐ICP‐MS multi‐elemental analysis. The P spots exist as dispersed apatite inclusions and micron‐sized grains in the subsoil. P in these spots and grains were up to 600 times greater than the bulk soil P concentrations.