Combining spectroscopic and flux measurement techniques to determine solid-phase speciation and solubility of phosphorus in agricultural soils

Abstract

Soil solid-phase phosphorus (P) speciation and chemical conditions exert a crucial role in both aqueous P speciation and concentration—the most available and mobile pool for biota in environments. However, the interplay between solid-phase speciation and the aqueous phase of P in soil systems is unresolved. Herein, we investigated the speciation and solubility of P in tropical agricultural soils, which are often poor in P availability. Phosphorus K-edge X-ray absorption near edge structure (XANES) spectra of acidic soils identified dominant Al-hydroxide-phosphates (∼58%: P adsorbed to gibbsite 19%, P adsorbed to kaolinite 14%, and variscite 16%), followed by Fe-hydroxide-phosphates (∼36%: P adsorbed to ferrihydrite 14% and strengite 22%), and trace organic P (∼6%). Calcium phosphates (∼52%) prevailed in the alkaline/calcareous soils with moderate and minor contributions from Al-phosphates (∼37%: P adsorbed to kaolinite 28%, P adsorbed to gibbsite 1%, and variscite 7%) and Fe phosphate (∼11%), respectively. The time-averaged P concentration in the aqueous phase using diffusive gradients in thin films (CDGT) was highly variable from 4 to 123 μg P L–1, with the highest and the lowest values observed in acidic-coarse- and alkaline-fine-textured soils, respectively. The CDGT concentration was positively related to belowground P uptake. The CDGT concentration had negative relationships with pH, Ca-P XANES, and total and extractable Ca, indicating that Ca phosphates were the primary constituents decreasing the dissolvable P concentration. This study highlighted the importance of solid-phase P species in determining P fluxes and is relevant to diverse soils rich in Fe/Al/Ca compounds under extremely acidic to alkaline conditions.