Stepwise redox changes alter the speciation and mobilization of phosphorus in hydromorphic soils

Sustainable engineering and management of hydromorphic arable soils need deep knowledge about the redox-mediated interactions between nutrients and soil colloids. Consequently, we examined the redox-mediated interactions of P with metal oxides and organic carbon (OC) in toe-, mid-, and upper-slope arable soils under dynamic redox changes using geochemical (biogeochemical microcosm), spectroscopic (XANES), and molecular (quantum chemical calculations (QCC)) approaches. We controlled the redox potential (E_H) in two directions i.e., 1) slowly oxidizing direction (SOD; E_H increased from -286 to +564 mV); and 2) slowly reducing direction (SRD; E_H decreased from +564 to -148 mV). In the SOD of all soils, P, Fe²⁺ and OC mobilized at E_H ≤ 200 mV, due to the pH decrease from 7.2 to 4.1 and dissolution of Fe-oxyhydroxides/carbonates, as indicated by the decrease of Fe-P and Ca-P determined by P-K-edge-XANES. At E_H > 200 mV, P immobilized due to the strong P binding with Fe³⁺ as suggested by QCC. In the SRD of mid-slope-soil, P immobilized with decreasing E_H, due to pH increase and P retention by aromatic carbon and/or precipitation by carbonates, as supported by increase of organic-P and Ca-P. These findings help for management of P in arable soils.