Unlocking fixed soil phosphorus upon waterlogging can be promoted by increasing soil cation exchange capacity

Phosphorus (P) adsorbed by iron (Fe) oxyhydroxides in soil can be released when the Fe(III) minerals are reductively dissolved after soil flooding. However, this release is limited in tropical soils with large Fe contents and previous studies have suggested that P sorbs or precipitates with newly formed Fe(II) minerals. This hypothesis is tested here by scavenging Fe2+ in flooded soils by increasing the cation exchange capacity (CEC) of soil through resin application (30 cmolc kg-1; Na-form). Three soils from rice paddies with contrasting properties were incubated in aerobic and anaerobic conditions with or without resin and with or without addition of organic matter (OM) to stimulate redox reactions. Dissolved Fe was 0.1–1.1 mm in unamended anaerobic soils and decreased to less than 0.07 mm with resin addition. Anaerobic soils without resin and aerobic soils with or without resin had marginal available P concentrations (<2 mg P kg-1; anion-exchange membrane P). In contrast, available P increased 3- to 14-fold in anaerobic soils treated with resins, reaching 16 mg P kg-1 in combination with extra OM. Application of Ca-forms of resin did not stimulate P availability and dissolved Ca concentrations were larger than in unamended soils. Resin addition can increase P availability, probably by a combination of reducing solution Fe2+ (thereby limiting the formation of Fe(II) minerals) and increasing the OM solubility and availability through reducing dissolved Ca2+. The soil CEC is a factor controlling the net P release in submerged soils.