Phosphorus Pools in Acid Sulfate Soil Are Influenced by pH, Water Content, and Addition of Organic Matter

The purpose of this study is to determine the effect of pH, soil water content, and organic matter availability on soil P pools and Fe minerals in acid sulfate soils which are common in coastal areas and paddy rice fields. An acid sulfate soil (original pH 3.2 or adjusted to pH 5.5) was amended with mangrove root pieces to achieve an organic carbon addition of 50% or 150% of native soil organic carbon. Then, the soil was incubated for 12 weeks with 4 weeks each in submerged, and then moist and then again submerged conditions. At the end of each 4-week period, soil P pools (labile P, moderately labile P, non-labile P, and residual P), oxalate extractable Fe/Al, and potential phosphate P sorption were measured. During the submerged periods, addition of mangrove roots decreased the redox potential and increased oxalate extractable Fe, but only at pH 5.5, indicating that reducing microbes were constrained by the low pH of the original soil. Labile phosphate was up to twofold higher with mangrove roots than in the unamended control, with greatest increase at 150% OC and pH 5.5. The increase in labile phosphate was likely due to release of bound P by Fe reduction and of P released from mangrove roots. Mangrove root addition enhanced phosphate sorption during the first 8 weeks, suggesting that mangrove roots provided more P binding sites in cell walls. Raising the pH to 5.5 and addition of organic matter can enhance P availability in acid sulfate soils by providing P and enhancing soil P release.