Influence of pH, phosphorus addition and soil water content on phosphorus pools in acid sulfate soils

Abstract

Acid sulfate soils (ASS) are common in wetlands and important for crop production. However, little is known about P availability in ASS which is constrained due to high Fe content and changing redox potential. This thesis aimed to investigate the effect of P addition, pH and soil water content on P pools in ASS. A sandy sulfuric ASS (pH 3.2) was used in the experiments. In the first experiment, the ASS was amended with NaOH or Ca(OH)2) to achieve soil pH 4 or 5.5, without or with addition of inorganic P equivalent to 20 kg ha−1. Soils were incubated in either submerged or moist conditions (100% of maximum water holding capacity). After two weeks, P pools and Fe /Al oxides were determined. P pools were higher with P addition than without. The increase in pH enhanced Fe oxides but had little effect on labile P. Added soluble P was available after two weeks of addition under submerged conditions. In Experiment 2, the ASS (pH adjusted to 5.5) was amended with phosphate at 0, 96, 385, 578 and 770 mg P kg−1 and incubated under submerged conditions. P pools were measured after two and four weeks. Sorption and release of P were determined in incubated soils and jarosite, separately. The maximum sorption capacity of the soil was 350 mg P kg-1. Jarosite had a high capacity to bind P and contributed about 50% to P binding in this soil. Labile P represented more than 50% of added P at P rates above 350 mg P kg-1. The third chapter included two experiments. In the first experiment, the ASS (pH adjusted to 5.5) was amended with 400 mg P kg-1 as inorganic P, manure or combinations of inorganic P/manure and was incubated submerged. After two and four weeks, labile P was measured. Labile P in treatments with 50% of P added as manure was about half of that in treatments with a greater proportion of inorganic P. In the second experiment, treatments (inorganic P alone: 100F, manure alone: 100M, combination of manure and inorganic P: 50M/50F and unamended control) were kept submerged, then moist, followed by resubmergence. At the end of first submergence, labile P was two-fold higher in 100F than in manure treatments. But in the moist period and after resubmergence, labile P did not differ between 100F and 100M. Oxalate extractable Fe was higher in 100M than 100F and remained unchanged until the end of the experiment. In experiment 4, the ASS (without or with pH increase to 5.5) was amended with mangrove roots to achieve an organic carbon (OC) addition of 50% or 150% of native soil OC and incubated under submerged, moist and then resubmerged conditions. At the end of each four-week period, P pools, amorphous Fe/Al and potential P sorption were measured. Mangrove roots addition decreased the redox potential and increased labile P under submerged conditions, but only at pH 5.5. P binding was enhanced by root addition during the first 8 weeks, but not after re-submergence.