EFFECTS OF ANAEROBIC DECOMPOSITION OF ORGANIC MATTER ON SORPTION AND TRANSFORMATIONS OF PHOSPHATE IN DRAINED SOILS

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Abstract
This paper reports the results of a study of the effects of organic matter additions to anoxic soils on P transformations and sorption. We examined four soils alternately cropped to rice, where upland crops develop severe P-deficiency problems. The soils were amended with 0, 0.5, and 1.0% cellulose and kept flooded for 85 d. The P sorption, as affected by anaerobic decomposition of organic matter, oxalate extraction, and some anion additions, was investigated in the drained soils. The effect of pH, changed by ±2.0 pH units on soils flooded for the same time and then drained was studied for P sorption. Soil flooding and draining, with or without added organic matter, increased the P sorption significantly over unflooded soils. The added organic matter, however, significantly enhanced conditions for P sorption in drained soils. The increased P sorption in drained soil due to prior flooding with or without added organic matter was removed by extracting the soil with 0.2 M ammonium oxalate acidified to pH 3.0. Extracting amorphous Fe by this treatment significantly reduced P sorption by the drained soil. Both pH adjustment and the introduction of citrate or silicate ions had some effect on P sorption, but their effects were much smaller than those caused by the anaerobic decomposition of organic matter. The anaerobic decomposition of organic matter, which is associated with accelerated soil reduction, increased amorphous soil Fe and P sorption. © Williams & Wilkins 1986. All Rights Reserved.