EFFECTS OF LOW MOLECULAR WEIGHT ORGANIC ACIDS ON PHOSPHORUS ADSORPTION CHARACTERISTICS IN A BLACK CLAY SOIL IN NORTHEAST CHINA

The decrease of P adsorption induced by low molecular weight organic acids (LMWOA) is considered as an important mechanism for enhancing P mobilization in soils. But how LMWOA specifically alter soil P adsorption capacity is not well understood, much less the mechanisms involved. In this study, the effects of three common LMWOA (oxalic acid, citric acid and malic acid) on P adsorption in a black clay soil were studied in batch experiments, followed by sequential P fractionation to elucidate the mechanisms of P adsorption perturbed by LMWOA. The adsorption data of the studied soil were well fitted to Freundlich equation (r2=0.821-0.878, P<0.01). Oxalic acid, citric acid and malic acid at 10 mmol kg-1 soil decreased Freundlich parameters Kf and n, which represent P adsorption capacity and energy. The mechanism involved is that LMWOA significantly decreased the amounts of P adsorbed in soil NaHCO3-Pi (soil soluble and exchangeable Pi, 11.6-29.3%) and NaOH-Pi (Fe/Al-Pi, 18.3-41.2%) fractions.