Sustained release and the kinetics of phosphorus from dark brown forest soils by organic acids.

By simulating oxalic/oxalate and citric acid/citrate concentrations in forest litter leachates of northeast China, we prepared 0.5 and 2.0 mmol/L oxalic acid/oxalate and citrate/citrate solutions. Dark brown forest soils of A1 horizon were extracted with different concentrations of oxalic acid/oxalate and citrate/citrate solutions, in which different extraction time and frequencies were set, and the effects of extraction time and frequency on phosphorus release of dark brown forest soils by organic acids were studied, and the optimum extraction time was determined. The aims of this study were to ascertain the effects of oxalic acid and citric acid on phosphorus sustained activation from dark brown forest soils, and clarify the degrees and kinetics of phosphorus activation by organic acids from dark brown forest soils. The results showed that, when extracting A1 horizon of dark brown forest soils with oxalic acid/oxalates and citric acid/citrates, phosphorus release firstly increased and then decreased, and the optimum extraction time was 12 h. As for cumulative phosphorus release, within1--72 h, phosphorus release kinetics by the above extraction agents could be well fitted by a logarithmic equation. When extracting dark brown forest soils of A1 horizon with oxalic acid/oxalates and citric acid/citrates for 10 times and 18 times respectively, phosphorus release of the second extraction was greater than that of the first one, and after the third extraction, it generally decreased with increasing extraction times. And there were still some high concentrations of phosphorus release till the tenth or eighteenth extraction, particularly in citric acid/citrate treatments, indicating that phosphorus release caused by organic acids was cumulative. Release of cumulative phosphorus increased with increasing extraction times, and those from A1 horizon caused by oxalic acid/oxalates and citric acid/citrates and extraction times could all be well fitted by a logarithmic equation. Our study provides a theoretical basis for the better understanding of the sustained effects of organic acids on phosphorus activation under field conditions, and is of significance in practice.