Decarboxylation of organic anions to alleviate acidification of red soils from urea application

Purpose Decarboxylation of organic anions in crop straw is recognized as one of the mechanisms for increasing pH in acidified soils. However, the effectiveness of specific compounds in alleviating soil acidification from nitrification has not been well determined. This study examined three organic anions commonly found in crop straws and their effect on soil acidity and N transformation processes following urea application to a red soil (Ferralic Cambisol). Materials and methods A 35-day incubation experiment was conducted using soil after receiving 26 years of two different nutrient treatments: (1) chemical nitrogen, phosphorus, and potassium fertilization (NPK, pH 4.30) and (2) NPK plus swine manure (NPKM, pH 5.88). Treatments included three rates (0.25, 0.5, and 1.0 g C kg−1) of calcium citrate, 0.5 g C kg−1 calcium oxalate, 0.5 g C kg−1 calcium malate, urea-only (control) soil, and a non-treated soil as a reference. Soil acidity, mineral N species, decarboxylation, and their correlations were determined. Results and discussion All three organic anions significantly increased pH in both soils and the effectiveness was positively correlated with application rate. The change in total exchangeable soil acidity was dominated by aluminum concentration in the NPK soil, but by proton concentration in the NPKM soil. At ≥ 0.5 g C kg−1, the anions decreased soil exchangeable acidity by 25–68% in NPK soil and by 63–88% in NPKM soil as compared with control. Oxalate was the most effective in increasing soil pH by 0.70 and 1.31 units and reducing exchangeable acidity by 3.79 and 0.33 cmol(+) kg−1 in NPK and NPKM soils, respectively, and also resulted in the highest CO2 production rate. Addition of organic anions led to a lower nitrification rate in NPKM soil relative to the NPK soil. Conclusions These results imply that crop straws rich in organic anions, especially oxalate, would have a higher potential to alleviate soil acidification.