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

Authors: CAI, ZEJIANG - Chinese Academy Of Agricultural Sciences; XU, MINGGANG - Chinese Academy Of Agricultural Sciences; ZHANG, LU - Chinese Academy Of Agricultural Sciences; WANG, BOREN - Chinese Academy Of Agricultural Sciences; WEN, SHILIN - Chinese Academy Of Agricultural Sciences; MISSELBROOK, TOM - Rothamsted Centre For Sustainable Pest And Disease Management; CARSWELL, ALISON - Rothamsted Centre For Sustainable Pest And Disease Management; Gao, Suduan

Submitted to: Journal of Soils and Sediments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/13/2020
Publication Date: 5/20/2020
Citation: Cai, Z., Xu, M., Zhang, L., Wang, B., Wen, S., Misselbrook, T.H., Carswell, A.M., Gao, S. 2020. Decarboxylation of organic anions to alleviate acidification of red soils from urea application. Journal of Soils and Sediments. 20:3124–3135. https://doi.org/10.1007/s11368-020-02630-2.
DOI: https://doi.org/10.1007/s11368-020-02630-2

Interpretive Summary: Accelerated soil acidification from chemical fertilizer applications have resulted in reduced crop yield in the red soil region of Southern China. Organic anions released from decomposition of crop residues could increase soil pH, but the potential and effective compounds have not been well determined. This study examined three organic anions (citrate, malate, and oxalate) commonly found in crop straws and their effect on soil acidity and N transformation processes following urea application to a red soil in a laboratory incubation experiment. Results show that all three organic anions significantly increased soil pH, and the effectiveness was positively correlated with application rate. Among the anions, oxalate was the most effective in increasing soil pH and reducing soil acidity. However, to release a sufficient amount of organic anions, a much higher amount of crop straws is required than can be produced in the field. The results imply that straw return can play some role in reducing soil acidity and should be encouraged as a management practice, but by itself cannot prevent soil acidification if chemical fertilizer is the major nitrogen source so other means should be incorporated.

Technical Abstract: Decarboxylation of organic anions within incorporated crop straw is recognized as a major driver 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). A 35-day incubation experiment was conducted using soil after receiving 26 years of two different nutrient treatments at a long-term experimental station in Qiyang, Hunan Province, China: 1) chemical nitrogen, phosphorus, and potassium fertilization only (NPK soil, pH 4.30), and 2) NPK plus swine manure (NPKM soil, pH 5.88). For the incubation study, urea was added to the soils at 150 mg N kg-1 and treatments applied were: 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), and a nontreated soil as a reference. Soil acidity, mineral N species, CO2 production, and their correlations were determined. All three organic anions significantly increased soil pH under both nutrient treatments, 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 acidity by 25–68% in NPK soil and by 63–88% in NPKM soil as compared to control. Oxalate was the most effective treatment, increasing soil pH by 0.70 and 1.31 units, reducing exchangeable acidity by 3.79 and 0.33 cmol (+) kg-1 in NPK and NPKM soils, respectively, and also resulting in the highest respiration or CO2 production rate. Addition of organic anions led to a lower nitrification rate in NPKM soil relative to the NPK soil. These results imply that crop straws rich in organic anions, especially oxalate, have a good potential to alleviate soil acidification. Straw return should be a management practice for soil acidity.