Organic acid accumulation under flooded soil conditions in relation to the incorporation of wheat and rice straws with different C:N ratios

Crop residues are important agricultural resources and incorporating wheat straw into rice fields is becoming an alternative to open-field burning after wheat harvest in rice-wheat rotation systems. Organic acids are important soil constituents that are believed to be linked to many biological and environmental processes in the soil, including nutrient cycling, phytotoxicity, metal solubility and greenhouse gas formation. A number of studies have focused on the impact of wheat straw incorporation on organic acid accumulation under flooded soil conditions, but information on the relationship between organic acid accumulation and the properties of wheat straw, especially the C:N ratio, and the response of organic acid accumulation to N application is rare. In this study, incubation experiments were conducted to investigate the difference in organic acid (formic, acetic, propionic and butyric acids) accumulation in soil solution under flooded conditions between wheat and rice straw incorporation, and the relationship between the C:N ratio of the straw materials and organic acid accumulation. Results showed that the concentration of organic acids in the soil solution increased with the rate of straw incorporated. The overall accumulation of organic acids was higher when soil was incorporated with wheat straw than with rice straw. The NH4+ concentration in soil solution decreased more with wheat straw incorporation because of its higher C:N ratio. The addition of urea-N significantly enhanced CH4 flux and reduced the concentration of organic acids in soil solution in the wheat straw treatment, whereas CH4 flux and the concentration of organic acids in the rice straw treatment were less sensitive to the addition of urea-N. The practical implication of the results obtained in this study to N management in rice fields is also discussed.