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

Abstract

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 NH⁺ ₄ 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.

Key words:

• C:N ratio
• flooded soil conditions
• gas emission
• nitrogen addition
• organic acids
• rice straw
• wheat straw

ACKNOWLEDGMENTS

This study was supported by National Basic Research Program of China (also called 973 Program), the international collaborative project “Managing Crop Residues for Healthy Soils in Rice Ecosystems” sponsored by the German Ministry of Economic Cooperation and Development and a fund from the Key Lab of Environmental Materials and Engineering in Jiangsu Province, China. The authors thank Ms O.R. Angeles and Mr S. Pantoja from the Division of Crop, Soil and Water Science of the International Rice Research Institute for their technical support in soil solution sampling and organic acid measuring.