Accumulation of organic acids and their phytotoxicity in paddy rice fields incorporated with crop straws have been widely concerned.Laboratory incubation was conducted to investigate the accumulation of organic acids,including formic,acetic,propionic and butyric acid in soil solution under flooded conditions and the effects of C N ratios of straw materials and N application.Results showed that the concentration of organic acids in soil solution increased with the rate of straws incorporated.The overall accumulation of organic acids (especially propionic acid) was significantly higher in soil incorporated with wheat straws than with rice straws,while NH₄⁺ concentration in the soil solution was the other way round due to wider C N ratio in wheat straw.Addition of urea significantly reduced concentration of organic acids in soil solution and enhanced CH₄ flux but had no obvious influence on CO₂ flux in the wheat straw treatment,while in the rice straw treatment,the concentration of organic acids and CH₄ flux were less sensitive to N addition.Results obtained in this study indicated that wider C N ratio of wheat straw enhanced the immobilization of inorganic N and inhibited the transformation of organic acids to CH₄ whichled to higher accumulation of organic acids.N application can serve as one of the effective ways to reduce the accumulation of organic acids in rice fields incorporated with wheat straw,but may enhance CH₄ emission