Abstract: The effect of CO₂ enrichment on the quality and quantity of wheat straw, wheat straw decomposition and anaerobic soil microbial biomass activity was examined in rice season. Analysis shows that elevated CO₂ significantly enhances C/N and lignin/N rations of wheat straw due primarily to decreased N content. However, elevated CO₂ has no effect on biochemical compositions (soluble fractions, semi-cellulose, cellulose and lignin) and phenol content of wheat straw. Alteration of wheat straw quality caused by elevated CO₂ has insignificant effect on wheat straw and wheat straw C decomposition, but rather significantly retardes wheat straw N decomposition. When all produced wheat straw under ambient and elevated CO₂ are added to the soil, elevated CO₂ alteration of wheat straw quantity has no obvious impact on subsequent wheat straw decomposition. Change in wheat straw quality from elevated CO₂ insignificantly affects subsequent soil microbial biomass C. This implicitly explains any lack of effect of elevated CO₂ on wheat straw quality, decomposition and C content.