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In order to make an investigation on the effect of synchronously burying and returning straw and biogas slurry to the soil on straw decomposition, the research was conducted under normal condition at 20℃ by simulating burying and returning to soil by net bag method. In the 105d experimental period, effects of biogas slurry produced by anaerobic fermentation of cattle manure or tape water on straw decomposition were systematically studied; influences of different soil types and different straw lengths on the decomposition rate of straw, the contents of cellulose, hemicelluloses, and lignin in straw decomposition after adding biogas slurry were also explored. The experimental results showed that biogas slurry could greatly accelerate straw decomposition. The straw decomposition rate in groups of tape water added was 33.61%, while that in groups of biogas slurry added was 42.37%. The straw decomposition mainly occurred in the first 70d after adding biogas slurry. When the experiment was over, the straw decomposition rate of each experimental group was higher than 42%. The straw decomposition rate in the experimental groups of straw length being 20mm was higher than that of being 10mm. When the experiment was over, the straw decomposition rate in the experimental groups of straw length being 20mm could reach 52%. Hemicelluloses decomposition could apparently be affected by straw lengths, and the shorter the straw length was, the quicker hemicelluloses decomposition was. However, cellulose and lignin decomposition could obviously be influenced by soil types. Compared with black loam soil, sandy loam soil was more active for the decomposition of cellulose and lignin in straw after its returning to soil. This research may lay a theoretical foundation for quick decomposition of straw in its returning to soil, as well as promote the widespread application of technique in straw returning to soil.