Abstract:In order to compare the effects of different amounts with straw returning in different soil layers on the composition and stability of micro-aggregates, organic carbon and the humus components of micro-aggregates, situ-simulation experiment using nylon bag was conducted. In the experiment, four levels of R0 (0), R1 (0.44%), R2 (0.88%), R3 (1.32%) and three different returning depths of 0—15(S1), 15—30(S2), 30—45(S3) cm designd. The results showed that the micro-aggregates of 0.25~0.02 mm were the predominant class, and the aggregates of 0.25~0.02 and >0.25 mm content in the soil layers increased with the straw returning, while the aggregates of 0.02~0.002 and <0.002 mm were reduced, and the largest change among the particle content occured in the S1 layer. By incorporating the straw into the soil, the mean weight diameter (MWD) of different size micro-aggregates in different soil layers increased compared with those without straw. At the same time, different amounts of straw returning to the soil could increase the content of humic acid and fulvic acid in each fraction of micro-aggregates, and in the same soil layer, the content of organic carbon and humic acid increased with the decrease of particle size, while in different soil layers, the content of organic carbon and humic acid showed as S1 > S2 > S3. The ratio of humic acid to fulvic acid showed different trends for different micro-aggregates in different soil layers. As a result, it can be seen that straw returning can promote the transformation of soil micro-aggregates to larger aggregates, thus increase the soil fertility and improving the structure of the soil.