Abstract: Soil aggregate is the main carrier of soil organic carbon. Its stability is the key index of soil quality and fertility, which has great significance to maintaining the stability of soil organic carbon pool. In this study, the responses of soil organic carbon and stability of soil aggregates to thinning were investigated based on an experiment with four stand densities \[1100 trees·hm^-2 (high thinning, HT), 1415 trees·hm^-2 (moderate thinning, MT), 1850 trees·hm^-2 (light thinning, LT), and 2100 trees·hm^-2 (CK)\] of 35-year-old Larix principis-rupprechtii plantation in Haodifang Forest Farm, Shanxi Province. The stability of soil aggregates \[mean weight diameter (MWD), geometric mean diameter (GMD), fractal dimension (D)\] and the content of soil organic carbon in aggregates with different particle sizes were measured by dry-sieving method. A regression model was used to analyze the relationship between soil aggregate stability and soil organic carbon content. The results showed that: (1) Thinning had significant influence on the stability of soil aggregates. At the depths of 0-10 cm and 20-30 cm, the orders of MWD and GMD were MT>LT>HT>CK and LT>MT>HT>CK, respectively. The CK stands had the highest D value in each soil layer, but the difference was statistically significant only in the soil depth of 10-30 cm. Aggregates with particle size of >2 mm mainly existed in MT stands. (2) In each soil layer, the content of soil organic carbon was the highest in MT stands. The contribution rate of soil organic carbon in MT stands with particle size >2 mm was the highest, up to 45.48%. The content of soil organic carbon in deep soil layer (20-30 cm) was positively correlated with aggregate stability. Therefore, proper thinning was conducive to increasing soil organic carbon content and improving soil aggregate stability. Soil organic carbon was mainly stored in >2 mm particle size aggregates. From the perspective of improving soil organic carbon and aggregate stability, it is suggested that the density of 35-year-old L. principis-rupprechtii plantation should be controlled at 1415 trees·hm^-2.

Key words: Larix principis-rupprechtii plantation, soil organic carbon, mean weight diameter, geometric mean diameter, stand density.