[Effects of Land Use Type on the Content and Stability of Organic Carbon in Soil Aggregates]

The distribution and stability of soil aggregates under different land use types, and the distribution of organic carbon in various aggregates, were explored in order to provide a basis for the improvement of soil structure and the maintenance and improvement of soil organic carbon in Chongqing. Focusing on six representative subtropical land use types in the Beibei District of Chongqing (coniferous and broadleaf mixed forest, bamboo forest, orchard, dryland, paddy field, and waste grassland), soils were sorted using the wet sieving method and the six soil types were compared and analyzed. The distribution of aggregates and their organic carbon content were determined at soil profile depths of 0-20, 20-40, 40-60, and 60-100 cm for each land use type. The results showed that there were significant differences in soil structure and fertility levels under the different land use types. Among the particle size aggregates of the 0-100 cm soil layer, the aggregate particle size of the six land use types was mainly >0.25 mm; the content of >0.25 mm aggregate fraction of bamboo forest soil is the highest, followed by the grassland soil, and the dryland and orchard soils is the lowest. Granular aggregates in the size class 0.25-2 mm were mainly distributed in the 0-20 cm soil layers (28.78%-50.08%), while the aggregate size fractions 0.053-0.25 mm and<0.053 mm were mainly concentrated in the 40-60 cm soil layers. Across the entire profile depth, the aggregate MWD and GMD of the bamboo forest and barren grassland soils were higher than the other land use types, i. e., the soil aggregates in these soils were more stable. The stability of soil aggregates was significantly positively correlated with soil aggregate organic carbon content (r=0.569, P<0.01). In the 0-100 cm soil layer, the organic carbon content of soil aggregates was higher for the 0.25-2 mm and<0.053 mm fractions, with an average content of 56.54 g·kg^-1 in the 0.25-2 mm fraction. Except for the organic carbon content of the soil aggregates with different particle size of dryland was the highest in the 20-40 cm soil layer, while under the other land use types, the organic carbon content in soil aggregates decreased with depth, showing significant surface enrichment. Overall, under the six different land use types, the soil aggregates in bamboo forest soils and waste grassland soils showed good stability, and at each soil depth, the organic carbon content of aggregates was highest in the bamboo forest soil.