[Effects of Land Use Intensity on Organic Carbon, Pb, and Cd Contents and Occurrence Forms of Soil Aggregates]

The change of land use intensity will affect the structural stability and soil properties of soil aggregates, which will have an important impact on the content and morphology of aggregate organic carbon and heavy metals in soil. In order to elucidate the changes of organic carbon and heavy metal content and occurrence forms of soil aggregates driven by land use intensity, six treatments were set up by field experiment, including abandonment (CK), low-intensity utilization (P1), medium-low-intensity utilization (P2), medium-intensity utilization (P3), medium-high-intensive utilization (P4), and high-intensity utilization (P5), to explore the effects of different land use intensities on the composition of soil aggregates, organic carbon content, and heavy metal Cd and Pb content and occurrence forms. The results showed that: ① With the increase of land use intensity, soil pH, soil organic matter, and cation exchange decreased significantly (P<0.05), and the contents of various nutrients such as total nitrogen and total phosphorus increased as a whole, and the nutrients in the 0-20 cm soil layer were higher than those in the 20-40 cm soil layer. ② The proportion of large particle size aggregates decreased significantly with the increase of land use intensity, and the proportion of small particle size aggregates increased significantly, while the organic carbon content of each particle size aggregate increased first and then decreased with the increase of use intensity. ③ The content of Cd and Pb in the soil layer of 0-20 cm was generally higher than that in the soil layer of 20-40 cm and showed a decreasing trend with the increase of land use intensity, with the highest soil ω(Cd) reaching 0.61 mg·kg^-1 under the P2 treatment and 88.1 mg·kg^-1 of ω(Pb) under the P3 treatment. The occurrence morphology of Cd and Pb was significantly affected by land use intensity (P<0.05), and with the increase of use intensity, the proportion of exchangeable state and carbonate state increased significantly, whereas the proportion of residual state decreased, and the highest proportion of residual state was approximately 75.3% under the P4 treatment. In general, the soil environmental factors (P<0.05) were significantly changed by the synergistic effect of physical disturbance and chemical fertilizer input, which not only reduced the stability of soil aggregate structure but also led to significant changes in the contents of heavy metals cadmium (Cd) and lead (Pb) (P<0.05) and affected their occurrence morphology.