Response of soil organic carbon and soil aggregate stability to changes in land use patterns on the Loess Plateau

Land use change can significantly alter the proportion of soil aggregates, thereby influencing aggregate stability and distribution of soil organic carbon (SOC). However, there is minimal research on the variations in the distribution of soil aggregates, aggregate stability, and SOC in soil aggregates following land use change from farmland (FL) to forest and grassland in the Loess Plateau region of China. Select six land use patterns (farmland (FL), abandoned cropland (ACL), Medicago sativa (MS), natural grassland (NG), Picea asperata Mast. (PA), Platycladus orientalis (L.) Franco (PO)) on the Loess Plateau in China and collect undisturbed soil samples. These six land use patterns have similar geographical characteristics. The distribution of aggregates and the aggregate-associated SOC contents under the six land use patterns were measured at the 0-10 cm, 10-30 cm and 30-50 cm depths. The results showed that forestland and grasslands converted from FL significantly increased the aggregates (> 5 mm) content, mean weight diameter (MWD), and geometric mean diameter (GMD) but decreased the aggregates (< 0.25 mm) content. Compared with FL, the values at the 0-50 cm depth under PA, NG, MS, PO and ACL increased by 473.71-732.55%, 283.98-724.60%, 179.06-634.12%, 142.31-413.50% and 110.25-213.34%, respectively, for MWD and by 244.04-607.77%, 141.68-666.67%, 52.39-483.33%, 50.49-214.43%, and 35.23-64.29%, respectively, for GMD. Land use patterns and soil aggregate size had obvious influences on SOC content, SOC content in soil and aggregates decreased under ACL. In other forestland and grasslands, The SOC content in bulk soil, > 5 mm, 2-5 mm, 1-2 mm, 0.5-1 mm, 0.25-0.5 mm, and < 0.25 mm aggregates at the 0-50 cm depth after afforestation increased by 20.75-125.87%, 14.50-163.64%, - 11.86-118.18%, 9.65-150.95%, 38.28-126.49%, 51.26-165.87% and - 15.59-163.37%, respectively, Compared to FL. The contributions of different aggregates particle sizes to the increase in SOC content in bulk soil were 104.74%, 7.86%, 4.76%, 6.23%, 5.37%, and - 21.97%, respectively. MWD and GMD were positively correlated with SOC content in aggregates (1 mm), SOC content in bulk soil and aggregates. Although SOC content in bulk soil and different aggregates particle sizes under NG and PA were significantly higher that than under MS and PO, the soil macroaggregate content, MWD, and GMD under PO and NG were higher than that under PA and MS. These findings suggest that converted FL into PO and NG significantly improved soil structure and also increased SOC content. Therefore, in the process of transforming land use patterns on the Loess Plateau, the proportion of forest land and grassland should be appropriately increased to improve soil carbon storage and quality. The results of this study provides a theoretical basis and scientific basis for the scientific evaluation and understanding of soil organic carbon accumulation and distribution under different land use patterns in the Loess Plateau region of China.