Soil organic carbon (SOC) and soil aggregate stability as effected by returned farmland to forestland or grassland in the Loess Plateau Centre Gansu region of China

Abstract Land use change can significantly alter the proportion of soil aggregates, thereby influencing aggregate stability and distribution of soil organic carbon (SOC). However, there needed to be more 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 central Gansu, China. The current research aimed to explore the dynamics of soil aggregates, aggregate stability, and SOC distribution in the Anjiagou watershed in the hilly and gully areas of the Loess Plateau in central Gansu, China. Six land use types (farmland (FL), abandoned cropland (ACL), Medicago sativa (MS), natural grassland (NG), Picea asperata Mast. (PA), Platycladus orientalis (L.) Franco (PO)) were selected as the research objects. By measuring the water-stable aggregates and SOC content of soil aggregates at 0–10 cm, 10–30 cm, and 30–50 cm soil depths, the composition, stability, and SOC characteristics of soil aggregates were analyzed. The results showed that the conversion of FL to forestland and grasslands significantly increased the content of aggregates > 5 mm, mean weight diameter (MWD), and geometric mean diameter (GMD) but decreased the content of aggregates < 0.25 mm. Compared with FL, the increases of MWD and GMD in the soil depth of 0–50 cm under PA, NG, MS, PO and ACL were 473.71% ~ 732.55%, 283.98% ~ 724.60%, 179.06% ~ 634.12%, 142.31% ~ 413.50%, 110.25% ~ 213.34% and 244.04% ~ 607.77%, 141.68% ~ 666.67%, 52.39% ~ 483.33%, 50.49% ~ 214.43%, 35.23% ~ 64.29% respectively. Compared with FL, after FL was returned to forestland and grasslands, SOC content in soil and aggregates decreased under ACL. In other forestland and grasslands, SOC contents 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 soil depth 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. After returning FL to forestland and grasslands, the contributions of aggregates with sizes of > 5 mm, 2–5 mm, 1–2 mm, 0.5-1 mm, 0.25–0.5 mm, and < 0.25 mm 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 of > 5 mm, 2–5 mm, and 1–2 mm, as well as in bulk soil and aggregates. Although SOC content in bulk soil and different particle size aggregates was significantly higher under NG and PA conditions than under MS and PO conditions, soil macroaggregate content, MWD, and GMD were higher under PO and NG treatments than under PA and MS treatments, indicating that soil structure was more reasonable. The ability to resist soil erosion was strongest under PO and NG treatments. Therefore, to improve soil and water conservation capacity and increase soil C sequestration in the Loess Plateau, converting FL to PO and NG may be the best choice for improving the ecological environment.