Effects of Long-Term Mulching on Soil Aggregation and Organic Carbon Sequestration in Sloping Croplands of the Loess Plateau

Sloping cropland on the Loess Plateau faces severe challenges from soil organic carbon (SOC) depletion and structural instability due to erosion and intensive tillage. Although mulching can enhance SOC sequestration, its long-term effects on the spatial distribution of SOC and aggregates across slopes remain unclear. A 15-year field experiment evaluated five practices—conventional tillage (T), no tillage (NT), straw mulching (SM), plastic film mulching (PM), and ridge–furrow plastic film mulching (RPM)—on SOC storage, aggregate stability, and their variation with different slope positions. Compared to T, all mulching treatments significantly increased SOC concentration by 4.19% to 83.48% in the 0–30 cm layer. SM and RPM notably increased macro-aggregates (>2 mm) and their associated SOC (24.04–56.49% higher than T) by adding organic matter and optimizing micro-topography. Different slope positions strongly influenced SOC redistribution: lower slopes accumulated more SOC than upper slopes due to erosion–deposition processes. Mulching reduced SOC spatial variability and minimized differences between slope positions. Although mulching increased cumulative SOC mineralization compared to T, the long-term net SOC gain was positive, driven by improved aggregate protection and reduced erosion. SM and RPM are recommended for sustainable slope farmland management due to their dual benefits in enhancing carbon sinks and soil stability. This study offers practical strategies for improving soil health and SOC sequestration in vulnerable sloping landscapes.