Land Uses Impacts on Soil Organic Carbon Dynamics: Isotopic Evidence From Stabilisation and Decomposition in Alluvial Soils

ABSTRACT Stable carbon isotope signatures (δ 13 C) of natural abundance in soil provide insights into the impacts of land use change on soil organic carbon (SOC) dynamics. This study investigated the effects of land use conversion from undisturbed forest to pasture, orchard and agricultural systems with varying cultivation histories (cultivated with Oat, Maize and three‐carbon plants including wheat and soybean (C3), which are indicated by the plant names of Oat, Maize + C3 and Maize + C3 (R) ( R = rotationally)) on δ 13 C and its controlling factors at different soil depths (0–30, 30–60 and 60–100 cm). Our results demonstrate that land use changes significantly influenced δ 13 C signatures in both surface and subsurface soils. Cation bridging (Ca 2+ , Fe 3+ ) and aggregate stability emerged as critical SOC stabilisation mechanisms. The highest δ 13 C values were observed in the 0–30 cm layer under Maize+C3, Maize+C3(R) and orchards, exceeding those in undisturbed forest soils by 5.46%, 3.77% and 5.14%, respectively. In deeper layers, pasture and orchards exhibited the greatest δ 13 C enrichment. Soil under pasture had the lowest β and highest α values. Soil mean weight diameter (MWD) was positively correlated with SOC and negatively correlated with δ 13 C at 0–30 cm, indicating the crucial role of soil aggregation in SOC stabilisation and isotopic signatures. It showed larger aggregate size (higher MWD) is associated with greater SOC storage and a more negative (lighter) isotopic signature. Cations (Al, Fe, K, Ca) influenced SOC stabilisation and δ 13 C through the formation of organo‐mineral complexes and cation bridging. These findings provide valuable insights into carbon cycling dynamics under various land use systems, contributing to the development of sustainable land management practices such as reduced tillage, C 3 ‐C₄ rotations and agroforestry to enhance carbon sequestration in alluvial soils.