Conservation Agriculture Enhances Labile and Mineral‐Associated Organic Carbon in Mediterranean Durum Wheat Under Legume‐Based Rotation

Conservation agriculture (CA) is widely recognized as an effective strategy for restoring soil organic carbon (SOC), but the mechanisms through which tillage intensity, residue quality and crop rotation regulate SOC stabilization remain poorly understood in Mediterranean cropping systems. This study evaluated the long‐term effects of CA, tillage intensity and wheat–faba bean rotation on SOC quantity, quality and vertical distribution. Two long‐term field experiments were conducted on clay‐loam soils under Mediterranean conditions in southern Italy. Experiment 1 compared conservation agriculture and conventional tillage under continuous durum wheat or wheat–faba bean rotation, whereas Experiment 2 compared no‐tillage and minimum tillage within CA. SOC was monitored over time, while mean weight diameter (MWD) of water‐stable aggregates, permanganate‐oxidizable carbon (POX‐C) and SOC pools, including coarse (cPOM‐C), fine particulate (fPOM‐C) and mineral‐associated organic carbon (MAOM‐C), were determined at the final sampling. In Experiment 1, CA increased aggregate stability and SOC relative to conventional tillage, with gains largely confined to the 0–10 cm layer. The combination of CA and wheat–faba bean rotation promoted the greatest SOC accumulation through increased MAOM‐C despite lower carbon inputs. Higher cPOM‐C and fPOM‐C contents and distinct C/N ratios indicated enhanced physical protection of plant‐derived carbon within aggregates. In Experiment 2, minimum tillage maintained SOC comparable to no‐tillage during the continuous wheat phase despite lower carbon inputs. Following legume introduction, only no‐tillage increased SOC, POX‐C and, to a lesser extent, MAOM‐C in the surface layer, highlighting a management‐dependent effect of residue quality. Overall, SOC stabilization under CA relies on two complementary processes: physical protection of particulate organic carbon within aggregates and formation of mineral‐associated organic carbon from high‐quality residues. However, SOC gains remained largely confined to surface soil, underscoring the need for management strategies that enhance carbon stabilization throughout the soil profile.