The effects of tillage and residue management on labile soil organic carbon fractions and soil organic carbon stocks can vary spatially and temporally, and for different soil types and cropping systems. Surface soil (0–15 cm) was fractionated into aggregate sizes (>4.76 mm, 4.76–2.00 mm, 2.00–1.00 mm, 1.00–0.25 mm, 0.25–0.053 mm, <0.053 mm) under different tillage regimes. Tillage significantly reduced the proportion of macro-aggregate fractions (>2.00 mm) and thus aggregate stability was reduced by 35% compared with (ridge with no tillage) RNT, indicating that tillage practices led to soil structural change for this subtropical soil. The highest SOC was in the 1.00–0.25 mm fraction (35.7 and 30.4 mgkg^-1 for RNT and CT, respectively), while the lowest SOC was in micro-aggregate (<0.025 mm) and silt + clay (<0.053 mm) fractions (19.5 and 15.7 mg⁄ kg for RNT and CT, respectively). Labile C fractions: particulate organic C (POC), microbial biomass C (MBC) and dissolved organic C (DOC) were all significantly higher in NT and ST than in CT in the upper 15 cm. The portion of 0.25– 2 mm aggregates, mean weight diameter (MWD) and geometric mean diameter (GMD) of aggregates from ST and NT treatments were larger than from CT at both 0–15- and 15–30-cm soil depths. Positive significant correlations were observed between SOC, labile organic C fractions, MWD, GMD, and macro-aggregate (0.25–2 mm) C within the upper 15 cm. Tillage did not influence the patterns in SOC across aggregates but did change the aggregate-size distribution, indicating that tillage affected soil fertility primarily by changing soil structure. The average concentration of particulate organic carbon (POC), dissolved organic carbon (DOC) and microbial biomass carbon (MBC) in organic manure plus inorganic fertilizer treatments (NP+S and NP+FYM) in 0–60 cm depth were increased by 64.9–91.9%, 42.5–56.9%, and 74.7–99.4%, respectively, over the CK treatment. Accumulation of crop residues and organic matter in the surface layer under conservation tillage creates favourable feeding conditions and also provides physical protection to various soil organisms, thereby increasing their abundance as well as diversity.

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