Conservation tillage exerts diverse pathways to sequester organic carbon in fluvo-aquic soils of the North China Plain

Conservation tillage (minimal tillage + straw return) significantly enhances soil organic carbon (SOC) accumulation, but the underlying pathways of C sequestration remain unclear. Using a 16-year conservation tillage experiment in the North China Plain, this study investigated how conservation tillage affects SOC quantity and lability, and identified the mechanisms by which tillage and straw management regulate SOC sequestration in the 0–10 cm surface layer. It was found that straw practice exerted a significantly stronger effect on the SOC pool than tillage regime. Compared to continuous tillage, reduced and no-tillage on average increased SOC by 0.55 g kg⁻¹ (with 12.6 % lower lability) under straw removal and by 0.63 g kg⁻¹ (with 15.3 % lower lability) under straw return. By contrast, across all tillage regimes, straw return significantly increased SOC concentration by 3.46–3.62 g kg⁻¹ and SOC lability by 34.9–40.8 % compared to straw removal. Reducing tillage frequency significantly decreased SOC mineralization, promoting the accumulation of relatively stable organic C through increased bulk density and inactive pore volume, along with decreased aeration and water-holding pore volumes. Whereas, straw return significantly increased organic C accumulation in physical subfractions, particularly fine intra-particulate organic matter and mineral-associated organic matter within macroaggregates, which predominantly accounted for the enhancements in SOC concentration and lability. Soil physicochemical properties differentially influenced microbial community abundance under conservation tillage. Specifically, bacteria responded positively to reduced and no-tillage, while both bacteria and fungi were stimulated by straw return, collectively stabilizing SOC. Reducing tillage frequency primarily increased bacterial necromass contribution to SOC, while straw return significantly enhanced necromass C derived from both bacteria and fungi. We conclude that conservation tillage enhances SOC sequestration through diverse pathways in the tested fluvo-aquic soils.