Mystery of Deep Soil Organic Carbon Dynamics Uncovered through Long-term Conservation Tillage Experiments

The boreal ecosystem in China serving as a crucial store of soil organic carbon (SOC). However, agricultural soil degradation threatens this carbon reservoir. Conservation tillage has been shown to enhance soil fertility and biodiversity, with the potential to increase SOC stocks. Yet, the composition and origins of SOC in boreal regions—particular in deep soil profiles (i.e., 20–100 cm) —remain unclear. Here, we present findings from a 15-year field trial in a typical boreal agroecosystem, comparing no-till with straw mulching (NT) and conventional tillage (CT) under a maize-soybean crop rotation. We found that NT significantly increased active SOC fractions in deep soil, including microbial biomass carbon (MBC, by 104%), particulate organic carbon (POC, by 112%), light fraction organic carbon (LFOC, by 42%) and dissolved organic carbon (DOC, by 22%). NT increased the stability of microbial necromass carbon (MNC) in the deep soil by increasing the ratio of fungal necromass C to bacterial necromass C (by 55% compared to topsoil). Plant-derived carbon showed increased stability in the deep soil, as evidenced by decreased (Ac/Al)s values (-58%) and (Ac/Al)v values (-28%) and a 84% rise in V-type phenols under NT. In the 20-100 cm layer, FNC and MBC emerged as key factors influencing SOC contents. Our findings suggest that long-term NT in boreal agroecosystems affects SOC dynamics in both topsoil and deep soil, providing new insights into carbon sequestration under long-term conservation tillage. These results contribute to a more comprehensive understanding of the carbon cycle in sustainable agriculture.