Unraveling the Distribution and Stability of Deep Soil Organic Carbon after Long-Term Conservation Tillage Practices

The dynamics of soil organic carbon (SOC) in deep soil layers (below 20 cm) represent a critical uncertainty in assessing the carbon sequestration potential of conservation agriculture. This study unravels the response of SOC distribution and stability to 15 years of no-till with straw mulching (NT) versus conventional tillage (CT) in a boreal agroecosystem. Our findings indicate that NT notably enhanced active SOC fractions in deep soil (DS), with increases observed in 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 %. More importantly, NT fundamentally altered the composition and stability of the deep SOC pool. It enhanced the stability of microbial necromass carbon (MNC) in DS by elevating the fungal-to-bacterial necromass carbon ratio. Additionally, plant-derived carbon (PDC) demonstrated increased stability in DS under NT, indicated by a 58 % reduction in (Ac/Al)s values, a 28 % decrease in (Ac/Al)v values, and an 84 % increase in V-type phenols. In the 20-100 cm layer, FNC and MBC emerged as key factors influencing SOC contents. Our findings suggest that longterm NT farming fundamentally transforms the distribution and stability of SOC in DS. It not only enriches labile carbon pools but also promotes a shift towards a more persistent carbon pool dominated by fungal necromass and physically protected plant-derived compounds in deep soil. This demonstrates that long-term NT fundamentally affects SOC fractions, origins and stability in DS, providing new insights into carbon biogeochemical cycling under long-term conservation tillage and highlighting its potential to enhance carbon sequestration beyond the topsoil in agricultural systems.