Subsoil Tillage With Straw Mulching Enhances Top‐Down Control of Deep Soil Carbon Accumulation via Fungal Pathways

ABSTRACT Enhancing soil organic carbon (SOC) sequestration, especially in deep soil layers, is essential for agricultural sustainability. Conservation tillage practices, such as no‐tillage and subsoil tillage with straw mulching, can increase SOC accumulation, yet their depth‐specific effects remain unclear. This study aimed to investigate how different conservation tillage practices influence SOC distribution and microbial contributions to SOC accumulation across the entire 0–100 cm soil profile. We assessed microbial biomass and necromass using biomarkers—phospholipid fatty acids (PLFAs) and amino sugars—in a 6‐year conservation tillage experiment (no‐tillage without straw, NT; no‐tillage with straw mulching, NTS; subsoil tillage with straw mulching, STS) spanning the 0–100 cm soil profile. Our findings revealed that straw mulching treatments (NTS and STS) sustain higher SOC storage, total PLFAs, bacterial PLFAs, and microbial necromass carbon in the topsoil (0–10 cm). Both NTS and STS boosted SOC contents in the topsoil by 17% and 14%, respectively, while STS enhanced the proportion of SOC in deeper soil layers through a top‐down effect. Specifically, STS maintained the highest SOC storage, SOC content, and fungal necromass carbon content, with greater fungal necromass contributing to SOC in the deep soil (40–60 cm) compared with NTS. Structural equation modeling revealed that a bacterial pathway dominated SOC buildup in topsoil under NTS, while a fungal‐mediated pathway drove deeper SOC accumulation under STS. Our results demonstrate that both NTS and STS can improve SOC content, but their effects vary with soil depth, associated with different microbial regulatory mechanisms. Thus, subsoil tillage with straw mulching offers a top‐down effect to align with the goal of carbon sequestration across the entire soil profile and supports sustainable agriculture.