No‐till reduced subsoil organic carbon due to decreased microbial necromass in micro‐aggregates

Microbial necromass is a crucial contributor to the formation of soil aggregates and serves as a key source of stable soil organic carbon (SOC). However, how tillage practices regulate microbial necromass accumulation within aggregate fractions at different soil depths remains unclear. Thus, a 20‐year field experiment was carried out to identify how microbial necromass carbon (MC) is distributed in aggregates and its contribution to SOC under no‐till (NT) and plow tillage (PT) in a rice‐wheat rotation system. The results showed that NT significantly increased bacterial and fungal necromass carbon (C) concentrations in both macro‐ and micro‐aggregates (>0.25 and <0.25 mm), and enhanced the contribution of MC to SOC compared with PT at 0–5 cm topsoil depth. However, NT decreased SOC concentration in micro‐aggregates by 12.4% mainly attributed to lower bacterial and fungal necromass C in micro‐aggregates by 21.6% and 27.6% compared with PT at 5–15 cm subsoil depth. There was no significant difference for SOC mineralization per unit SOC concentration between NT and PT, which can be attributed to the equilibrium between microbial necromass C and labile C (e.g., dissolved organic C). Random forest and partial least squares path model demonstrated that soil total nitrogen, available NH 4 + , and NO 3 − positively regulated β‐glucosidase activity, and thus affected MC and SOC concentration. NT decreased soil nitrogen concentration and β‐glucosidase activity in micro‐aggregates, and thus lowered MC formation at 5–15 cm depth. Our findings highlights that NT reduced subsoil organic carbon due to decreased microbial necromass in micro‐aggregates.