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 NH4+, and NO3- positively regulated beta-glucosidase activity, and thus affected MC and SOC concentration. NT decreased soil nitrogen concentration and beta-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.