Contribution of microbial necromass to soil organic carbon in profile depths exhibited opposite patterns across ecosystems: A global meta-analysis

Abstract

Microbial necromass carbon (MNC) is a crucial component of soil organic carbon (SOC) and plays a significant role in long-term carbon sequestration. However, the distribution patterns of MNC across soil profiles remain poorly understood. Here, we compiled a dataset of 1447 observations from depths of up to 1 m across cropland, forest, and grassland ecosystems to evaluate the distribution and key influencing factors of MNC. MNC content consistently decreased from 9.3 g kg⁻¹ in topsoil (0–20 cm) to 2.4 g kg⁻¹ in deep soil (60–100 cm), regardless of ecosystem types. Fungal necromass contributed more to SOC in topsoil, whereas bacterial necromass dominated deeper soil profiles. The MNC/SOC ratio decreased from 50.9 % in topsoil to 34.2 % in deep soil for croplands. In contrast, the MNC/SOC ratio increased from 26.2 % in topsoil to 44.2 % in deep soil for forestland and increased from 39.7 % in the topsoil to 52.2 % in the subsoil (40–60 cm) for grassland. Cropland topsoil exhibited a low proportion of large macroaggregates, which were negatively correlated with the MNC/SOC ratio and microbial quotient. This suggests possible destabilization of plant-derived carbon due to intensified microbial activity from macroaggregate disruption. The microbial necromass accumulation coefficient (MNC/microbial biomass C) decreased with soil depth in croplands but increased in forests and grasslands. Nitrogen availability and clay content were the primary regulators of the MNC/SOC ratio in cropland topsoil, whereas the C/N ratio was the dominant controlling factor in forests and grasslands. These results suggest that the MNC/SOC ratio in cropland topsoil is associated with rapid microbial turnover due to high nitrogen availability, coupled with MNC stabilization promoted by high clay content. Overall, our results indicated that MNC/SOC ratio was more important in deep soil than in topsoil and showed an opposite pattern in cropland. This highlights a potential strategy for enhancing SOC in croplands by promoting the stabilization of plant-derived carbon through macroaggregate formation.