Root activity creates a unique microbial hotspot in the rhizosphere and profoundly regulates soil carbon (C) dynamics, but empirical assessments of the soil microbial carbon pump (MCP, the iterative accumulation of necromass after microbial anabolism) and associated ecological consequences on soil C storage based on insight of the rhizosphere are still neglected, especially for different root functional modules.

We assessed the soil MCP efficacy (i.e., the contribution of microbial necromass to SOC) by investigating the divergent contribution of microbial necromass based on amino sugar extrapolations to soil organic C (SOC) in the rhizosphere of two root functional modules (i.e., absorptive roots and transport roots) and the bulk soil in an alpine coniferous forest.

The results showed that the MCP efficacy in both rhizosphere and bulk soil was more than 50%, suggesting that microbial necromass plays a key role in SOC formation. More importantly, absorptive roots drove a greater MCP efficacy (56%) in the rhizosphere than transport roots (51%).

Synthesis. These observations suggest that the microbial necromass is a major contributor to SOC storage in both rhizosphere and bulk soil in alpine coniferous forests. The magnitude of the contribution of microbial necromass to rhizosphere SOC depends on root functional differentiation. Our study provides novel and direct empirical evidence for the active soil MCP functions in SOC sequestration from the perspective of the rhizosphere.