Shifts in microbial mechanism regulate carbon priming effect under two simulated root exudate and nitrogen addition in coniferous and broad-leaved forest soils

Priming effect (PE) plays a crucial role in driving soil organic carbon (SOC) decomposition and it is strongly affected by C addition types and nitrogen (N) addition. However, the understanding of the strength and microbial mechanisms of PE in response to specific root exudates (glucose and oxalic acid) and N addition remains inadequate.

In this study, we carried out a 60-day incubation experiment using simulated root exudates (i.e., glucose and oxalic acid) and inorganic N in coniferous and broad-leaved forest soils to estimate their effects on PE and microbial mechanisms.

Oxalic acid addition resulted in positive PE through “co-metabolism” (i.e., the accelerated microbial decomposition of native SOC), which was supported by an increase in microbial biomass C (MBC), and the activities of enzyme involved in the C and N metabolism in both forest soils. In contrast, N addition significantly lowered positive PE by moderating N mining, as supported by the decreased ratios of fungi: bacteria (F: B), oxidase activities: hydrolase activities (O: H), and C: N enzyme activities, and increased CO₂ derived from root exudate per MBC. These results indicated that stoichiometric decomposition increased with the partial preferential use of the root exudate. The pattern of increased ratios of F: B, O: H, and C: N enzymes with incubation time revealed the dominance of microbial N mining.

Collectively, our results demonstrate that shifts in driving PE from stoichiometric decomposition to microbial N mining over time predominantly depend on N availability, thereby providing insightful evidence for accurately assessing soil C dynamics for future climate change.