Exudate components mediate soil C dynamic through different priming mechanisms in forest soils

While accumulating evidence suggests that root exudate inputs can significantly affect soil organic carbon (SOC) dynamic and stability through rhizosphere priming effects, our understanding of the effects of specific exudate components on soil C storage and the underlying mechanisms remains largely limited. In a well-controlled rhizosphere simulation system, we added three exudate components (i.e., glucose, glycine, and oxalic acid) through artificial roots to soils collected from a ~200-year-old spruce-fir forest for an incubation period of 25 days. The results showed that oxalic acid addition induced a net soil C loss (−1.12%) through two synergistic priming processes. On the one hand, the addition of oxalic acid accelerated SOC mineralization by stimulating microbial activity and enzyme production (biotic process); on the other hand, the previously mineral-protected SOC was released for microbial utilization through the disruption of mineral-organic associations after oxalic acid addition (abiotic process). In contrast, glucose addition increased soil C accumulation (1.28%) by two antagonistic priming processes. Glucose not only enhanced microbial activity and enzyme production and consequently accelerated SOC mineralization (biotic process), but also promoted the formation of mineral-organic associations to inhibit the microbial accessibility of SOC (abiotic process). Compared to glucose, glycine addition induced a greater soil C accumulation (2.52%) mainly via inhibiting microbial degradation (biotic process). These observations suggested that distinct components of root exudates profoundly mediate soil C dynamics and soil C storage in forests through different biotic and abiotic priming mechanisms.