Higher priming effect induced by stems than leaf and root litters

Soil organic carbon (SOC) decomposition can be influenced by plant litters via the priming effect (PE). While plant litters largely differ in biomass allocation, and chemical properties and thus decomposability, if and how PE varies with litter type is still unknown. We aimed to synthesize geographical generalities of the litter-induced PE across ecosystem types, and explore the relative importance of regulatorssuch as environmental conditions, litter and soil properties.We conducted a meta-analysis with 621 observationsfrom 118 publications, using multiple statistical approaches such as mixed-effects meta-regressions, variation partitioning analysisand structure equation modelling.Plant litter addition significantly enhanced SOC decomposition by on average 42.33% globally, i.e., an overall positive PE. Stem litters induced a higher PE than leaf and root litters, especially in soils from croplands, indicating the higher susceptibility of SOC decomposition in response to plant litter inputs than soils in natural ecosystems. Microbial biomass Cwas also significantly stimulated by average 45.40% acrossecosystem types, thus supporting the microbial co-metabolism hypothesis. Although environmental conditions had the highest relative importance followed by soil and litter properties, the effect size and direction varied with litter type. There was a higher positiveeffect and a lower negative effect of temperature on root and leaf litter-induced PE, respectively. Soil properties were more important for the root litter-induced PE than leaf litter-induced PE, with soil C:nitrogen ratio being a negative effect and soil pH being a positiveeffect, likely due to the lower decomposability of roots.We emphasize the importance of the specificity of litter type in regulating PE. Future studies should comprehensively investigate litter properties and interactions with environmental condition and soil property with long-term experiments to improveglobal predictions of SOC decomposition via PE.