Priming effect and its regulating factors for fast and slow soil organic carbon pools: A meta-analysis

The priming effect (PE) plays a critical role in the control of soil carbon (C) cycling and influences the alteration of soil organic C (SOC) decomposition by fresh C input. However, drivers of PE for the fast and slow SOC pools remain unclear because of the varying results from individual studies. Using meta-analysis in combination with boosted regression tree (BRT) analysis, we evaluated the relative contribution of multiple drivers of PE with substrate and their patterns across each driver gradient. The results showed that the variability of PE was larger for the fast SOC pool than for the slow SOC pool. Based on the BRT analysis, 67% and 34% of the variation in PE were explained for the fast and slow SOC pools, respectively. There were seven determinants of PE for the fast SOC pool, with soil total nitrogen (N) content being the most important, followed by, in a descending order, substrate C:N ratio, soil moisture, soil clay content, soil pH, substrate addition rate, and SOC content. The directions of PE were negative when soil total N content and substrate C:N ratio were below 2 g kg-1 and 20, respectively, but the directions changed from negative to positive with increasing levels of this two factors. Soils with optimal water content (50%-70% of the water-holding capacity) or moderately low pH (5-6) were prone to producing a greater PE. For the slow SOC pool, soil pH and soil total N content substantially explained the variation in PE. The magnitude of PE was likely to decrease with increasing soil pH for the slow SOC pool. In addition, the magnitude of PE slightly fluctuated with soil N content for the slow SOC pool. Overall, this meta-analysis provided new insights into the distinctive PEs for different SOC pools and indicated knowledge gaps between PE and its regulating factors for the slow SOC pool.