Positive soil priming effects are the rule at a global scale

Priming effects of soil organic matter decomposition are critical to determine carbon budget and turnover in soil. Yet, the overall direction and intensity of soil priming remains under debate. A second-order meta-analysis was performed with 9296-paired observations from 363 primary studies to determine the intensity and general direction of priming effects depending on the compound type, nutrient availability, and ecosystem type. We found that fresh carbon inputs induced positive priming effects (+37%) in 97% of paired observations. Labile compounds induced larger priming effects (+73%) than complex organic compounds (+33%). Nutrients (e.g., N, P) added with organic compounds reduced the intensity of priming effects compared to compounds without N and P, reflecting "nutrient mining from soil organic matter" as one of the main mechanisms of priming effects. Notably, tundra, lakebeds, wetlands, and volcanic soils showed much larger priming effects (+125%) compared to soils under forests, croplands, and grasslands (+24…+32%). Our findings highlight that positive priming effects are predominant in most soils at a global scale. Optimizing strategies to incorporate fresh organic matter and nutrients is urgently needed to offset the priming-induced accelerated organic carbon turnover and possible losses.