Data S1

Priming effects of soil organic matter decomposition are essential phenomena of terrestrial carbon cycling and affect the capacity of soils to serve as sources or sinks for atmospheric CO₂ in response to fresh carbon inputs. Yet, the general direction and intensity of soil priming in terrestrial ecosystems 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 prevalently result in positive priming effects (+37%) in 97% of paired observations. Labile compounds induced higher priming effects (+73%) than complex organic compounds (+33%). Compound inputs with nutrient additions (N, P alone or together) reduced the intensity of priming effects compared to compounds without N and P. Notably, tundra, lake-beds, wetlands, and volcanic soils have much larger priming effects (+125%) compared to forests, croplands, and grasslands, suggesting that SOC decomposition in these terrestrial ecosystems is more easily stimulated by fresh carbon inputs. Our findings highlight that positive priming effects are predominant at a global scale, indicating that priming commonly accelerates SOC decomposition.