Asymmetry in litter-induced priming effects between dry- and humid-region forests

Litter input can alter soil carbon (C) dynamics through priming effects (PE), but whether litter-induced PE differs between forests in dry and humid regions remains unclear. Here, we synthesized 1568 paired observations from global forest litter addition experiments to evaluate variation in litter-induced PE between dry- and humid-region forests. Mean litter-induced PE was negative in dry-region forests (-1.24%) but positive in humid-region forests (+6.5%). However, individual effect sizes showed substantial within-group variability and considerable overlap between the two groups. Forest attributes further differentiated PE patterns: PE was generally negative across forest categories in dry-region forests, but mostly positive in humid-region forests, except in secondary forests. In dry-region forests, litter-induced PE showed no significant relationship with either the aridity index or experimental duration. In humid-region forests, PE was higher in longer-duration experiments, increasing from approximately 5% in experiments lasting ≤6 months to nearly 9% in those lasting > 12 months, and was negatively associated with the aridity index. Random Forest models further showed that the relative importance of environmental predictors differed between dry- and humid-region forests. Overall, our results indicate a significant difference in mean litter-induced PE between the two forest groups while also revealing substantial variation within each group. These findings highlight the importance of considering regional aridity, forest attributes, and experimental duration when interpreting litter-induced PE and assessing forest soil C dynamics.