Climate Zone Overrides Litter Input in Shaping Forest Soil Carbon Fractions: A Global Meta-Analysis

Litter input, including aboveground and belowground plant residues such as leaves, branches, and roots, is a major pathway of carbon return to forest soils. The prevailing paradigm in forest carbon management emphasizes litter input as the primary driver of soil organic carbon (SOC) sequestration. Here, litter input refers specifically to experimental litter manipulation, including litter-addition and litter-removal treatments. Although numerous experimental studies have examined the effects of litter manipulation on SOC, several limitations remain. By synthesizing 1555 global observations, we demonstrate that climate zone, not litter manipulation per se, is the dominant moderator of SOC fraction responses. Litter addition significantly increased labile fractions (light fraction: +60%) but left MAOC largely unchanged. Conversely, litter removal depleted labile pools yet failed to destabilize MAOC. This universal inertia of MAOC challenges the assumption that litter management directly enhances long-term carbon stability. Furthermore, we reveal a critical climate dependency: tropical forests show attenuated carbon gains under litter addition, while temperate systems are more responsive. Our findings necessitate a paradigm shift from uniform litter-based strategies to climate-zone-specific forest management, prioritizing the protection of existing soil carbon in vulnerable biomes over indiscriminate litter augmentation.