Topography Dependence of Terrestrial Litter-Derived Soil Respiration

Carbon (C) released from decomposing plant litter constitutes a major component of soil CO₂ efflux at the land surface, yet its contribution is rarely constrained separately from heterotrophic respiration of stable soil organic matter in global C budgets and Earth system model evaluations. We combined a global dataset with high-resolution field observations to quantify the contribution of litter-derived soil respiration (R_s) using litter-input and litter-removal experiments, and to project its current distribution and future dynamics with machine-learning models. On average, litter-derived R_s accounted for 30.9% of total R_s. Higher contributions were estimated for experiments of shorter duration, highlighting the important role of fast-cycling C alongside soil organic matter turnover. Litter-derived R_s varied substantially among ecosystems (grasslands > croplands > forests > wetlands) but did not differ significantly between tropical and temperate climates. Land surface slope exerted a stronger control than climatic or edaphic factors across both mountain and non-mountain regions, suggesting a pronounced topographic regulation. High-frequency field measurements further confirmed this pattern, with litter-derived R_s at mountain ridges being 1.5 times that in valleys. Global projections indicated a higher litter-derived R_s at low latitudes and greater vulnerability in cold climates under SSP 1-2.6 and SSP 5-8.5 scenarios. These findings demonstrate that litter decomposition is a substantial source of soil CO₂ flux that is strongly controlled by terrain. Accounting for this CO₂ pathway improves our understanding of how landscape heterogeneity influences terrestrial C cycle and enhances future predictions of ecosystem responses to climate change.