Litter carbon (C) release refers to the amount of litter C lost via microbial respiration or leaching, while the net litter C sink represents the C retained in soil following decomposition. Both are critical but understudied processes in forest C sequestration, particularly under variable water availability driven by climate and land-cover changes. Here, we modeled litter C decomposition rates () using 668 field observations with the Random Forest (RF) algorithm at 1 km resolution across Chinese forests. The RF model demonstrated good performance (R 2 = 0.86), with predicted exhibiting a clear latitudinal zonation. Using modeled , we then estimated the spatial and temporal patterns of litter C release and net litter C sink. Litter C release increased toward lower latitudes, whereas net litter C sink peaked at mid-latitudes and declined toward both lower and higher latitudes. Over 2000-2018, litter C release increased (0.49 +/- 0.07 g C m-2 yr-2), while the net litter C sink decreased (-0.10 +/- 0.04 g C m-2 yr-2), driven primarily by changing water availability mediated by climate and canopy dynamics. Reduced water limitation in persistently arid zones enhanced decomposition, whereas increased water limitation in seasonally dry regions suppressed productivity. In humid regions, a reduction in water surplus led to asynchronous increases in litter input and decomposition. Our findings highlight the central role of water availability in regulating litter C fluxes and underscore the sensitivity of forest C cycling to climate variability, with important implications for projecting terrestrial carbon-climate feedback under future environmental change.