Substrate and climate determine terrestrial litter decomposition

Litter decomposition is a fundamental biogeochemical process for carbon flux and nutrient cycling in terrestrial ecosystems, yet the global variation in decomposition rates and their covariations with climate and substrate are not fully understood. Here, we synthesized a global dataset of 6,733 independent observations across six continents to illustrate the climatic and substrate controls over litter decomposition. The average decomposition rates of various litter types ranged from 0.74 to 4.01 y^-1 across polar to tropics, showing a large geographical span. Litter substrate and climate directly explained 36 and 30% of the variations in decomposition rates, with the carbon-to-nitrogen ratio identified as the best predictor. In the absence of climate variables, litter substrate can effectively explain the variation, while the model's predictive capacity decreased significantly after litter substrate was excluded. Our synthesis highlights that a fundamental constraint on litter substrate leads to predictable global-scale patterns of terrestrial litter decomposition rates. Integrating litter chemistry parameters should be prioritized for parameter optimization in Earth system models.