Litter position and stoichiometry regulate plant litter decomposition and nitrogen release in terrestrial ecosystems

Litter decomposition is a biogeochemical process that is crucial in ecosystem carbon (C) and nutrient cycling. Litter decomposition commonly occurs both on and below the soil surface, but how the position affects litter decomposition and associated C and nitrogen (N) turnover rates globally remains unclear. We collected literature-based data of 500 paired aboveground and belowground decomposition rates (k values) of the same litter material and ancillary traits, climate, and soil property data from 184 sites spanning five major biomes globally. Analysis of variance and hierarchical partitioning were used to examine how litter position and stoichiometry affected litter decomposition and N release rates and their relationships. Across all biomes, the belowground k value was 77.7% higher than the aboveground k value, and the belowground N release rate was 65.5% higher than the aboveground N release rate. The effects of position on litter decomposition rate depended on biomes and litter types. Litter C:N ratio was a dominant driver of net N immobilization and release of decomposing litter. The coordination between decomposition and N release rates for litters with a low C:N ratio was stronger than that with a high C:N ratio. Litter decomposition and N release rates were faster for higher-quality litter and increased towards favorable climate conditions with increasing temperature. Litter quality was the crucial predictor of the position effect on litter decomposition and recalcitrant litter was more susceptible to litter position. Together these findings indicate that soil burial enhances soil carbon and nutrient turnover rates globally, and that the litter position and stoichiometry shift the patterns of litter N immobilization and release. Incorporating the position effect and litter stoichiometry could therefore reduce the uncertainty of biogeochemical cycle models in the context of global changes that may affect the relative litter production, stoichiometry and decomposition of litter aboveground versus belowground.