Differential decomposition rates along the vertical continuum: Standing leaf to deep root litter in alpine ecosystems

Plant litter exists in various positions, ranging from high above the ground to deep within the soil, and its decomposition is essential for carbon storage and nutrient cycling in terrestrial ecosystems. However, the position-dependence of litter decomposition remains poorly understood, as most studies typically examine leaf and root decomposition in isolation. To address this gap, we conducted a 3-year decomposition experiment on leaf and root litter across three typical ecosystems with contrasting hydrological conditions on the Tibetan Plateau. We found that leaf litter decomposed more rapidly than root litter across all studied ecosystems, primarily due to the higher lignin content in roots. Moreover, the position of the litter played a significant role in influencing decomposition rates for both leaf and root litter. Specifically, surface leaf litter (∼0 cm on the ground) decomposed faster than standing leaf litter (+10 cm above the ground), while deep root litter (buried at soil depth of 30 cm) decomposed more quickly than shallow root litter (buried at soil depth of 10 cm). The decomposition of standing and surface leaf litter was primarily driven by leaf quality and microbial activity, respectively, whereas the decomposition of shallow and deep root litter was more strongly influenced by soil quality and plant traits (e.g., belowground biomass), respectively. These findings highlight the importance of incorporating the position dependence of litter decomposition into terrestrial ecosystem models to improve the prediction of carbon and nutrient cycling.