Mixture of plant functional groups inhibits the release of multiple metallic elements during litter decomposition in alpine timberline ecotone

Mixed litter decomposition is a common phenomenon in nature and is very important for the circulation of material through an ecosystem. Different plant functional groups (PFGs) are likely to interact during decomposition. It is unclear how mixed decomposition influences the release of multiple metallic elements, and the biogeochemical circulation mechanism in the alpine ecosystem remains elusive. In this study, a two-year experiment on decomposition of mixed litter from six dominant PFGs was conducted at two elevations in an alpine timberline ecotone using the litterbag method. First, the results suggested that PFG identity had greater impacts on the release of all metallic elements than elevation. The release rates of potassium (K), calcium (Ca), magnesium (Mg) and copper (Cu) in graminoid, deciduous shrub and forb litter were significantly higher than those in evergreen conifer, evergreen shrub and mixed litter. Second, the release of metallic elements showed non-additive effects during mixed litter decomposition. K, Ca, Mg, sodium (Na), Cu, and aluminium (Al) exhibited antagonistic effects, while Fe exhibited a synergistic effect. The antagonistic effects on Na, K, Ca and Cu release increased with increasing elevation, while the antagonistic effects on Mg, Al and Mn release decreased with increasing elevation. Third, Al and Fe showed high levels of accumulation. The K release rate decreased while Al and Fe accumulation increased with plant litter upward shift. In conclusion, mixtures of PFGs inhibits the release of multiple metallic elements during litter decomposition in the alpine timberline ecotone. We speculate that an upward shift in PFGs in response to climate warming will slow the release of K and accelerate the enrichment of Fe and Al in alpine timberline ecotones.