Immobilization of heavy metals during aquatic and terrestrial litter decomposition in an alpine forest

Plant litter decomposition is an important pathway of heavy metal cycling in forested soil and watershed ecosystems globally, but is so far an overlooked aspects in the existing literature. To investigate the temporal dynamics of heavy metals in decomposing litter, we conducted a two-year field experiment using litterbag method across aquatic and terrestrial ecosystems in an alpine forest on the eastern Tibetan Plateau. Using multigroup comparisons of structural equation modeling with different litter mass-loss intervals, we assessed the direct and indirect effects of several biotic and abiotic factors on the release rates of lead (Pb), cadmium (Cd), and chromium (Cr). Results suggested that both the concentrations and amounts of Pb, Cd, and Cr increased during litter decomposition regardless of ecosystem type and litter species, showing an immobilization pattern. The release rates of Pb, Cd, or Cr shared a common hierarchy of drivers across aquatic and terrestrial ecosystems, with environmental factors and initial litter quality having both direct and indirect effects, and the effects of initial litter quality gained importance in the late decomposition stages. However, litter chemical dynamics and microbial diversity index have significant effects on release rates throughout the decomposition process. Our results are useful for better understanding heavy metal fluxes in aquatic and terrestrial ecosystems, and for predicting anthropogenic heavy metal pollution impacts on ecosystems. In addition, our results indicated that not only spatial but also temporal variability should be taken into consideration when addressing heavy metal dynamics accompanying litter decomposition process.