1. Litter decomposition is fundamental to carbon (C) and nutrient cycling in ecosystems, which could be altered by plant invasion. The impacts of plant invasion on litter decomposition are generally predicted by traits difference between leaf litters of invasive and non-invasive species. However, plant invasion not only changes litter composition, but might also increase the activity or change the functional diversity of decomposers to alter litter decomposition, which is barely studied, and the effect could be different under varied climate conditions.

2. We studied decomposition of litters from non-invasive and invasive native plants, as affected by litter treatments (in a mixture or alone) and decomposer organisms of different functional groups (by controlling the mesh size of litterbags), in sites with or without an invasive woody grass, Moso bamboo (Phyllostachys edulis), at seven locations across a climate gradient.

3. We show that greater decomposer functional diversity, particularly the presence of macrofauna, accelerated the cycling of litter C and nitrogen (N), increased the climatic sensitivities of decomposition rates, but decreased the N use efficiency of decomposers (represented by litter C to N loss ratio). Litter decomposed in mixtures decomposed faster (by 9.5%) and had more N loss (by 28.9%) than that of in monoculture, regardless of the functional diversity of decomposers. In contrast, the invasion of Moso bamboo slowed decomposition and decreased N use efficiency; this negative effect could be reversed when macrofauna was excluded from the decomposition process, which challenges the nutrient facilitation hypothesis. Bamboo invasion depressed the climatic sensitivity of decomposer functional groups when macrofauna was present but not when macrofauna was excluded.

4. Synthesis. We found that the functional diversity of decomposer organisms modulates and largely determines litter decomposition affected by a woody grass invasion along a climate gradient. These results suggest that, under current and future climate, including the changes in decomposer functional groups, particularly macrofauna, and their interaction with litter traits, would provide a mechanistic and more reliable prediction on ecosystem functions altered by invaders than a functional trait-based framework.

Litter decay rates (k), N loss (% of initial), C loss (% of initial) of Moso bamboo (Phyllostachys edulis) and other species that incubated as single litter species and mixtures in different forest types (bamboo forest, non-invaded forest, and bamboo invaded forest) at seven locations along climatic gradient.