Contrasting mass-ratio vs. niche complementarity effects on litter C and N loss during decomposition along a regional climatic gradient

1. The mass-ratio and niche complementarity mechanisms drive the influence of litter trait diversity on decomposition. However, the implications of these mechanisms remain poorly understood, as few studies have evaluated their importance relative to environmental conditions and soil decomposers when assessing different processes during decomposition (e.g. C dynamics and N transformations). 2. We measured litter C and N losses during decomposition in 10 litter mixtures and calculated community-weighted means (CWM) and dissimilarity (Rao's Q) of 13 litter traits as metrics of the mass-ratio and niche complementarity mechanisms respectively. The 10 litter mixtures were incubated along a regional gradient (10 sites) in southern France over a year, where local environmental conditions (climate and soil parameters), and soil decomposers (microbes, nematodes, and macro-fauna) were considered as drivers of decomposition. 3. Sites and litter mixtures represented a wide range of environmental conditions, decomposers, and litter diversity. Litter mixture was the major factor affecting litter C and N loss. N was immobilized during the early decomposition stages (up to c. 40% mass loss), especially in mixtures of low N concentrations. Litter CWM traits were the key drivers of litter C and N loss, but litter trait dissimilarity emerged as a major driver of litter N loss. Macrofauna played a significant role on C and N loss, but environmental conditions exerted a minor influence. 4. Our results highlight that distinct aspects of trait diversity in mixed species litter can play a more important role than environmental conditions and decomposers, which can affect the cycling of litter C and N at regional scales. Rather than opposing mass-ratio and niche complementarity mechanisms, our study shows that both can play a critical role simultaneously in litter decomposition. 5. Synthesis. Our study emphasizes that, in the current context of global biodiversity decline, the mass-ratio and niche complementarity mechanisms should be considered to assess litter decomposition dynamics under global change. As litter diversity mechanisms distinctly affected the cycling of C and N, we recommend taking into account such element-specific effects to improve the prediction of the impacts of biodiversity change on biogeochemical cycles.