Abstract. Loss of biodiversity impacts ecosystem functions, such as carbon (C) cycling. Soils are the largest terrestrial C reservoir, containing more C globally than the biotic and atmospheric pools together. As such, soil C cycling, and the processes controlling it, has the potential to affect atmospheric CO 2 concentrations and subsequent climate change. Despite the growing evidence of links between plant diversity and soil C cycling, there is a dearth of information on whether similar relationships exist between soil biodiversity and C cycling. This knowledge gap occurs even though there has been increased recognition that soil communities display high levels of both taxonomic and functional diversity and are key drivers of fluxes of C between the atmosphere and terrestrial ecosystems. Here, we used meta-analysis and regression analysis to quantitatively assess how soil biodiversity affects soil C cycling pools and processes (i.e., soil C respiration, litter decomposition, and plant biomass). We compared the response of process variables to changes in diversity both within and across groups of soil organisms that differed in body size, a grouping that typically correlates with ecological function. When studies that manipulated both within-and across-body size group diversity were included in the meta-analysis, loss of diversity significantly reduced soil C respiration (−27.5 %) and plant tissue decomposition (−18 %) but did not affect above-or belowground plant biomass. The loss of within-group diversity significantly reduced soil C respiration, while loss of across-group diversity did not. Decomposition was negatively affected both by loss of within-group and across-group diversity. Furthermore, loss of microbial diversity strongly reduced soil C respiration (−41 %). In contrast, plant tissue decomposition was negatively affected by loss of soil faunal diversity but was unaffected by loss of microbial diversity. Taken together, our findings show that loss of soil biodiversity strongly impacts on soil C cycling processes, and highlight the importance of diversity across groups of organisms (e.g., primary consumers and secondary decomposers) for maintaining full functionality of C cycle processes. However, our understanding of the complex relationships between soil biodiversity and C cycling processes is currently limited by the sheer number of methodological concerns associated with these studies, which can greatly overestimate or underestimate the impact of soil biodiversity on soil C cycling, challenging extrapolation to natural field settings. Future studies should attempt to further elucidate the relative importance of taxonomic diversity (species numbers) versus functional diversity.

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Soil food webs play a key role in the cycling of carbon and nutrients and in sustainably provisioning ecosystem services. Despite the tremendous diversity of organisms that soil food 25 webs harbor, we still know surprisingly little about the role of biodiversity in influencing the 26 processes and services provided by soil food webs. To guide future research in this area, we outline a conceptual framework linking hierarchical levels of soil biodiversity to ecosystem processes and services. Here, we distinguish among different hierarchical levels of diversity: trophic, functional, taxonomic and genetic diversity. We conclude that the levels of food web diversity that matter most vary with the processes or services considered, with functional trait diversity being the most universally influential level of diversity. Increased research emphasis on manipulating diversity across hierarchical levels of biodiversity organization, with an explicit focus on the functional role of the component species, is critical for enhancing our understanding of the role of soil food web diversity in driving ecosystem processes and services.