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we tested effects of biodiversity on the functioning of methanotrophs, a specialized group of soil 27 bacteria that plays a key role in mediating greenhouse gas emissions from soils. Using pure 28 strains of methanotrophic bacteria, we assembled artificial communities of different diversity 29 levels, with which we inoculated sterile soil microcosms. To assess the functioning of these 30 communities, we measured methane oxidation by gas chromatography throughout the 31 experiment and determined changes in community composition and community size at several 32 time points by quantitative PCR and sequencing. We demonstrate that microbial diversity had a 33 positive overyielding-effect on methane oxidation, in particular at the beginning of the 34 experiment. This higher assimilation of CH 4 at high diversity translated into increased growth 35 and significantly larger communities towards the end of the study. The overyielding of mixtures 36 with respect to CH 4 consumption and community size was positively correlated. The temporal 37 CH 4 consumption profiles of strain monocultures differed, raising the possibility that temporal 38 complementarity of component strains drove the observed community-level strain richness 39 effects; however, the community niche metric we derived from the temporal activity profiles did 40 not explain the observed strain richness effect. The strain richness effect also was unrelated to 41 both the phylogenetic and functional trait diversity of mixed communities. Overall, our results 42 suggest that positive biodiversity-ecosystem function relationships show similar patterns across 43 different scales and may be widespread in nature. Additionally, biodiversity is probably also 44 important in natural methanotrophic communities for the ecosystem function methane oxidation. 45 3 Therefore, maintaining soil conditions that support a high diversity of methanotrophs may help 46 to reduce the emission of the greenhouse gas methane. 47 48

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