Biodiversity-ecosystem functioning (BEF) experiments have predominantly focused on communities of higher organisms, in particular plants, with comparably little known to date about the relevance of biodiversity for microbiallydriven biogeochemical processes. Methanotrophic bacteria play a key role in Earth's methane (CH 4 ) cycle by removing atmospheric CH 4 and reducing emissions from methanogenesis in wetlands and landfills. Here, we used a dilution-to-extinction approach to simulate diversity loss in a methanotrophic landfill cover soil community. Combining analyses of CH 4 flux and community structure, we found a linear decrease of CH 4 oxidation rates with the number of taxonomic units lost. This effect was independent of community size, consistent over the three-month study, and occurred in relatively diverse communities, challenging the notion of high functional redundancy mediating high resistance to diversity erosion in natural microbial systems. The effects we report resemble the ones for higher organisms, suggesting that BEF-relationships are universal across taxa and spatial scales.