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The decomposition of dead organic matter is a major determinant of carbon and nutrient cycling in ecosystems, and of carbon fluxes between the biosphere and the atmosphere^,,. Decomposition is driven by a vast diversity of organisms that are structured in complex food webs^,. Identifying the mechanisms underlying the effects of biodiversity on decomposition is critical^,, given the rapid loss of species worldwide and the effects of this loss on human well-being^,,. Yet despite comprehensive syntheses of studies on how biodiversity affects litter decomposition^,,,, key questions remain, including when, where and how biodiversity has a role and whether general patterns and mechanisms occur across ecosystems and different functional types of organism^,,,,. Here, in field experiments across five terrestrial and aquatic locations, ranging from the subarctic to the tropics, we show that reducing the functional diversity of …