Functionally or phylogenetically distinct neighbours turn antagonism among decomposing litter species into synergy

1. Plant species coexisting in direct contact produce patches of mixed litters. Mixing litter sometimes synergistically accelerates and sometimes antagonistically decelerates litter decomposition, but we insufficiently understand why. 2. Here, we hypothesize that antagonism or synergy within a mixed-litter patch depends on the neighbouring litter matrix. Specifically, phylogenetical or functional dissimilarity within neighbouring litter, or among patch and neighbouring litter, may favour complementarity and thereby within-patch synergy. 3. From a pool of 20 grassland species, we created 120 mixed-litter patches of two species, and exposed these patches to neighbourhoods in long-term grassland mesocosms of different functional and phylogenetic compositions. 4. We found 60% less (antagonism) to 80% more (synergy) decomposition than expected from single-species litters. Functionally similar, and grass-dominated, mixed-litter patches decomposed most synergistically. Synergy was most strongly favoured by phylogenetic distance among neighbours and functional dissimilarity between neighbours and patch. 5. Synthesis. Our results show that the relationship between biodiversity and ecosystem functioning was context-dependent. We suggest that the coexistence of grasses and the formation of phylogenetically diverse, functionally distinct, patchy vegetation may be reinforced by synergistic nutrient recycling.