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 decomposer organisms and plant litter types slowed the cycling of litter carbon and nitrogen. Moreover, we found evidence of nitrogen transfer from the litter of nitrogen-fixing plants to that of rapidly decomposing plants, but not between other plant functional types, highlighting that specific interactions in litter mixtures control carbon and nitrogen cycling during decomposition. The emergence of this general mechanism and the coherence of patterns across contrasting terrestrial and aquatic ecosystems suggest that biodiversity loss has consistent consequences for litter decomposition and the cycling of major elements on broad spatial scales. Main textBiological diversity that directly influences litter decomposition exists at multiple trophic levels 4 . This diversity includes plants producing litter mixtures of varying quality, microbial decomposers, and invertebrate consumers of varying body size, which selectively exploit the heterogeneous resources provided by litter mixtures 4,13 . Efforts to derive generalities about biodiversity effects on litter decomposition have been elusive, since both pioneering work 14 and recent syntheses have highlighted contrasting effects of litter species richness on 3 decomposition [4][5][6]15,16 . In part, this variation appears to be due to site-specific conditions, including contrasts between aquatic and terrestrial ecosystems as well as geographic settings.Further differences may arise from variation in experimental protocols, selected plant species, and the types of decomposers included in a given experiment. Such methodological discrepancies have complicated syntheses across studies, hindering the emergence of common patterns and mechanisms.Here we report on the results from the first concerted biodiversity experiments on decomposition by manipulating diversity across trophic levels and distinct biomes in both forest floor and stream habitats (Extended Data Table 1). We hypothesised that functional diversity of decomposers (variation in body size) and leaf litter (variation in litter quality) promote C and N cycling across contrasting locations (subarctic to tr...

Plant litter decomposition is a key process in terrestrial carbon cycling, yet the relative importance of various control factors remains ambiguous at a global scale. A full reciprocal litter transplant study with 16 litter species that varied widely in traits and originated from four forest sites covering a large latitudinal gradient (subarctic to tropics) showed a consistent interspecific ranking of decomposition rates. At a global scale, variation in decomposition was driven by a small subset of litter traits (water saturation capacity and concentrations of magnesium and condensed tannins). These consistent findings, that were largely independent of the varying local decomposer communities, suggest that decomposer communities show little specialisation and high metabolic flexibility in processing plant litter, irrespective of litter origin. Our results provide strong support for using trait-based approaches in modelling the global decomposition component of biosphere-atmosphere carbon fluxes.

SummaryProper estimates of decomposition are essential for tropical forests, given their key role in the global carbon (C) cycle. However, the current paradigm for litter decomposition is insufficient to account for recent observations and may limit model predictions for highly diverse tropical ecosystems. In light of recent findings from a nutrient-poor Amazonian rainforest, we revisit the commonly held views that: litter traits are a mere legacy of live leaf traits; nitrogen (N) and lignin are the key litter traits controlling decomposition; and favourable climatic conditions result in rapid decomposition in tropical forests. Substantial interspecific variation in litter phosphorus (P) was found to be unrelated to variation in green leaves. Litter nutrients explained no variation in decomposition, which instead was controlled primarily by nonlignin litter C compounds at low concentrations with important soil fauna effects. Despite near-optimal climatic conditions, tropical litter decomposition proceeded more slowly than in a climatically less favourable temperate forest. We suggest that slow decomposition in the studied rainforest results from a syndrome of poor litter C quality beyond a simple lignin control, enforcing energy starvation of decomposers. We hypothesize that the litter trait syndrome in nutrientpoor tropical rainforests may have evolved to increase plant access to limiting nutrients via mycorrhizal associations.

Summary Plant leaf litter comprises the major common source of energy and nutrients in forested soil and freshwater ecosystems world‐wide. However, despite the similarity of physical and biochemical processes, generalizations across aquatic and terrestrial ecosystems regarding litter decomposition drivers remain elusive. We re‐analysed data from a published field decomposition experiment conducted in two ecosystems (forest floors and streams) across five biomes (from the tropics to subarctic) with increasing decomposer community complexity (microbes, microbes and mesofauna, microbes and meso‐ and macrofauna). Using a wide litter quality gradient (15 litter combinations), we aimed to disentangle the roles of decomposer community complexity from that of leaf litter traits (18 traits encompassing four broad trait categories: nutrients, C quality, physical structure and stoichiometry) on litter C and N loss. Comparisons of decomposition drivers between ecosystems were evaluated across and within biomes. Differences in environmental conditions (e.g. climate, soil/water fertility) and litter nutrients – with a particular focus on Mg and Ca – across biomes were the major drivers of litter C loss in both ecosystems, but decomposer complexity also played a prominent role in streams. Within biomes, we observed consistent effects of litter nutrients and stoichiometry on litter C and N loss between ecosystems, but the effects of decomposer complexity differed between streams and forest floors in the temperate, Mediterranean and tropical biomes. Our results highlight that, beyond the litter traits commonly identified as controlling decomposition (e.g. C, N and lignin), micronutrients (e.g. Mg and Ca) can also play an important, and globally consistent, role in both aquatic and terrestrial ecosystems. In addition, in forest streams the complexity of decomposer communities had similar importance as litter traits for predicting litter C and N turnover across all five biomes. The identification of common drivers in our large‐scale ecosystem comparison suggests a basis to develop common models across aquatic and terrestrial ecosystems for C and N dynamics during decomposition. Future modelling efforts should account for the global similarities (litter micronutrient effects) and biome‐level differences (contingent decomposer effects) found between ecosystems.