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...

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Summary The complexity of soil food webs and the cryptic habitat hamper the analyses of pools, fluxes and turnover rates of carbon (C) in organisms and the insight into their interactions. Stable isotope analysis has been increasingly used to disentangle soil food web structure, yet it has not been applied to quantitatively characterize C dynamics at the level of the entire soil food web. The present study employed 13CO2 pulse labelling to investigate the incorporation of maize root‐derived C into major soil compartments and food web players in an arable field for 25 days. Bulk tissue and compound‐specific (lipids) C isotope ratios were used to quantify pool sizes and 13C incorporation in bacteria and fungi as primary decomposers, nematodes as key drivers of the microfood web and decomposers and predators among the meso‐ and macrofauna. About 20% of the C assimilated by maize was transferred to below‐ground pools. 13C was predominantly incorporated into rhizosphere micro‐organisms rather than in those of the bulk soil. 13C in phospholipid fatty acid biomarkers revealed that root‐derived C was incorporated into the soil food web mainly via saprotrophic fungi rather than via bacteria. Only small amounts of 13C were transferred to higher trophic levels, predominantly into fungal‐feeding nematodes and macrofauna decomposers. Most importantly, C pool size and 13C incorporation did not match closely. Although the fungal C stock was less than half that of bacteria, C transfers from fungi into higher trophic levels of the fungal energy pathway, that is fungal‐feeding nematodes and meso‐ and macrofauna decomposers, by far exceed that of bacterial C. This challenges previous views on the dominance of bacteria in root C dynamics and suggests saprotrophic fungi to function as major agents channelling recent photoassimilates into the soil food web.

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Synergistic effects on decomposition in litter mixtures have been suggested to be due to the transfer of nitrogen from N‐rich to N‐poor species. However, the dominant pathway and the underlying mechanisms remain to be elucidated. We conducted an experiment to investigate and quantify the control mechanisms for nitrogen transfer between two litter species of contrasting nitrogen status (15N labeled and unlabeled Fagus sylvatica and Fraxinus excelsior) in presence and absence of micro‐arthropods. We found that 15N was predominantly transferred actively aboveground by saprotrophic fungi, rather than belowground or passively by leaching. However, litter decomposition remained unaffected by N‐dynamics and was poorly affected by micro‐arthropods, suggesting that synergistic effects in litter mixtures depend on complex environmental interrelationships. Remarkably, more 15N was transferred from N‐poor beech than N‐rich ash litter. Moreover, the low transfer of 15N from ash litter was insensitive to destination species whereas the transfer of 15N from labeled beech litter to unlabeled beech was significantly greater than the amount of 15N transferred to unlabeled ash suggesting that processes of nitrogen transfer fundamentally differ between litter species of different nitrogen status. Microbial analyses suggest that nitrogen of N‐rich litter is entirely controlled by bacteria that hamper nitrogen capture of microbes in the environment supporting the source‐theory. In contrast, nitrogen of N‐poor fungal dominated litter is less protected and transferable depending on the nitrogen status and the transfer capacity of the microbial community of the co‐occurring litter species supporting the gradient‐theory. Thus, our results challenge the traditional view regarding the role of N‐rich litter in decomposing litter mixtures. We rather suggest that N‐rich litter is only a poor nitrogen source, whereas N‐poor litter, can act as an important nitrogen source in litter mixtures. Consequently both absolute and relative differences in initial litter C/N ratios of co‐occurring litter species need to be considered for understanding nitrogen dynamics in decomposing litter mixtures.