Root litter decomposition is suppressed in species mixtures and in the presence of living roots

Plant species diversity and identity can significantly modify litter decomposition, but the underlying mechanisms remain elusive, particularly for root litter. Here, we aimed to disentangle the mechanisms by which plant species diversity alters root litter decomposition. We hypothesised that (1) interactions between species in mixed communities result in litter that decomposes faster than litter produced in monocultures; (2) litter decomposition is accelerated in the presence of living plants, especially when the litter and living plant identities are matched (known as home-field advantage). Monocultures and a mixture of four common grassland species were established to obtain individual litter and a ‘natural’ root litter mixture. An ‘artificial’ mixed litter was created using litter from monocultures, mixed in the same proportions as the species composition in the natural litter mixtures based on qPCR measurements. These six root litter types were incubated in four monocultures, a four-species mixture and an unplanted soil. Root decomposition was strongly affected by root litter identity and the presence, but not diversity, of living roots. Mixed-species litter decomposed slower than expected based on the decomposition of single-species litters. In addition, the presence of living roots suppressed decomposition independent of the match between litter and living plant identities. Decomposition was not significantly different between the ‘natural’ and ‘artificial’ root litter mixtures, indicating that root-root interactions in species mixtures did not affect root chemical quality. Synthesis. Suppressed decomposition in the presence of living roots indicates that interactions between microbial communities associated with living roots and root litter control root litter decomposition. As we found no support for the importance of home-field advantage or interspecific root interactions in modifying decomposition, suppressed decomposition of mixed-species litter seems to be primarily driven by chemical rather than biotic interactions.