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.