Data from: Plant species richness negatively affects root decomposition in grasslands

Plant diversity enhances many ecosystem functions, including root biomass
production, which drives soil carbon input. Although root decomposition
accounts for a large proportion of carbon input for soil, little is known
about plant diversity effect on this process. Plant diversity may affect
root decomposition in two non-exclusive ways: by providing roots of
different substrate quality (e.g. root chemistry) and/or by altering the
soil environment (e.g. microclimate). To disentangle these two pathways,
we conducted three decomposition experiments using a litter-bag approach
in a grassland biodiversity experiment. We hypothesized that: (i) plant
species richness negatively affects substrate quality (indicated by
increased C:N ratios), which we tested by decomposing roots collected from
each experimental plot in one common plot; (ii) plant species richness
positively affects soil environment (indicated by increased soil water
content), which we tested by decomposing standardized roots in all
experimental plots; (iii) the overall effect of plant species richness on
root decomposition, due to the contrast between quality and environmental
effects, is neutral, which we tested by decomposing community roots in
their ‘home’ plots. Plant species richness negatively affected root
decomposition in all three experiments. The negative effect of plant
species richness on substrate quality was largely explained by increased
root C:N ratios along the diversity gradient. Functional group presence
explained more variance in substrate quality than species richness. Here,
the presence of grasses negatively affected substrate quality and root C:N
ratios, while the presence of legumes and small herbs had positive
effects. Plant species richness had a negative effect on soil environment
despite its positive effect on soil water content which is known to
stimulate decomposition. We argue that – instead of soil water content – a
combined effect of soil temperature and seasonality might drive
environmental effect of plant diversity on decomposition in our plant
communities, but this remains to be tested. Synthesis. Our results
demonstrate that both substrate quality and soil environment contribute to
the net negative effect of plant diversity on root decomposition. This
study promotes our mechanistic understanding of increased soil carbon
accumulation in more diverse grassland plant communities.