Synergistic effects of grassland plant species on soil microbial biomass and activity: Implications for ecosystem-level effects of enriched plant diversity

1. The effects of the grass species Lolium perenne L. and nine dicotyledonous grassland species, grown in monocultures and two-species mixtures, on (i) the soil microbial biomass, (ii) the respiration:biomass ratio and (iii) plant litter decomposition was investigated in a glasshouse experiment. 2. Microbial biomass was sometimes greater and sometimes less in the two-species mixtures than could be explained in terms of the additive effects of the two component species grown singly; this variation was independent of differences in below-ground plant productivity between monoculture and mixture treatments. 3. The microbial respiration:biomass ratios and plant litter decomposition rates in the two-species mixture treatments were either greater or less than expected based on the monoculture treatments; these differences were dependent on the combinations of species present. Because the respiration:biomass ratio is a measure of ecosystem stability, it is here proposed that stability does not respond predictably to shifts in species diversity. 4. These results provide evidence that increasing plant species richness (from one to two species) has the potential to influence soil processes positively or negatively in a non-additive way. The possible ecological implications of this are discussed.