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 dicotyl edonous grassland species, grown in monocultures and two-species mixtu res, on (i) the soil microbial biomass, (ii) the respiration:biomass r atio and (iii) plant litter decomposition was investigated in a glassh ouse experiment. 2. Microbial biomass was sometimes greater and someti mes less in the two-species mixtures than could be explained in terms of the additive effects of the two component species grown singly; thi s variation was independent of differences in below-ground plant produ ctivity between monoculture and mixture treatments. 3. The microbial r espiration:biomass ratios and plant litter decomposition rates in the two-species mixture treatments were either greater or less than expect ed based on the monoculture treatments; these differences were depende nt on the combinations of species present. Because the respiration:bio mass ratio is a measure of ecosystem stability, it is here proposed th at stability does not respond predictably to shifts in species diversi ty. 4. These results provide evidence that increasing plant species ri chness (from one to two species) has the potential to influence soil p rocesses positively or negatively in a non-additive way. The possible ecological implications of this are discussed.