Transgressive overyielding of soil microbial biomass in a grassland plant diversity gradient

In a grassland plant diversity experiment, we studied if plant species richness induces transgressive overyielding in soil microbial biomass, the definition of which is greater microbial biomass with a plant mixture than in each of the plant monocultures. In June and November of 2012 soil microbial biomass increased significantly with plant species richness. Overyielding occurred in more than 61% of 24 plots containing six species, and transgressive overyielding occurred in 13–21% of the plots depending on season. Plots with nine species showed overyielding in all cases, and 25–50% showed transgressive overyielding. Transgressive underyielding occurred in a few plots with six species in June. Our results indicate plant complementarity effects on soil microbial biomass in diverse plant communities across seasons and stress the relevance of simplification of plant communities for soil processes.