Increasing plant diversity effects on productivity with time due to delayed soil biota effects on plants

Plant diversity experiments in temperate grasslands have served as model system for exploring the consequences of biodiversity loss and understanding the relationship between biodiversity and ecosystem functioning. Recently, ecologists have emphasized that plant diversity effects on plant productivity increase with time; however, the mechanisms responsible for delayed ecosystem responses to diversity manipulations are poorly understood. The present paper briefly reviews temporal changes in grassland biodiversity experiments and proposes antagonistic and facilitative soil effects on plant performance as a mechanistic explanation for increasing plant diversity effects on plant productivity with time. We show that the mechanisms previously proposed to be responsible for the increasing influence of plant diversity effects with time rely in part or entirely on soil biota. Moreover, we show that significant plant diversity effects on soil organisms increase considerably with experimental duration and that the number of respective long-term studies is extremely low. We posit that in the long-term species-rich plant communities experience predominantly facilitative net effects by soil biota promoting plant community growth, such as decomposers, arbuscular mycorrhizal fungi and plant growth promoting rhizobacteria, whereas species-poor plant communities are subject to antagonistic net soil effects due to the accumulation of pathogens, such as nematodes. We therefore suggest that soil biota and their net effect on plants need to be considered in biodiversity experiments to understand changes in time of the effects of plant diversity on the productivity and functioning of plant communities.