Interactive effects of plant and soil community history on ecosystem temporal stability
It has been hypothesized that pathogenic soil microbes drive asymptotic relationships between biodiversity and ecosystem functioning. If such pathogens are largely species-specific, they could decrease productivity under high host density in low diversity communities, resulting in a positive but saturating biodiversity–ecosystem functioning relationship. The degree of shared history between plant populations and the soil microbes may determine the strength of plant–soil feedbacks, with soil communities sharing a long history of coexistence with plant assemblages exerting stronger feedback control.
We test these hypotheses in a long-term grassland biodiversity experiment using species-specific aboveground biomass data collected bi-annually from 2011–2014. Plant diversity ranged from monocultures to 8-species mixtures. We manipulated plant–soil feedbacks by using soils with different histories of coexistence with plant assemblages within each plot. We additionally manipulated plant selection history by using seeds collected from 8-year old plots of the same species composition or seeds without such selection history. We compared the aboveground primary production between diversity levels and soil treatments, and furthermore analyzed temporal trends.
Mixtures always outperformed monocultures, in particular when they were composed of plants with a mixture selection history. However, contrary to our predictions, these plant assemblages were more responsive to soil history. This result suggests that plants selected in monocultures invest more in pathogen defense, while plants from mixtures have been selected in a low pathogen pressure environment for less investment in defense mechanisms, namely due a diluting effect of diversity on species-specific pathogens. Plant assemblages without selection history did respond in the expected way to soil treatments: in this case, monocultures were more negatively affected by soil with history than were mixtures, leading to an increased biodiversity effect compared with that on soils without history. The observed interactions of plant selection and soil history increased over time, perhaps due to further co-evolution of the aboveground and belowground communities. Furthermore, we observed a positive effect of plant selection and soil coexistence history on community assembly, resulting in a more stable and resistant ecosystem. In conclusion, we propose that plant–soil feedbacks can play an important role in defining the performance and structure of grassland communities. This could have widespread implications on how we view biodiversity–ecosystem functioning relationships.