Are Plant-Soil Feedbacks Caused by Many Weak Microbial Interactions?

We used high-throughput sequencing and multivariate analyses to describe soil microbial community composition in two four-year field plant-soil feedback (PSF) experiments in Minnesota, USA and Jena, Germany. In descending order of variation explained, microbial community composition differed between the two study sites, among years, between bulk and rhizosphere soils, and among rhizosphere soils cultivated by different plant species. To try to identify soil organisms or communities that may cause PSF, we correlated plant growth responses with the microbial community composition associated with different plants. We found that plant biomass was correlated with values on two multivariate axes. These multivariate axes weighted dozens of soil organisms, suggesting that PSF was not caused by individual pathogens or symbionts but instead was caused by 'many weak' plant-microbe interactions. Taken together, the results suggest that PSFs result from complex interactions that occur within the context of a much larger soil microbial community whose composition is determined by factors associated with 'site' or year, such as soil pH, soil type, and weather. The results suggest that PSFs may be highly variable and difficult to reproduce because they result from complex interactions that occur in the context of a larger soil microbial community.

Plant-soil feedbacks help explain plant community productivity.

Plant productivity often increases with species richness, but the mechanisms explaining this diversity-productivity relationship are not fully understood. We tested if plant-soil feedbacks (PSF) can help to explain how biomass production changes with species richness. Using a greenhouse experiment, we measured all 240 possible PSFs for 16 plant species. At the same time, 49 plant communities with diversities ranging from one to 16 species were grown in replicated pots. A suite of plant community growth models, parameterized with (PSF) or without PSF (Null) effects, was used to predict plant growth observed in the communities. Selection effects and complementarity effects in modeled and observed data were separated. Plants created soils that increased or decreased subsequent plant growth by 25% ± 10%, but because PSFs were negative for C3 and C4 grasses, neutral for forbs, and positive for legumes, the net effect of all PSFs was a 2% ± 17% decrease in plant growth. Experimental plant communities with 16 species produced 37% more biomass than monocultures due to complementarity. Null models incorrectly predicted that 16-species communities would overyield due to selection effects. Adding PSF effects to Null models decreased selection effects, increased complementarity effects, and improved correlations between observed and predicted community biomass. PSF models predicted 26% of overyielding caused by complementarity observed in experimental communities. Relative to Null models, PSF models improved the predictions of the magnitude and mechanism of the diversity-productivity relationship. Results provide clear support for PSFs as one of several mechanisms that determine diversity-productivity relationships and help close the gap in understanding how biodiversity enhances ecosystem services such as biomass production.

Plant-soil feedbacks help explain biodiversity-productivity relationships.

Species-rich plant communities can produce twice as much aboveground biomass as monocultures, but the mechanisms remain unresolved. We tested whether plant-soil feedbacks (PSFs) can help explain these biodiversity-productivity relationships. Using a 16-species, factorial field experiment we found that plants created soils that changed subsequent plant growth by 27% and that this effect increased over time. When incorporated into simulation models, these PSFs improved predictions of plant community growth and explained 14% of overyielding. Here we show quantitative, field-based evidence that diversity maintains productivity by suppressing plant disease. Though this effect alone was modest, it helps constrain the role of factors, such as niche partitioning, that have been difficult to quantify. This improved understanding of biodiversity-productivity relationships has implications for agriculture, biofuel production and conservation.

Live long and prosper: plant-soil feedback, lifespan, and landscape abundance covary.

Plant soil feedbacks (PSFs) are thought to be important to plant growth and species coexistence, but most support for these hypotheses is derived from short-term greenhouse experiments. Here we use a seven-year, common garden experiment to measure PSFs for seven native and six nonnative species common to the western United States. We use these long-term, field-based estimates to test correlations between PSF and plant landscape abundance, species origin, functional type, and lifespan. To assess potential PSF mechanisms, we also measured soil microbial community composition, root biomass, nitrogen cycling, bulk density, penetration resistance, and shear strength. Plant abundance on the landscape and plant lifespan were positively correlated with PSFs, though this effect was due to the relationships for native plants. PSFs were correlated with indices of soil microbial community composition. Soil nutrient and physical traits and root biomass differed among species but were not correlated with PSF. While results must be taken with caution because only 13 species were examined, these species represent most of the dominant plant species in the system. Results suggest that native plant abundance is associated with the ability of long-lived plants to create positive plant-soil microbe interactions, while short-lived nonnative plants maintain dominance by avoiding soil-borne antagonists, increasing nitrogen cycling and dedicating resources to aboveground growth and reproduction rather than to belowground growth. Broadly, results suggest that PSFs are correlated with a suite of traits that determine plant abundance.