Can plant-soil feedbacks drive biodiversity-productivity relationships in grasslands?

Background/Question/Methods

The positive relationship between biodiversity and productivity is well documented experimentally, yet the underlying mechanisms driving the relationship are still poorly understood. Recently, a number of studies have pointed to feedbacks between plant species and soil microbial communities as a potentially important mechanism. Further experimental support, especially regarding the interaction between diversity and plant-soil feedbacks (PSF), is still needed. We conducted a greenhouse experiment utilizing soil from an 18-year grassland biodiversity experiment at Cedar Creek (MN), addressing the following hypotheses: 1) The presence, strength, and direction of plant-soil microbe feedbacks would vary among the eight focal species, 2) the magnitude or direction of these feedbacks would change when, holding soil source constant, the focal plant is grown in single- or multi-species communities, and 3) soils collected from high diversity plots would differentially affect the growth of single or multi-species communities relative to soil from monoculture plots. We planted 663 communities of one, six or eight species in soil from each of eight monoculture and three high diversity plots at Cedar Creek. Each greenhouse pot contained 93% identical, sterilized “base” soil and 7% live soil inoculum from each soil source to minimize abiotic differences among soils while maintaining distinct microbial communities.

Results/Conclusions

The way in which diversity interacted with plant-soil feedbacks highlights a potential mechanism by which plant diversity leads to increased productivity. Species exhibiting decreased growth on soil from their own monoculture (negative PSF) showed a less negative PSF when grown in a high diversity mixture on their own soil, hinting at a “dilution effect” of diversity on PSFs. Growing in a community with higher diversity decreases conspecific density and therefore also decreases the negative impacts a species has on itself through feedbacks with its microbial community. Interestingly, the one species with a positive PSF also showed a “dilution” of this positive effect when grown in a high diversity mixture. Importantly, there was no significant difference among soil types when the soil inoculum was sterilized before being added to the pot, suggesting that the differences among soil types are due to differing microbial communities. Furthermore, pots inoculated with soil from high diversity plots had higher germination success for all species (p=0.04) and increased realized species richness at each planted diversity level (p=0.013). Combined, these results suggest multiple mechanisms by which species diversity can lead to increased productivity and highlight the importance of considering plant-soil feedbacks in driving this relationship.