Effects of plant-soil feedbacks on intra- and inter-specific competition in desert grasses

Background/Question/Methods

Plant-soil microbe interactions have high potential as a stabilizing mechanism of coexistence in plant communities. For example, density-dependent pathogen accumulation in plant roots will result in negative feedback which keeps species from outcompeting others. Past research has shown that the effects of soil microbes on individual plant fitness are correlated with patterns of relative species abundance and succession in old field and tropical forest communities. However, few studies have explicitly tested the role of density-dependence in microbe-mediated intra- and interspecific plant competition, which makes it difficult to incorporate plant-microbe interactions into the species coexistence framework.

We investigated the role of density-dependence in effects of plant-microbe interactions on intra- and interspecific competition between two co-occurring desert grasses in a greenhouse experiment. The presence of biological soil crust (BSC) was fully crossed with a plant-soil feedback experiment comparing soil inocula consisting of rhizospheric soil collected from conspecific or heterospecific experimental field monocultures planted in 2007. To isolate the microbial contribution to feedback, we sterilized half of the inocula.  To investigate density-dependent and independent effects of soil inocula on competition, plants were grown in pots in a response-surface design, which allowed us to directly model treatment effects on intra- and interspecific competition coefficients.

Results/Conclusions

Under greenhouse conditions, Bouteloua gracilis was the competitive dominant, achieved higher biomass, and experienced negative plant-soil feedbacks with rhizospheric microbes. For B. gracilis, negative effects of conspecific rhizosphere microbes increased with intraspecific density (R² = 0.16, F_{1, 72} = 15.4, p < 0.001), directly demonstrating density-dependent negative plant-soil feedbacks. The competitive inferior, Bouteloua eriopoda, showed no differences between live and sterile treatments of the two types of rhizopheric inocula. The presence of BSC increased above and belowground biomass of both species. However, for B. eriopoda, benefits of BSC to both itself and its competitor resulted in more intense interspecific competition (interaction between B. gracilis density and BSC presence, F = 3.87, p = 0.05). These results show that, in a semi-arid grassland system, plant-BSC microbe interactions are generally beneficial to the plant host, while plant-rhizospheric microbe interactions showed evidence of density-dependent negative feedbacks that may increase coexistence.