Plant neighborhood is a strong driver of plant performance, yet how plants perceive the identity of neighbors remains a central question in plant ecology. Through the release of root exudates, plants can culture unique rhizosphere microbial communities to aid in defense and nutrient acquisition. Recent evidence suggests that plants can respond to and regulate microbial quorum signaling compounds, such acylated homoserine lactones (AHL), and that these compounds may play a key role in defining plant-microbe interactions. Geum rossii and Deschampsia caespitosa are two codominant plants in the alpine tundra of the Rocky Mountains that have strong and divergent effects on the soil microbial community. Our previous work has shown that these plant species respond to each other by increasing N uptake. Here we ask whether this response could be mediated by increased AHL production when the species interact. To test this prediction, we isolated monocultures and mixtures of the two species and repeatedly treated the soils with AHL. We measured soil nitrogen (N) pools, extracellular enzymes, stomatal conductance, and plant N uptake rate. We hypothesized that the addition of AHL could induce increased microbial biomass, soil N availability, and plant N uptake in monoculture.

Results/Conclusions

Contrary to our expectations, we found that AHL did not affect the size of the microbial N pool. AHL decreased soil inorganic N availability, increased the activity of oxidative enzymes in the rhizosphere and reduced phosphatase and leucine amino peptidase activity. Again, contrary to our predictions, we found that plant N uptake rates were reduced with the addition of AHL and this response was consistent across neighbor treatments. The role of AHL in the rhizosphere is largely unexplored and these results are some of the first to suggest that plants can respond to AHL in field settings. Our findings suggest that plants may regulate AHL signaling in the soil to reduce bacterial population size and disease risk, rather than to promote nutrient acquisition. Plant nutrient uptake may have been reduced in the presence of AHL due to allocation costs associated with the production of inference compounds. Our results suggest that the dynamics between plants and AHL can influence nutrient cycling and may be critical to maintaining a balance between symbiotic and pathogenic microbes in the rhizosphere.