PS 39-12 - Bromus tectorum (cheatgrass) modifies the soil microbial community

Background/Question/Methods

Entire landscapes of sagebrush steppe are being displaced by the invasive annual grass Bromus tectorum (cheatgrass). Our research investigates a potential belowground feedback mechanism that drives B. tectorum invasion in which root exudation increases nitrogen (N) availability. We hypothesize that drought stress triggers root exudation in B. tectorum, which stimulates microbes in the rhizosphere. Increased microbial activity leads to increased decomposition of soil organic matter and therefore N mineralization. To test this mechanism and the influence of genetic variation on root exudation, we selected seeds from a local population (Washoe County, NV) and another that experiences cooler winter temperatures (Siskiyou County, CA). We germinated 60 plants in a greenhouse under uniform conditions for 2 months before implementing a drought treatment for half of the plants in each population. Droughted plants underwent 4 weeks of progressive drying and remained at 25% water holding capacity for 2 weeks. After harvest, root exudates were extracted and aggregated for each treatment. Soil samples taken from all plants were used in an N mineralization and respiration incubation experiment where incubations treated with resuspended root exudates were compared with field moist and watered controls. Microbial biomass and nitrification potential were measured for pre-treatment and post-harvest soils.

Results/Conclusions

Plants from Washoe were larger and had higher percent germination compared to Siskiyou individuals. Compared to pre-treatment soil, soil collected from the greenhouse after the experiment exhibited decreases in heterotrophic biomass and increases in nitrifier abundance. Contrary to our prediction, estimated microbial biomass and nitrifier abundance did not significantly differ with population or drought treatment. These results suggests that B. tectorum promoted conditions that released nitrifying microbes from competition with heterotrophic microbes for NH₄, and these effects occurred regardless of B. tectorum population or drought conditions. B. tectorum water + root exudate amendments suppressed microbial respiration relative to wetted controls, particularly in the Washoe population. Heterotrophic respiration had a greater response to Siskiyou root exudates compared to Washoe exudates within the first 24 hours, but after 8 days incubations treated with Washoe exudates had higher respiration rates. These findings suggest that Bromus tectorum root exudates alter the microbial community in favor of nitrifiers by suppressing (rather than stimulating) heterotrophic microbes via root exudates. While there are differences among B. tectorum populations, drought treatments did not trigger greater root exudation. Future research will examine how microbial shifts depend on the degree of invasion and soil N availability.