COS 186-1 - Cheatgrass (Bromus tectorum) invasion: root exudation and controls on biogeochemical cycling

Abstract: Cheatgrass (Bromus tectorum) is an invasive annual grass capable of converting sagebrush steppe to annual grassland. Nitrogen (N) access has been hypothesized as a key factor in cheatgrass invasion. Furthermore, cheatgrass may be able to access N that is unavailable to other plant species, possibly by manipulating carbon (C) and nitrogen (N) cycling through root exudation. Our research investigates whether root exudation by cheatgrass increases N availability in the rhizosphere. We hypothesize that drought stress triggers root exudation, which stimulates soil microbes that accelerate decomposition of soil organic matter and mineralize excess N. Because plants adapted to arid conditions may cope with drought differently, we investigated the effects of cheatgrass from two source populations on plant traits, soil microbial function, and soil biogeochemical cycling.





A factorial greenhouse experiment compared how a warm-sourced and cold-sourced population of cheatgrass influenced soil processes under different watering treatments. A second year compared warm-sourced cheatgrass to squirreltail (Elymus elymoides), a native grass commonly used for restoration. We compared biogeochemical cycling, microbial functional groups, and extracellular enzymes of post-greenhouse experiment soils to soils pre-treatment. Root exudates were extracted and amended to greenhouse soils in a follow-up laboratory incubation to measure direct effects on biogeochemical cycling.





Compared to pre-treatment soils, cheatgrass soils collected from the greenhouse experiment in year 1 exhibited a 60% decrease in microbial biomass and an increase in nitrifying microbes by a factor of 13.6. Exudates from cold-sourced cheatgrass showed an increase in N mineralization and all cheatgrass root exudates suppressed respiration compared to watered control incubations. In the second year, nitrification potential increased from the pre-treated soils for both the warm cheatgrass and squirreltail greenhouse soils. Respiration for droughted greenhouse soils was higher than their watered counterparts for both squirreltail and cheatgrass. All root exudate additions stimulated respiration compared to controls. Root exudates reduced immobilization of N compared to wetted controls, but there were no differences in mineralization between exudate amendments.


Our findings suggest that cheatgrass root exudates, particularly from the cold-sourced population, alter the microbial community in favor of nitrifiers by suppressing (rather than stimulating) heterotrophic microbes via root exudates. However, a second year of investigation suggests that cheatgrass may also stimulate heterotrophic microbes under certain conditions. Contextual factors such as stress conditions may determine whether root exudates suppress or stimulate microbial activity.