Grazer and exotic plant species modifications of bacterial populations and organic matter processing in Yellowstone National Park winter range grasslands

Background/Question/Methods Understanding the magnitude of the interdependence of above and belowground communities in controlling and maintaining ecosystem function is a central question related to the consequences of plant invasions. Gardiner Basin (GB) located at the north entrance to Yellowstone National Park (YNP) is an arid grassland ecosystem that provides important winter range for bison, elk, and the declining YNP pronghorn population.  GB  is the most extensively invaded grassland in the park and is dominated by a variety of invasive grasses and mustards.   Due to the importance of GB as winter range habitat, the restoration of native vegetation in GB is a priority for YNP.   We investigated how bacterial community responses to simulated grazer urine deposition may be altered by the presence of exotic plants by comparing GB to native YNP grasslands. Polymerase chain reaction-based methods utilizing bacterial primers quantified bacterial diversity and abundance as well as ammonia oxidizing and nitrite oxidizing bacteria (AOB and NOB respectively). We determined in situ soil respiration and KCl extractable N to quantify C and N cycling processes.

Results/Conclusions The presence of the exotic mustard Alyssum desertorum significantly reduced bacterial diversity and abundance compared to soils of the native grass Poa secunda and other winter range soils. The abundance of AOB and NOB were reduced by 3-fold in Alyssum soils compared to Poa soils. Soils from the other winter range site were not different from Poa soil in GB with regard to AOB and NOB.  Soil respiration and N was significantly lower in Alyssum soils and there were no differences between Poa soils and the other YNP soils. Application of simulated urine in Alyssum soils did not have an effect on bacteria parameters. However urine application increased soil respiration by 25% and soil N by 30%. In Poa and the other northern range soils simulated urine significantly altered bacterial diversity and abundance. AOB and NOB bacterial abundance decreased over time following the urine treatment. AOB abundance was significantly correlated with soil respiration and soil N concentration.  Our results suggest that the presence of Alyssum has uncoupled C and N processes.  This response in part appears related to shifts in bacterial diversity and abundance.  However, the invasion by Alyssum in GB has not affected the soils in which the native grass Poa still occurs.  Our results indicate that restoration efforts in GB can be successful due to the persistence of native soil bacterial communities within invaded soils.