Smooth brome invasion influences nitrogen cycling and soil bacterial community structure in a fescue grassland
Exotic plant invasions represent a significant threat to the integrity of native grasslands. Across the Northern Great Plains, grasslands invaded by smooth brome (Bromus inermis Leyss) support lower plant diversity, potentially resulting in important consequences for ecosystem function. Previous research on smooth brome has primarily focused on aboveground changes in plant communities, but there is growing evidence that the soil ecosystem can be significantly altered with invasion. The two objectives of this thesis were to examine whether smooth brome invasion alters soil nitrogen cycling, and to determine if changes in plant community diversity or productivity influence soil bacterial communities. Relationships between smooth brome and the soil ecosystem were assessed using data collected from a Festuca hallii Vasey (Piper) (plains rough fescue) grassland located near Macrorie, SK. Gross rates of nitrogen cycling and community productivity from smooth brome invaded and native grassland sites were compared to determine the potential influence of smooth brome invasion on the soil nitrogen cycle. The relationship between increasing smooth brome abundance and soil bacterial structure and composition was also studied. Gross mineralization rates and total soil nitrogen were significantly higher in smooth brome-invaded areas relative to native grassland. Bacterial and archaeal amoA, used as indicators of ammonia-oxidizer population sizes, were altered by smooth brome cover. Higher gross mineralization rates were likely due to stimulated microbial activity caused by increased litter and root production in areas invaded by smooth brome. Smooth brome decreased plant species richness through increased litter production, but had the opposite effect on bacterial communities. Bacterial communities had higher species richness and evenness in soils invaded by smooth brome, and smooth brome invasion was also associated with bacteria important for soil nitrogen cycling. As bacteria dominate microbial biomass and are important for decomposition processes, a more even bacterial community may have supported increased mineralization rates in smooth brome-invaded soils. Specifically, a more even bacterial community may have increased mineralization rates through greater resource utilization and niche partitioning. The responses observed in these studies suggest that belowground changes with smooth brome invasion have the potential to have important consequences for ecosystem processes.
DegreeMaster of Science (M.Sc.)
SupervisorLamb, Eric G.; Siciliano, Steven D.
CommitteeRomo, James T.; Bai, Yuguang; Coulman, Bruce
Copyright DateMay 2013