The impact of smooth brome on nitrogen cycling processes in a fescue grassland

Background/Question/Methods

Invasive plant species are a major threat to today’s ecosystems, as they can dramatically change plant community structure, with potential domino effects on other ecosystem processes. A variety of invasion mechanisms have been proposed, including the ability of an invasive species to alter nutrient cycling. To investigate the role of altered nutrient cycling in plant species invasions, we studied nitrogen cycling rates in a smooth brome (Bromus inermis) invaded grassland, near Saskatoon, SK, Canada. The goal of this study was to examine whether nitrogen cycling rates differ between smooth brome invaded and native soil, and if aboveground smooth brome abundance is related to soil ammonia-oxidizing bacteria (AOB) population size. Soil mineralization and nitrification rates were compared in invaded and native grassland using ¹⁵N stable isotope chemistry, and AOB abundance was quantified using RT-PCR. We used general linear models to assess the significance of these relationships.   

Results/Conclusions

Mineralization rates were significantly higher in smooth brome invaded soils compared to native fescue grassland soil, however, no significant difference was found in nitrification rates between treatments. Ambient ammonium and nitrate levels did not differ significantly between invaded and native soils. Despite no significant difference in nitrification levels between treatments, the presence of smooth brome had a significant effect on both archael and bacterial AOB population sizes.  These results suggest that the primary influence of smooth brome on the nitrogen cycle is through changes in the rate of organic matter mineralization, potentially through changes to plant biomass quality or quantity. This hypothesis is supported by evidence that plant community biomass in smooth brome invaded areas was significantly higher compared to uninvaded grassland. It appears though that these changes do not have downstream effects on the rest of the nitrogen cycle. In conclusion, this study showed that invasive species such as smooth brome can potentially alter nutrient cycling patterns.