Exotic plant invasion alters nitrogen dynamics in an arid grassland

The introduction of normative plant species may decrease ecosystem stabilit y by altering the availability of nitrogen (N) for plant growth. Invasive s pecies can impact N availability by changing litter quantity and quality, r ates of N-2-fixation, or rates of N loss. We quantified the effects of inva sion by the annual grass Bromus tectorum on N cycling in an arid grassland on the Colorado Plateau (USA). The invasion occurred in 1994 in two communi ty types in an undisturbed grassland. This natural experiment allowed us to measure the immediate responses following invasion without the confounding effects of previous disturbance. Litter biomass and the C:N and lignin:N r atios were measured to determine the effects on litter dynamics. Long-term soil incubations (415 d) were used to measure potential microbial respirati on and net N mineralization. Plant-available N was quantified for two years in situ with ion-exchange resin bags, and potential changes in rates of ga seous N loss were estimated by measuring denitrification enzyme activity. B romus invasion significantly increased litter biomass, and Bromus litter ha d significantly greater C:N and lignin:N ratios than did native species. Th e change in litter quantity and chemistry decreased potential rates of net N mineralization in sites with Bromus by decreasing nitrogen available for microbial activity. Inorganic N was 50% lower on Hilaria sites with Bromus during the spring of 1997, but no differences were observed during 1998. Th e contrasting differences between years are likely due to moisture availabi lity; spring precipitation was 15% greater than average during 1997, but 52 % below average during spring of 1998. Bromus may cause a short-term decrea se in N loss by decreasing substrate availability and denitrification enzym e activity, but N loss is likely to be greater in invaded sites in the long term because of increased fire frequency and greater N volatilization duri ng fire. We hypothesize that the introduction of Bromus in conjunction with land-use change has established a series of positive feedbacks that will d ecrease N availability and alter species composition.