The introduction of nonnative plant species may decrease ecosystem
stability by altering the availability of nitrogen (N) for plant
growth. Invasive species can impact N availability by changing litter
quantity and quality, rates of N2-fixation, or rates of N loss. We
quantified the effects of invasion 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 community 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 ratios
were measured to determine the effects on litter dynamics. Long-term
soil incubations (415 d) were used to measure potential microbial
respiration 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 gaseous N loss were estimated by measuring
denitrification enzyme activity. Bromus invasion significantly
increased litter biomass, and Bromus litter had significantly greater
C:N and lignin:N ratios than did native species. The 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. The contrasting differences between years are
likely due to moisture availability; spring precipitation was 15%
greater than average during 1997, but 52% below average during spring
of 1998. Bromus may cause a short-term decrease in N loss by
decreasing substrate availability and denitrification enzyme 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
during fire. We hypothesize that the introduction of Bromus in
conjunction with land-use change has established a series of positive
feedbacks that will decrease N availability and alter species
composition. Published in Ecological Applications, volume 11, issue 5,
on pages 1301 - 1310, in 2001.