Abstract:
Within A rtemisia tridentata ssp. wyomingensis Nutt. (Wyoming big sagebrush)
communities of the Great Basin, lands dominated by the exotic annual grass Bromus
tectorum L. (cheatgrass) are increasing at an alarming rate. Carbon applications, which
reduce plant-available soil N, have been suggested as a way to give native vegetation a
competitive advantage over exotic annual grasses when reseeding after a fire. The main
objectives of my study were to 1) compare N pools in intact A. tridentata- and B.
tectorum-dominated communities to look for evidence of ecosystem changes associated
with annual grass invasion, and 2) quantify and compare the effects of sugar and
nitrogen additions on N pools in each of these communities. Research occurred at six
sites in eastern Oregon and southwestern Idaho, each containing pairs of A. tridentata
and exotic annual grass communities in close proximity. Pairs were carefully selected
with similar soil types, precipitation, elevation, aspect, slope and ecological sites (i.e.
potential vegetation and production). At the beginning of the cheatgrass growing season
(late fall), three treatments (sugar, nitrogen or control) were applied. For soil pools,
only one difference between untreated plots of annual and native communities was
detected. In autumn. NO3 in the native community was about two-thirds the level in
the annual community. The sugar treatment decreased inorganic N to near undetectable
levels one week after application, and levels remained low six months later (during peak
B. tectorum biomass). Although the sugar treatment did not increase microbial biomass
N from chloroform fumigation extraction, we found higher '5N in microbial biomass
and soil organic matter, suggesting that more N remained in the microbial pooi over the
growing season. The reduction of aboveground plant biomass by sugar and the increase
of aboveground plant biomass from nitrogen addition were more pronounced for B.
tectorum than for native plants. Plant responses indicated that treatment with labile
carbon like sugar may be useful tools for restoration of native plants and for prevention
of B. tectorum dominance, but additional research is necessary to quantify dose
responses of B. tectorum to sugar.
