Bromus tectorum, a successful and damaging exotic annual grass throughout the western U.S., has also been a successful invader of C₃-C₄ perennial grasslands located within the low elevation, open ponderosa pine forests of Colorado's Front Range. Successful invasion can result in replacing a perennial grassland, which is active throughout most of the growing season, with an annual grassland, which is active only from early fall through early summer of the following year. To date, the results of studies that attempt to document how B. tectorum invasion into perennial grasslands changes soil nutrient and carbon (C) pools have been mixed, perhaps because most studies do not control for (1) daily and/or seasonal variation in soil pools and plant phenology and/or (2) variation in water availability, which is often a limiting resource in these semi-arid environments. We hypothesized that examining how C and nitrogen (N) pools beneath these plant communities change with time and water availability would shed light on these disparate results. We measured pools of inorganic N and microbial and dissolved organic C and N beneath stands of B. tectorum and C₃-C₄ perennial grass communities over a period of four days during June, July, and August of 2003. Half our plots received small water additions prior to the beginning of the experiment. We found that all pools of labile C and N were greater beneath stands of B. tectorum than perennial grass communities (P<0.05). While accounting for variation in water availability was only important for uncovering between-community differences in microbial pools, incorporating seasonal variation was important for detecting differences in all soil pools. Our results suggest that B. tectorum invasion increases labile C and N availability, and that accounting for temporal variation is crucial for uncovering the effects of B. tectorum invasion.