The quantity and quality of plant OM inputs are key controls on soil microbial C availability and N demand, yet in semiarid ecosystems the timing of plant inputs may also be important since soil temperature and moisture conditions restrict periods of active growth. In this study, we attempted to stimulate microbial C availability by increasing plant detrital inputs in situ prior to the dry season, when soil was still moist, to examine the effects on C mineralization and gross N cycling compared to undisturbed soils. We created a C-pulse treatment by herbiciding vegetation in small plots in annual (cheatgrass), perennial (crested wheatgrass), and sagebrush communities. For perennial vegetation this treatment likely increased the total detritus pool, but for annual grass plots only the timing of plant inputs was affected. We hypothesized that increasing C availability before summer dry-season would increase microbial N demand and limit NO₃^- accumulation observed in annual grass soils during summer. Untreated and C-pulse soils were sampled before and after soil NO₃^- accumulation typically occurs in annual grass soils. Carbon mineralization rates were greater in C-pulse compared to control soils, suggesting that microbial C availability increased in the C-pulse treatment. However, soil inorganic-N pools, and gross N cycling rates also increased strongly in C-pulse compared to control soils, and these changes occurred earlier in summer than the typical NO₃^- accumulation observed in annual grass soils. Greater C turnover in C-pulse treated soils, in conjunction with elevated gross N cycling rates relative to control soils, point to increased microbial consumption of substrates with low C:N ratios. Interestingly, the stimulatory effects of the C-pulse treatment were most persistent in annual grass versus perennial dominated soils, suggesting that future shifts in timing of plant inputs through climate or vegetation change may have substantial affects on microbial C and N cycling.