Technical Abstract: Rainfall in deserts occurs in discrete events or pulses. The frequency and magnitude of precipitation pulses may differentially drive ecosystem components, such as soil microbes, plants, and microbial crusts. As significant components of desert ecosystems, microbial crusts are photosynthetic communities of lichen, bacteria, cyanobacteria, and fungi. On the Jornada Experimental Range in southern New Mexico, rainfall addition was used to manipulate the frequency and magnitude of rain events. Plots either received small (5mm) weekly pulses, large (20mm) monthly pulses, or ambient conditions (no additional rainfall). Crust CO2 exchange was measured two days after all plots received water treatments. Measurements were made prior to and after addition of 2mm of water, to measure CO2 flux in the native state and in moist conditions with stimulated activity, respectively. Prior to 2mm of water application, the dominant process was respiration on the control and monthly plots, while photosynthesis was the dominant process on the weekly plots. After the 2mm water application, all three treatments showed a photosynthetic response, with the weekly-pulsed plots showing the greatest response. Crusts that received more frequent, small events contributed more to ecosystem carbon gain than crusts that received larger, infrequent events or that experienced ambient conditions.