In semi-arid environments, precipitation-driven spatial and temporal patterns of soil water content are a key control on plant and ecosystem function. Temporal variation in water availability results from discrete storms that provide a pulse of soil water that drives biotic activity until available water is depleted. Climate variability and change from seasons to decades and longer are likely to alter the frequency and intensity of precipitation pulses in aridland ecosystems. Moreover, ecosystem structure and function may be altered by deviations in pulse regime. We investigated the impact of altered precipitation frequency on soil CO2 and moisture availability and the ecophysiological response of the dominant Chihuahuan Desert grass, Bouteloua eriopoda. To do so, we manipulated rainfall event size and timing in a monsoon rainfall manipulation experiment at the Sevilleta LTER site in central New Mexico, USA. Treatments (n=5) included ambient rainfall plus one 20mm rain event each month (July-Sept) and ambient plus four 5mm rain events each month. Soil processes were measured by in situ sensors that measured soil CO2, moisture and temperature at 2, 8, and 16 cm depths at 30-minute intervals. Pre-dawn and midday water potential, leaf level photosynthesis and stomatal conductance of B. eriopoda were measured in association with experimental rainfall events.
Results/Conclusions Altered patterns of precipitation affected seasonal mean soil CO2 and mean daily soil water content. In general, large monthly rainfall treatments decreased surface CO2 relative to ambient plots, but significantly increased soil CO2 at 8 and 16cm depths relative to ambient, with the largest increase at 16cm. All rain events led to a distinct spike in soil CO2 at 2cm most likely due to microbial activity. Mean daily soil moisture at 8 and 16 cm was higher in the monthly compared to the ambient and weekly rainfall treatments. These patterns are weakly reflected in the physiological responses of B. eriopoda. Leaf level gas exchange and stomatal conductance were both significantly higher in the small and large rainfall treatments relative to ambient plots, and net photosynthesis was highest overall in the plots with a small number of large events. These patterns persisted until mid-August when large ambient rain events recharged soil moisture throughout the soil profile. Overall, our results suggest that a small number of large events (more extreme events) will increase soil moisture content leading to a positive production response by B. eriopoda, along with an increase in soil CO2 fluxes to the atmosphere.
