Precipitation pulses and soil CO2 flux in a Sonoran Desert ecosystem

Precipitation is a major driver of biological processes in arid and semiarid ecosystems. Soil biogeochemical processes in these water-limited systems are closely linked to episodic rainfall events, and the relationship between microbial activity and the amount and timing of rainfall has implications for the whole-system carbon (C) balance. Here, the influences of storm size and time between events on pulses of soil respiration were explored in an upper Sonoran Desert ecosystem. Immediately following experimental rewetting in the field, CO2 efflux increased up to 30-fold, but generally returned to background levels within 48 h. CO2 production integrated over 48 h ranged from 2.5 to 19.3 g C m(-2) and was greater beneath shrubs than in interplant spaces. When water was applied on sequential days, postwetting losses of CO2 were only half a large as initial fluxes, and the size of the second pulse increased with time between consecutive events. Soil respiration was more closely linked to the organic matter content of surface soils than to rainfall amount. Beneath shrubs, rates increased nonlinearly with storm size, reaching an asymptote at approximately 0.5 cm simulated storms. This nonlinear relationship stems from (1) resource limitation of microbial activity that is manifest at small time scales, and (2) greatly reduced process rates in deeper soil strata. Thus, beyond some threshold in storm size, increasing the duration or depth of soil moisture has little consequence for short-term losses of CO2. In addition, laboratory rewetting across a broad range in soil water content suggest that microbial activity and CO2 efflux following rainfall may be further modified by the routing and redistribution of water along hillslopes. Finally, analysis of long-term precipitation data suggests that half the monsoon storms in this system are sufficient to induce soil heterotrophic activity and C losses, but are not large enough to elicit autotrophic activity and C accrual by desert shrubs.