Responses of Soil Respiration to Precipitation in a Savannah Ecosystem: Fine Temporal Measurements of Soil CO2 Efflux

Precipitation pulses are the primary drivers of carbon cycling in deserts, and many components, such as soil respiration, may be a function of plant community composition. Drylands are experiencing substantial vegetation change, such as the encroachment of woody plants into historic grasslands, potentially altering soil biogeochemistry through litter inputs and microclimate. How precipitation will drive fine temporal responses of soil respiration in the different microclimates created by developing shrub islands is unclear. This study asks: how does the response of soil respiration to precipitation differ in shrub, grass, and open microsites? What is the contribution from each microsite to whole ecosystem CO2 exchange? At a medium density shrub community along a gradient of woody plant encroachment at the San Pedro River Valley in southeastern Arizona, we measured soil CO2 efflux, soil moisture, and soil temperature every half hour in the dry premonsoon and rainy monsoon periods. Prior to the monsoon, daily fluctuations in respiration coincided with soil temperature and microsites behaved similarly. During the monsoon, soil moisture best predicted day to day variation in respiration rates. Respiration beneath the mesquites was the highest and respiration in the open spaces was the lowest. However, the greatest diurnal trends occurred in the open spaces, due to greater soil moisture and temperature fluctuations. Differential responsiveness of the micosites to precipitation played an important role in determining ecosystem carbon balance as woody plant abundance increases in the San Pedro River Valley.