Springtime water supplement in a wet year increases soil respiration but not photosynthetic rates in a semiarid ecosystem on the Colorado Plateau
Semiarid ecosystems have the potential to annually gain and lose a substantial fraction of total ecosystem carbon storage (plant+soil) due to interannual variation in water availability. Previous work at our Colorado Plateau study site has shown that supplementing monsoonal water late in the summer does not have a large effect on plant growth or biomass. We hypothesized that this lack of effect may have been due to supplemental water arriving outside of the peak growing season, which is spring. To test the responsiveness of the ecosystem to supplemental water during the growing season, we added the equivalent of a 10 mm rain event three times during the spring of 2014 to plots that were both ambient and warmed (+2 °C) by infrared heating lamps. We measured photosynthetic rates of a dominant grass, Acnatherum hymenoides, and a dominant shrub, Atriplex confertifolia using an LI-6400 Portable Photosynthesis system. We also measured soil resporation rates using an LI-8100 soil respiration chamber.
While somewhat variable, the photosynthesis measurements did not show a strong response to the watering treatment for either A. hymenoides or A. confertifolia. These plants also did not show substantial responses in their stem water potentials in either ambient or warmed plots. However, measurements of soil respiration did show that watered plots (warmed and unwarmed) exhibited substantially higher respiration rates for 2-3 days after the treatment. These data suggest that in an already wet year, these plants that are adapted to dry ecosystems, were already operating at their maximum physiological potential, and unable to take advantage of extra soil water, but that soil microbes are more sensitive to day-to-day variation in conditions. Overall, these findings suggest that during wet years, carbon gains in semiarid ecosystems may reach a wetness threshold after which increased respiration reduces net carbon gain by increasing soil respiratory losses.