David Robinson, Byron J. Adams, Zachary T. Aanderud1,
1Department of Plant and Wildlife Sciences, Brigham Young University, Provo, Utah 84602, USA
2Department of Biology, Brigham Young University, Provo, Utah 84602, USA
The availability of water as soil moisture is a key driver determining the temporal variability of microbial respiratory processes. For example, in the spring, summer, and fall pulses of rainfall stimulate soil respiration dramatically increasing rates up to 500% higher than pre-wetting conditions and pulses collectively accounting for 5-10% of annual net ecosystem production and up to 90% of the late-season ecosystem respiration. In addition, bacteria are metabolically active for most of the year, especially as freeze-thaw events, in the absence of snow cover stimulate wintertime respiration with wintertime activity accounting for more than 50% of the carbon sequestered by higher plants during the growing season. However, all pulses are not created equal and the fundamental metabolics and bacteria traits facilitating enhanced bacterial activity once dry soils are wetted or frozen soils thaw is rarely compared. We simulated three drying-rewetting and freeze-thaw cycles in three cold desert soils across the globe (i.e., Antarctica’s Taylor Valley, USA’s Great Basin Desert; and China’s Junggar Basin) and measured soil respiration in real-time with Vaisala GMP 220 CO2 sensors and microbial gene expression through the sequencing of messenger RNA.
Soil rewetting and thawing created distinctly different CO2 pulse that varied by continent. For example, in China and USA, soil rewetting rapidly elevated CO2 flux within one day to at least 980 ppm and pulses continued for 8-14 days. However, in the same two desert soils, thawing slowly elevated CO2 over 3-5 days as fluxes reached a maximum of at most 598 ppm. In contrast, Antarctica soils generated cycles with higher maximums following soil thawing (335 – 265 ppm at -2°C) than rewetting (295 – 230 ppm at 5°C) and freeze-thaw pulses peaked more rapidly in Antarctica (2 days) than China and USA soils. Antarctica microorganisms pass through a more robust extreme environmental filter allowing high levels of metabolic activity under subzero conditions. Regardless of metabolic activity being stimulated by rewetting or thawing, soil respiration incrementally declined from the first cycle to the last cycle in deserts, suggesting that the accessibility of labile carbon substrates to microorganisms declined with each cycle. Our results suggest that the effects of soil rewetting and thawing on soil respiration are unique but not universal among ecosystems.