Quantifying microbial growth and carbon use efficiency in dry soil environments via 18O water vapor equilibration

As the global hydrological cycle intensifies with future warming, more severe droughts will alter the terrestrial biogeochemical carbon (C) cycle. As soil microbial physiology controls the large fluxes of C from soil to the atmosphere, improving our ability to accurately quantify microbial physiological parameters in soil is essential. However, currently available methods to determine microbial C metabolism in soil require the addition of water, which makes it practically impossible to measure microbial physiology in dry soil samples without stimulating microbial growth and respiration (namely, the "Birch effect").We developed a new method based on in vivo 18O water vapor equilibration to minimize soil re-wetting effects. This method allows the isotopic labelling of soil water without any liquid water or dissolved substrate addition to the sample. This was compared to the main current method (18O-water application method) in soil samples either at near-optimal water holding capacity or in air dry soils. We generated time curves of the isotopic equilibration between liquid soil water and water vapor and calculated the average atom percent 18O excess over incubation time, which is necessary to calculate microbial growth rates. We tested isotopic equilibration patterns in nine different soils (natural and artificially constructed ones) covering a wide range of soil texture and organic matter content. We then measured microbial growth, respiration and carbon use efficiency in three natural soils (either dry or at near-optimal water holding capacity). The proposed 18O vapor equilibration method provides similar results as the currently widely used method of liquid 18O water addition to determine microbial growth when used a near-optimal water holding capacity. However, when applied to dry soils the liquid 18O water addition method overestimated growth by up to 250%, respiration by up to 500%, and underestimated carbon use efficiency by up to 40%.Finally, we applied the new method to undisturbed biocrust samples, at field water content (1-3%), and show for the first time real microbial growth rates and CUE values in such arid ecosystems. We describe new insights into biogeochemical cycling of C that the new method can help uncover and consider the wide range of questions regarding microbial physiology and its response to global change that can now be proposed and addressed.