Soil microbes increase investment into storage compounds during drought conditions.

The rise of atmospheric CO₂ concentrations, with subsequent increase in global warming and the frequency and duration of severe droughts, is altering the terrestrial carbon (C) cycling, with potential feedback to climate change.  Microbial growth, turnover and carbon use efficiency, are major controls of soil carbon fluxes to the atmosphere. Given the prominent role of soil microbial physiology for C cycling, quantifying microbial physiological responses to climate change is essential. Advances in the field now permit the quantification of community-level microbial growth and carbon use efficiency in dry conditions, by introducing stable isotopes in soil water via a water vapor equilibration technique. This has recently allowed, for the first time, to evaluate microbial physiology under drought conditions.

We here used the water vapor equilibration technique to measure deuterium (²H) incorporation into phospholipid and neutral fatty acids (PLFA and NLFA) and polyhydroxybutyrate (PHB). We applied this approach to soil samples collected from a long-term climate change experiment (ClimGrass) where warming, elevated atmospheric CO₂ (eCO₂) and drought are manipulated in a full factorial combination. Samples were taken in the field at four time points: before drought, one month and two months into drought, and few days after rewetting. We used a high-throughput method to extract PLFAs and NLFAs from soil, as well as a newly developed method to extract PHB, and measured ²H enrichment in these compounds via GC-IRMS.

We showed that during drought, bacterial growth rates are reduced, except for Actinobacteria, which maintain similar mass specific growth rates as compared to control conditions. Similarly, fungi growth rates are not affected by drought. Production of NLFAs (belonging to fungi and gram-negative bacteria) increased up to 4 to 6 folds when compared to production of membrane lipids. PHB production rates did not change compared to control conditions, revealing a higher production per unit of active bacteria. Our study demonstrates that climate change can have strong effects on microbial physiology. Investment into storage compounds is a major strategy present across different soil microbial groups in response to drought. Soil fungi and actinobacteria are key taxa in the microbial response to drought, maintaining most of the growth rates of the soil microbial community.