COS 276-6 - Drought rewet effects on soil microbial metabolism and activity

Abstract: Environmental stress, such as drought, is increasing in frequency with unknown outcomes for soil microbiomes. Microbial metabolic activity is generally reduced during heat and moisture stress or drought periods, either due to cell death or transformation to a dormant, metabolically inactive state. Upon wetting of dry soils, there is a rapid physiological response of the soil microbiome, most often measured as a burst of respiration (CO₂) known as the Birch effect. Yet the details of this phenomenon remain elusive. Here we aim to understand how rewetting of a dry soil impacts microbial physiology and metabolism immediately following moisture addition when bursts of respiration occur.



Soil samples were collected from our irrigated field experiment, in Prosser, WA. Samples were incubated to simulate 1 and 2 weeks of drought followed by a rewet event. We evaluated the response of the soil microbiome using a transcriptomic and metabolomic approach, coupled with community respiration and biomass measurements. Our results revealed that microbial biomass carbon decreased through the dehydration period and additionally through the rewetting process, with similar results for both the 1-week and 2-week drought exposure. The reduction of microbial carbon after rewetting was accompanied by a burst of CO₂ production within the first 10 minutes upon rewetting, with a gradual increase through the subsequent 90 minutes. Transcriptomic analysis revealed upregulation of genes associated with increased bacterial activity and respiration after rewet, including ribosomal proteins, NADH dehydrogenase, citric acid cycle, F-type ATP synthase, and transporters for amino acids, peptides, and oligosaccharides. Total CO₂ respired from soil exposed to two weeks of drought was 75.6% higher than soil exposed to one week of drought, consistent with the Birch effect. Dry and wet conditions had distinct metabolic profiles of the 336 metabolites identified over the course of the experiment. However transcriptomic data revealed that the 1- and 2-week drought samples did not differ in metabolic pathways after rewet, despite large and significant differences in respiration. These findings suggest similar pathways, but different quantities of carbon fluxing in response to drought periods, perhaps due to greater lysis of microbial biomass during extended drought. Together our results suggest that soil microbial communities react rapidly to changing moisture conditions with important implications for carbon allocation and fluxes within the soil environment.