Unraveling the Molecular Mechanisms Underlying the Soil Microbiome Response to Soil Moisture

Abstract:

Soil microorganisms are highly sensitive to changes in their environment, and rapidly measuring their physiological responses is necessary to fully understand the biological processes that they mediate. Drought is an increasing relevant environmental stress that soil microbiomes experience and understanding the mechanisms by which the soil microbiome responds to soil dehydration is vital. Here, we used ¹³C as a tracer of nutrient fluxes through microbiomes in desiccated soil after rewetting by measuring aerobic respiration using Real Time Mass Spectrometry (RTMS) and tracking the metabolic fate of the substrate using Isotope Ratio Mass Spectrometry.

The molecular mechanism(s) underlying the rapid mineralization of soil organic matter and increased rate of CO₂ release upon rewetting dry soil (termed the ‘Birch Effect’) are yet to be fully defined. One known microbial mechanism to protect against dehydration is the production of intracellular compounds known as osmolytes. We hypothesize that the rapid release of CO₂ arises from the microbial processing of these intracellular compounds rather than extracellular carbon. RTMS allows for the simultaneous, highly temporally resolved (every 2 sec) evaluation of the production and consumption of multiple gasses including ¹²CO₂,¹³CO₂, O₂, N₂ and H₂O. We compared the hydration response (real time CO₂ production) between water and ¹³C labeled glucose dissolved in water amendments. The initial burst of ¹²CO₂ (5 min) followed by a leveling off was identical in both treatments. An increase in ¹³CO₂ production was observed later (2-4 h) in the ¹³C glucose experiment. When solid phase ¹³C glucose was amended to the soils, the ¹³CO₂ production delay was much longer, potentially indicating that the solubilization rate is important to the respiration process. By contrast, amending the soil with ¹³C labeled alanine, either in solution or solid phase, produced negligible amounts of ¹³CO₂, potentially indicating usage of the amino acid is not being used for energy production and respiration, but diverted to another pathway. We are currently exploring this divergence between substrates using IRMS to track the ultimate fate of the ¹³C label. Together these results demonstrate the importance of substrate form in regulating metabolic pathways that contribute to soil respiration.