Effects of altered soil moisture on respiratory quotient in the Edwards Plateau

Abstract:

Climate change is expected to alter precipitation patterns around the world. The impacts of altered precipitation on ecosystem function will be partly controlled by soil microbes because of their primary role in soil carbon cycling. However, microbial responses to drought remain poorly understood, particularly local responses that might partly reflect specialization based on historical conditions. Here, we investigated the respiratory response of microbial communities originating from historically wetter and drier sites to both low and high soil moistures. We focused on the respiratory quotient (RQ= moles of CO₂ produced per mole of O₂ consumed), which varies with the oxidation state of organic carbon being respired and/or the compounds being synthesized by soil microbes. We hypothesized that there would be a shift in RQ across the gradient of soil moisture. Soils were collected from 13 sites across a steep precipitation gradient on the Edwards plateau in central Texas, air-dried, rewet at low or high soil moisture (6% or 24% gravimetric, respectively), and incubated in an atmosphere of 21% O₂, 1% Ar, and balance He. After eight weeks, CO₂, O₂ and Ar in the headspace of incubation vials were measured by gas chromatography after separation of Ar and O₂ at subambient temperature. Because of the high calcite content in soils on the Edwards plateau, we corrected the RQ values by assuming pH was buffered at 8 and then adding the calculated amount of CO₂ dissolved in water in the incubations vials to the measured CO₂ in the headspace.

We found that uncorrected RQ values were slightly less than one and increased significantly with increasing mean annual precipitation. In contrast, corrected RQ values were greater than one and decreased with increasing mean annual precipitation. In both cases, we see a shift in RQ across the gradient, suggesting that differences in substrate utilization may vary based on origin across the gradient and with current level of soil moisture. This could provide insight into how microbial communities respond physiologically to shifts in environmental conditions, such as precipitation.