Through the Eye of the Needle: Microbial Processing of Carbon in Soils in Response to Drought and Rewetting.

Recent research into saturated systems has largely focused on methanogenesis, and indeed, the balance between CO₂ and CH₄ emissions is important to represent in global models of the Earth system. However, increases in the extent and duration of drought are projected across the globe, as climate continues to change. As soils move from dry to wet conditions, several things happen with respect to biogeochemical system. We focus herein on two elements, i) hydrologic connectivity among soil pores, ii) change in local ionic strength as water evaporates and solutes concentrate in discontinuous water films. Hydrologic connectivity is a determinant of the transport potential of both SOC species and of any planktonic soil microorganisms. It couples with ionic strength in that the local ionic strength is a factor in controlling the relative desorption of complex versus simple SOC species as well as changing local abiotic habitat qualities in ways that fundamentally alter the bioavailability of soil organic C and the ability of the soil microbial communities to decompose or recycle native SOC. We sought to measure the effect of drought vs saturation on the mobilization of soil C, and to better understand the effects of antecedent soil moisture conditions on the pore-water associated microbial community. Study soils were sampled from Caribou-Poker Flats Research Watershed (Alaska), a tidal wetland near the mouth of the Columbia River (Washington), and the Disney Wilderness Preserve (Florida). Cores from these disparate sites were subjected to a 30-day simulated drought or flood in the lab, after which soils and soil pore waters were sampled. We used high-resolution mass spectrometry to assess the chemical forms of C in soils and soil pore waters, coupled with transcriptomics and metagenomics to assess the native microbial community and their activities (e.g. osmotolerance, community diversity, motility).