Abstract
Soil microbes vigorously produce and consume gases that reflect active soil biogeochemical processes. Soil gas measurements are therefore a powerful tool to monitor microbial activity. Yet, the majority of soil gases lack non-disruptive subsurface measurement methods at spatiotemporal scales relevant to microbial processes and soil structure. To address this need, we developed a soil gas sampling system that uses novel diffusive soil probes and sample transfer approaches for high-resolution sampling from discrete subsurface regions. Probe sampling requires transferring soil gas samples to above-ground gas analyzers where concentrations and isotopologues are measured. Obtaining representative soil gas samples has historically required balancing disruption to soil gas composition with measurement frequency and analyzer volume demand. These considerations have limited attempts to quantify trace gas spatial concentration gradients and heterogeneity at scales relevant to the soil microbiome. Here, we describe our new flexible diffusive probe sampling system integrated with a modified, reduced volume trace gas analyzer and demonstrate its application for subsurface monitoring of biogeochemical cycling of nitrous oxide (N2O) and its site-specific isotopologues, methane, carbon dioxide, and nitric oxide in controlled soil columns. The sampling system observed reproducible responses of soil gas concentrations to manipulations of soil nutrients and redox state, providing a new window into the microbial response to these key environmental forcings. Using site-specific N2O isotopologues as indicators of microbial processes, we constrain the dynamics of in situ microbial activity. Unlocking trace gas messengers of microbial activity will complement -omics approaches, challenge subsurface models, and improve understanding of soil heterogeneity to disentangle interactive processes in the subsurface biome.
Highlights
Later approaches used sampling devices that pre-equilibrated sampling volumes with soil gas by diffusion into open tubes[16,17] or traps[18,19] with sufficient volume to withdraw sample gas while reducing contamination by advective mass flow. These devices have the disadvantage of a larger spatial footprint, with few exceptions[20], and potential for bulk water contamination. This approach was improved by allowing soil gas to diffuse into an internal sampling volume across barrier membranes composed of silicone21, polypropylene22–24, polyethylene[25], or expanded PTFE26–29
We constructed novel diffusive probes from porous sintered PTFE (Fig. 1a)—a material with superior hydrophobicity, inertness, cleanability, microfiltration, and pore distribution uniformity to previous probe m embranes[43]. We coupled these probes to multiple Tunable Infrared Laser Direct Absorption Spectroscopy (TILDAS) analyzers (Fig. 1c) to target trace gas messengers of nitrogen and carbon cycling (N2O, Nitric oxide (NO), nitrogen dioxide ( NO2), CH4, CO2), and the site-specific stable isotopologues of N 2O, whose signatures reflect N 2O production p athways[44,45]
The performance of the diffusive sampling system based upon hydrophobic sintered PTFE (sPTFE) subsurface gas probes (Fig. 1) was demonstrated in two mesocosm experiments: 1) addition of fertilizer to three columns of identical topsoil under ambient redox conditions, resolving different temporal responses among N 2O, NO, CH4 and C O2 trace gas messengers; and 2) addition of fertilizer to two topsoil columns under different redox environments, clocking the onset of anaerobic methanogenesis and aerobic nitrification, and identifying methane “hot moments”
Summary
Many biologically interactive small molecules and their isotopologues (e.g., N 2O, CH4) are routinely monitored in the gas phase by direct absorption spectroscopy and online mass spectrometry with excellent precision and time resolution These recently developed analytical tools can be modified and coupled with optimized subsurface diffusive probe designs to yield new and deeper insights into a growing range of biogeochemical processes. We constructed novel diffusive probes from porous sintered PTFE (sPTFE) (Fig. 1a)—a material with superior hydrophobicity, inertness, cleanability, microfiltration, and pore distribution uniformity to previous probe m embranes[43] We coupled these probes to multiple Tunable Infrared Laser Direct Absorption Spectroscopy (TILDAS) analyzers (Fig. 1c) to target trace gas messengers of nitrogen and carbon cycling (N2O, NO, nitrogen dioxide ( NO2), CH4, CO2), and the site-specific stable isotopologues of N 2O, whose signatures reflect N 2O production p athways[44,45]. These techniques represent a general approach to monitor trace gases as messengers of soil microbial processes to help interpret and model the effects of critical soil microbiomes
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