Abstract
Abstract. Gas concentrations and isotopic signatures can unveil microbial metabolisms and their responses to environmental changes in soil. Currently, few methods measure in situ soil trace gases such as the products of nitrogen and carbon cycling or volatile organic compounds (VOCs) that constrain microbial biochemical processes like nitrification, methanogenesis, respiration, and microbial communication. Versatile trace gas sampling systems that integrate soil probes with sensitive trace gas analyzers could fill this gap with in situ soil gas measurements that resolve spatial (centimeters) and temporal (minutes) patterns. We developed a system that integrates new porous and hydrophobic sintered polytetrafluoroethylene (sPTFE) diffusive soil gas probes that non-disruptively collect soil gas samples with a transfer system to direct gas from multiple probes to one or more central gas analyzer(s) such as laser and mass spectrometers. Here, we demonstrate the feasibility and versatility of this automated multiprobe system for soil gas measurements of isotopic ratios of nitrous oxide (δ18O, δ15N, and the 15N site preference of N2O), methane, carbon dioxide (δ13C), and VOCs. First, we used an inert silica matrix to challenge probe measurements under controlled gas conditions. By changing and controlling system flow parameters, including the probe flow rate, we optimized recovery of representative soil gas samples while reducing sampling artifacts on subsurface concentrations. Second, we used this system to provide a real-time window into the impact of environmental manipulation of irrigation and soil redox conditions on in situ N2O and VOC concentrations. Moreover, to reveal the dynamics in the stable isotope ratios of N2O (i.e., 14N14N16O, 14N15N16O, 15N14N16O, and 14N14N18O), we developed a new high-precision laser spectrometer with a reduced sample volume demand. Our integrated system – a tunable infrared laser direct absorption spectrometry (TILDAS) in parallel with Vocus proton transfer reaction mass spectrometry (PTR-MS), in line with sPTFE soil gas probes – successfully quantified isotopic signatures for N2O, CO2, and VOCs in real time as responses to changes in the dry–wetting cycle and redox conditions. Broadening the collection of trace gases that can be monitored in the subsurface is critical for monitoring biogeochemical cycles, ecosystem health, and management practices at scales relevant to the soil system.
Highlights
The impact of the biosphere’s soils on atmospheric composition is typically measured at the soil surface, yet belowground approaches may provide a more mechanistic perspective into trace gas cycling
We evaluated how measured soil gas concentrations changed in response to the following: probe sample flow rate (Experiment 4), environmental manipulation of the soil matrix (Experiment 5), and forced changes to the soil redox state (Experiment 6)
We found that the isotopic ratios of all three N2O isotopomers (δ448, δ546, δ456), site preference, and N2O concentration responded to the wetting over the subsequent 36 h period
Summary
The impact of the biosphere’s soils on atmospheric composition is typically measured at the soil surface, yet belowground approaches may provide a more mechanistic perspective into trace gas cycling. Gil-Loaiza et al.: Versatile soil gas concentration and isotope monitoring otic and biotic processes including microbial metabolism and soil environmental conditions (Conrad, 2005; Karbin et al, 2015; Jiao et al, 2018) that vary in space (i.e., soil aggregate, Schimel, 2018, to field, Wang et al, 2014) and time (e.g., rain-driven emission pulses) (Jiao et al, 2018) Environmental drivers such as soil moisture and oxygen availability modulate rates of aerobic and anaerobic processes that influence gas cycling including N2O emissions (Groffman et al, 2009) and VOC fluxes (Raza et al, 2017; Abis et al, 2020). While surface flux chambers remain a dominant, integrative tool to constrain soil gas fluxes, new capabilities are needed to unearth spatiotemporal variations in belowground processes
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