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.