Coupling new soil probes and high-resolution trace gas isotopomer measurement systems: evaluating feasibility in controlled soil columns

Microbial communities in soil are dominant drivers of plant-soil-microbe interactions and ecosystem dynamics. Current soil sampling methodologies and protocols face challenges for addressing mismatches between feasible frequency and intensity of sample collection versus the extent and duration of the study system. As a result, many soil analyses give only "snapshots" of biogeochemical cycles processes mediated by microorganisms. Therefore, new "online" sampling approaches for tracking soil microbial activity are needed.

Here, we present a new system for measuring real-time isotopic fingerprints of soil microbial communities that improve the spatial and temporal limitations of other instrumentation and methodologies. Central to our approach are novel diffusive soil probes that we couple to high resolution Tunable Infrared Laser Direct Absorption Spectrometers (TILDAS) to analyze in situ trace gas isotopomers. This high-resolution quantitative system was evaluated using controlled soil "mesocosms" containing artificial soils (silica sand) with controlled gas composition to mimic isotopic composition of soil trace gases (CO₂, CH₄, NO₂).

We conducted a series of tests to: 1) Evaluate the behavior of the column; 2) Determine the ability of the probe to achieve fully equilibrated and representative samples in short period of time; and 3) Assess the performance of the system for measuring isotopic ratios of nitrous oxide (δ¹⁸O, δ¹⁵N, and the ¹⁵N site- preference of N₂O), methane (δ¹³C), and CO₂ in soil. We additionally demonstrated the versatility of the system by coupling it with a second platform—the Vocus mass spectrometer (high resolution volatile organic compound gas analyzer) for volatile organic compounds (VOCs).

Our results show that coupling gas analyzers to these diffusive probes provides quantitative recovery of isotopic signatures, and demonstrate the feasibility of fast, high-resolution quantitative subsurface trace gas sampling of N₂O, CH₄, and VOC concentrations and isotopic signatures. These capabilities have the potential to inform soil-plant-microbe interactions, additionally through integration with -omics approaches.