Versatile soil gas concentration and isotope monitoring: optimization and integration of novel soil gas probes with online trace gas detection

Abstract. Gas concentrations and isotopic signatures can unveil microbial metabolisms and their responses to environmental changes in soil. Currently, few methods measure soil trace gases such as the products of nitrogen and carbon cycling, or volatile organic compounds (VOCs), that could 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 measurements resolving spatial (centimeter scale) and temporal (minutes) variations in concentrations and isotopic signatures of in situ soil gases. We developed a system that integrates new 15 cm long sintered PTFE diffusive soil gas probes with various infrared spectrometers and a VOC mass spectrometer. The system is based on porous and hydrophobic soil probes that non-disruptively collect and transport gas from multiple probes to one or more central gas analyzers. Here, we demonstrate the feasibility and versatility of an automated multi-probe 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 probe flow rate, we optimized recovery of representative soil gas samples while reducing sampling artifacts on subsurface concentrations. Second, we forced environmental manipulations in soil-filled columns to demonstrate real-time detection of subsurface gas dynamics in response to irrigation and soil redox conditions. In addition, we developed a new laser spectrometer to recover isotope ratios for 14N14N16O ( δ446 ), 14N15N16O ( δ456 ), 15N14N16O ( δ546 ), and 14N14N18O ( δ448 ) with high precision and low concentration dependence. We captured temporal subsurface gas pulses in CO2, N2O, and VOCs. This demonstrated the potential for diffusive-based probes to couple to trace gas sensors for soil health and fertility studies, and to inform high-throughput meta-omics, leading to the development of a suite of powerful new tools for soil analysis.