Soil Gas Monitoring.pdf

Soil gas concentration and isotope monitoring system integrates novel soil gas probes with online trace gas detection. The system uses porous and hydrophobic sintered PTFE diffusive soil gas probes to non-disruptively collect soil gas samples. The probes are connected to a transfer system that directs gas from multiple probes to one or more central gas analyzers, such as laser and mass spectrometers.

The system measures isotopic ratios of nitrous oxide (δ18O, δ15N, and the 15N site-preference of N2O), methane, carbon dioxide (δ13C), and volatile organic compounds (VOCs). It was tested using an inert silica matrix to challenge probe measurements under controlled gas conditions. The system was optimized by changing and controlling system flow parameters, including the probe flow rate, to recover representative soil gas samples while reducing sampling artifacts on subsurface concentrations.

The system was used to monitor the impact of environmental manipulations, such as irrigation and soil redox conditions, on in situ N2O and VOC concentrations. A new high-precision laser spectrometer was developed to measure the stable isotope ratios of N2O with a reduced sample volume demand. The system successfully quantified isotopic signatures for N2O, CO2, and VOCs in real-time as a response to changes in dry-wetting cycle and redox conditions.

Soil gases, such as N2O, CO2, CH4, and VOCs, are important messengers of belowground biogeochemical processes and microbial activity. Isotopic signatures of these gases can help identify and quantify gas processes, such as microbial pathways driving CH4 production. The ratio of 15N to 14N and the position of the 15N relative to the O in N2O can depend on the N2O production pathway, reflecting the microbial pathway and not substrate isotopic signature.

VOCs are signals for diverse microbial and chemical interactions in soils, involved in microbial and plant-microbe interactions, and may reflect soil health, stress responses, and microbial identity. Inert tracers, such as Helium, can help distinguish physical from chemical mechanisms affecting soil gas concentrations. The system can elevate the understanding of the role of microbial communities and their metabolism in soil by tracking microbial activity using trace gas messengers.

Traditional soil gas sampling approaches have limitations, such as being time-consuming, creating artifacts, and disturbing the probe surroundings. Diffusive probes sample soil gases by non-advective gas exchange driven by molecular diffusion, but have relatively large volumes and long equilibration times. The new system addresses these limitations by using smaller probes with enhanced diffusion, reducing disturbance to the soil environment, and providing real-time measurements of soil gas concentrations and isotopic signatures.