Development of Spatially and Temporally-Resolved Subsurface Measurements of Hydrogen, its Isotopes, and other Trace Gases

The soil microbiome is a complex system that plays a central role in biogeochemical processing of nutrients and impacts the exchange of trace species between the atmosphere and subsurface environment. Understanding how soil microbes thrive, under what conditions, and through the utilization of which resources, is a critical part of any assessment of ecosystem health and stability. Diatomic hydrogen is a byproduct of biological nitrogen fixation, an important process that contributes to soil nitrogen content, and is a nutrient for certain subsurface microbes. Consumption by microbes is the largest sink of atmospheric hydrogen, and the largest source of uncertainty in the global hydrogen budget. Identifying its production and consumption pathways, and how they are externally influenced, would inform our understanding of this important soil gas, and reveal new insights into the role of biological nitrogen fixation in biogeochemical nutrient cycling, soil health, and agricultural productivity. The concentration and isotopic signature of hydrogen are valuable indicators of the pathways responsible for its production and consumption in soil, however, in situ measurements of soil hydrogen are complicated by the large concentration gradients of hydrogen near nitrogen-fixing centers. This DOE Phase II SBIR proposal is aimed at continuing the design and development of a commercially viable system to map subsurface hydrogen and its isotopic signature by leveraging recent advances in soil gas sampling and well-established laser-based detection technologies. During Phase I a catalytic converter was developed that quantitatively converted hydrogen and its isotopes to water vapor, which was then readily detected using high-resolution infrared spectroscopy. The conversion system was coupled to soil gas probes buried in mesocosm columns containing agricultural soils. Testing of the compiled system successfully demonstrated quantitative recovery of soil hydrogen, and the ability to temporally resolve hydrogen production “hot moments”. During Phase II the water isotope infrared analyzer will be modified for improved performance, including developing a heated absorption cell to minimize water-surface interactions. A commercial hydrogen converter package and an associated calibration system will be designed, built, and tested. The total soil hydrogen sampler will be tested and optimized in the laboratory, then field-deployed to an agricultural location to demonstrate real-world performance. The proposed system will enable new measurements of an important subsurface gas to refine nutrient cycling models, understand biological processes, and inform atmospheric budgets and models. This capability will be attractive to soil management and agricultural research markets due to the importance of hydrogen as a messenger of biological nitrogen fixation, a process that is crucial to sustainable agriculture practices. It will also find value in the academic and governmental soil research community, where the role of hydrogen in subsurface processes and its impact on ecosystem health and stability is actively being explored.