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 microbes thrive, under what conditions, and through the utilization of which resources is an important part of any assessment of ecosystem health and stability. Within this context, diatomic hydrogen is an important gas both as a byproduct of nitrogen fixation and as a nutrient for certain subsurface microbes. Hydrogen is produced at nitrogen fixing nodules on plant roots, then diffuses into soil, where it is released to the atmosphere or consumed by 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 its role in biogeochemical nutrient cycling. The concentration and isotopic signature of hydrogen are valuable indicators of the pathways responsible for its production and consumption in soil. Measurements of hydrogen are difficult due to its non-reactive nature and lack of an optical signature. In situ measurements are further complicated by the large concentration gradients of hydrogen near nitrogen-fixing centers. This DOE SBIR proposal is aimed at designing and developing a commercially viable system to measure hydrogen and its isotopic signature by leveraging recent advances in subsurface soil gas sampling and well-established laser- based detection technologies. The resulting product will be able to map subsurface hydrogen and its isotopic variant to determine production and consumption pathways in real-world soils and ecosystems. During Phase I, components of the proposed system will be developed and eventually combined for a benchtop demonstration of the system capabilities. These components include a sample preparation step to remove interfering species, a reaction step to quantitatively and selectively convert hydrogen to a molecule that can be detected using infrared laser spectroscopy, and a high-precision accurate detection platform that is field-deployable. These developments will aid in the design of a Phase II system for eventual commercialization. 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. 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.