Novel Probe of Oxygen and its Isotopes for Millimeter-Scale Measurements of Soil Dynamics

One of the major challenges facing the world is the creation of sustainable energy sources. Biofuels and bioproducts have great potential. But the development of sustainable and efficient agricultural production methods requires deep understanding of the dynamics of plant-microbe-mineral interactions within the rhizosphere. Technologies that probe soil microbial activity in real time with high spatial fidelity are needed to better understand these dependencies. Oxygen (O2) is central to a number of processes in the rhizosphere including microbial and root respiration, nitrogen-cycling processes such as nitrification and denitrification, and the biodegradation and oxidation of organic contaminants. The subsurface oxygen isotopic signature can help to determine the drivers of soil oxygen dynamics. This project will develop an instrument to detect subsurface oxygen and its isotopes with millimeter-scale spatial resolution. This relies on two innovations. The first is a small volume soil probe that can extract very small volumes (10s of µl) of soil gas. The second innovation is a novel detection method for molecular oxygen and its isotopic composition. The sampling system will catalytically convert molecular oxygen to water vapor since water can be detected with much greater sensitivity. The water composition and isotopic abundance will be determined by tunable infrared laser differential absorption spectroscopy (TILDAS), a well-established method. The extraction and catalytic conversion techniques will be demonstrated during Phase I and integrated with a TILDAS water isotope monitor. This integrated system will be optimized to assure conversion of oxygen to water without isotopic fractionation, thus allowing the isotopic composition of water to be used to establish the O2 isotopic signatures of the soil gases. A suite of field and laboratory application experiments will also be planned, for execution during Phase II. The proposed technology will be adopted by the soil science research community to study the spatial and temporal variability of soil biogeochemical activity. Bioenergy and bioproduct research is a growing field whose success depends on the improved understanding of subsurface ecosystems. The use of this novel technology will enhance our understanding of difficult to observe soil process and will result in sounder agricultural decisions, leading to better food security.