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

Understanding the dynamics of plant-microbe-mineral interactions within the rhizosphere is important to developing methods for sustainable and efficient agricultural production. 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 identify the different oxygen consumption or loss pathways, and its diffusion in and out of the rhizosphere region.

Fine scale spatial heterogeneity in soil O₂ is thought to allow the co-occurrence of aerobic and anaerobic biogeochemical processes in the soil. Thus, technologies that probe soil microbial activity in real time with high spatial fidelity are needed to better understand the dynamics of processes in the rhizosphere. We will present here recent progress in our development of an instrument to detect subsurface oxygen and its isotopes with millimeter-scale spatial resolution. This novel system 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 conversion of oxygen to carbon dioxide (CO₂) followed by sensitive, selective detection with an Aerodyne Tunable Infrared Laser Direct Absorption Spectrometer (TILDAS). This integrated system is optimized to assure conversion of oxygen to carbon dioxide without isotopic fractionation, thus allowing the isotopic composition of CO₂ to be used to establish the O₂ isotopic signatures of the soil gases.