Novel Probe of oxygen and its isotopes for millimeter-scale measurements of soil dynamics

Sustainable agricultural approaches to improve energy biofuel production and the production of chemical feedstocks are needed. Understanding the dynamics of plant-microbe-mineral interactions within the rhizosphere, the region in the soil closest to the roots, is particularly important in addressing agricultural sustainability. Oxygen (O2) is key to processes in the rhizosphere including microbial and root respiration, nitrogen-cycling including nitrification and denitrification, and the biodegradation and oxidation of organic contaminants. The subsurface oxygen concentration and isotopic signature can be leveraged to identify the different oxygen consumption or loss pathways, and its diffusion in and out of the rhizosphere region. The overall objective of this SBIR project is to develop a technique to detect subsurface oxygen and its isotopes with millimeter-scale spatial resolution, enabling direct interrogation of the rhizosphere. A microvolume probe is coupled with a reaction module and an infrared spectrometer, for precisely measuring the concentrations and isotopic signatures of oxygen. In Phase I a microvolume probe, reactor for conversion of O2 to carbon dioxide (CO2), and an interface connecting these units with a tunable infrared laser direct absorption spectrometer (TILDAS) were designed and built. The TILDAS instrument was configured for simultaneous measurement of the three most abundant oxygen isotopes of carbon dioxide, C16O16O, C16O18O and C16O17O. Laboratory tests of the combined probe, mixing volume, reactor, and TILDAS in a benchtop system successfully demonstrated measurement of oxygen and its isotopic signature in microvolume samples. The conversion parameters (e.g. oven temperature, heating time, reaction pressure) were optimized, achieving >97% conversion of oxygen while minimizing isotopic fractionation. Tests in soil surrounding plant roots were successful, with a shift in the 18O isotopic ratio from ambient air observed. During the Phase II project the conversion technology and microvolume probes will be further refined. A field deployable prototype system consisting of probes, reactor, and TILDAS will be designed and built. The compatibility of the microvolume probes for other trace gas and VOC monitors will also be studied. The overall proposed system will be field demonstrated in soil experiments in a laboratory, greenhouse and at a DOE biofuel agricultural research facility. These experiments will provide proof of concept for the use of these mm-scale measurements to elucidate drivers of soil biogeochemical processes. Understanding how to work with and not against soil organisms is important for agricultural health and productivity. The proposed system will serve as a window into soil nutrient cycling, plant-microbe-mineral interactions, and effects of external forces (drought, flooding climate change) on soil processes. With this information, biofuel and soil research scientists, agronomists, and “smart” and precision farmers, will be able to identify and address key parameters to improve sustainable production at agricultural and bioenergy sites.