Spatially-Resolved Microbial Activity Probe Using Infrared Measurements of Nitrous Oxide and Methane Isotopes in Soil

Soil health is an important factor in ensuring long term agricultural production and food security- Soil microbial activity is crucial to nutrient cycling and soil fertility, but it is strongly dependent upon environmental variables, such as temperature, humidity, and nutrient and oxygen content- Technology that probes soil microbial activity in real time with spatial fidelity is needed to better understand these dependences, and thus inform biogeochemical models upon which land management strategies are based- Recent work has shown that the isotopic ratios of certain soil gases are signatures of their biochemical production pathways- Specifically, nitrous oxide and methane can be produced by a variety of microbial populations and processes, each with its own isotopic signature- Infrared spectroscopy has recently proven to be a valuable tool to precisely measure trace gas isotopic ratios- In this proposal, that technology will be improved upon and coupled to recently-developed soil probes, enabling rapid, spatially-precise soil gas measurements- The resulting system will be able to measure the 18O and site-specific 15N isotopic ratios of nitrous oxide at precisions of 4 and 3 per mil, respectively, and the 13C isotopic ratio of methane with a precision of 1 per mil- In addition to these analytical capabilities, the dual methane-nitrous oxide isotopomer instrument will be developed to interface with a soil probe array, allowing for subsurface mapping of microbial activity in real time- It will also be designed to run unattended in the field for extended deployments- During Phase I a prototype instrument will be built, and its noise performance optimized- Efforts will be made to minimize or eliminate any concentration dependence of the measurement- In parallel with this effort, a soil probe will be developed that best interfaces with the instrument- Finally, an overall dual methane-nitrous oxide isotopomer instrument and soil probe package will be designed, to be built and field-tested during Phase II-