Novel Isotope Sensor for Spatially-resolved Analysis of Nutrient Exchange in the Rhizosphere TechNologies for Characterizing the Rhizosphere

Ensuring adequate agricultural productivity in the face of an increasing human population is a major concern with implications for both production of bioenergy products and global food security. Increasing agricultural productivity is needed, which requires a stronger understanding of soil biogeochemical processes that impact plant nutrient availability. The project is focused on developing an innovative isotope sensor that will enable high spatial-resolution analysis (down to the microbe level) to target nutrient exchange in the rhizosphere. The concept uses a novel, ultra-low volume isotope analyzer based on laser absorption spectroscopy, called a Capillary Absorption Spectrometer (CAS). In this project, the CAS is being developed to analyze stable carbon isotope ratios (13C/12C) at high spatial resolution with laser ablation (LA) sampling. The resulting LA- CAS tool will enable studies designed to track uptake of isotope labeled 13CO2 from fixation into the plant and eventually through the roots and into the soil as root exudates. The concept builds on successful work in which laser ablation has been coupled to isotope ratio mass spectrometers (IRMS). The innovative aspect is that the IRMS is being replaced by the more sensitive CAS, which can analyze orders of magnitude smaller sample sizes, enabling smaller ablation spot sizes and improved spatial resolution. Furthermore, as compared to IRMS (as well as other isotope analyzers) the CAS is more compact, lower power, and more amenable to potential field deployment as a stand-alone gas analyzer. In Phase I, a breadboard LA-CAS system was assembled and used to prove the concept with spatially resolved measurements of 13C/12C in the rhizosphere of switchgrass. Measurements with the breadboard LA-CAS were compared directly to measurements with LA-IRMS and demonstrated to enable smaller spot sizes (~ 10 µm) and better spatial resolution. In Phase II, the technology readiness level (TRL) will be increased with the goal of developing a tool useful for DOE applications and related commercial products. An LA-CAS prototype will be produced and demonstrated in scientifically relevant experiments with the goal of identifying locations of increased transfer of carbon into the rhizosphere – i.e., potential hotspots of plant carbon input into the rhizosphere microbial community. The software and packaging of the CAS platform will be advanced to facilitate commercialization for a range of applications. The proposed device will provide a new, valuable tool to specifically enhance fundamental understanding of key abiotic and biotic controls on nutrient exchange through the rhizosphere. In addition, the tool will have further, general commercial applications by enabling high throughput isotope analysis with minimal sample preparation. The CAS can replace more costly and labor-intensive devices such as isotope ratio mass spectrometers (IRMS) and nanoSIMS devices.