Exploring Real-time Oxygen Dynamics in the Rhizosphere of Sorghum with High Spatial and Temporal Resolution

The presence of oxygen in soil controls the occurrence and rates of biogeochemical processes underlying soil nutrient transformations and greenhouse gas dynamics. Oxygen (O2) levels within the rhizosphere are heavily modulated by both root and microbial respiration. Thus, a microscopic environment near the root may not be well represented by broader, non-rhizosphere soil. Here we examine on the millimeter-scale oxygen consumption or loss processes in the rhizosphere of sorghum, how they are influenced by sorghum genotype and irrigation practices, and the relationship between oxygen dynamics and nitrification in the rhizosphere.

A field study has been conducted at a research farm at the University of Illinois Urbana-Champaign. Several sorghum genotypes were grown under a rainout shelter with plants of each genotype undergoing one of 2 irrigation treatments. Soil O2 concentration and isotopic ratios, nitrous oxide (N2O), and carbon dioxide (CO2) were measured in the rhizosphere of sorghum via an array of novel microvolume probes coupled to an Aerodyne TILDAS (Tunable Infrared Laser Direct Absorption Spectrometer). Probes were placed within the rhizosphere or outside the root zone with the aid of root windows installed at the site.

We collected real-time in situ measurements of O2, O2 isotopes, CO2 and N2O over the 2022 sorghum growing season. The high spatial and temporal resolution of the measurements allowed us to observe spatiotemporal heterogeneity of biogeochemical activity in the rhizosphere as a function of agricultural activity.

The novel microvolume sampling system coupled with the O2 detection method can provide insights into fine scale gradients driven by higher microbial consumption near the roots. Measurements on this mm-scale has further applications for monitoring other trace soil gases and their spatial and temporal heterogeneity in soil systems.