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 be a microbial hotspot and not well represented by broader, non-rhizosphere soil.  Here we examine the millimeter-scale oxygen consumption or loss processes in the rhizosphere of sorghum, how they are influenced by irrigation practices, and the relationship between oxygen dynamics and nitrification in the rhizosphere.

In a field study at a research farm at the University of Illinois Urbana-Champaign, sorghum was grown under a rainout shelter with plants 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 the 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 continuous, 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.  

We will also report on controlled laboratory incubations to quantify the impact of soil microbial oxygen consumption on ¹⁸O enrichment as compared to water displacement in the soil; and controlled greenhouse experiments to measure fine scale gradients of oxygen concentrations and isotopic composition near roots.

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