Oxygen and redox potential gradients in the rhizosphere of alfalfa grown on a loamy soil

Abstract

Oxygen (O2) supply and the related redox potential (EH) are important parameters for interactions between roots and microorganisms in the rhizosphere. Rhizosphere extension in terms of the spatial distribution of O2 concentration and EH is poorly documented under aerobic soil conditions. We investigated how far O2 consumption of roots and microorganisms in the rhizosphere is replenished by O2 diffusion as a function of water/air‐filled porosity. Oxygen concentration and EH in the rhizosphere were monitored at a mm‐scale by means of electroreductive Clark‐type sensors and miniaturized EH electrodes under various matric potential ranges. Respiratory activity of roots and microorganisms was calculated from O2 profiles and diffusion coefficients. pH profiles were determined in thin soil layers sliced near the root surface. Gradients of O2 concentration and the extent of anoxic zones depended on the respiratory activity near the root surface. Matric potential, reflecting air‐filled porosity, was found to be the most important factor affecting O2 transport in the rhizosphere. Under water‐saturated conditions and near field capacity up to –200 hPa, O2 transport was limited, causing a decline in oxygen partial pressures (pO2) to values between 0 and 3 kPa at the root surface. Aerobic respiration increased by a factor of 100 when comparing the saturated with the driest status. At an air‐filled porosity of 9% to 12%, diffusion of O2 increased considerably. This was confirmed by EH around 300 mV under aerated conditions, while EH decreased to 100 mV on the root surface under near water‐saturated conditions. Gradients of pO2 and pH from the root surface indicated an extent of the rhizosphere effect of 10–20 mm. In contrast, EH gradients were observed from 0 to 2 mm from the root surface. We conclude that the rhizosphere extent differs for various parameters (pH, Eh, pO2) and is strongly dependent on soil moisture.

Impact of Small-Scaled Differences in Micro-Aggregation on Physico-Chemical Parameters of Macroscopic Biopore Walls

“…For measuring pH and Eh the manual approach applied by Uteau et al (2015) was refined to a semi-automatic procedure using a computer numerical controlled (CNC) mill to which a twisted drill was attached. Therewith, we were able to scrape off the uppermost soil material with a high spatial resolution (increments of the used stepper motors equals 8 µm).…”

“…(I) Combining a micromanipulator and a force sensor resulted in a (completely automated) measurement system enabling us to determine oxygen diffusivities or concentrations (pO 2 ) with coupled measurements of the soil penetration resistances in increments of 100 µm in distance from the biopore surface. (II) An enhanced approach of Uteau et al (2015) enabled us to determine both, pH and Eh, with a spatial resolution of 100 µm by using a computer numerical controlled (CNC) mill for small-scaled soil displacement. (III) Differences in aggregation were visualized with the help of a scanning electron microscope and the soil structure was classified.…”

Soil aeration: the relation between air‐filled pore volume and redox potential

“…Uteau et al . () reported an increase in O 2 diffusion at 0.09–0.12 cm 3 cm −3 air‐filled pore volume that accompanied an increase in E H . However, their results were derived from homogenized and repacked soil columns that do not necessarily match any internal structure that occurs under natural conditions.…”

Relating soil organic matter composition to soil water repellency for soil biopore surfaces different in history from two Bt horizons of a Haplic Luvisol

“…Consequently, the decay of organic particles should be enhanced in biopores colonized by both EW and R, because such biopores show pH values more favourable for microorganisms than biopores colonized by R or EW alone (Whalley et al, 2005). Biopores colonized only by roots will be more acidic due to CO 2 and H + release, whereas those colonized solely by EW will result in a more alkaline soil due to calcite release from specialized oesophageal glands (Lankester, 1865).According to DRIFT data, the zone of the biopore walls influenced either by EW, R, or REW is limited to a thickness of about 0-2 mm that is in accordance with Koebernick et al (2017), Uteau et al (2015), and Leue, Ellerbrock, Bänninger, et al (2010) and . For biopores created by EW and REW, the PWI values became relatively constant (i.e., small standard variations) for zones that are in distances above 2 mm from the biopore wall (Figure 3), whereas the PWI for <2 mm in distance from the pore wall surface indicated the formation of a kind of lining.…”