Temperature-induced diurnal redox potential in soil

With the capabilities to measure redox potentials (E_H) at a high temporal resolution, scientists have observed diurnal E_H that occur in a distinct periodicity in soils and sediments. These patterns have been disregarded for a long time because minor fluctuations of the E_H in the tens of mV range are difficult to interpret. Various explanations have been proposed for the origin of diel E_H but a cohesive assessment of the temperature-dependency for field- and laboratory-based investigations is missing at present. In this study, we investigated spatiotemporal diel E_H of previous long-term (up to 10 years) field- and lab-based monitoring data collected at high-temporal (every hour) and spatial (up to 6 depths) resolution. In addition, we set up a redox experiment where we manipulated the soil temperature (ST) by diel temperature cycles to assess the E_H response. Diel fluctuations were absent for laboratory experiments with ΔE_H of a few mV (daily E_H-max - daily E_H-min), but we found pronounced fluctuations up to ∼100 mV for field investigations. The spatiotemporal pattern in E_H fluctuations was amplified in the topsoil during the summer months concomitant with ST. We showed for the first time that changes in ST during an incubation experiment altered the E_H by -3.3 mV °C^-1 and inferred that the diel E_H were driven by the thermal conditions of the soil itself. This is particularly important when E_H is measured close to the soil surface and underlines that minor fluctuations of the E_H with a recurring periodicity should be carefully checked for its dependency with the soil and reference electrode temperature. Redox measurements should not be considered a routine determination and cautious handling of E_H data by physical sound corrections is urgently needed in order to link ΔE_H to daily biogeochemical cycling in soils.