(eq 1.)
In fully aerobic soils, oxygen serves as the electron acceptor, resulting in the production of CO2 and H2O. The redox potential is controlled by the rates of reduction and oxidation processes in soil and is connected to soil respiration, for example in rewetted agricultural soils, where redox is a strongly correlated predictor of CO2 flux (Bartolucci et al., 2021). Based on this, redox could be a promising indicator for soil carbon cycle (labile carbon pools) and structure (oxygen availability), two key components of soil health (Kibblewhite et al., 2008).
Redox can also serve as an indicator for nutrient availability and pest regulation (Husson, 2013). Redox can be thought of as a parallel to pH, which measures proton availability, as redox (Eh) measures electron availability. And as pH, redox can influence nutrient availability considerably. Redox is managed to avoid toxic As and Cd buildup in rice paddies (Evans et al., 2021) and to improve Mn supply (Husson, 2013; Zhang and Furman, 2021). A key challenge for using redox as an indicator is however, that is changes rapidly with soil water and oxygen conditions (Zhang and Furman, 2021). Husson et al. proposed to use redox potential as a soil test from dried soil samples. (Husson et al., 2016). In the test, dried soil would be rewetted and the redox would be measured for 2 min. In theory, the test is similar to the CO2 burst test, but taking only 2 min instead of 24 h. It therefore has great potential for a high-throughput indicator for soil health, but it should be tested in different soils and under different management to see how it correlates with established soil tests and how it reacts to management.
To evaluate redox as a soil biological health indicator, we compared it to existing measures of soil health (CO2burst, visual evaluation of soil structure VESS and soil organic matter). We used 18 sites from an ongoing carbon sequestration experiment (Carbon Action, 2019-; (Mattila et al., 2022)), where each site had a carbon farming trial plot and a control plot. We measured CO2burst and redox from dried and rewetted samples and compared the results with each other and other analyses of soil properties. The results were used to classify soils and to evaluate the change in soil health from three years of carbon farming. This allowed the evaluation of the redox potential as an indicator of soil health.