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A redox gradient is a series of reduction-oxidation (redox) reactions sorted according to redox potential. The redox ladder displays the order in which redox reactions occur based on the free energy gained from redox pairs. These redox gradients form both spatially and temporally as a result of differences in microbial processes, chemical composition of the environment, and oxidative potential. Common environments where redox gradients exist are Marsh, , contaminant plumes, and .
The Earth has a global redox gradient with an oxidizing environment at the surface and increasingly reducing conditions below the surface. Redox gradients are generally understood at the macro level, but characterization of redox reactions in heterogeneous environments at the micro-scale require further research and more sophisticated measurement techniques.
Eh can be measured by collecting samples in the field and performing analyses in the lab, or by inserting an electrode into the environment to collect in situ measurements. Typical environments to measure redox potential are in bodies of water, soils, and sediments, all of which can exhibit high levels of heterogeneity. Collecting a high number of samples can produce high spatial resolution, but at the cost of low temporal resolution since samples only reflect a singular a snapshot in time. In situ monitoring can provide high temporal resolution by collecting continuous real-time measurements, but low spatial resolution since the electrode is in a fixed location.
Redox properties can also be tracked with high spatial and temporal resolution through the use of induced-polarization imaging, however, further research is needed to fully understand contributions of redox species to polarization.
The following is a list of common reactions that occur in the environment in order from oxidizing to reducing (organisms performing the reaction in parentheses):
Benthic zone sediments exhibit redox gradients produced by variations in mineral composition, organic matter availability, structure, and sorption dynamics. Limited transport of dissolved electrons through subsurface sediments, combined with varying pore sizes of sediments creates significant heterogeneity in benthic sediments. Oxygen availability in sediments determines which microbial respiration pathways can occur, resulting in a vertical stratification of redox processes as oxygen availability decreases with depth.
Soil Eh generally ranges from −300 to +900 mV. The table below summarizes typical Eh values for various soil conditions:
| Waterlogged | Eh < +250 |
| Aerated – moderately reduced | +100 < Eh < +400 |
| Aerated – reduced | −100 < Eh < +100 |
| Aerated – highly reduced | −300 < Eh < −100 |
| Cultivated | +300 < Eh < +500 |
Redox gradients form along contaminant plumes, in both aquatic and terrestrial settings, as a function of the contaminant concentration and the impacts it has on relevant chemical processes and microbial communities. The highest rates of organic pollutant degradation along a redox gradient are found at the oxic-anoxic interface. In groundwater, this oxic-anoxic environment is referred to as the capillary fringe, where the water table meets soil and fills empty pores. Because this transition zone is both oxic and anoxic, electron acceptors and donors are in high abundance and there is a high level of microbial activity, leading to the highest rates of contaminant biodegradation.
Benthic zone sediments are heterogeneous in nature and subsequently exhibit redox gradients. Due to this heterogeneity, gradients of reducing and oxidizing chemical species do not always overlap enough to support electron transport needs of niche microbial communities. Cable bacteria have been characterized as sulfide-oxidizing bacteria that assist in connecting these areas of undersupplied and excess electrons to complete the electron transport for otherwise unavailable redox reactions.
, found in Mudflat, , hydrothermal vents, and at the bottoms of aquatic environments, also exhibit redox gradients. The community of microbes—often metal- or sulfate-reducing bacteria—produces redox gradients on the micrometer scale as a function of spatial physiochemical variability.
See sulfate-methane transition zone for coverage of microbial processes in SMTZs.
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