Using fixed-potential electrodes to quantify iron and manganese redox cycling in upland soils

Area of Science:

• Soil Science
• Environmental Chemistry
• Geochemistry

Background:

• Metal redox reactions significantly influence soil processes like carbon mineralization.
• Temporal variations in soil redox cycling, particularly for iron and manganese, are poorly understood.
• Potentiostatically poised electrodes (fixed-potential electrodes) show potential for in situ redox measurements in soils.

Purpose of the Study:

• To investigate fine-scale temporal redox fluctuations of iron and manganese in unsaturated soils using fixed-potential electrodes.
• To explore the influence of environmental conditions on these redox dynamics.
• To quantify the frequency, period, and amplitude of oxidation and reduction events in unsaturated soils.

Main Methods:

• Deployment of three-electrode systems with working electrodes fixed at 100 mV (mimicking iron oxides) and 400 mV (mimicking manganese oxides) in unsaturated soil.
• Real-time calculation of reduction and oxidation rates from temporal electric current variations.
• Comparison of electrode measurements with soil porewater chemistry, gas partial pressures (pCO2, pO2), groundwater levels, resistivity, and precipitation data.

Main Results:

• Fixed-potential electrodes detected redox fluctuations over timescales from minutes to weeks.
• A negative current at 100 mV indicated consistent iron oxidation, while the 400 mV electrode showed fluctuating iron and manganese oxidation/reduction.
• Rising water tables promoted reduction, whereas rainfall above the water table stimulated oxidation, with precipitation frequency driving multi-day redox events (5-10 day intervals).

Conclusions:

• This study presents the first direct detection of oxidation and reduction event dynamics in unsaturated soils using fixed-potential electrodes.
• Fixed-potential electrodes are a promising tool for accurately studying fast-changing biogeochemical impacts of metal redox cycling.
• The findings advance the quantification of soil redox reactions, which have historically been challenging to measure.