Oxidation and reduction (redox)

Redox or reduction-oxidation is a chemical reaction in which one substance (the reduced agent or electron donor) loses electrons and another substance (the oxidized agent or electron acceptor) gains them. These reactions have a voltage or difference in electric potential, which can be measured in volts (v). The redox in soils is a good indicator of oxygen availability. When redox is very low (< -100 mV) the soil is anaerobic (very little oxygen) and when it is high ( >300 mV) the soil is aerobic (high in oxygen).

Enzymatic browning

is an example of a redox reaction that takes place in most fruits and vegetables[4].

Wetland soils are usually saturated with water, resulting in anaerobic conditions with low oxygen and low redox values[2]. Redox varies along the sediment column with deeper soils having lower redox compared to surface soils. As a result, different reactions take place at different soil depths. Soil aerobic respiration or decomposition occurs mostly in the aerobic surface layer (> 300 mV), while removal of nitrates due to denitrification occurs in the facultative aerobic layer (100- 300 mV). Deeper in the sediment column, where the soil is mostly anaerobic, methanogenesis occurs (< - 200 mV)[3].

Thus, redox is a good indicator of processes occurring within wetlands soils, some of which are core to delivering key ecosystem services (e.g nutrient removal) and others which can be problematic. For instance, improvement of water quality in mangroves can only occur if soils are within -100 to 200 mV as this is the range where denitrification (removal of nitrates as gas) occurs. Similarly, if soil sediment redox is very low (<-300mV) the wetland is likely to produce methane, a powerful greenhouse gas[3].

Soil redox vary naturally depending on water table fluctuations, temperature and pH[3]. But it can also be modified due to anthropogenic activities such as water pollution[1]. Excessive nutrients can result in increased organic matter production, increased flooding, and increased microbial consumption, resulting in oxygen depletion in the water. The low oxygen can be detected as low redox values in the water and sediment[1].

References

1. ^ a b Marchand, C, Lallier-Vergès, E & Allenbach, M (April 2011), 'Redox conditions and heavy metals distribution in mangrove forests receiving effluents from shrimp farms (Teremba Bay, New Caledonia)', Journal of Soils and Sediments. [online], vol. 11, no. 3, pp. 529-541. Available at: http://link.springer.com/10.1007/s11368-010-0330-3 [Accessed 1 September 2023].
2. ^ Mitsch, WJ & Gosselink, J (2015), Wetlands, p. 213, Wiley, New Jersey, USA.
3. ^ a b c Reddy, KR & DeLaune, RD (2008), Biogeochemistry of Wetlands, p. 774, CRC Press, Taylor and Francis Group, Boca Raton, Florida.
4. ^ Redox Facts for Kids. [online] Available at: https://kids.kiddle.co/Redox [Accessed 23 October 2023].

Last updated: 29 August 2023