Reduction Processes in Soil

Reduction is caused by a decrease in oxygen concentration in the soil atmosphere for example, by waterlogging or compaction. Oxygen 

As each of these reduction processes occurs, the Eh is buffered, or poised, at a particular value. So, for example, while there is nitrate present, the system will be poised at around +0.22 V once the nitrate has been used up, reduction of Mn will poise the system at around +0.2 V, and so on. En tn oxidized systems is difficult to measure because there is not a specific reaction controlling it. Generally, when oxygen is present, the Eh is greater than +0.3 V. 

The phase changes that occur are important the gaseous products can be lost from the soil to the atmosphere Mn and Fe become soluble and can be moved within or out of the system. 

Reduction of N03, Mn, and Fe can occur under moderately reduced conditions, such as intermittently waterlogged soil or sediment. Reduction of nitrate causes the loss of a major nutrient. Often the reduction does not proceed through to the formation of N2 gas, and nitrous oxide (N2O) is formed, which is a potent greenhouse gas and can 

Source, adapted from Bohn, McNeal and O Connor, Soil Chemistry, Wiley Interscience, 1985. 

---