The most conspicuous chemical change that take place when soil is flooded is reduction of

Flooded soils are a condition in which an area of soil is oversaturated with water, often due to natural occurrence or with the intended purpose of agricultural reasons like the cultivation of rice.
The most conspicuous chemical change that takes place in the flooded soil is the reduction of iron (Fe).
Competition generally limits the use of weaker electron acceptors, iron reducers may exist in the soil but will be dominated by the presence of denitrifiers, who will grow faster with access to their stronger electron acceptor.