This is a very common occurrence in drained coastal wetland in which subsidence is common even in undrained soils. I learned from a geomorphology project last year that Louisiana has had some very serious engineering problems because of this phenomenon. A large land area has been drained in the delta and is now subsiding in areas that have been used for building. Subsidence is a fairly rapid process once it starts, but land may remain stable for many decades before signs of subsidence appear. Aeration quickly changes the chemistry of the soil.
Aerobic activities take over so that the reduced compounds in the soil become less stable.
Sulfate is preferred to sulfide and iron becomes oxidized. The characteristic gleying and mottling of saturated soil, disappears over time. Denitrification stops so that nitrate is now the stable nitrogen compound in the soil. The rate of decomposition increases so that carbon dioxide is released into the atmosphere. The oxidizing conditions of the drained soil allow the accumulation nutrients such as nitrates, which are often ideal for growing crops.