Please follow the following links and pdf attachments for the answer to your query.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4009779/http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4009779/

https://microbewiki.kenyon.edu/index.php/Central_Metabolism_(Flooded_soils)

Nitrogen mobility steps from Amonium to nitrate and other microbial required oxygen for performance of their task

consider this in your evaluation.  I do not have the authors name, but obtained on internet so it may be in document properties.

I dont have in this computer, but the US Corps of Engineers has a manual on wetland identification and delineation manual (1987), and various hydric soil listings and hydrophytic plant lists and hydrology indicators for various areas and habitats in the US.  Whether or not you have the actual soil types or plant species may not be important, but you may recognize similar soil characteristics or plant genera that favor wetlands.  You will eventually see changes in the soil and plant community in the conversion of an aerated soil to a wetland.  Some plants may adapt, others not.  Some of the indicators shown in this book discuss what the anaerobic conditions do as far as increasing organics and reducing soils, stripping, altering soil color, iron concretions, etc.  I will try to get back and attach manual for you, but it is on the internet.  

Here is wetland delineation manual still used.  I added wetland plant listing and regional supplement for Atlantic Coastal Plain to suggest further information and detail sometimes available on delineation for specific areas.  

Sorry your question is not clear  did you mean why O2 increasingly depleted by time