@ Buddha, because in case of Fe oxidation of ferrous to ferric iron by the reduction of oxygen in the presence of water, therefore microbial involvement is not necessary. It is generally occur in the upland soil. For more details please have a look of the attached file.
I would add to the response by JC Tarafdar that it is the microbial activity that decreases the O2 content to the point that oxides of, e.g., Fe and Mn, are reduced.
I dont know ,, how to compare the redox reaction of eleemnts like N versus S, its like meeting two ends on a redox scale , probably S is the last in the series to be oxidised ...