Abstract
Flooding and desiccation of soil environments mainly affect the availability of water and oxygen. While water is necessary for all life, oxygen is required for aerobic microorganisms. In the absence of O(2), anaerobic processes such as CH(4) production prevail. There is a substantial theoretical knowledge of the biogeochemistry and microbiology of processes in the absence of O(2). Noteworthy are processes involved in the sequential degradation of organic matter coupled with the sequential reduction of electron acceptors, and, finally, the formation of CH(4). These processes follow basic thermodynamic and kinetic principles, but also require the presence of microorganisms as catalysts. Meanwhile, there is a lot of empirical data that combines the observation of process function with the structure of microbial communities. While most of these observations confirmed existing theoretical knowledge, some resulted in new information. One important example was the observation that methanogens, which have been believed to be strictly anaerobic, can tolerate O(2) to quite some extent and thus survive desiccation of flooded soil environments amazingly well. Another example is the strong indication of the importance of redox-active soil organic carbon compounds, which may affect the rates and pathways of CH(4) production. It is noteworthy that drainage and aeration turns flooded soils, not generally, into sinks for atmospheric CH(4), probably due to the peculiarities of the resident methanotrophic bacteria.