Anaerobic oxidation of methane (AOM) is a microbial process of methane (CH4) consumption under anoxic conditions with various terminal electron acceptors (other than oxygen), e.g. sulfate, nitrate, nitrite, some metals (Fe, Mn) or organic compounds. AOM is common in marine ecosystems, where microbial sulfate reduction consumes most of the CH4 produced in sediments. Despite the global significance of AOM, the mechanisms (specific electron acceptors, microbial groups, etc.), optimal conditions and AOM relevance in terrestrial ecosystems are almost unknown. Therefore, there is a strong need for investigation of AOM in terrestrial ecosystems, especially those exposed to prolonged anaerobic conditions such as natural or restored peatlands and rice paddies. This proposal focuses on the AOM mechanisms and intensity within a large climate gradient in four research sites: one natural peatland in Sweden, one restored peatland in Germany and one rice paddy field in China. To estimate the in situ AOM rate, the belowground 13C-CH4 labeling is efficiently applied. The product of AOM - released 13CO2 - together with the dynamics of porewater electron acceptors allows assessing the AOM intensity and links it to existing environmental conditions at each of the sites. Along with the field studies, the specific mechanisms and the microbial groups driving AOM is thoroughly investigated in a set of laboratory experiments with the soils from all research sites. Application of novel methods of electrochemical analysis (measurement of electron exchange capacities of the electron acceptors), 13C tracing in 13CO2 produced by 13CH4 oxidation and in PLFA and GDGT biomarkers (proxies for bacterial and archaeal communities) will reveal the optimal conditions for the highest potential AOM rates and determine microbial groups responsible for AOM. To our knowledge, the current proposal is the first attempt to estimate the in situ AOM rate and to determine the responsible microbial groups in a range of ecosystems with sustained CH4 production on Eurasian continent. Finally, understanding of AOM mechanisms may change the existing concept of CH4 cycling in terrestrial ecosystems and will improve current process-based models of regional and global carbon balance.