The projected increase of extreme precipitation and freeze-thawing events may lead to frequent occurrence of anaerobiosis in upland soils, which has significant impacts on biogeochemical processes affecting soil carbon loss. However, compared to mineralization, the impacts of anaerobiosis (potentially accompanied by fermentation) on soil organic carbon (SOC) leaching is limited. Here we conducted microcosm and intact soil column incubation experiments to examine processes influencing SOC leaching from four typical Chinese grassland soils under simulated anaerobiosis. Compared to aerobiosis, non-fermenting anaerobiosis increased the pH, dissolved phenol concentrations and aromaticity of soil leachates. In contrast, fermenting anaerobiosis induced acetate accumulation, lowered pH, stimulated phenol oxidative activity and generally decreased aromaticity in soil leachates in both microcosm and soil column experiments relative to aerobiosis. Both anaerobiosis potentially induced a strong release of dissolved organic carbon (DOC) accompanied by iron and nitrate reduction, especially with fermentation. However, DOC in soil leachates decreased in alpine subsoils under fermentation relative to aerobiosis. This interesting phenomenon was mainly attributed to (i) minimal iron reduction and dissolution in the alpine subsoils and (ii) enhanced DOC oxidation by elevated phenol oxidative activity in the fermentation relative to aerobiosis treatments. These results collectively indicate that anaerobiosis may increase SOC leaching and its magnitude is dependent on the extent of iron reduction and pH variations. Fermentation-enhanced release of ferrous iron and acetate may have an even stronger influence on the downstream biogeochemistry. Hence, temporary anaerobiosis warrants better recognition and investigation in the Mongolian (relative to Qinghai-Tibetan) grasslands that show high soil iron reduction potentials and are predicted to experience increased extreme precipitation in the future.