Abstract: The following mechanisms of nitrogen fertilizers suppressing atmospheric methane oxidation in soils by methanotrophs have been reviewed:(1) competition between ammonium and methane for MMO’s active sites;(2) toxicity of metabolites such as organic acids and NO ₂ ^- to methanotrophs;(3) methanotrophs suffering physiological lack of water due to addition of nitrogen fertilizer into the soil fixing free soil water; and (4) nitrogen turnover in soils.Based on the research reports available in the world, the mechanism of ammonium suppressing atmospheric methane oxidation is supposed to be that both methanotrophs and ammonium oxidizing bacteria compete for limited amount of oxygen in soil air, if the amount of nitrogen added to the soil exceeds the soil capacity.What we have proposed successfully explains the reason why exogenous nitrogen can suppress atmospheric methane oxidation, that is, there exists a dynamic balance between ammonium oxidizing bacteria and methanotrophs in non fertilized soils called natural soils, but a large amount of nitrogen added to the soil will stimulate the activity and growth of ammonium oxidizing bacteria, which, in turn, outdoes methanotrophs in competition for limited oxygen in the soil and becomes dominant bacteria as compared with methanotrophs. So it is very difficult for methanotroph to outgrow ammonium oxidizing bacteria again and maintain the previous balance in fertilized soils, and however they can arrive at new balance,at which there are less methanotrophs but more ammonium oxidizing bacteria compared with that before nitrogen application. This may be the reason why soils fertilized with ammonium will gradually lose the ability to oxidize atmospheric methane.

Key words: ammonium oxidizing bacteria, atmospheric methane oxidization, methanotrophs, nitrogen fertilizer, suppression mechanism