An increase in atmospheric nitrogen (N) deposition can promote soil acidification,which may increase the release of ethylene (C2H4) under forest floors.Unfortunately,knowledge of whether increasing N deposition and C2H4 releases have synergistic effects on soil methane (CH4) uptake is limited and certainly deserves to be examined.We conducted some field measurements and laboratory experiments to examine this issue.The addition of (NH4)2SO4 or NH4Cl at a rate of 45 kg N ha-1 reduced the soil CH4 uptake under a temperate old-growth forest in northeast China,and there were synergistic effects of N amendments in the presence of C2H4 concentrations equal to atmospheric CH4 concentration on the soil CH4 uptake,particularly in the NH4Cl-treated plots.Effective concentrations of added C2H4 on the soil CH4 uptake were smaller in NH+4-treated plots than in KNO3-treated plots.The concentration of ca 0.3 μl C2H4 L-1 in the headspace gases reduced by 20% soil atmospheric CH4 uptake in the NH4Cl-treated plots,and this concentration was easily produced in temperate forest topsoils under short-term anoxic conditions.Together with short-term stimulating effects of N amendments and soil acidification on C2H4 production from forest soils,our observations suggest that knowledge of synergistic effects of NH+4,rather than NO-3,amendments and C2H4 on the in situ soil CH4 uptake is critical for understanding the role of atmospheric N deposition and cycling of C2H4 under forest floors in reducing global atmospheric CH4 uptake by forests.Synergistic functions of NH+4-N deposition and C2H4 release due to soil acidification in reducing atmospheric CH4 uptake by forests are discussed.