Nitrogen deposition accelerates greenhouse gas emissions at an alpine steppe site on the Tibetan Plateau

The increase in atmospheric N deposition can alter the exchange of greenhouse gas (GHG) between the biosphere and atmosphere. The effect of N deposition on GHG is poorly understood in alpine steppe. Therefore, we conducted a 4-year experiment with multilevel N addition of 0, 10, 20, 40, 80, 160 kg N ha^-1 using the static chamber and gas chromatography method to quantify responses of GHG fluxes and characterize the relationship between GHG fluxes and N addition rate in Namco located in the alpine steppe in the central Tibetan Plateau (TP). The mean CO₂ efflux, CH₄ uptake, N₂O emission during growing seasons across four years were 33.59 ± 16.41 mg C m^-2 h^-1, -56.42 ± 29.20 μg C m^-2 h^-1, 0.67 ± 4.40 μg N m^-2 h^-1 respectively. CO₂ efflux increased 1.55-22.6%, CH₄ uptake decreased 5.96-20.1%, N₂O emission increased 37.3-133.5% with N addition. We also observed a critical threshold of 40 kg N ha^-1 a^-1 of N addition above which the increase rate of N₂O emission would diminish. Our current results implied that under the background of future N deposition, N induced C sequestration might be offset by GHG balance causing a heating effect to the climate.