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 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 CO efflux, CH uptake, NO emission during growing seasons across four years were 33.59 ± 16.41 mg C m h, -56.42 ± 29.20 μg C m h, 0.67 ± 4.40 μg N m h respectively. CO efflux increased 1.55-22.6%, CH uptake decreased 5.96-20.1%, NO emission increased 37.3-133.5% with N addition. We also observed a critical threshold of 40 kg N ha a of N addition above which the increase rate of NO 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.