Effects of simulated nitrogen deposition on greenhouse gas emissions from alpine meadows in northern Tibet

The alpine meadow is the largest and most important ecosystem in northern Tibet.Recently,nitrogen deposition in Tibet has had a profound impact on carbon and nitrogen cycles in grassland ecosystems.To assess the impact of nitrogen deposition on greenhouse gas emissions from alpine meadows in northern Tibet,we conducted a nitrogen fertilizing experiment,using four fertilizing levels of 0,7,20,and 40 kg hm~(-2) a~(-1),in Nagqu County in the growing season in 2014.Our results showed that the impact on greenhouse gas emission varied with different levels of nitrogen deposition.Regarding daily variation,nitrogen significantly enhanced CO_2 emission but had no effect on the absorption of CH_4,or the emission of N_2O.Overall,nitrogen deposition significantly increased greenhouse gas emissions.The highest emission occurred with the N_2O treatment,and there was a positive correlation among CO_2,net primary productivity(NPP),and total organic carbon(TOC)(P0.05).The total nitrogen(TN),NH_4~+-N,and NO_3~--N were not correlated with CO_2(P0.05),and the same pattern was observed in the relationship between CH_4 and TN/NPP/TOC/NH_4~+-N/NO_3-N(P0.05).N_2O levels showed a significant linear correlation with NPP/TOC/NO_3-N(P0.05),but were not correlated with TN/NH_4~+-N(P0.05).With additional nitrogen deposition expected in the future,greenhouse gas emissions would increase significantly and have a positive or negative feedback on climate change in Northern Tibet.