Opposite responses of global warming potential to ammonium and nitrate addition in an alpine steppe soil from Northern Tibet

Deposit of inorganic nitrogen (N) in different forms may divergently impact soil greenhouse gas (GHG) emissions and then aroused different feedback of global warming potential (GWP). Nevertheless, little information is available in alpine grassland ecosystems, it is far from elucidated. In this study, the effects of ammonium (NH4+-N) and nitrate (NO3--N) addition on GHG emissions were evaluated by using laboratory incubation for an alpine steppe soil in northern Tibet. Six-level N gradient was set for the incubation: 14 (2T), 28 (4T), 56 (8T), 112 (16T), 224 (32T), 448 (64T) kg.ha(-1).year(-1). Our results showed that NH4+-N and NO3--N addition significantly increased and decreased soil CO2, respectively. In general, NH4+-N addition had a stronger promotion to N2O emission and a stronger inhibition to CH4 uptake compared with NO3--N addition at same level. Besides, GHG emissions increased or decreased linearly with inorganic N addition gradient (P < 0.05), and sharp N2O emissions were observed when NH4+-N input amounts more than 400 kg ha(-1).year(-1) (64T). On the whole, there was an opposite responses of global warming potential (GWP) to NH4+-N and NO3--N addition, in which NH4+-N addition increased, but NO3--N addition decreased. The GWP was mainly contributed by CO2 emission (>84%) in alpine steppe soil from Northern Tibet. The results suggested that NO3--N input likely to be more eco-friendly than NH4+-N input in alpine steppe area, it may have implications in environmental policy related to the anthropogenic emissions of the two N forms. (C) 2020 The Authors. Published by Elsevier B.V.