Atmospheric deposition of inorganic nitrogen in a semi-arid grassland of Inner Mongolia, China

Due to increasing global demand for crop production and energy use, more and more reactive nitrogen (Nr) has been generated and emitted to the environment. As a result, global atmospheric nitrogen (N) deposition has tripled since the industrial revolution and the ecological environment and human health have been harmed. In this study, we measured dry and wet/bulk N deposition from July 2013 to December 2015 in a semi-arid grassland of Duolun County, Inner Mongolia, China. The samples of dry and wet/bulk N deposition were collected monthly with a DELTA (DEnuder for Long Term Atmospheric sampling) system and with Gradko passive samplers and a precipitation gauge. The measured results show that the annual mean concentrations of NH3, NO2, HNO3, particulate NH4+ (pNH(4)(+)) and particulate NO3- (pNO(3)(-)) in atmosphere were 2.33, 1.90, 0.18, 1.42 and 0.42 mu g N/m(3), respectively, and that the annual mean volume-weighted concentrations of NH4+-N and NO3--N in precipitation were 2.71 and 1.99 mg N/L, respectively. The concentrations of Nr components (including NH3, NO2, HNO3, pNH(4)(+), pNO(3)(-), NH4+-N and NO3--N) exhibited different seasonal variations. Specifically, NO2 and HNO3 exhibited higher concentrations in autumn than in summer, while the other Nr components (NH3, pNH(4)(+), pNO(3)(-), NH4+-N and NO3--N) showed the highest values in summer. Based on measured concentrations of Nr components and their deposition velocities estimated using the GEOS-Chem global atmospheric chemical transport model, the calculated annual mean dry deposition fluxes were 3.17, 1.13, 0.63, 0.91 and 0.36 kg N/(hm(2).a) for NH3, NO2, HNO3, pNH(4)(+) and pNO(3)(-), respectively, and the calculated annual mean wet/bulk deposition fluxes were 5.37 and 3.15 kg N/(hm(2).a) for NH4+-N and NO3--N, respectively. The estimated annual N deposition (including dry N deposition and wet/bulk N deposition) reached 14.7 kg N/(hm(2).a) in grassland of Duolun County, approaching to the upper limit of the N critical load (10-15 kg N/(hm(2).a)). Dry and wet/bulk deposition fluxes of all Nr components (with an exception of HNO3) showed similar seasonal variations with the maximum deposition flux in summer and the minimum in winter. Reduced Nr components (e.g., gaseous NH3 and pNH(4)(+) in atmosphere and NH4+-N in precipitation) dominated the total N deposition at the sampling site (accounted for 64% of the total N deposition), suggesting that the deposited atmospheric Nr mainly originated from agricultural activities. Considering the projected future increases in crop and livestock production in Inner Mongolia, the ecological and human risks to the negative effects of increased N deposition could be increased if no mitigation measures are taken.