Temporal and spatial variations of atmospheric reactive nitrogen deposition and impacts in China

China is experiencing intense air pollution caused in large part by anthropogenic reactive nitrogen (Nr) emissions, which result in the deposition of Nr from the atmosphere to terrestrial and aquatic ecosystems with implications for human and environmental health. But information on the magnitude and impact of N deposition in China is still limited. Here we quantify Nr deposition through two different approaches: (1) summarizing all published data on Nr emissions, bulk deposition and plant/crop N data to show the historical change of Nr deposition from 1980 to 2010; (2) using a Nationwide Nitrogen Deposition Monitoring Network (NNDMN) to evaluate the current spatial distribution of dry and wet Nr deposition. These datasets are robust for us to have a systematic analysis on China's atmospheric Nr pollution, deposition and impacts. We find that average annual bulk deposition of Nr has significantly increased (p<0.001) by approx. 60% from the 1980s (13 kg N ha-1) to 2000s (21 kg N ha-1). NH4-N is the dominant form of Nr in deposition, but the rate of increase is largest for NO3-N deposition, in agreement with decreased ratios of NH3 to NOx emissions since 1980. We also observe significant impacts of elevated Nr deposition on vegetation, including increased plant foliar N concentrations in semi-natural ecosystems and crop N uptake from long-term unfertilized croplands (all p<0.01 or 0.05). According to the NNDMN monitoring results and the simulated deposition velocity (Vd) of Nr species, we find equal importance of dry deposition (estimated by measured Nr concentrations with modeled Vd) and wet (i.e. bulk) deposition in China. Average dry and wet Nr deposition fluxes were 20.6±11.2 and 19.3±9.2 kg N ha-1 yr-1 across China, with distinct spatial variations of total dry plus wet Nr deposition: North China>Southeast China>Southwest China>Northeast China>Northwest China> Tibetan Plateau. Significant positive relationships between Nr emissions and deposition were found as well, based on the regional emission and deposition datasets. Our results suggest that China is facing an ongoing challenge to lower Nr emissions, N deposition and their negative impacts on sensitive ecosystems and the environment.