Temporal variation of atmospheric wet/dry reactive nitrogen deposition in Taiyuan rainfed farming area of Shanxi Province

Nitrogen (N) deposition has a series of negative effects on ecosystem, therefore the atmospheric reactive N (Nr) pollution and deposition induced by excessive anthropogenic Nr have become a global environmental concern. Until recent years, researches on Nr deposition and precipitation were conducted at different scales, but still there are few continuous monitoring data about N deposition in rainfed farming areas. Here, the temporal variation of atmospheric wet and dry N deposition was quantified using the DELTA system, passive samplers and rain gauge at Hecun Village (which is Yangqu County, Taiyuan City, Shanxi Province) in a four-year period spanning from April 2011 to March 2015. The results showed the average deposition rates of NH₃, HNO₃, NO₂, particle form of NO₃^- (pNO₃^-) and particle fore of NH₄⁺ (pNH₄⁺) in the four-year period were 4.50 kg (N)·hm^-2·a^-1, 3.54 kg (N)·hm^-2·a^-1, 2.56 kg (N)·hm^-2·a^-1, 1.62 kg (N)·hm^-2·a^-1 and 2.75 kg (N)·hm^-2·a^-1, respectively. This led to total dry deposition value of 12.38-18.95 kg (N)·hm^-2·a^-1, with N dry deposition flux being the highest in 2011, and the lowest in 2014. From April 2011 to March 2015, there was a positive linear correlation between monthly dry N deposition and ammonia deposition (with a correlation coefficient range of 0.809 8-0.937 1), indicating that ammonia had a significant influence on N deposition of the study area. Total wet N precipitation range was 11.67-41.31 kg (N)·hm^-2·a-1 in the region and the average concentrations of NO₃^--N and NH₄⁺-N in rainfall were 3.20 mg (N)·L^-1 and 2.43 mg (N)·L^-1, respectively. There were considerable differences in annual wet N deposition fluxes in the four-year period, of which it was the highest in 2012 and the lowest in 2014. Moreover, wet N deposition accounted for over 50% of total N deposition. Furthermore, there was a positive linear correlation between NO₃^--N and NH₄⁺-N concentrations in rainwater in the four-year period, and there was also a positive linear correlation between NO₃^--N concentration and rainfall and then a quadratic correlation was also observed between NH₄⁺-N concentration and rainfall. This indicated that rainfall had a significant impact on wet depositions of NO₃^--N and NH₄⁺-N. Such high deposition rates of Nr in Taiyuan showed a severe Nr pollution from anthropogenic sources and a significant atmospheric N input in croplands. Overall, there were bigger differences in annual wet N deposition than annual dry N deposition for the investigated four-year period in Taiyuan area. Although tested in dry land, the rate of dry deposition was lower than that of wet precipitation. This study was important for N management referring to Nr deposition in farmlands and for monitoring N cycle in the farmland ecosystem.