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As an important component of global nitrogen (N) biogeochemical cycle, atmospheric N deposition refers to the removal process of reactive N, including oxidized N (NO y ) and reduced N (NH x ), from the atmosphere to earth surface through dry and wet deposition. Nitrogen deposition can exert important impacts on the structure and functioning of both terrestrial and aquatic ecosystems. Increasing N deposition poses a potential threat to natural ecosystems and human health. It is a challenge to accurately monitor the composition and flux of dry and wet N deposition in different ecosystems, using a unified technology. Here, we reviewed the research progress on monitoring methods of dry and wet N deposition in China and aboard, including micrometeorology, inferential method, model estimation, surrogate surface, precipitation collection, and ion exchange resin methods. We further discussed the advantages and disadvantages of each method in terms of its applications at regional, national, and global scales. This review would provide a methodological support to establish national monitoring network for atmospheric N deposition.

China is one of the global hotspots of atmospheric nitrogen (N) deposition. This is especially true for eastern China, an economically developed and densely populated area, which emits large quantities of anthropogenic reactive N species to the air and experiences high levels of atmospheric N deposition. Few studies, however, have examined spatial variations of N deposition across different ecosystems in northern and southern regions of eastern China. Using the traditional rain gauge (SDM6A) method, we conducted a three-year (2011-2013) study to quantify wet N deposition fluxes at twelve sites (six northern and six southern monitoring sites, covering urban, rural and background land use types) in China. The annual volume-weighted mean (VWM) concentrations of NH 4 + -N, NO 3 - -N and total inorganic N (TIN) atmonitoring sites were in the ranges of 0.62-2.76, 0.54-2.50 and 1.25-4.92 mg N·L -1 , and averaged 1.4, 1.5 and 2.9 mg N·L -1 , respectively. Northern sites showed higher concentrations of N species in precipitation than southern sites. Annual deposition fluxes of NH 4 + -N, NO 3 - -N and TIN were in the ranges of 7.0-18.3, 6.9-18.9 and 14.9-32.6 kg N·hm -2 ·a -1 , averaging 11.5, 12.2 and 23.7 kg N·hm -2 ·a -1 , respectively. There were significant differences in total N deposition fluxes among different land use types in the north, showing the decreasing order of urban sites (26.3±6.4 kg N·hm -2 ·a -1 )> rural sites (21.8±3.5 kg N·hm -2 ·a -1 )> background (15.5±1.3 kg N·hm -2 ·a -1 ). Total N deposition fluxes at urban, rural and background sites in the south were (26.8±2.7), (25.5±2.9) and (20.5±2.4) kg N·hm -2 ·a -1 , respectively, showing no significant spatial variation. While urban sites did not show obvious regional differences, total N wet deposition fluxes at rural and background sites in the south were significantly higher than those at corresponding sites in the north. Our results revealed that both south and north regions of eastern China (including background areas) are receiving high levels of wet N deposition and associated ecological and environmental risks should be considered.

N deposition has increased significantly with economic development and intensive human activities in China and has affected natural ecosystems in remote areas. To evaluate the atmospheric N deposition of the Bayinbuluk alpine grassland of the central Tianshan Mountains, the dry and wet N depositions were monitored from May 2010 to December 2011. Fluxes of HNO3, NH3, NO2, particulate ammonium and nitrate (pNH4+ and pNO3-) averaged at 1.47, 0.68, 0.13, 0.23 and 0.25 kg N x hm(-2) x a(-1), respectively. Wet depositions of NH(4+)-N and NO(3-)-N were 2.47 and 1.59 kg N x hm(-2) x a(-1), respectively. Total atmospheric inorganic N deposition fluxes averaged at 6.82 kg N x hm(-2) x a(-1), and the wet and dry depositions were 4.06 and 2.76 kg N x hm(-2) x a(-1), respectively. Nitrogen deposition fluxes showed a significant seasonal change, with 72.1% of dry N deposition occurring in spring and summer, and 78.3% of wet N deposition concentrating in summer and autumn.

Excessive N fertilization in intensive agricultural areas of China has resulted in serious environmental problems because of atmospheric, soil, and water enrichment with reactive N of agricultural origin. This study examines grain yields and N loss pathways using a synthetic approach in 2 of the most intensive double-cropping systems in China: waterlogged rice/upland wheat in the Taihu region of east China versus irrigated wheat/rainfed maize on the North China Plain. When compared with knowledge-based optimum N fertilization with 30-60% N savings, we found that current agricultural N practices with 550-600 kg of N per hectare fertilizer annually do not significantly increase crop yields but do lead to about 2 times larger N losses to the environment. The higher N loss rates and lower N retention rates indicate little utilization of residual N by the succeeding crop in rice/wheat systems in comparison with wheat/maize systems. Periodic waterlogging of upland systems caused large N losses by denitrification in the Taihu region. Calcareous soils and concentrated summer rainfall resulted in ammonia volatilization (19% for wheat and 24% for maize) and nitrate leaching being the main N loss pathways in wheat/maize systems. More than 2-fold increases in atmospheric deposition and irrigation water N reflect heavy air and water pollution and these have become important N sources to agricultural ecosystems. A better N balance can be achieved without sacrificing crop yields but significantly reducing environmental risk by adopting optimum N fertilization techniques, controlling the primary N loss pathways, and improving the performance of the agricultural Extension Service.