It is well known that diffuse source pollution typically come from unlicensed sources and dispersed land-use activities. Major sources of diffuse water pollution include contaminated run-off from roads, drainage from housing estates, surplus nutrients from farmland, livestock wastes, as well as atmospheric deposition mainly due to the emissions of gases into air from transport, industry and agriculture. As the air pollution in China is serious in the recent years, scientists worry that the deposition of air pollutants may exacerbate diffuse pollution.

Former studies had shown that nitrogen (N) deposition onto a watershed contributed a large proportion to riverine N input to the downstream lakes or bays. For example, the contribution of N deposition to the N load to Chesapeak Bay was estimated to be 32% and even to 49%, showing very high values. In the past two decades, that annual bulk N deposition increased by 60% over China during the 1980s (13.2 kg N/hm²) and 2000s (21.1 kg N/hm²), largely due to the increasing NH3 emissions from cropland N fertilization and livestock waste and NOx emissions from industry and transport accompanied with the strong economic growth in the recent decades. Meanwhile, diffuse pollution has also been increasingly more serious in China since the 1980s. However, the contributions of atmospheric N deposition to diffuse N pollution in major watersheds of China are not clear at present.

By monitoring atmospheric dry and wet N deposition at three sites located in the main land uses as well as riverine N exports at the outlets of a forest and an agricultural sub-catchment in a typical hilly red soil catchment in southern China during September 2010 and August 2012, a team of researchers from the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) and China Agricultural University studied the correlations between N deposition and riverine N export from the sub-catchments and estimated the contribution of atmospheric deposition to riverine N export from the catchment.

The researchers found that atmospheric N deposition was as high as 26.1 to 55.8 kg N/(hm2.yr) across different land uses in the studied catchment, while the riverine N exports ranged from 7.2 to 9.6 kg N/(hm2.yr) in the forest sub-catchment and 27.4 to 30.3 kg N/(hm2.yr) in the agricultural sub-catchment. The correlations between both wet N deposition and riverine N export and precipitation were highly positive, and so were the correlations between NH4+-N or NO3--N wet deposition and riverine NH4+-N or NO3- -N exports except for NH4+-N in the agricultural sub-catchment. "These results indicated that N deposition contributed to riverine N export", said lead researcher Dr SHEN Jianlin from ISA.

The researchers also found that the monthly export coefficients of atmospheric deposited N from land to river in the forest sub-catchment (with a mean of 14%) presented a significant positive correlation with precipitation, while the monthly contributions of atmospheric deposition to riverine N export (with a mean of 18.7% in the agricultural sub-catchment and a mean of 21.0% in the whole catchment) were significantly and negatively correlated with precipitation. “The relative high contribution of N deposition to diffuse N pollution in the catchment suggests efforts should be done to control anthropogenic reactive N emissions (e.g. from N fertilizer applications, animal production and fossil fuel combustion) to the atmosphere for reduced atmospheric N deposition as well as diffuse pollution in hilly red soil regions in southern China”, Dr SHEN added.

This research was financially supported by the National Basic Research Program of China (2012CB417105), the Key Deployment Program of the Chinese Academy of Sciences (KZZD-EW-11), the 100-Talents Program of the Chinese Academy of Sciences for Dr. Yong Li and the National Natural Science Foundation of China (41101247, 41071151).

The study entitled “Contribution of atmospheric nitrogen deposition to diffuse pollution in a typical hilly red soil catchment in southern China” has been published in Volume 26, Issue 9, September 2014 of Journal of Environmental Sciences, details could be found at http://www.sciencedirect.com/science/article/pii/S1001074214000722.