Wet and dry nitrogen deposition in the central Sichuan Basin of China

Accepted Manuscript Wet and dry nitrogen deposition in the central Sichuan Basin of China Fuhong Kuang, Xuejun Liu, Bo Zhu, Jianlin Shen, Yuepeng Pan, Minmin Su, Keith Goulding PII:

S1352-2310(16)30625-2

DOI:

10.1016/j.atmosenv.2016.08.032

Reference:

AEA 14815

To appear in:

Atmospheric Environment

Received Date: 11 March 2016 Revised Date:

7 August 2016

Accepted Date: 8 August 2016

Please cite this article as: Kuang, F., Liu, X., Zhu, B., Shen, J., Pan, Y., Su, M., Goulding, K., Wet and dry nitrogen deposition in the central Sichuan Basin of China, Atmospheric Environment (2016), doi: 10.1016/j.atmosenv.2016.08.032. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Wet and dry nitrogen deposition in the Central Sichuan

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Basin of China

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Fuhong Kuanga, b, Xuejun Liub, Bo Zhua, *, Jianlin Shenc, Yuepeng Pand, Minmin Sub,

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Keith Gouldinge

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Key Laboratory of Mountain Surface Processes and Ecological Regulation, Chinese

Academy of Sciences, Chengdu, Sichuan, 610041, China b

College of Resources and Environmental Sciences, China Agricultural University,

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Beijing, 100193, China c

Key Laboratory of Agro-Ecological Processes in Subtropical Regions, Institute of

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Subtropical Agriculture, Chinese Academy of Sciences, Changsha, Hunan, 410125,

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China

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State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric

Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing,

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100029, China

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The Sustainable Soil and Grassland Systems Department, Rothamsted Research,

West Common, Harpenden, Hertfordshire, AL5 2JQ, UK

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Corresponding author:

Bo Zhu, Email: [email protected]

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Address: No. 9, Block 4, Renminnan Road, Chengdu, Sichuan, China.

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Tel: +8628 85232090, Fax: +8628 85235869

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Abstract:

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Reactive nitrogen (Nr) plays a key role in the atmospheric environment and its

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deposition has induced large negative impacts on ecosystem health and services.

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Five-year continuous in-situ monitoring of N deposition, including wet (total nitrogen

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(WTN), total dissolved nitrogen (WTDN), dissolved organic nitrogen (WDON),

ACCEPTED MANUSCRIPT ammonium nitrogen (WAN) and nitrate nitrogen (WNN)) and dry (DNH3, DHNO3,

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pNH4+, pNO3- and NO2) deposition, had been conducted since August 2008 to

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December 2013 (wet) and May 2011 to December 2013 (dry) in Yan-ting, China, a

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typical agricultural area in the central Sichuan Basin. Mean annual total N deposition

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from 2011 to 2013 was 30.8 kg N ha-1 yr-1, and speculated that of 2009 and 2010 was

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averaged 28.2 kg N ha-1 yr-1, respectively. Wet and dry N deposition accounted for

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76.3% and 23.7% of annual N deposition, respectively. Reduced N (WAN, NH3 and

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pNH4+) was 1.7 times of oxidized N (WNN, HNO3, NO2 and pNO3-) which accounted

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for 50.9% and 30.3% of TN, respectively. Maximum loadings of all N forms of wet

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deposition, gaseous NH3, HNO3 and particulate NH4+ in dry deposition occurred in

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summer and minimum loadings in winter. Whether monthly, seasonal or annual

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averaged, dissolved N accounted for more than 70% of the total. N deposition in the

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central Sichuan basin increased during the sampling period, especially that of

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ammonium compounds, and has become a serious threat to local aquatic ecosystems,

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the surrounding forest and other natural or semi-natural ecosystems in the upper

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reaches of the Yangtze River.

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Key words: Air pollution; Wet deposition; Dry deposition; Nitrogen; Agricultural

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activities

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1. Introduction

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Atmospheric nitrogen (N) deposition is an important component of N biogeochemical

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cycling (Galloway et al., 2004), with recent studies providing evidence for a positive

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relationship between increased bulk N deposition and anthropogenic reactive N

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emissions (NH3 and NOX) (Liu and Zhang, 2009; Liu et al., 2013). The main sources

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of N deposition are emissions from increasing N fertilizer consumption in agriculture,

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industry, the export trade (Zhang et al., 2013) and fossil fuel combustion (Compton et

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al., 2011). Nitrogen fertilizer produced by the Haber-Bosch process feeds almost 50%

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of the ever increasing world population (Erisman et al., 2008), but NH3 and NOX

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emissions from the overuse of N fertilizer and other anthropogenic sources have

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increased atmospheric N deposition such that it now plays a key role in air pollution

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around the world. They have also caused large negative impacts on ecosystem health

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ACCEPTED MANUSCRIPT and services such as biodiversity loss (Stevens et al., 2004; Clark and Tilman, 2008),

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eutrophication (Bergström and Jansson, 2006), changes in N transformation process

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(Hart et al., 1994; Pilkington et al., 2005) and global acidification (Gao et al., 2013;

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Bates and Peters, 2007).

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China produced 37.1 Tg N fertilizer and agriculture consumed 28.1 Tg N fertilizer in

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2010, accounting for > 30% of total world production and consumption, and

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exceeding the combined N fertilizer use in North America (11.1 Tg N) and the

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European Union (10.9 Tg N) in 2009 (Zhang et al., 2013), but less than half of the N

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applied was taken up by crops (Zhang et al., 2008a). The Sichuan Basin (2.6×105 km2)

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is one of the most intensive agricultural regions in Southwest China, comprising 7%

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of China’s cropland, producing 10% the country’s total agricultural feed and food, and

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accounting for 60% of Sichuan’s total grain output

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was applied as ammonium bicarbonate and urea at average rates of 108, 218, 283 and

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323 kg N ha-1 yr-1 in 1980, 1990, 2000 and 2010 (Sichuan Statistical Yearbook,

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1981-2011), respectively, reflecting a continuously increasing trend. Ammonium

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bicarbonate is highly susceptible to NH3 loss (Cai et al., 1986), and even though it is

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being replaced by urea, agriculture is the dominant source of NH3 emissions when

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compared with industry, vehicle emissions and human waste (Bouwman et al., 1997).

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The relatively dispersed agricultural production and decentralized agricultural

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management, lack of scientific fertilizer application guide and partly the result of

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topography, causes farmers to apply N fertilizer in accordance with their own

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subjective ideas, leading to over-fertilization. The continual increases in agricultural

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production and fertilizer use, industrial development, and population growth are likely

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to cause increasing reactive N emissions from this area, where the so called ‘Purple

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Soil’ (a local Chinese name, and for entisols (USA classification)) (Zhu et al., 2009)

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with its high pH (6.0 to 8.5) occurs widely. Nitrogen pollution is a serious issue in the

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Sichuan Basin, even in the upper reaches of the Yangtze River, with its widely

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distributed natural and semi-natural ecosystems. Liu and Zhang (2009) have shown

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that bulk N deposition (i.e. wet plus some dry deposition, measured in open collectors)

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in the Sichuan Basin is ≥ 25 kg N ha-1 yr-1. High N deposition will not only provide N

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(Zhu et al., 2009). Fertilizer N

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and semi-natural ecosystems (Liu et al., 2011).

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Previous research in the Sichuan Basin measured only ammonium and nitrate N in

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bulk deposition (e.g. Liu and Zhang, 2009; Yang and Sun, 2008), and Xu et al. (2015)

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only evaluated total inorganic N deposition nationally. No systematic research has

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been conducted to provide for detailed information on atmospheric deposition that

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includes both wet and dry deposition of inorganic and organic N species in the

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Sichuan Basin. Therefore we continuously monitored in-situ N deposition in Yan-ting

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County of the Sichuan Basin, with the objectives being 1) to provide continuous

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monitoring data for N deposition for national evaluation; 2) to quantify forms and

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fluxes, seasonal and inter-annual variations of N deposition in order to better

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understand the current status of N deposition and impacts in this area.

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2. Materials and Methods

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2.1 Site descriptions

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The experimental site (N31°16’, E105°28’ at an altitude of 400-600m) was located at

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the ‘Yan-ting Agro-Ecological Station of Purple Soil’, a member station of the

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Chinese Ecosystem Research Network (CERN), Chinese Academy of Sciences, in the

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central Sichuan Basin, southwestern China (Fig. 1). It has a moderate subtropical

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monsoon climate with annual mean precipitation of 836 mm, distributed 5.9%, 65.5%,

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19.7% and 8.9% in the spring, summer, autumn and winter, respectively. The average

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air temperature is 17.3

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distributed in this region. The site is located in a typical intensive agricultural area

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with mostly rice-wheat, rice-oilseed rape and maize-wheat cropping systems and an

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average synthetic N fertilizer application rate of ≥280 kg N ha-1 yr-1.

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. The calcareous ‘Purple Soil’, which has a pH >8, is widely

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Fig. 1 Location and land use of the Yan-ting site, Sichuan Basin, China

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2.2 Measurements of wet and dry deposition

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2.2.1 Sampling and analysis for wet deposition

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Wet deposition was collected and quantified at Yan-ting from August 2008 to

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December 2013. Precipitation after each event was collected using wet-dry automatic

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collectors (UNS 130/E & DSC HD/PE, produced by Eigenbrodt, Germany). The

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cover of the collection funnel opened automatically without delay when the

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precipitation sensor was activated and closed automatically when precipitation ceased

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and no water remained on the sensor surface. A thermostatic device in the sensor

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controlled evaporation in order to ensure the cover closed quickly once precipitation

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ceased. Precipitation was collected in a 5000 ml sample bottle during the event and

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taken to the laboratory within 2 hours of each event. After each sampling, the

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container was cleaned with deionized water. All samples were refrigerated at 4

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and analyzed within 48 hours.

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Each sample was filtered through a 0.45 μm membrane filter (Tianjin, China), and 50

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ml of filtrate stored in plastic bottles. The species and concentrations of inorganic N

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(IN) including NH4+-N (AN) and NO3--N (NN) were measured using an Automatic

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ACCEPTED MANUSCRIPT Analyzer (AA3 continuous-flow analyzer, Seal Corporation, Germany). The detection

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limit of N for this instrument was 3 μg N L-1. It should be noted that NO3--N was

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converted to NO2--N during the analysis, so NO2--N was included in our analysis, and

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the ‘NO3--N’ recorded equal to the sum of NO2--N and NO3--N. Total N (TN) and total

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dissolved N (TDN) were measured via oxidation by alkaline potassium persulfate and

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measured using UV spectrophotometry (UV-1810 PC, Persee Corporation, China).

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Dissolved organic N (DON) was calculated as DON=TDN-DIN.

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2.2.2 Sampling and analysis for dry deposition

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Dry deposition was collected and quantified from May 2011 to December 2013.

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Gaseous NH3, HNO3 and particulate NH4+ and NO3- (pNH4+ and pNO3-) were

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collected using a DELTA system, designed by the Center for Ecology and Hydrology,

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Edinburgh, UK. The particle cut-off size for the DELTA system was estimated to be

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4-5 µm (Tang et al., 2009). The DELTA system comprised a denuder filter sampling

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train, which contained two long glass denuders (each 15 cm long) and two short glass

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denuders (each 10 cm long) to collect gaseous HNO3 and NH3, respectively, and two

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aerosol filters of diameter 37 mm to capture pNH4+ and pNO3-. A mini air pump (6V)

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provided low sampling flow rates of 0.2-0.4 L min-1, with a high sensitivity dry gas

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meter to record the sampled volume. When a laminar air stream passes through the

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denuder, coated on the inside with acid or alkaline solution, HNO3 or NH3,

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respectively, were captured, while aerosols pass through to be captured by the filters.

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The mixed air inlet is set at 1.6 m above the ground (Tang et al., 2009; Luo et al.,

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2013).

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The denuder filter sampling train was collected on the last day of each month. It was

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immediately refrigerated at 4

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solution were used to extract the denuders for HNO3 and the filters for pNO3-, and 10

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ml high purity water were used to extract the denuders for NH3 and the filters for

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pNH4+. Ammonium and nitrate in the solutions were measured with an Automatic

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Analyzer, as described above.

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Atmospheric NO2 was collected using Gradko passive diffusion tubes as in the UK

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Environmental Change Network (http://www.ecn.ac.uk). The samplers comprised

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and analyzed within one week. 10 ml 0.05% H2O2

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in a gray cap and 30 µl 20% triethanolamine pipetted into the caps, which were then

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placed on the tubes; the other end of each tube was capped with a white cap; three

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tubes were prepared for each month. The samplers were fixed 1.5 m above the ground,

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the white caps removed, and the tubes exposed for one month. At the end of each

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month, the white caps were replaced for transport and the disks extracted with a

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solution

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N-1-Naphthylethylene-diaminedihydrochloride. NOx was analysed using visible

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spectrophotometry with a colorimetric method at a wavelength of 542 nm (Luo et al.,

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2013).

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2.3 Calculation and statistical analysis

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Monthly wet N deposition was obtained by multiplying the concentration of N in an

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event by the amount of precipitation in that event as measured in a standard rain

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gauge, using the equation:

and









C = (   )/(  )

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sulphanilamide,

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Pi is the amount (mm) of precipitation in event i, Ci is the measured concentration

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(mg N L-1) of N in that event, and n is the number of precipitation events in a month.

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The monthly, seasonal and annual wet depositions were calculated according to the

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equation:

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DW=10-2∑ C P

Pi is the amount (mm) of precipitation in event i, Ci is the concentration (mg N L-1) of

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N forms in that event, n is the number of precipitation events in the monthly, seasonal

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or annual period, and 10-2 is a unit conversion factor.

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The monthly fluxes (F) of gaseous and particulate dry deposition N forms are the

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product of the concentration of N species at a specific height ( ) and the deposition

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velocity ( ):

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F = − 

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As it is difficult to measure the real  , well-tested  models with empirical

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parameters cited from the literature, combined with the on-site meteorological data,

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were used to estimate  . For Nr gases, the dry deposition velocity is calculated as

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the reciprocal of the sum of three different deposition resistances according to the

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big-leaf resistance analogy model (Wesely and Hicks, 1977) as  = 1/( +  +  )

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 is the aerodynamic resistance,  is the quasi laminar boundary layer resistance, and  is the canopy resistance.

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(1)

 is parameterized according to Erisman & Draaijers (1995) as Ra ( z ) = ( ku* ) −1[ln(

z−d z−d z0 ) −ψ h ( ) + ψ h ( )] z0 L L

(2)

z is the measuring sensor height, k is the von Karman constant (0.41), u* is the friction

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velocity, d is the zero-plane displacement height, z0 is the roughness length, ψh is the

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integrated stability function for entrained scalars and L is the Monin-Obukhov length.

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d and z0 were set to be 0 m and 0.01 m respectively for bare soil and 0.67 and 0.1

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times of the plant height, respectively, when the soils were fully covered with plants

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(Li et al., 2000; Zhang et al., 2003).

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 is also parameterized after Erisman & Draaijers (1995) as

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(3)

k is the von Karman constant, u* is the friction velocity, Pr is the Prandtl number, and

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Sc is the Schmidt number.

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 is calculated according to Wesely (1989) as

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 = %&



' (&)



+& +& *+



,- (&-*

+&



.- (&/'

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(4)

2 is stomatal resistance, 3 is the leaf mesophyll resistance, 45 is the leaf

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cuticles and otherwise the outer surfaces in the upper canopy resistance,  is the

gas-phase transfer resistance affected by buoyant convection in canopies, 4 is the leaves, twig, bark or other exposed surfaces resistance in the low canopy,  is the

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transfer resistance that depends only on canopy height and density, and 62 is the

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soil, leaf litter, etc., at the ground surface.

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For particulate N species, the dry deposition velocity is parameterized according to

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Slinn (1982) using the following equation:

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&. (&'+78

+ 6

(5)

 is the aerodynamic resistance, 25!9 is the surface resistance, and 6 is the

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gravitational settling (or sedimentation) velocity. Details of the parameterization of

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Rsurf and 6 can be found in Zhang et al. (2001).

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Seasonal N deposition was calculated as the sum of wet and dry N deposition per

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month, summed for the appropriate three-month period. The seasons were divided

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into spring (March to May), summer (June to August), autumn (September to

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November) and winter (December to February). All statistical analyses were made

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using SPSS 13.0 (SPSS Inc., USA) and figures drawn using SPSS 13.0 and Office

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Standard 2010 (Microsoft, USA). A one-way analysis of variance (ANOVA) was used

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to describe seasonal variation of N forms (ɑ=0.05).

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3

Results

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3.1 Precipitation

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Annual precipitation from 2008 to 2013 averaged 1026.4 mm, and increasing

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gradually over the experimental period, it was 23% higher than the long-term average.

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The most intense rain events occurred in the rainy season from May to September.

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Precipitation in the rainy season accounted for 76.4%, 81.0%, 85.6%, 83.9%, 84.4%

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and 89.3% of total annual precipitation from 2008 to 2013, respectively.

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3.2 Wet deposition

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During the monitoring period from August 2008 to December 2013, concentrations of

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total N (CWTN), total dissolved N (CWTDN), NH4+-N (CWAN) and NO3--N (CWNN) in

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all wet deposition averaged 2.90, 2.55, 1.48 and 1.05 mg N L-1, respectively (Fig. 2A);

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CWAN

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dissolved N (WTDN) in wet deposition averaged 1.85 and 1.57 kg N ha-1, respectively

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(Fig. 2C). On average, monthly WTDN accounted for 86.4

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dominant component of monthly loadings. Monthly amounts of ammonium (WAN)

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and nitrate (WNN) in wet deposition averaged 0.87 and 0.52 kg N ha-1, respectively

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(Fig. 2C). Monthly amounts of total dissolved organic N in wet deposition (WDON)

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was 1.4 times that of

CWNN.

Monthly amounts of total N (WTN) and total

of

WTN,

and was the

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averaged 0.18 kg N ha-1 (Fig. 2C), accounting for 11.5

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WTN, WTDN, WAN, WNN

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kg N ha-1 (Fig. 3A), in summer were 10.48, 8.98, 5.37, 2.46 and 1.15 kg N ha-1 on, in

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autumn were 5.19, 4.34, 2.09, 1.66 and 0.59 kg N ha-1, and in winter were 1.37, 1.22,

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0.50, 0.59 and 0.13 kg N ha-1, respectively. Seasonal WTDN accounted for 85.0% of

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WTN

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N deposition had significant seasonal differences. N loadings in summer were

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significant higher than that in other seasons, and the least loadings were usually in

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winter.

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3.3 Dry deposition

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3.3.1 Gaseous dry deposition (NH3, HNO3, NO2)

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During the monitoring period from May 2011 to December 2013, concentrations of

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gaseous NH3 (CDNH3), HNO3 (CDHNO3) and NO2 (CDNO2) averaged 3.71, 0.56 and

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2.62 µg N m-3, respectively (Fig. 2B). CDNH3 was 6.6 times that of CDHNO3. Monthly

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dry depositions of gaseous ammonia (DNH3), gaseous nitric acid (DHNO3) and

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nitrogen dioxide (DNO2) averaged 0.31, 0.11 and 0.07 kg N ha-1, respectively (Fig.

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2D). Seasonal average DNH3, DHNO3, DNO2 in spring were 0.67, 0.32 and 0.27 kg N

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ha-1 (Fig. 3B), in summer were 2.00, 0.40 and 0.24 kg N ha-1, in autumn were 0.45,

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0.28 and 0.10 kg N ha-1, and in winter were 0.19, 0.17 and 0.19 kg N ha-1. DNH3 in

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summer was significantly higher than those in other seasons; DHNO3 in winter and

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DNO2

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3.3.2 Particulate dry deposition (pNH4+, pNO3-)

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Concentrations of pNH4+ (CDpNH4+) and pNO3- (CDpNO3-) averaged 3.15 and 1.15 µg

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N m-3 respectively (Fig. 2B). The mean

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Monthly amounts of particulate ammonium (DpNH4+) and particulate nitrate (DpNO3-)

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deposited averaged 0.15 and 0.05 kg N ha-1, respectively (Fig. 2D). Seasonal average

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DpNH4

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0.18 kg N ha-1 in spring, summer, autumn and winter, respectively (Fig. 3B). DpNH4+

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and DpNO3- did not vary significantly through the seasons.

of WTDN. Seasonal average

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and WDON in spring were 5.42, 4.42, 2.48, 1.54 and 0.40

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on average; and WAN was about 1.5 times that of WNN. All components of wet

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in autumn were significant lower than those in other seasons.

+

CDpNH4

+

was 2.7 times that of

CDpNO3

-

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and DpNO3- were 0.33 and 0.14, 0.52 and 0.11, 0.40 and 0.16, and 0.37 and

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Fig. 2 Concentrations (A. wet deposition; B. dry deposition) and monthly fluxes (C.

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wet deposition; D. dry deposition) of N forms in wet deposition (n=169) and dry

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deposition (n=32) in the Yan-ting area during the monitoring periods of September

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2008 to December 2013 (wet deposition) and May 2011 to December 2013 (dry

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deposition), respectively. Error bar in each column (A and B) denotes standard

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deviation (error) of means of all single measured values during monitoring period.

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Error bar in each column (C and D) denotes standard deviation (error) of means of 64

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(C) and 32 months (D).

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and dry N deposition (B) from Summer 2011 to Autumn 2013 in the Yan-ting area.

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Seasonal variation of WDON was calculated using WTDN, WAN and WNN data. Error

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bar in each column denotes standard deviation (error) of means of seasons in

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five-years (A) and three-years (B).

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3.3.3 Annual N deposition

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Annual N deposition in 2009 and 2010 comprised only wet deposition, which in 2011,

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2012 and 2013 included wet and dry deposition. From 2011 to 2013, annual N

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deposition was 29.6

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deposition was 22.5 kg N ha-1 yr-1 on average, respectively; dry N deposition was 6.0,

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7.8 and 8.0 kg N ha-1 yr-1 from 2011 to 2013, averaged 7.3 kg N ha-1 yr-1. According to

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the average amount of dry N deposition from 2011 to 2013, it was speculated that

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annual total N deposition in 2009 and 2010 were 30.5 and 25.9 kg N ha-1 yr-1,

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respectively. Annual average

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DpNO3

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respectively. The main form of N deposition was inorganic, with ammonia-containing

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substances (WAN, NH3 and pNH4+) and nitrate-containing substances (WNN, HNO3,

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NO2 and pNO3-) accounting for 50.9% and 30.3% of TN, respectively. Annual total

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DON accounted for 11.9% of WTDN or 8.4% of TN, respectively.

WTDN, WAN, WNN, WDON, DNH3, DHNO3, DpNH4

+

,

and DNO2 were 19.0, 10.4, 6.3, 2.3, 3.3, 1.1, 1.6, 0.6 and 0.8 kg N ha-1 yr-1,

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33.9 and 28.8 kg N ha-1 yr-1, respectively (Fig. 4). Wet N

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ACCEPTED MANUSCRIPT Fig. 4 Annual variation of N forms in atmospheric wet and dry deposition in the

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Yan-ting area accumulated from January to December. The data shown were

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measured from January 2009 to December 2013 for wet deposition and from May

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2011 to December 2013 for dry deposition. Error bar in each column denotes standard

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deviation (error) of means of five-years (WTN, WTDN, WNN, WAN and WDON) and

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three-years (DNH3, DHNO3, DpNH4+, DpNO3- and DNO2).

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4

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4.1 Reduced N compared with oxidized N in wet and dry deposition

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Concentrations of AN and monthly, seasonal and annual amounts of AN in wet

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deposition were 1.4, 1.7, 1.5 and 1.7 times those of NN, respectively, and the ratios of

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the concentrations and monthly, seasonal and annual amounts of gaseous NH3 to

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HNO3 in dry deposition were 6.6, 2.8, 2.5 and 2.9, respectively, analogous ratios of

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pNH4+ and pNO3- were 2.7, 3.0, 2.9 and 2.8, respectively. Reduced N was more than

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oxidized N. The ratio of all reduced N (WAN, NH3 and pNH4+) to oxidized N (WNN,

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HNO3, NO2 and pNO3-) in total N deposition was about 1.7, higher than that at

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Chengdu, which was measured 1.3 in 2008 (Wang & Han, 2011). It was also higher

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than those of rural areas of China in recent years, e.g. 1.5 in bulk deposition in rural

323

sites in North China in 2011 (Luo et al., 2013), and 1.5 in wet deposition in the Tai

324

Lake region (Tab. 2) (Xie et al., 2008). In general, concentrations of AN were usually

325

higher than those of NN (Tab. 1), no matter what the ecosystem and when it was

326

measured. Recently, some studies have confirmed that urban areas also have local

327

NH3 sources, contradicting the traditional view that NH3 is transported to urban areas

328

from agricultural regions (Pan et al., 2016). Concentrations of all components of dry

329

deposition (DNH3, DHNO3, DpNH4+, DpNO3- and NO2) in this study were much lower

330

than those measured in North China from 2006 to 2008 (Shen et al., 2009) or 2010 to

331

2012 (Luo et al., 2013).

333

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Discussion

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ACCEPTED MANUSCRIPT Tab. 1 Concentrations of AN and NN in bulk N precipitation at various sites in China. Ecosystem

AN

Observation period

City

NN

Location

-type

-1

mg N L

mg N L-1

References

Urban

Sichuan, Chengdu

30°39.2´N,104°0.7´E

3.03±2.60

2.36±1.99

(Wang & Han, 2011)

2006.01~2006.12

Urban

Beijing

39°56´ N,116°23´E

3.10±2.08

1.54±1.11

(Xu & Han, 2009)

2006.01~2006.12

Urban

Guangdong, Guangzhou

--

0.99±0.16

0.75±0.12

(Cao et al., 2009)

1998~2000

Urban

Xizang, Lhasa

--

0.20±0.27

0.10±0.15

(Zhang et al., 2003)

2005.01~2005.12

Urban

Shanghai

31°18´ N, 121°30´ E

1.91±0.20

1.09±0.15

(Huang et al., 2008)

1999.01~2004.09

Suburban

Beijing, Haidian

39º57´N, 116º18´E

4.67±1.33

2.24±0.54

(Liu et al., 2006)

2004~2008

Suburb

Shenyang, Liaoning

41°32´N,123°23´E

1.82±0.51

0.89±0.21

(Yu et al., 2011)

2005.09~2007.10

Suburb

Zhejiang, Hangzhou

30°18´N,119°04´E

0.81±0.20

0.59±0.10

(Li et al., 2010)

2006.04~2007.10

Suburb

Heilongjiang, Wuchang

44°44´N, 127°36´E

1.39±0.31

0.55±0.06

(Li et al., 2010)

2003.04~2004.02

Rural

Xinjiang, Urumqi river -valley 30°38´N~31°11´N, Rural

Hubei, Zigui

0.32±0.26

0.44±0.82

(Zhao et al., 2008)

1.44±0.76

1.53±1.44

(Wu and Han, 2015)

M AN U

2009.06~2010.07

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2008.03~2008.10

SC

334

110°18´N ~111°0´E

2008.04~2008.06

Rural

Jiangxi,Yingtan

2009.01~2012.12

Rural

Sichuan,Yanting

2012.01~2012.12

Natural

Yunnan, Lijiang

28º12´N,116º55´E

2.58±3.19

0.83±0.65

(Cui et al., 2011)

31º16´N, 105º28´E

1.48±1.05

1.05±0.91

(This study)

0.29±0.03

0.01±0.01

(Niu et al., 2014)

0.19

0.18

(Qiao et al., 2015)

100°07´-100°10´ E, 27°10´-27°40´N

2010.04~2011.05

Natural

Sichuan, Jiu Zhaigou

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335

33°02´N,103°56´E

Concentrations of WAN and WNN in wet deposition (Fig. 5A) decreased from January

337

to May, and then increased from October to December. Precipitation washes out

338

pollutants from the atmosphere, and concentrations in precipitation depend on

339

pollutant load and the amount of precipitation. We measured minimum values in July,

340

August or September, which are the rainy months. It should also be noted that crops

341

are planted and fertilized in the Yan-ting area from late May to June, and again in

342

October to November, when concentrations of all ammonium and nitrate forms in wet

343

N deposition rapidly increased.

344

Gaseous NH3 in dry deposition (Fig. 5B) increased rapidly from April to July, which

345

are the months with higher temperatures and in which fertilizers are applied, leading

346

to considerable ammonia volatilization from the ammonium bicarbonate and urea

347

applied to this alkaline soil. The concentration of gaseous NH3 then decreased, but

348

increased a little in October when fertilizer is again applied but at relatively lower

349

temperatures. Concentrations of monthly pNH4+ in dry deposition (Fig. 5B) varied

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ACCEPTED MANUSCRIPT 350

only slightly through the year, indicating that emission sources or the formative

351

environment for aerosols were stable and that precipitation did not wash out pNH4+

352

effectively. Pan (2012) also showed that, in northern China, the proportion of all

353

samplers was only 45% which

354

Concentrations of HNO3 and pNO3- increased from January to March (Fig. 5C), then

355

decreased gradually from April to September. When the rainy season ended,

356

concentrations of these two oxidized pollutants increased again because these

357

oxidized pollutants are washed out effectively. Our results contrast with those of

358

Northern China (Pan et al., 2012), where amounts of

359

correlated with rainfall. Strong local anthropogenic sources (industry and high density

360

transportation) in Northern China could cause these very different results.

changed as the changes of the rainfall.

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+

WNO3

-

were not significantly

EP

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WNH4

Fig. 5 Monthly concentration of reduced and oxidized N forms in wet and dry

363

deposition. (A) concentrations of NH4+-N and NO3--N in wet deposition, (B)

364

concentrations of gaseous NH3 and particulate NH4+ in dry deposition, and (C)

365

concentrations of gaseous HNO3 and particulate NO3- in dry deposition. Error bar in

366

each column denotes standard deviation (error) of means of all single measured values

367

per months during monitoring period.

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368 369

4.2 Temporal variation of N deposition in the central Sichuan Basin

370

4.2.1 Annual variation of wet N deposition

371

Precipitation and average concentrations of

372

2013, but the ratio of the concentrations of

WAN

and

WAN

WNN

and

increased from 2002 to

WNN

decreased from 2.3 in

ACCEPTED MANUSCRIPT 2002-2004 (Yang and Sun, 2008) to 1.4 in 2008-2013 (our data) (Fig. 6A). Amounts

374

of WAN and WNN deposition were 5.8 and 2.7 kg N ha-1 yr-1 at Yan-ting from 2002 to

375

2004 (Yang and Sun, 2008), which accounted for 44.8% and 20.8% of wet N

376

deposition, respectively, and had increased to 10.4 and 6.3 kg N ha-1 yr-1 from 2009 to

377

2013, accounting for 47.0% and 27.8% of wet N deposition, respectively. However,

378

the ratio of ammonium N to nitrate N decreased over that period, despite the amounts

379

of both WAN and WNN having increased (Fig. 6B). This shows that emissions of WAN

380

and WNN were stable or increasing, those of reduced forms more than oxidized forms.

381

Thus agricultural, transport and industrial sources of N in the Yan-ting area increased,

382

but those of transport and industry increased faster than those of agriculture. This was

383

because, after the earthquake of May 2008, the local construction industry (brickyard,

384

cement works and timber mills) developed rapidly.

385

N deposition in the Sichuan Basin has increased rapidly and Sichuan has become one

386

of the most polluted areas in China. Mean wet N deposition at our monitoring site

387

from 2002 to 2004 was 13.1 kg N ha-1 yr-1 (Yang and Sun, 2008) and 22.5 kg N ha-1

388

yr-1 from 2009 to 2013 (our data), i.e. an increase of > 70% in the last 10 years. Large

389

quantities of studies have shown that N deposition has been increasing recently (Liu

390

et al., 2013), but over a very long period. Wet N deposition at Rothamsted

391

Experimental Station in 1855 was 5.2 kg N ha-1 yr-1 but increased to 18.0 kg N ha-1

392

yr-1 in 1980 then decreased gradually (Goulding et al., 1998). Total N deposition in

393

Northern China averaged 60.6 kg N ha-1 yr-1 from 2008 to 2010 (Pan et al., 2012) and

394

76.4 kg N ha-1 yr-1 from 2010-2012 (Luo et al., 2013). Highly significant (P<0.001)

395

increases in bulk N deposition have also been found in southwest China, with an

396

annual average increase rate of 0.4 kg N ha-1 yr-1 from 1980 to 2010 (Liu et al., 2013).

397

From 2011 to 2013, the inter-annual variation coefficient of WTN, WAN and WNN was

398

large at 28.1%, 29.3% and 43.7%, respectively. Yang et al. (2015) showed that in East

399

Asia, the inter-annual variations of aerosols were dominated by variations in

400

meteorological conditions, and the decadal trends of this variation were driven by

401

variations of anthropogenic emissions. Such high inter-annual variations in our study

402

show that long-term, continuous and uniform standard research is necessary and

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ACCEPTED MANUSCRIPT important to better understand the changes of N deposition.

404

Liu et al. (2013, 2016) concluded that NH4+-N from agricultural sources still

405

dominates wet/bulk N deposition but the contribution has decreased drastically

406

between the 1980s and 2000s. This contrasts with the N deposition pattern in the US,

407

where reduced N was of increasing importance compared with oxidized N (Li et al.,

408

2016). The absolute amount of NH4+-N deposited is still increasing. In addition to

409

agricultural sources there are emissions from cities. Ammonia is a key to the

410

formation of secondary particles, being easily absorbed by acid materials such as

411

H2SO4 and HNO3. However, the importance of ammonium N in the Sichuan Basin is

412

not appreciated. Indeed in China, where the annual emission of ammonia was 8.4 Tg

413

from 2005 to 2008 (Paulot et al., 2014), ammonia induced pollution, especially that

414

from agriculture, is not yet taken into accounted in air pollution legislation.

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415

Fig. 6 Temporal variations of ammonium and nitrate N in wet deposition from 2002 to

417

2013. The wet depositions of ammonium and nitrate from 2002 to 2004 were

418

measured at the same site as this study and were cited from Yang and Sun, (2008). (A)

419

variations of precipitation, concentrations of AN and NN, and the concentration ratio;

420

(B) variations of annual amount of AN and NN, and the annual load ratio.

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421 422

4.2.2 Seasonal and inter annual variation of N deposition

423

Concentrations and amounts deposited of all N forms in wet deposition (WTN, WTDN,

424

WAN, WNN

425

and their minima always occurred in winter (Fig. 3A), because abundant rainfall in

and

WDON)

in summer were significantly higher than in other seasons

ACCEPTED MANUSCRIPT 426

summer washes out pollutants. The inter-annual coefficients of variation of

427

WTDN, WAN, WNN

428

respectively.

429

that, in Yan-ting, agriculture sources were relatively stable, but industrial and traffic

430

sources variable, perhaps because of their rapid development. Amount of DNH3 was

431

5.0 times that of DHNO3 and 8.4 times that of DNO2 in summer. DNH3 in summer was

432

significantly higher than in winter, spring and autumn (Fig. 3B), in agreement with

433

research in Northern China. Thus summer fertilization is a dominant factor

434

influencing the composition of N in the atmosphere in this area. DpNH4+ showed no

435

significant seasonal variation, but the highest value also occurred in summer, showing

436

the importance of fertilizer use, especially as livestock emissions and human excretion

437

are relatively stable over the whole year. This is contrary to research in Northern

438

China (Pan et al., 2012), where DpNH4+ was higher in winter than in other seasons not

439

only because this is the dry season, but also because of the complex local emissions

440

from industry or transportation. DHNO3 in summer was higher than in other seasons

441

and significantly different to that in winter. The inter-annual coefficients of variation

442

of DNH3, DHNO3, DpNH4+, DpNO3- and DNO2 were 11.6%, 10.9%, 17.2%, 39.9% and

443

23.6%, respectively. The variations of reduced N in dry deposition were lower than

444

those of oxidized N because annual industrial and traffic sources varied considerably,

445

in agreement of variations in wet N deposition.

446

4.3 N deposition in Yan-Ting compared with other regions of China

447

Annual N deposition at Yan-Ting, including wet and dry deposition, was 30.8 kg N

448

ha-1 yr-1 on average, higher than in some agricultural areas such as Changsha,

449

Shenyang and the Tai Lake region, but lower than in Fengqiu, Jiangsu, Ying-tan and

450

Northern China (Tab.2). Xu et al. (2015) reported annual N deposition in rural areas

451

of the Northeast Plain, the North China Plain including the Weihe Plain and the

452

Sichuan Basin of 40.1, 56.4 and 35.4 kg N ha-1 yr-1, respectively, in these major

453

agricultural regions of China. Our data are slightly lower than the average values for

454

the Sichuan Basin, indicating that the Yan-ting area experiences moderate N

455

depositions for China. Wet and dry N deposition accounted for 76.3% and 23.7% of

WDON

were 12.8%, 12.9%, 9.9%, 23.4% and 35.0%,

varied the least but

WNN

and

WDON

varied the most indicating

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WAN

and

WTN,

ACCEPTED MANUSCRIPT annual N deposition, respectively. Dentener et al. (2006) used a multi-model to

457

evaluate and predict dry N deposition on a regional scale in 2000 and 2030. Their

458

results showed that NHX and NOY dry deposition in East Asia were 7.4 and 3.0 kg N

459

ha-1 yr-1 in 2000, respectively, increasing to 8.0 and 3.7 kg N ha-1 yr-1in 2030. Our

460

results for NHX and NOY in dry deposition from 2011 to 2013 were 4.8 and 2.5 kg N

461

ha-1yr-1, respectively, lower than Dentener (2006) modelled average values for 2000.

462

NHX deposition in our study was approximately the same as in other major

463

agricultural regions around the world in 2000, which in Southeast Asia, South Asia

464

and the East European Plain were 4.6, 4.8 and 4.9 kg N ha-1 yr-1, respectively, but

465

higher than in South America (2.2 kg N ha-1 yr-1), the USA (3.0 kg N ha-1 yr-1) and

466

Western Europe (3.9 kg N ha-1 yr-1). Thus, although its deposition is not the highest of

467

the agricultural areas, the Sichuan Basin is still a hot spot of NHX emission.

468

Tab. 2 Reported N deposition at various sites in China

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456

N deposition

Location

Observation time 2009~2013 2010.09~2011.08

Changsha, Hunan

2010.09~2011.08

Changsha, Hunan Jiu zhai National nature reserve Northern China

Jiangsu

References

30.8 (w+d)

1.4~2.3

(This study)

Paddy field

22 (w+d)

1.1~1.8

(Shen et al., 2013)

Tea field

34 (w+d)

1.1~1.8

(Shen et al., 2013)

forest

55 (w+d)

1.1~1.8

(Shen et al., 2013)

2010.04~2011.05

Forest

--

1.1

(Qiao et al. 2015)

2008~2010

complex

60.6 (w+d)

3.83

(Pan et al., 2012)

2008.06~2009.05

dry farmland

40.3 (w+d)

4.3

(Huang et al., 2011)

40 (w+d)

1.2

(Yang et al., 2010)

Agricultural

2007.10~2008.09 catchment

2004~2008

Farmland

14.5 (w)

2.1

(Yu et al., 2011)

Yingtan, Jiangxi

2004.01~2005.12

Paddy field

82.5 (w+d)

2.7~5.0

(Fan et al., 2009)

Tai Lake region

2003.06~2005.07

Paddy field

27 (w)

1.5

(Xie et al., 2008)

North China Plain

2003~2004

complex

28 (b)

2.1

(Zhang et al., 2008b)

Subtropical South China

2001~2003

Forest

14.8 (w)

1.3~2.1

(Chen et al., 2004)

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Shenyang, Liaoning

AN/ NN

2010.09~2011.08

EP

Fengqiu, Henan

(kg N ha-1 yr-1)

Dry farmland

TE D

Yanting, Sichuan Changsha, Hunan

Land use type

469 470

w: wet deposition; d: dry deposition; b: bulk deposition

471

On average, the amounts of AN and NN contributed 62.9% and 37.1% of inorganic N

472

in wet deposition, in contrast to that at Ying-tan Ecological Experimental Station

473

(Jiangxi, South China), at which AN and NN contributed almost exactly the reverse at

474

38.8% and 61.2%, respectively, from 2004 to 2005 (Fan et al., 2009). Our results

ACCEPTED MANUSCRIPT showed that DON deposition from 2009 to 2013 was 2.3 kg N ha-1 yr-1 on average,

476

ranging from 1.1 to 3.3 kg N ha-1 yr-1. Previous studies in Sichuan (sampling sites

477

were in Wenjiang, Jianyang and Chengdu) from 2005 to 2009 measured annual DON

478

deposition at 4-5 kg N ha-1 yr-1 (Zhang et al., 2012), higher than in our study. Previous

479

research also showed that DON accounted for 20.8% of N deposition over ten

480

monsoon seasons at Ying-tan from 2003 to 2012 (Cui et al., 2014), so DON is not a

481

negligible component of N deposition.

482

4.4 Some ecological effects of N deposition in the upper reaches of the Yangtze

483

River

484

Precipitation is the only way by which surface water and ground water are replenished.

485

Rainwater containing high concentrations of N is bound to affect surface and ground

486

water quality. Also critical loads for N deposition of the main lakes and reservoirs in

487

China are small and those of some lakes and reservoirs have been exceeded in recent

488

years. So, even if N deposition is the only source of pollution, some lakes and

489

reservoirs will become eutrophic (Ye et al., 2002). In the Environmental Quality

490

Standards for Surface Water (GB3838-2002) in PR China, surface waters have been

491

divided into five categories based on features and ecological functions. Agricultural

492

water and general landscape water is in the fifth category, which requires

493

concentrations of TN, AN and NN of ≤ 2, 2 and 20 mg N L-1, respectively.

494

Unfortunately 67.0% of TN and 26.3% of AN in the wet deposition sampled in our

495

study exceeded these requirements. This shows that, although the contribution of NN

496

from precipitation to surface water is small and it will be diluted when entering

497

surface and groundwater, the contribution of AN cannot be ignored. In addition, the

498

secondary protection zones for centralized surface water for drinking, the third

499

category, has required concentrations of TN, AN and NN of ≤ 1, 1 and 10 mg N L-1,

500

respectively. 91.6% of TN and 61.5% of AN in the wet N deposition sampled in our

501

study exceeded this standard. Research has shown that AN concentrations in

502

precipitation in China attain the required standards of the third category only in nature

503

reserves or sparsely populated areas (Tab. 1), although some urban and rural areas

504

such as Lhasa and Hangzhou achieved the standards in the years before agriculture

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ACCEPTED MANUSCRIPT was greatly intensified (Tab. 1). NN in all areas can meet the standard for the third

506

category.

507

The concentration of gaseous NH3 in the central Sichuan basin was 3.7 µg N m-3 on

508

average and ranged from 0.43 to 13.15 µg N m-3, 87.5% of samples were > 1 µg N m-3

509

and 31.3% of samples > 4 µg N m-3. Cape et al. (2009) showed that the annual critical

510

level of NH3 for higher plants was 2-4 µg N m-3, and only 1 µg N m-3 for some

511

sensitive species such as mosses and lichens. Thus gaseous NH3 concentrations at our

512

site exceed the critical level. Indeed, previous research showed that Betula Luminifera

513

and Pleioblastus Amarus forest, two local dominant tree species in the ecological

514

barrier constructed in the upper reaches of Yangtze River, have been negatively

515

affected by gradually increasing N deposition (Tu et al.,2009; Luo et al., 2010). The

516

hilly area of Sichuan is not only important for agriculture, but also the major

517

ecological barrier area in the upper reaches of the Yangtze River due to its location at

518

the frontier of the Three Gorges Reservoir. Large amounts of N deposition into this

519

area would cause great stress to local ecosystems, especially the fresh water

520

ecosystem and environment of the Three Gorges Reservoir. Fortunately, Eshleman et

521

al. (2013) have found that N deposition reduction could very effectively reduce

522

surface water pollution in a forest watershed. Controls on reactive N emissions could

523

relieve the pollution pressure on fresh water ecosystems in the environment of the

524

Three Gorges Reservoir.

525

4.5 Contrasting methods for measuring dry deposition and uncertainties in N

526

deposition calculations

527

4.5.1 Comparing measurement methods

528

Two approaches can be used to measure dry deposition. One is the surface analysis

529

method, which estimates contaminant accumulation on natural or manmade surfaces,

530

and the other is the atmospheric flux method, which estimates the areal average rate

531

of depletion from the atmosphere to all of the surfaces beneath (Yi et al., 1997); the

532

collection bucket method represents the former, and the DELTA system the latter.

533

Total dry N deposition collected with the DELTA system (DELTA-T) averaged 7.3 kg

534

N ha-1 yr-1, while that collected by the bucket averaged 6.0 kg N ha-1 yr-1 in this study

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ACCEPTED MANUSCRIPT (unpublished data of Yang-ting), 21.7% lower than that of the DELTA system (Fig. 7).

536

Tang (2009) showed that the particle cut-off size for the DELTA system was 4-5 µm.

537

Large particulate matter of diameter >10 µm is deposited by gravity, defined as

538

sedimentary deposition (Tang et al., 2006) and will be captured by the bucket. Also,

539

some of the small particles will be captured by the bucket lining through interception,

540

but it is hard to quantify the amount and the cut-off size of this material, although the

541

quantity is very small. In addition, the relative humidity in Yan-ting was > 75%

542

(Meteorological data of Yan-ting, auto-observed by Milos 520, VAISALA, made in

543

Finland), and the dissolution ratio of gaseous NH3 in water is 700:1, moreover, HNO3

544

could be dissolved with water in any proportion, indicating that a lot of dew could

545

form on the bucket lining gaseous NH3 or HNO3 dissolve in it; this is also difficult to

546

quantify. However, because dry deposition was collected once a month, the effect of

547

dew capture could be relatively small because the dew only absorbed limited N

548

content due to much smaller volume of dew compared with that of rainfall or snowfall.

549

In general, dry N deposition collected by the bucket contained particles of diameter >

550

10 µm, some gaseous pollutants and some aerosols but of unknown quantity and

551

cut-off size. The amount of particulate matter collected by the DELTA system

552

(DELTA-P) was much lower than that collected by the bucket (Fig. 7). Thus the

553

bucket is more effective for larger particles and the DELTA system for gaseous

554

material.

555

4.5.2 Uncertainties of N deposition collection and calculation

556

Dry deposition fluxes were estimated from measured concentrations and calculated

557

 using the meteorological data from Yan-ting and the deposition model. The  of

558

NH3, HNO3, NO2 and particulate matter (pNH4+ and pNO3-) in Yan-ting were

559

estimated at 0.28, 0.77, 0.11 and 0.17 cm s-1, respectively. Variations of  was large

560

because of differences in the meteorological conditions and the underlying surfaces.

561

Previous research had shown that the  of NH3, HNO3, NO2 and particulate matter

562

in agricultural systems in China varied from 0.06 to 0.74, 0.63 to 2.00, 0.05 to 0.59

563

and 0.03 to 0.25 cm s-1, respectively (Xu et al., 2015; Shen et al., 2009; Pan et al.,

564

2012; Yang, 2010). The  in Yan-ting was in the lower range. Even so, some

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ACCEPTED MANUSCRIPT uncertainties still exist, such as how the underlying surface parameters (e.g., crop

566

types and growth differences between plots) influenced  and the ability of the

567

ground surface to capture dry N deposition (Loubet et al., 2008).

568

In addition, Sutton et al. (1994) showed that NH3 is both emitted from and deposited

569

to land and water surfaces, i.e. NH3 fluxes near canopies are bi-directional and the net

570

direction of this flux is often uncertain. The compensation point (Farquhar et al., 1980;

571

Sutton et al., 1994) has been used to determine bi-directional fluxes of NH3 near a

572

plant canopy: when NH3 concentrations in air are lower than the NH3 compensation

573

point, emission occurs; if higher, NH3 is deposited. Since bi-directional NH3 exchange

574

was not considered in this study, NH3 deposition may be overestimated because of the

575

relatively high NH3 compensation point of 5 µg N m-3 in fertilized cropland (Sutton et

576

al., 1993).

577

In contrast, total N deposition may be underestimated due to the particle cut-off size

578

of dry deposition. Tang et al. (2009) showed that the particle cut-off size for the

579

DELTA system was 4-5 µm, and so the DELTA sampler does not capture the larger

580

diameter particles. Our results showed that particulate dry N deposition in the bucket

581

was 2.8 times that of the DELTA system. Another complication is that previous

582

research used wet-only (Jiang et al., 2013; Luo et al., 2002) or bulk (Qi et al., 2013;

583

Yang et al., 2010) collectors to estimate wet N deposition or total N deposition, and

584

only a few reports distinguished wet and sedimentary dry deposition (Shen et al.,

585

2013; Anatolaki and Tsitouridou, 2007). Thus measurement methods are inconsistent

586

and likely to lead to very different estimations of N deposition. Liu et al. (2006)

587

showed that the ratio of wet-only to bulk deposition was 0.7 on average, and wet-only

588

deposition of inorganic N was 8.3-8.4 kg N ha-1 less than that of bulk deposition

589

during 2003 and 2004 in the Dongbeiwang area of Beijing. However, the ratio was

590

different in other areas (Zhang et al., 2008b). In our study, the ratio of wet-only to

591

bulk deposition was 0.8 on average, some large particles were measured in bulk

592

deposition but not contained in wet-only deposition, so if wet-only deposition was

593

used to calculate the total N deposition, it might be underestimated.

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Fig. 7 Annual amount of dry deposition estimated using the DELTA system and a

596

simple collection bucket. DELTA-T = total dry N deposition estimated with the

597

DELTA system; DELTA-P = particulate dry N deposition estimated with the DELTA

598

system; Bucket = total dry N deposition estimated with the bucket. Error bar in each

599

column denotes standard deviation (error) of means of five-years (Bucket) and

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three-years (DELTA-T and DELTA-P).

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5 Conclusions

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Five years of continuous observations at Yan-ting showed that dissolved N was the

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main component of N deposition in this typical agricultural area of the central

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Sichuan Basin. Reduced N and oxidized N accounted for 50.9% and 30.3% of TN,

606

ammonia induced atmospheric pollution, especially that from agriculture, should be

607

arousing attention. N deposition is increasing and may lead to regional environmental

608

degradation. Rainwater containing high concentrations of N has become a threat to

609

surface water quality and so to the natural and semi-natural ecosystems of the upper

610

reaches of the Yangtze River. It is therefore urgent to improve N management in

611

agro-ecosystems, especially to prevent emissions of gaseous ammonia. Finally,

612

although there has been considerable research into N deposition in China,

613

uncertainties still exist in the current estimation of N deposition, especially in hilly or

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mountainous areas where topography strongly affects deposition. Nationwide,

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long-term, continuous and uniform standard research into N deposition needs to be

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implemented urgently in China.

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Acknowledgements

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This work was supported by the National Program on Key Basic Research Projects of

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China (Grant No. 2012CB417101) and National Natural Science Foundation of China

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(Grants No. 40901144, 31421092 and 41405144). The authors would like to thank

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Luo Xiaosheng, Xu Wen, Tang Jialiang, Luo Youcai and Zhang Rong for their

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cooperation during sample collection and analysis.

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ACCEPTED MANUSCRIPT Figure Legends

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Fig. 1 Location and land use of the Yan-ting site, Sichuan Basin, China

887

Fig. 2 Concentrations (A. wet deposition; B. dry deposition) and monthly fluxes (C.

888

wet deposition; D. dry deposition) of N forms in wet deposition (n=169) and dry

889

deposition (n=32) in the Yan-ting area during the monitoring periods of September

890

2008 to December 2013 (wet deposition) and May 2011 to December 2013 (dry

891

deposition), respectively. Error bar in each column (A and B) denotes standard

892

deviation (error) of means of all single measured values during monitoring period.

893

Error bar in each column (C and D) denotes standard deviation (error) of means of 64

894

(C) and 32 months (D).

895

Fig. 3 Seasonal variation of wet N deposition (A) from Autumn 2008 to Autumn 2013

896

and dry N deposition (B) from Summer 2011 to Autumn 2013 in the Yan-ting area.

897

Seasonal variation of WDON was calculated using WTDN, WAN and WNN data. Error

898

bar in each column denotes standard deviation (error) of means of seasons in

899

five-years (A) and three-years (B).

900

Fig. 4 Annual variation of N forms in atmospheric wet and dry deposition in the

901

Yan-ting area accumulated from January to December. The data shown were

902

measured from January 2009 to December 2013 for wet deposition and from May

903

2011 to December 2013 for dry deposition. Error bar in each column denotes standard

904

deviation (error) of means of five-years (WTN, WTDN, WNN, WAN and WDON) and

905

three-years (DNH3, DHNO3, DpNH4+, DpNO3- and DNO2).

906

Fig. 5 Monthly concentration of reduced and oxidized N forms in wet and dry

907

deposition. (A) concentrations of NH4+-N and NO3--N in wet deposition, (B)

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concentrations of gaseous NH3 and particulate NH4+ in dry deposition, and (C)

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concentrations of gaseous HNO3 and particulate NO3- in dry deposition. Error bar in

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each column denotes standard deviation (error) of means of all single measured values

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per months during monitoring period.

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Fig. 6 Temporal variations of ammonium and nitrate N in wet deposition from 2002 to

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2013. The wet depositions of ammonium and nitrate from 2002 to 2004 were

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measured at the same site as this study and were cited from Yang and Sun, (2008). (A)

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ACCEPTED MANUSCRIPT variations of precipitation, concentrations of AN and NN, and the concentration ratio;

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(B) variations of annual amount of AN and NN, and the annual load ratio.

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Fig. 7 Annual amount of dry deposition estimated using the DELTA system and a

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simple collection bucket. DELTA-T = total dry N deposition estimated with the

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DELTA system; DELTA-P = particulate dry N deposition estimated with the DELTA

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system; Bucket = total dry N deposition estimated with the bucket. Error bar in each

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column denotes standard deviation (error) of means of five-years (Bucket) and

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three-years (DELTA-T and DELTA-P).

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ACCEPTED MANUSCRIPT Highlights (for review): Wet and dry N deposition was quantified in central Sichuan basin for 5 years.




N forms in N deposition showed significant seasonal and inter-annual variations.




All reduced N forms contributed 50.9% of total N deposition.




N deposition (averaged 30.8 kg N ha-1 yr-1) is a significant N nutrient input.




Long term and standardized research is urgent for evaluate N deposition.

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