Source of nitrogen in wet deposition to a rice agroecosystem at Tai lake region

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Atmospheric Environment 42 (2008) 5182–5192 www.elsevier.com/locate/atmosenv

Source of nitrogen in wet deposition to a rice agroecosystem at Tai lake region Yingxin Xiea,b, Zhengqin Xionga,c,, Guangxi Xinga, Xiaoyuan Yana, Shulian Shia, Guoqing Suna, Zhaoliang Zhua a

State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, CAS, Nanjing 210008, China b National Engineering Research Center for Wheat, Henan Agricultural University, Zhengzhou 450002, China c College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing, Jiangsu 210095, China Received 23 March 2007; received in revised form 20 February 2008; accepted 1 March 2008

Abstract Nitrogen (N) in wet deposition can provide significant nutrients to algae, which potentially contributes to eutrophication in waterbodies, and to nutrient surplus of rice–wheat crops in the Tai lake region, Jiangsu Province, China. Quantifying the N compounds in wet deposition and determining their sources is important to understand how to control eutrophication in lakes and to improve recommendations for fertilizer use. In this study, the concentrations of inorganic N, molar ratios of NH+ 4 / + 15 15 NO N abundance of NH+ 3 and the natural 4 (d NH4 ) in wet deposition were determined for 78 precipitation events between June 2003 and July 2005. Samples were collected at two sites in Tai lake region, a watershed rice agroecosystem currently experiencing eutrophication. The average N wet deposition amounted to 27 kg N ha1 yr1, with 60% in the form of 15 + NH+ 4 . Annually, two cycles of depleted and enriched d NH4 indicate the shifting of main source of NH3 volatilization between chemical fertilizer and excreta of local residents and domestic animals, surface waters, and other organic N. The peak  + 15 in NH+ 4 /NO3 ratio, coupled with depleted d NH4 in mid-June, coincided with the rice-transplanting period, which is indicative of surplus fertilizer application. Enriched d15NH+ 4 values in August–October and in April–May were indicative of enhanced emissions from excreta and polluted waterbodies in the warmer seasons. Findings suggest that d15NH+ 4 could be  used to indicate the sources of NH3 volatilization, and the NH+ 4 /NO3 ratio to indicate the intensity of wet N deposition. r 2008 Elsevier Ltd. All rights reserved. Keywords: Nitrogen isotope; Atmospheric wet deposition; Rice agriculture; Watershed ecosystem; Eutrophication

1. Introduction The amount of anthropogenic reactive nitrogen (N) has increased due to the rapid development of Corresponding author. Current address: College of Resources

and Environmental Sciences, Nanjing Agricultural University, Nanjing, Jiangsu 210095, China. Tel.: +86 25 84395148; fax:+86 25 84396027. E-mail addresses: [email protected], [email protected] (Z. Xiong). 1352-2310/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2008.03.008

agriculture and industry, and the expansion of the global population (Galloway et al., 2004; Zhu et al., 2005; Xiong et al., 2007). High emissions produce high deposition. Consequently, deposition rates of N compounds increased considerably over the last two decades (Vitousek et al., 1997; Goulding et al., 1998; Kaiser, 2001; Ohara et al., 2007). Globally, atmospheric N deposition has reached 70 Tg N yr1 (Holland et al., 1999; Galloway et al., 2004). N in precipitation is present mainly as ammonium

ARTICLE IN PRESS Y. Xie et al. / Atmospheric Environment 42 (2008) 5182–5192  (NH+ 4 ) and nitrate (NO3 ) as a result of dissolution of atmospheric NH3 (g) and HNO3 (g), which is primarily derived from NOx (NO+NO2), and scavenging of aerosols (Russell et al., 1998). The primary sources of NOx to the atmosphere include fossil fuel combustion, lightning, biomass burning, release from aircraft, soil emissions, and transport from the stratosphere (Galloway et al., 2004; Dise and Stevens, 2005). NHx is emitted primarily by activities associated with animal husbandry, emissions from synthetic fertilizers, , soils and oceans (Prospero et al., 1996; Galloway et al., 2004; Zhu et al., 2005). Emissions in China dominate the signature of pollutant concentrations in Asian region and China’s emissions were determined to be 11.4 Tg NOx and 13.6 Tg NH3 in the year 2000 (Streets et al., 2003). The emissions of ammonia to the atmosphere from agricultural activities are very large in China (Akimoto, 2003; Xiong et al., 2007). Nitrogen oxide emissions from Asia surpassed those from North America and Europe in the mid1990s, and should continue to exceed them for decades (Akimoto, 2003). Chinese NOx emissions showed a marked increase of 280% over 1980 levels, and growth in emissions since 2000 has been extremely high due to the rapid growth in vehicle ownership (Streets et al., 2003; Ohara et al., 2007). Increase in atmospheric N deposition can supplement the supply of available N to agricultural crops. Liu et al. (2006) reported annual bulk N deposition of up to 30 kg N ha1 yr1 in the Beijing region of China and even as high as 83 kg N ha1 yr1 when maize was the monitoring crop using the ITNI (integrated total nitrogen input) system (He et al., 2007). In intensive agricultural region, N application recommendation should take this deposited N addition into account for sustainable agriculture. Elevation in atmospheric N deposition also brings a series of environmental problems, such as eutrophied waterbodies, enhanced greenhouse effect, decreased soil net primary productivity, and diminished biological diversity (Sutton et al., 1998; Goulding et al., 1998; Matson et al., 2002; Rodhe et al., 2002; Fenn et al., 2003; Stevens et al., 2004; Solga et al., 2006). The total N deposition at the five IMPACT (Integrated Monitoring Program on Acidification of Chinese Terrestrial Systems) sites in south China ranges from 6 to 44 kg N ha1 yr1 in 2003, which is in the same range as that observed in Europe and North America (Larssen et al., 2006). Atmospheric N input of up to 50 kg N ha1 yr1

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may have already enhanced N leaching in these forests in south China (Aas et al., 2007; Chen and Mulder, 2007; Fang et al., 2007). Atmospheric deposition of N varies in space and time, as is shown by Network Center for EANET (2004– 2006), Table 1 and some initial studies in China (Li and Li, 1999; Larssen et al., 2006; Liu et al., 2006; Chen and Mulder, 2007). Little information on N deposition to the East China is available. Any means of differentiating sources of N in precipitation would be extremely useful. Deposition of NH+ is typically 2  the amount of NO 4 3 deposition in China (Larssen et al., 2006; Liu et al., 2006); this reflects the importance of NH3 emissions from agricultural sources for the total N load. Since the primary source of NH+ and NO was 4 3 associated with agriculture and industry, respectively, we can determine their relative contributions  by monitoring the molar ratio of NH+ 4 and NO3 (Fangmeier et al., 1994; Anderson and Downing, 2006). Isotope abundance data can sometimes aid in distinguishing sources (Moore, 1977; Freyer, 1978; Heaton, 1987; Russell et al., 1998; Yeatman et al., 2001; Xiao and Liu, 2002; Li et al., 2007). The available isotopic data for atmospheric N compounds have been summarized by Russell et al. (1998) and are typically found to lie between 15% and +28% with considerable overlap between data from different sources. Little information on further distinctions of wet deposited NH+ 4 within agricultural sources is available (Russell et al., 1998; Yeatman et al., 2001; Xiao and Liu, 2002; Li et al., 2007). Tai lake watershed is a unique ecosystem where we can study the influences of N deposition on nutrient recommendation to the agricultural crops and on eutrophication of natural surface waterbodies. As both the most developed region and the largest watershed in China, Tai lake region covers a continuum with substantial surface waters including lakes and rivers embedded by rice paddies. The rice–upland crop annual rotations have received synthesized fertilizer as high as 550–600 kg N ha1/yr1 (Xiong et al., 2006). Moreover, excreta from domestic animals and local residents are discharged directly into the water around their habitations. This region has had steadily declining water quality over the last two decades. In Tai lake, algae blooms occurred every year recently and worse case happened at the beginning of June in 2007, which drew highly attention from the Central Government of China. Studies in the past have tried to estimate

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Table 1 Fluxes of wet N deposition in Yangtze River Delta averaged from June 2003 to July 2005 as compared to the results from two EANET rural monitoring sites in China Site

This study Changshu Wuxi Mean EANET -Rurald Chongqing-Jinyunshan Chongqing-Jinyunshan Chongqing-Jinyunshan Xi’an-Weishuiyuan Xi’an-Weishuiyuan Xi’an-Weishuiyuan

Casesa

Precipitation (mm yr1)

Concentration (mg N L1) NO 3

NH+ 4

Total

 NH+ 4 /NO3 ratio

Wet N deposition (kg N ha1 yr1) NO 3

NH+ 4

Total

78 75

1222b 1180c 1201

0.90 0.90 0.90

1.38 1.32 1.35

2.28 2.23 2.26

1.53 1.47 1.50

11.0 10.7 10.9

16.9 15.6 16.3

27.9 26.3 27.1

2003 2004 2005 2003 2004 2005

1027 1181 1047 1668 1861 541

0.48 0.52 0.48 0.76 0.97 0.16

1.20 1.47 1.47 2.02 2.11 0.90

1.68 1.99 1.95 2.77 3.09 1.06

2.49 2.85 3.05 2.66 2.17 5.65

6.0 7.8 7.0 9.9 4.5 0.7

14.8 22.4 21.6 26.3 9.9 3.9

20.8 30.2 28.6 36.2 14.4 4.6

a

Precipitation event number for this study or year number for EANET sites. Data source: Suzhou statistical yearbook 2000–2005 (Anon, 2001–2006a). c Data source: Wuxi statistical yearbook 2000–2005 (Anon, 2001–2006b). d Data source: Data report on the acid deposition in the East Asian region 2003–2005 (Network Center for EANET (2004–2006)). b

the specific pollutant sources to the waterbodies (Xing et al., 2001; Xiong et al., 2002) indicating the main pollutant of NH+ 4 in the surface waters come from the direct discharge of manure and sewage. The emerging atmospheric deposition source seems to be increasing (Xie et al., 2007). To quantify the wet N deposition and to identify the source of N in precipitation, we monitored atmospheric wet N deposition continuously from June 2003 to July 2005 at two typical sites in Tai lake region. This paper reports the average wet N deposition and the seasonal variations of  + 15 NH+ 4 /NO3 ratios and d NH4 values in precipitation. The influence of N fertilizer application, season alteration, and NH+ 4 sources in precipitation from fertilizer, excreta and waterbodies has been discussed. 2. Material and methods 2.1. Study site The typical cropping system in Tai lake region is growing rice in the summer and winter wheat or oilseed rape in the winter, with both crops receiving high doses of chemical fertilizer (Xiong et al., 2006). Two monitoring sites were established in Tai lake region: the Changshu site (1201420 E, 311320 N), located at the Changshu Agro-ecological Experi-

mental Station, Chinese Academy of Sciences; and the Wuxi Site (1201280 E, 311360 N). Both monitoring sites are surrounded by paddy fields, about 15 and 10 km away from downtown Changshu city with a 0. 45 million habitants, and Wuxi city with a 1.32 million habitants, respectively. They belong to the rural monitoring type and are scattered by the rural residents. No point sources are adjacent to the site. This region has the subtropical humid marine monsoon climate, with an annual mean temperature and precipitation of 17 1C and 1300 mm, respectively. 2.2. Collection and treatment of rainwater samples At each monitoring site, two wet-only samplers (Fig. 1) were installed to collect rainwater samples, one of which was dedicated for the analysis of NH+ 4 + 15 and NO 3 concentration, and another for d NH4 analysis. The wet-only auto-sampler (APS-3, Wuhan Tianhong Intelligent Instrument Plant, China) is designed to collect a small amount of rainwater (Fig. 1a). This is a rainfall sensitive automatic sampling system. The top cover opens automatically to collect rainwater sample when it rains and closes automatically when the rain stops, keeping the rainwater samples from contamination. After each rain event, rainwater samples are immediately recorded, filtered and collected in

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 Fig. 1. Two samplers for rainwater collection: (a) Auto-sampler APS-3 collecting small samples for the analysis of NH+ 4 and NO3 concentrations and (b) Manual sampler collecting large samples for the analysis of d15NH+ . 4

250 ml plastics bottles, and then stored at 20 1C in  the refrigerator until analysis of NH+ 4 and NO3 concentrations. A customized manual rainwater sampler (Fig. 1b) is designed to collect a large amount of rainwater for analysis of the d15NH+ 4 . This collector, made of polyvinyl chloride plastic, is 0.5 m2 in area and 1.2 m above ground. A top cover is attached during the dry period to keep the device from contamination. A technician living on site will open the lid to collect rainwater right before each rain event predicted by the weather forecast. Since about 1–2 mg NH+ 4 -N is needed for precise mass spectrometric analysis, more than 2000 ml rainwater samples are collected for each rain event. The rainwater samples pass through a column of cation exchange resin to adsorb NH+ 4 in the rainwater immediately. The 1 NH+ HCl 4 adsorbed is then eluted with 2 mol l 15 into a 250 ml plastic bottle for d N analysis. 15 + 2.3. Analysis of NH+4 , NO 3 and d NH4

An ultraviolet spectrophotometer (Shimadzu UV mini-1240, with 70.005 absorbance of photometric accuracy) was used to measure the concentrations of   NH+ 4 and NO3 . NO3 is directly determined by the difference in absorbance at 220 and 275 nm wavelength. NH+ 4 is determined by as indo-phenol-blue colorimetric method at 625 nm (Lu, 2000). The procedure detection limits for both NH+ and 4 1 NO with relative error 3 is lower than 0.05 mg N l of 1%. An isotope mass spectrometer (MAT-251, USA) was used to determine d15N in the N2 derived from 15 15 NH+ 4 . Its analytic error is 0.2 d N. The d N was

calculated with the following equation: d15 Nð%Þ ¼ f½ð15 N=14 NÞ sample  ð15 N=14 NÞ standard  ð15 N=14 NÞ standardg  1000

where ‘‘standard’’ refers to the atmospheric N2, which by definition has a 15N atom% value of 0.3663 and a d15N value of 0% (Yeatman et al. (2001) and references therein). 2.4. Results presentation  The average concentrations of NH+ 4 and NO3 were volume weighted throughout the 26-month period by the precipitation recorded for each event.  The total wet deposition of NH+ 4 or NO3 was calculated from the volume-weighted average concentrations and the long-term annual precipitation in  the statistics. NH+ 4 /NO3 molar ratio was calculated according to their volume-weighted mean concentrations throughout the entire 26-month period.

3. Results and discussion 3.1. Annual atmospheric N wet deposition in Tai lake region As shown in Table 1, the amounts of NH+ 4 and NO 3 in the wet deposition amounted to 27.9 and 26.3 kg N ha1 yr1 at Changshu and Wuxi site, respectively, averaged 27 kg N ha1 (Table 1). The mean annual total N wet deposition is in a good agreement with results of EANET monitoring site of Chongqing in south-western China (Table 1) and five IMPACT sites in south China (Aas et al., 2007;

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Fang et al., 2007; Chen and Mulder, 2007). Another rural monitoring site from EANET at Xi’an, northwestern China showed larger annual variation from 4.6 to 36.2 kg N ha1 yr1 (Table 1). This annual wet-only deposition of 27 kg N ha1 from the current study is also comparable to the bulk deposition at Beijing agricultural region with an average of 30.6 kg N ha1, which include part of the dry deposition (Liu et al., 2006). The wet deposition of 27 kg N ha1 yr1 at Tai lake region has already been three to four times the wet N deposition at Rothamsted Experimental Station, UK (9 kg N ha1 yr1) (Goulding et al., 1998) or central New York, US (Fahey et al., 1999), and four times the annual dry and wet deposition at Midwestern US (7.7 kg N ha1 yr1) (Anderson and Downing, 2006). The total airborne N input into maize-based ITNI system amounted as high as 83 kg N ha1 yr1 (He et al., 2007), which is higher than the mean total N deposition in Germany which is estimated at between 25 and 50 kg ha1 yr1 (Russow and Bo¨hme, 2005). Although deposited N does not have a harmful effect on crops, it should be taken into account for better fertilizer management within intensive agroecosystem (Zhu and Chen, 2002). The atmospheric N deposition in Tai lake region is becoming a considerable burden on surface waters, which was indicated by the severe algae blooms. The preliminary result of annual wet deposition of 27 kg N ha1 has already far exceeded the empirical critical load of 5–10 kg N ha1 yr1 for N in permanent oligotrophic waters and in most natural or semi-natural ecosystems (UNECE, 2004). By hindcasting N deposition for alpine lakes, 1.5 kg N ha1 yr1 becomes the critical load defining the threshold for ecological change from eutrophication (Baron, 2006). Moreover, dry deposition, not included in this study, could further increase the total atmospheric N deposition, as suggested by many researchers (Goulding et al., 1998; Hill et al., 2005; Russow and Bo¨hme, 2005; Liu et al., 2006; Simpsona et al., 2006). Dry deposition of N to wet surfaces was even greater than to dry surface as revealed by Anderson and Downing (2006).  The atomic ratio of NH+ 4 /NO3 in wet deposition at Tai lake region reached 1.5 calculated from the volume-weighted mean concentration, and 60% of the total inorganic N was in the form of NH+ 4 (Table 1). This is comparable to published results from other agricultural and forest ecosystems in

China (Li and Li, 1999; Liu et al., 2006; Aas et al., 2007; Chen and Mulder, 2007) with greater contribution from NH+ 4 . This indicates that N deposition in Tai lake region is mainly dominated by NH+ 4 from agricultural activities and less by NOx from industrial activities. The NO 3 rate in the wet deposition could reflect the industrial development  level, as the mass ratio for NH+ 4 and NO3 was reversed in industrial developed regions (Fahey et al., 1999) or urban site at Beijng (Liu et al., 2006) or at Xiamen–Hongwen site (Network Center for EANET 2004–2006). We need to point out that the ratio in Tai lake region is lower than the ratios in other EANET rural sites in China (Table 1) and another agricultural region of Midwestern US (5.7) (Anderson and Downing, 2006). This reflects the greater influence of NO 3 from industry and vehicle in Tai lake region with higher mean NO 3 concentration in precipitation (Table 1). This is plausible since Tai lake region is one of the most developed regions in China as noted before. 3.2. Temporal variations of d15NH+4 value in precipitation as source indicator in Tai lake region NH+ 4 in precipitation mainly comes from NH3 emitted to the atmosphere from synthesized N fertilizers applied to soils, human and animal excreta, and some other N sources (Prospero et al., 1996; Freibauer and Kaltschmit, 2001). Different NH3 sources should exhibit their own d15N signature. The entire ranges of d15NH+ 4 in precipitation were from 16% to +22% in this study. The upper range agreed well with aerosol d15NH+ 4 next to animal sheds reported by Yeatman et al. (2001) with data ranged from +11% to +22%, and with animal excreta NH3 reported by Moore (1977) (data ranged from +22% to +28%) whereas the corresponding data for animal excreta NH3 from other literatures (data ranged from 15% to +8%) are controversy as reported by Heaton (1987) and Freyer (1978). The lower range of d15NH+ 4 in precipitation from this study is in agreement with many rainwater d15NH+ 4 results reported by Russell et al. (1998) (ranged from 8.3% to +8.6%), by Xiao and Liu (2002) (ranged from 22% to 1.7%, with a median of 12.276.7%), and by Li et al. (2007) (ranged from 13.4% to +2.3%). This data set of d15NH+ 4 value in precipitation is the widest set ranged from 16% to +22% indicating a wide range of NH3 sources in Tai lake region. This is plausible due to this unique

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The d15NH+ 4 values were depleted in June and February–March or earlier (Fig. 2). The 15N depleted period in June corresponded with rice base fertilization (i.e. the rate of N fertilizer applied reached 210 kg N ha1 in about 2 weeks). This depletion period in mid-June occurred regularly in all years at both sites. Another 15N-depleted period occurred either when the base fertilizer or the topdressing was applied to the winter wheat crop. This depletion period is less clear, probably because the duration and the specific time period are affected by the actual NH3 volatilization, which is dependent on the weather condition and crop growth, and fewer precipitation events in the winter dry season. The 15N-depleted periods indicate that NH+ 4 in the wet deposition comes from surplus fertilization to rice or wheat crops. The 15N-enriched period appeared during the warmer season interrupted only by the mid-June depleted period (Fig. 2). The 15N-enriched NH+ 4 in

watershed agroecosystem with developed economy and intensive agriculture. Seen in Fig. 2, a seasonal pattern in d15NH+ 4 value in precipitation was observed in Tai lake region. Two periods of 15N enrichment and depletion can be generally identified each year, though the specific period varied with the monitoring site and year. The 15N-enriched or -depleted period may reflect the dominant sources of NH3 volatilization though the selective wash-out of N compounds may influence d15NH+ 4 values (Freyer, 1978). Based on our earlier survey (Xing et al., 2001, 2002) and other literature sources (Moore, 1977; Yeatman et al., 2001), we may assume that the d15NH+ 4 values in organic manure is higher than the synthesized fertilizer which is usually near zero (Cao et al., 1991). This is the first attempt to distinguish agricultural associated sources in the wet NH+ 4 -N deposition between synthesized fertilizer and excreta by isotope composition measurement.

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Fig. 2. Temporal variations of d15NH+ 4 in precipitation from June 2003 to July 2005 at two monitoring sites in Tai lake region of China. The arrow denotes the timing of N fertilizer application for the wheat and rice crops with fertilizer rate of kg N ha1 in the box.

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the wet deposition should come from enhanced NH3 volatilization from human and livestock excreta during the warmer season. The large span of polluted surface waterbodies in this region is potential sources for NH3 emission and deposition, in particular, during the warmer season. As noted before (Xing et al., 2001; Xiong et al., 2002), the direct disposal of human and livestock excreta served as the major contributor to the severe surface water pollution in Tai lake region. N in excrement is mainly in the form of urea, which is hydrolyzed to ammonia. The hydrolysis of urea is associated with increasing pH and produced NH3 is easily lost by volatilization into the atmosphere. The typical d15N values for aerosol NH+ 4 are +22% for chickens, +14 to +21% for pigs, and +11 to +18% for cows (Yeatman et al., 2001). The d15N values of total N from pig dung, human waste and chicken manure are +7%, +13%, and +15%, respectively, and the d15NH+ 4 in human waste was +50% in Tai lake region (Xing et al., 2002). Mainly polluted by the excreta and sewage, the d15NH+ 4 values in the surface waters ranged from +10% to +28% in Tai

NH4+−N/NO3-−N ratio

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lake region (Xing et al., 2001). This warmer season except mid–June had enriched d15NH+ 4 values up to +22% with median at 6.5% in the wet deposition. Based on these measurements, we can infer that excretory wastes and polluted waterbodies in Tai lake region are the major burden for N wet deposition during the warmer season. The higher temperature and more active biological activities facilitated organic N mineralization and NH3 volatilization from excreta and N polluted water bodies, not from synthesized fertilizer. This also agreed well with the fact that the NH3 volatilization rate from chemical fertilizer is much lower for the topdressing at panicle initiation stage in August than that for the basal fertilization in June (Xing and Zhu, 2000; Zhu and Chen, 2002). 3.3. Temporal variations of NH+4 /NO 3 ratio in precipitation as wet deposition strength indicator in Tai lake region  The ratio of NH+ 4 /NO3 can reflect the relative + strength of NH4 and NO 3 in the wet deposition

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Fig. 3. Temporal variations of the ratios in rainfall from June 2003 to July 2005 at two monitoring sites in Tai lake region of China. The arrow denotes the timing of N fertilizer application for the wheat and rice crops with fertilizer rate of kg N ha1 in the box.

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(Fahey et al., 1999). The temporal variations of  NH+ 4 /NO3 ratio in precipitation were shown in  Fig. 3. The NH+ 4 /NO3 ratios peaked at the midJune right after rice transplanting, principally following the pattern of base fertilization. Two additional peaks appeared in February or March–

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May. We can see the main surplus was the basal fertilizer for rice crop and first topdressing and sometimes, basal fertilizer for wheat crop. Generally, transplantation of rice seedlings and application of base fertilizers (40%) take place simultaneously during 18–25 June in Tai lake region

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(Fig. 3). The first top dressing (30%) for the rice crop is applied 1–2 weeks after transplanting. Therefore, 70% of the total N required for rice crop, 300 kg N ha1 is applied within 2 weeks. The remaining 30% for the second topdressing takes place in mid-August, 2 months transplanting. Winter wheat is usually sown during 5–15 November, 2 weeks after rice harvest. Forty percent of the total N fertilizer for the wheat crop is applied as base fertilizer when wheat is sown. Two topdressings are applied equally during 25 December to 5 January, and during 25 February to 5 March. The total N fertilizer application rate for wheat is 250 kg N ha1, totaling 550 kg N ha1 yr1. NOx emission and NO 3 deposition rate should be relatively steady throughout the year in Tai lake region since the main sources for NOx emission from fossil fuel combustion in industry and motor vehicles (Dise and Stevens, 2005) are relatively constant in this region, whereas in Northern China, larger portion of NOx emission and NO 3 concentration in wet deposition occurred during the winter cold season when large amount of coal was consumed for heating (Liu et al., 2006). NH3 differs from NOx on migration in the air due to its relatively shorter transportation distance (Heaton, 1987; Akimoto, 2003) and NH+ 4 deposition plays a significant role in agricultural areas (Anderson and  Downing, 2006). Therefore, the NH+ 4 /NO3 ratio could potentially be used as an index for the wet N deposition strength in Tai lake region and similar agricultural regions with relatively constant NOx levels. We can use d15NH+ 4 value as NH3 volatilization  + source indicator and the NH+ 4 /NO3 ratio as NH4 deposition strength indicator. The relationship 15  + between NH+ 4 /NO3 ratio and d NH4 values at both sample sites are shown in Fig. 4, indicated by the arrows. Increased NH3 volatilization from chemical N fertilizer should be the main source  of the NH+ 4 /NO3 ratio peak coupled with depleted + 15 d NH4 in June. Judging by the depleted d15NH+ 4 value, top dressing for the winter crop in February at Changshu in 2005 and basal fertiliza tion at Wuxi in 2005 caused NH+ 4 /NO3 ratio peak. Enhanced NH3 volatilization from human and livestock excrement, and N polluted water bodies as indicated by the d15NH+ values are 4  responsible for the NH+ ratio peak in 4 /NO3 March–May (Fig. 4). The large quantity of N fertilizer used in China has caused significant deposition problem. On

average the loss from NH3 volatilization amounts to 18% of applied N (Xing and Zhu, 2000; Zhu and Chen, 2002), much higher for the rice crop (Ghosh and Ravi, 1998; Cai et al., 2002). NH3 volatilization was as high as 47% of the urea applied at rice transplanting, while it was only 11% of the urea applied at panicle initiation stage (Xing and Zhu, 2000). From our result, much of the total N applied was lost into the atmosphere, particularly the basal fertilizer (combining with the first topdressing) for rice crop. It reduces both the N efficiency and surface water quality due to surplus of nutrient load in Tai lake region. We would recommend optimizing fertilizer use for the rice and wheat crops and to better manage the disposal of human and animal excrement in the rural regions. 4. Conclusions The annual total wet deposition in Tai lake region is averaged at two sites covering 26 continuous measurements has reached 27 kg N ha1. The main +  form is NH+ 4 and the average NH4 /NO3 ratio is 1.5 in this region. Distinct temporal variations of 15  + NH+ 4 /NO3 ratio and d NH4 values in precipita tion are observed. The dynamics of the NH+ 4 /NO3 15 + ratio and d NH4 values are therefore closely related to the timing of N fertilization and seasonal climate changes. The d15NH+ 4 values could provide further information on the sources of NH3 volatilization and it may be distinguishable between synthesized fertilizer and excreta and N polluted water bodies in Tai lake region. NH3 volatilization from the basal fertilization to the rice crop and excreta and waterbodies during the warmer season is identified as the two main contributors to the current wet N deposition. It’s recommended to optimize fertilizer use and to better excreta management in the rural regions. Acknowledgments We thank Martha Shearer at Portland State University for proofreading. This manuscript benefited from precious comments of two anonymous reviewers. This study was funded by the State Key Laboratory of Soil and Sustainable Agriculture (Institute of Soil Science, Chinese Academy of Sciences), and by the National Natural Science Foundation of China (Grant no. 30390080).

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