Contribution of atmospheric nitrogen deposition to diffuse pollution in a typical hilly red soil catchment in southern China.

JO U R N A L OF EN V I RO N M EN T A L S CI E NC ES 2 6 ( 20 1 4 ) 1 7 9 7–1 8 0 5

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Contribution of atmospheric nitrogen deposition to diffuse pollution in a typical hilly red soil catchment in southern China Jianlin Shen1,2,⁎⁎, Jieyun Liu1,2,⁎⁎, Yong Li1,2,⁎, Yuyuan Li1,2 , Yi Wang1,2 , Xuejun Liu3 , Jinshui Wu1,2 1. Changsha Research Station for Agricultural & Environment Observation, Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha 410125, China 2. Key Laboratory of Agro-Ecological Processes in Subtropical Regions, Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha 410125, China 3. College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China

AR TIC LE I N FO

ABS TR ACT

Article history:

Atmospheric nitrogen (N) deposition is currently high and meanwhile diffuse N pollution is

Received 29 October 2013

also serious in China. The correlation between N deposition and riverine N export and the

Revised 31 March 2014

contribution of N deposition to riverine N export were investigated in a typical hilly red soil

Accepted 8 April 2014

catchment in southern China over a two-year period. N deposition was as high as 26.1 to

Available online 8 July 2014

55.8 kg N/(ha·yr) across different land uses in the studied catchment, while the riverine N exports ranged from 7.2 to 9.6 kg N/(ha·yr) in the forest sub-catchment and 27.4 to

Keywords:

30.3 kg N/(ha·yr) in the agricultural sub-catchment. The correlations between both wet N

Nitrogen deposition

deposition and riverine N export and precipitation were highly positive, and so were the

Wet deposition

correlations between NH+4-N or NO−3-N wet deposition and riverine NH+4-N or NO−3-N exports

Dry deposition

except for NH+4-N in the agricultural sub-catchment, indicating that N deposition

Riverine nitrogen export

contributed to riverine N export. The monthly export coefficients of atmospheric deposited

Non-point source pollution

N from land to river in the forest sub-catchment (with a mean of 14%) presented a significant positive correlation with precipitation, while the monthly contributions of atmospheric deposition to riverine N export (with a mean of 18.7% in the agricultural sub-catchment and a mean of 21.0% in the whole catchment) were significantly and negatively correlated with precipitation. The relatively high contribution of N deposition to diffuse N pollution in the catchment suggests that efforts should be done to control anthropogenic reactive N emissions to the atmosphere in hilly red soil regions in southern China. © 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Introduction It is well known that diffuse source pollution typically comes from unlicensed sources and dispersed land-use activities

(Environmental Agency, 2007). Major sources of diffuse water pollution include contaminated run-off from roads, drainage from housing estates, surplus nutrients from farmland, livestock wastes, as well as atmospheric deposition mainly due to the

⁎ Corresponding author. E-mail: [email protected] (Yong Li). ⁎⁎ These two authors contributed equally to this article.

http://dx.doi.org/10.1016/j.jes.2014.06.026 1001-0742/© 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

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emissions of reactive nitrogen gases into the air from transport, industry and agriculture (Vitousek et al., 1997; Liu et al., 2013). Former studies had shown that nitrogen (N) deposition onto a watershed contributed a large proportion to riverine N input to the downstream lakes or bays (Paerl, 1997; Boyer et al., 2002; Howarth, 2006). For example, in the study of N load to Chesapeake Bay by Howarth (2006), atmospheric N deposition to the Bay includes direct N deposition to the water surface of the Bay and indirect N deposition by input to the Bay from deposition onto watersheds. Using monitoring data of the Environmental Protection Agency of United States for N deposition and the retention rate of N deposition cited from a forest watershed, the author estimated that N deposition contributed 26% of the total N load to the Bay. Considering that the cited N deposition was underestimated and deposition retention rate overestimated, the author also calculated the total input to the Bay from deposition based on the scenarios that N deposition increased to 15.5 kg N/(ha·yr) and the retention rate for N deposition was 70%. Then the contribution of N deposition to the N load to Chesapeake Bay increased to 32% and even to 49%, showing very high values. Recently Liu et al. (2013) reported that annual bulk N deposition increased by 60% over China during the 1980s (13.2 kg N/ha) and 2000s (21.1 kg N/ha), largely due to the increasing NH3 emissions from cropland N fertilization and livestock waste and NOx emissions from industry and transport accompanied with the strong economic growth in the recent decades. Meanwhile, diffuse pollution has also been increasingly more serious in China since the 1980s. For example, Billen et al. (2013) investigated the distribution of net anthropogenic N input at the scale of world's watersheds and found very high (50 to 75 kg N/ (ha·yr) or even higher) N input in major watersheds (e.g., the Yangtze River, the Yellow River and the Pearl River watersheds) in China. The high N input to watersheds in China could be one of the major causes for the frequent occurrence of algae blooming in lakes or coasts in China in the recent years (Duan et al., 2009). However, the contributions of atmospheric N deposition to diffuse N pollution in major watersheds of China are less examined at present. In this study, a case study was conducted in a typical hilly red soil catchment in Central South China to systematically investigate the contribution of N deposition to diffuse N pollution. The objectives of this study were: (1) to quantify atmospheric dry and wet N depositions in a catchment scale; (2) to find out the correlation between N deposition and riverine N export from the catchment; and (3) to estimate the contribution of atmospheric deposition to riverine N export from the catchment.

1. Materials and methods 1.1. Study area This study was conducted in the 135-km2 Jinjing Catchment of the Xiangjiang River Watershed System in Changsha, the capital city of Hunan Province, China. The catchment is 70 km northeast of the city centre and has a population of 45,000. It is a typical hilly agricultural catchment in subtropical central China and has forest, paddy field and tea field as the three primary land use types, which account for 65.5%, 26.5% and 2.4% of the total catchment area, respectively. The other minor land uses in the catchment include reservoir/pond, residential area, river and road, accounting for 5.6% of the total catchment area. The climate in the catchment is subtropical monsoon and humid, with an average annual rainfall of 1330 mm (1955–2010), an annual mean air temperature of 17.5°C (1955–2010) and a prevailing wind from north

and northwest in the whole year. Three sampling sites, named Xishan, Huinong and Feiyue, were chosen to represent the three main land use types, i.e., forest, paddy field and tea field, respectively, in the Jinjing Catchment (Fig. 1) to conduct atmospheric N deposition monitoring. Besides, one forest sub-catchment (named Fuling; area: 0.6 km2) and one agricultural sub-catchment (named Tuojia; area: 56.9 km2) were chosen to conduct riverine/ditch N export monitoring at the outlets of the sub-catchments. As a mirror of the Jinjing Catchment, the agricultural sub-catchment had a similar land use pattern as the Jinjing Catchment with forest, paddy field, tea field and other land uses accounting for 58.3%, 31.7%, 4.3% and 5.7% respectively, of the total sub-catchment area.

1.2. Sampling 1.2.1. Atmospheric wet and dry N deposition monitoring In this study, the year-round wet and dry N deposition rates were monitored at the three sampling sites (Feiyue, Huinong and Xishan) from September 2010 to August 2012. At each site, daily (8:00 am to 8:00 am next day) rainfall or snowfall samples were collected by using wet-only samplers installed at a height of 1.5 m above the ground. For the dry N deposition sampling, the concentrations of five main Nr species (ammonia — NH3, nitrogen dioxide — NO2, nitric acid — HNO3, particulate ammonium — pNH+4 and particulate nitrate — pNO−3) were measured monthly at each site. NH3, HNO3, pNH+4 and pNO−3 were observed using the DELTA (DEnuder for Long-Term Atmospheric) sampling system designed by the Centre for Ecology and Hydrology, Edinburgh, UK. Detailed information on measuring the wet and dry N depositions at the sampling sites can be found in Shen et al. (2009, 2013).

1.2.2. Riverine N export monitoring A real-time hydrological monitoring system and water quality sampling point were set up at each outlet of the two selected sub-catchments. Instantaneous discharge data were automatically collected in every 10 min and were used to calculate the daily cumulative runoff. Water samples were periodically collected at a 10-day interval at the outlet of each sub-catchment. The collected water samples were immediately transported to the laboratory and stored at −18°C till analysis. More detailed information on riverine N export monitoring can be seen in Wang et al. (2014).

1.3. Analytical procedures The collected wet deposition and riverine N export samples were stored in the polypropylene bottles at − 18°C and were usually analyzed at one-month intervals. After thawing, the samples were filtered using 0.45 μm filter membranes. The NH+4-N and NO−3-N contents in the filtrates were analyzed with a flow-injection auto-analyzer (Tecator FIA Star 5000 Analyzer, Foss Tecator, Sweden). The total dissolved N content in the filtrate was measured by the potassium persulfate oxidation method (Zhang et al., 2008), with the transformed NO−3 also analyzed by the flow-injection auto-analyzer. The dissolved organic N content (DON) was calculated as the difference between the total N and the inorganic N contents.


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Fig. 1 – Map of sampling sites (left site: map of Hunan Province, China; right site: map of Jinjing Catchment).

The used sampling trains of DELTA systems and NO2 diffusion tubes were stored at 4°C and were analyzed at one-month intervals. Detailed analytical procedure for the dry deposition samples can also be seen in Shen et al. (2013).

1.4. Estimate of total N export load from land to river for diffuse sources in the agricultural sub-catchment and Jinjing Catchment The input of N for diffuse sources in the rural catchments mainly included atmospheric deposition, cropland fertilization, livestock and domestic wastes (Howarth, 2006; Chen et al., 2009; Yan et al., 2011). Export loads (EL) from land to river for different pollution sources were estimated as the product of application or generation N amounts in the catchment and export coefficients (Chen et al., 2009). Then the total N export load (EL, kg N/(ha·yr)) in the catchment can be estimated using the following equation: EL ¼ G f E f þ Gd Ed þ Gk Ek þ Ga Ea

ð1Þ

where, Gf (kg N/(ha·yr)) is the applied N fertilizer amount, Gd (kg N/(ha·yr)) is the generated N from livestock–poultry wastes, Gk (person/ha) is the population density and Ga (kg N/(ha·yr)) is the atmospheric N deposition rate in the catchment; Ef, Ed, Ek (kg N/(person·yr)) and Ea are the export coefficients from land to river for N fertilizer, livestock– poultry wastes, domestic wastes and atmospheric deposition, respectively. The mean applied fertilizer amount for N in the

catchment was estimated by summing the products of N fertilizer application rates in each land use (374 kg N/(ha·yr) for paddy fields, 450 kg N/(ha·yr) for tea fields and vegetable fields and 0 for forest based on our survey data in Jinjing Catchment) and the corresponding land use area ratio. The wastes (i.e., manure and urine) from production of three main kinds of livestock–poultry (i.e., pig, chicken and duck) were considered in this study. The mean generated N amount from animal wastes was estimated by summing the products of stock density for each kind of livestock–poultry (10.5, 3.4 and 4.7 head/(ha·yr) for pig, chicken and duck respectively based on our survey data in Jinjing Catchment) and the corresponding generation coefficient for waste-N (cited from Chen et al., 2009). The mean population density was 3.13 person/ha based on our survey data in Jinjing Catchment. The export coefficient from land to river for N fertilizer was cited from Zhu (2008) as 0.07, which is the mean loss rate of N fertilizer from croplands to rivers by runoff and leaching in China. The export coefficient for livestock–poultry was cited from Johnes (1996) as 0.15, which is comparable to those used in several other studies in China (Chen et al., 2009; Ding et al., 2010; Ma et al., 2011) because these coefficients do not vary greatly among areas (Ding et al., 2010). For atmospheric deposition, the export coefficient was 0.14 in forest as the mean of the ratios of monthly riverine N export to atmospheric N deposition found in this study; the export coefficients were set as 0.07 for cropland (assuming N input from atmospheric deposition is a kind of N fertilizers) and 0.14 for other land uses (the same as that used in forest).

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1.5. Calculation and statistics

2. Results and discussion

Wet deposition was calculated as the product of the precipitation amount and the concentrations of Nr species in rainwater. The monthly or annual wet deposition of Nr species (WDNm/y, kg N/ha) can be expressed using the following equation: WDNm=y ¼ 0:01

n X

Ci Pi

ð2Þ

i¼1

where, C (mg N/L) is the concentration of N species in rainwater; P (mm) is the water amount of a precipitation event; n is the total number of precipitation events; and the subscript i is the count for precipitation events. Due to no equipment to measure the in situ dry N deposition by the micrometeorological methods, we estimated the dry N deposition rates by multiplying the measured concentrations of Nr species by their deposition velocities (Vd) obtained from related studies in the literature. According to Nemitz et al. (2000) and Flechard et al. (2011), the dry deposition rate of each Nr species (F) can be expressed as: F¼

C Nr −C 0 ¼ C Nr V d Rt

ð3Þ

where, C Nr (μg N/m3) is the measured concentration of individual Nr species; C0 (μg N/m3) is the canopy compensation point of that Nr species (deposition occurs only when the measured concentration is higher than the canopy compensation point); Rt (sec/cm) is the total deposition resistance between the measuring height and the deposition surface; and Vd (cm/sec) is the deposition velocity, equal to the reciprocal of Rt when C0 is zero. Because we did not measure Vd (or Rt) in this study, the Vd of Nr species was obtained from Flechard et al. (2011) for simplification. For calculating the monthly riverine N (NH+4-N, NO−3-N and DON were considered in this study) export, the monthly cumulative discharge at the outlet of a sub-catchment, which was recorded by the hydrological monitoring system, was multiplied with the average NH+4-N (or NO−3-N and DON) concentration for the three samples collected in a month. The software package SPSS version 14.0 (SPSS Inc., Chicago, IL) was used to perform the descriptive statistics, Pearson correlation and linear regression analyses. For the Pearson correlation and the linear regression analyses, the significance was defined as the probability (p) value that is less than 0.05.

2.1. Nitrogen deposition 2.1.1. Annual nitrogen deposition The annual wet deposition rates of NH+4-N, NO−3-N and DON were summarized as 10.5–11.2, 6.5–7.5 and 2.2–4.2 kg N/ha, respectively, at the three sampling sites (Table 1). The annual total wet N deposition rates ranged from 20.6 to 21.5 kg N/ha at the three sampling sites, showing a small site variation. The inferred annual dry N deposition rates of NH3, NO2, HNO3, pNH+4 and pNO−3 were − 0.3–10.1, 1.4–3.1, 1.9–6.1, 1.6–11.0 and 0.5–4.6 kg N/ha, respectively. The estimated annual total dry N deposition rates were 5.5, 10.2 and 34.9 kg N/ha at Huinong, Feiyue and Xishan, respectively, showing an increasing trend from the moderate N input agricultural land (374 kg N/(ha·yr), paddy field), to the high N input agricultural land (ca. 450 kg N/(ha·yr), tea field) and then to the natural ecosystem (forest). Combining dry and wet depositions, the annual total N deposition rates at Feiyue, Huinong and Xishan were 31.7, 26.1 and 55.8 kg N/ha, respectively. Both wet and dry N deposition rates reported in this study were comparable with former studies conducted in other regions of South China (Aas et al., 2007). Using the measured wet and dry N deposition rates at the three sampling sites with contrasting land use, the mean total N depositions in the agricultural sub-catchment (Tuojia) and Jinjing Catchment were estimated by using the mean wet N deposition at the three sites and land use weighted dry N deposition. The estimated mean total N depositions at the agricultural sub-catchment and Jinjing Catchment were 44.3 and 46.3 kg N/(ha·yr) respectively, with wet and dry depositions contributing to 45.4%–47.4% and 52.6%–54.6% respectively.

2.1.2. Monthly N deposition Monthly N deposition rates in the forest and agricultural sub-catchments are presented in Fig. 2. The monthly N depositions ranged from 2.8 to 7.2 kg N/ha in the forest sub-catchment, while ranged from 2.3 to 6.0 kg N/ha in the agricultural sub-catchment. The lowest monthly N deposition in a year occurred in September during the two-year monitoring period in the two sub-catchments. The reason for this may be due to the air quality that was usually better during autumn around a year (Lu and Wu, 2003) and the precipitation that was lower in autumn than in spring and summer in

Table 1 – Mean annual wet and dry N deposition rates (kg N/ha) at the sampling sites and Tuojia sub-Catchment and Jinjing Catchment during September 2010 to August 2012. Site

Feiyue Huinong Xishan Tuojia Jinjing

Wet deposition

Dry deposition

NH+4

NO−3

DON

NH3

NO2

10.5 10.8 11.2 10.8 10.8

6.8 6.5 7.5 7.0 7.0

4.2 3.3 2.2 3.2 3.2

4.5 −0.3 10.1 6.2 6.9

1.6 1.4 3.1 2.3 2.4

HNO3

pNH+4

pNO−3

2.0 1.9 6.1 4.7 5.0

1.6 2.0 11.0 7.2 7.8

0.5 0.5 4.6 2.9 3.2

DON: dissolved organic nitrogen; WD: wet deposition; DD: dry deposition; TD: total deposition.

WD

DD

TD

21.5 20.6 20.9 21.0 21.0

10.2 5.5 34.9 23.3 25.3

31.7 26.1 55.8 44.3 46.3


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8

N deposition (kg N/ha)

Forest sub-catchment Agricultural sub-catchment 6

4

2

0

Oct Dec Feb Apr Jun Aug Oct Dec Feb Apr Jun Aug Month

Fig. 2 – Monthly atmospheric N deposition in the forest and agricultural sub-catchments.

The riverine TN exports in the forest sub-catchment in this study were higher than those measured in northeastern United States (Aber et al., 2003) and northern Japan (Ohte et al., 2001), where atmospheric N depositions were low, while comparable with those observed in Europe (MacDonald et al., 2002) and southern China (Chen and Mulder, 2007; Huang et al., 2011), where N depositions were also high. Though soil properties and climate may cause the differences, atmospheric N deposition, as the major N source of forest ecosystems, may affect riverine N export in a large degree. The much higher riverine export of NO−3-N than NH+4-N may be probably due to the soil surface absorption, plant uptake, microbe immobilization and nitrification of NH+4. The monthly riverine exports of total N ranged from 0.7 to 5.3 kg N/ha in the agricultural sub-catchment, higher than the monthly riverine N exports in the forest sub-catchment. The riverine N exports also generally showed high values in spring and summer when precipitation was high and relatively low values in autumn and winter when precipitation was low. Much higher N exports were found during April to July in the two sampling years. In the agricultural sub-catchment, paddy fields accounted for a large proportion (32%) of the total area, and the N fertilizers for the rice production were mainly applied during April to July at a total rate of approximately 374 kg N/ha. The synchronization of high riverine N export and N fertilizer applications suggests that N fertilizers may

Jinjing Catchment. The highest monthly deposition in a year occurred in April or March, which may be ascribed to the highest precipitation and severe air pollution in spring in Hunan Province (Lu and Wu, 2003). The temporal variation of N deposition in the agricultural sub-catchment was similar to that in the forest sub-catchment. Because forest was the major land use in the studied sub-catchment and N deposition in forest was the highest compared with other land uses, the mean N deposition in the agricultural sub-catchment is likely to be dependent on the N deposition in forest in a large content.

2.2. Riverine N exports Fig. 3 shows the monthly riverine N exports in the forest and agricultural sub-catchments. The monthly riverine exports of total N (TN, sum of NH+4-N, NO−3-N and DON) ranged from 0.1 to 2.9 kg N/ha in the forest sub-catchment, showing similar variation as the precipitation, that is the highest export was found in spring and summer and the lowest export found in autumn and winter. The monthly exports of NO−3-N ranged from 0.1 to 1.6 kg N/ha, which was much higher than (about 4 to 26 times of) the exports of NH+4-N and were similar as the findings of Huang et al. (2011). The estimated annual riverine exports of TN were 7.2 kg N/ha in 2010 to 2011 and 9.6 kg N/ha in 2011 to 2012, contributed predominantly by NO−3-N (57%).

3.0

a

NO3--N 6.0 Riverine N export (kg N/ha)

Riverine N export (kg N/ha)

DON 3.5 2.5 2.0 1.5 1.0 0.5 0.0


5.0

NH4+-N

b

4.0 3.0 2.0 1.0 0.0


Fig. 3 – Monthly riverine N exports in the forest (a) and agricultural (b) sub-catchments.

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27.4 kg N/ha in 2011 to 2012, contributed largely by NH+4-N (48%). The observed riverine N exports were compared with those determined in the watersheds with serious diffuse pollution in the United States (David et al., 1997; David and Gentry, 2000; Howarth, 2006).

have contributed to the riverine N export by runoff and leaching from paddy fields. High N exports were also found in months when precipitation was low and N fertilizers were not applied, for example in December 2010 and January 2012, indicating that the untreated animal wastes from the small-scaled animal production may also one of the important sources for riverine N export. In contrast to the forest sub-catchment, the riverine exports of NH+4-N were higher than those of NO−3-N in most of the months in the agricultural sub-catchment, indicating that excessive use of chemical N fertilizers (urea, ammonium bicarbonate, etc.) in croplands and discharge of untreated animal waste from small-scale animal husbandry may be important sources of riverine NH+4-N in the agricultural sub-catchment, which had been identified by many studies (Xie et al., 2007; Yan et al., 2011). The estimated annual riverine exports of TN were 30.3 kg N/ha in 2010 to 2011 and

2.3. Contribution of N deposition to riverine N exports 2.3.1. Correlations among precipitation, N deposition and riverine N export To show how riverine N export was affected by atmospheric N deposition in the studied catchment, a correlation analysis was done by using the two-year (from October 2010 to August 2012) monitoring data of wet N deposition at the forest and paddy field sites and the corresponding riverine N export of the forest and agricultural sub-catchments (Fig. 4). The results

a y = 0.298 + 0.00517x (R2 = 0.447**, n = 24)

2.0 1.5 1.0

y = 0.295 + 0.00272x (R2 = 0.314**, n = 24)

0.5 0.0 0


Wet N deposition (kg N/ha)

2.5

2.0

50

100

150 200 250 Precipitation (mm)

300

1.5 y = -0.042 + 0.00351x (R2 = 0.551**, n = 21) y = -0.003 + 0.000433x (R2 = 0.698**, n = 21)

0.5 0.0 0

50

100


300

3.0

b

2.5

y = 0.400 + 0.0505x (R2 = 0.375**, n = 24)

2.0 1.5 1.0

y = 0.322 + 0.0239x (R2 = 0.260**, n = 24)

0.5

350

c

1.0

NO3--N

0.0 0


Wet N deposition (kg N/ha)

NH4+-N

3.0

50

100


300

350

d

2.5

y = -0.136 + 0.00541x (R2 = 0.593**, n = 22)

2.0 1.5 1.0

y = 0.879 + 0.00201x (R2 = 0.006, n = 22)

0.5 0.0 0

350

50

100


300

350

e 1.5 1.0

y = -0.081 + 0.727x (R2 = 0.481**, n = 21)

0.5

y = 0.0038+0.0487x (R2 = 0.451**, n = 21)

0.0 0.0

0.5

1.0 1.5 2.0 Wet N deposition (kg N/ha)

2.5



2.0 2.5

f y = 0.222 + 0.765x (R2 = 0.288**, n = 22)

2.0 1.5 1.0 0.5

y = 0.772 + 0.364x (R2 = 0.064, n = 22)

0.0 0.0

0.5

1.0 1.5 2.0 2.5 Wet N deposition (kg N/ha)

3.0

Fig. 4 – Correlations between monthly precipitation and wet N deposition, between monthly precipitation and riverine N export and between monthly wet N deposition and riverine N export in the forest (a, c and e) and agricultural (b, d and f) sub-catchments. * and ** mean that the correlations are significant at the 0.05 level and 0.01 level respectively.

60

400

Precipitation Forest sub-catchment Agricultural

Precipitation (mm)

350 300

50 40

250

30

200 150

20

100 10

50 0

Oct Dec Feb Apr Jun AugOct Dec Feb Apr Jun Aug

0

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Export coefficient of AD in forest and contribution of AD to riverine N export (%)


Month Fig. 5 – Monthly precipitation, monthly export coefficients of atmospheric deposition (AD) in the forest sub-catchment and monthly contributions of AD to riverine N export in the agricultural sub-catchment.

deposition in the forest sub-catchment ranged from 2.9% to 52.6% with a mean of 14.0%. Low export coefficients usually occurred in months with low precipitation, and vice versa. The correlation analysis showed that there was a significant positive correlation (p < 0.01) between precipitation and the export coefficient of atmospheric N deposition (Fig. 6), indicating that N deposition contributed largely to riverine N export in the studied forest.

2.3.2. Export coefficient of atmospheric deposition in the forest sub-catchment

2.3.3. Contribution of atmospheric deposition to riverine N export in the agricultural sub-catchment and Jinjing Catchment

In the forest with high N deposition, atmospheric deposition usually is the dominant N source. Thus in this study, we considered all the N species in the riverine runoffs in the forest sub-catchment derive from atmospheric deposition. Then the export coefficient of atmospheric deposition from land to river in the forest sub-catchment can be calculated as the ratio of riverine N export to atmospheric N deposition. As shown in Fig. 5, monthly export coefficients of atmospheric N

The estimated annual total N export loads from land to river for the four kinds of diffuse sources in the agricultural sub-catchment and Jinjing Catchment were shown in Table 2. The annual total N export load from land to river in the agricultural sub-catchment was 29.40 kg N/ha, with atmospheric deposition contributing to 18.7%. These results were compared with the results in Chen et al. (2009), where the contribution of atmospheric deposition to the total N export load for diffuse

Export coefficient of AD in forest and contribution of AD to riverine N export (%)

showed that both monthly wet N deposition and riverine N export showed significant positive correlation with monthly precipitation (p < 0.01), except for riverine NH+4-N export in the agricultural sub-catchment. Monthly wet N deposition and riverine N export also showed significant positive correlation (p < 0.01), except for the NH+4-N in the agricultural sub-catchment as well. These results indicated that N deposition may contribute to riverine N export, especially for NO−3-N.

60

Forest sub-catchment Agricultural sub-catchment

50 40

y = 1.07 + 0.10x (R2 = 0.56**, n = 21)

30 20 10

y = 19.26 - 0.039x (R2 = 0.23*, n = 22)

0 0

50

100

150

200

250

300

350

400

Precipitation (mm) Fig. 6 – Correlations between monthly precipitation and monthly export coefficient of atmospheric deposition (AD) in the forest sub-catchment and monthly contribution of AD to riverine N export in the agricultural sub-catchment. * and ** mean that the correlations are significant at the 0.05 level and 0.01 level respectively.

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Table 2 – Estimated annual total N export loads from land to river for diffuse sources in the agricultural sub-catchment and Jinjing Catchment. Diffuse source

Applied or generated amount (kg N/ha)

Export coefficient

Export load (kg N/ha)

Riverine export (kg N/ha)

Agricultural sub-catchment Fertilizers Animal wastes Domestic wastes Deposition e Total

138.24 57.27 –c 44.30 –

0.07 a 0.15 b 1.80 d 0.14 f –

9.68 8.59 5.63 5.50 29.40

– – – – 28.83

Jinjing Catchment Fertilizers Animal wastes Domestic wastes Deposition Total

109.91 57.27 – 46.30 –

0.07 0.15 1.80 0.14 –

7.69 8.59 5.63 5.84 27.76

– – – – –

a

This value was cited from Zhu (2008) as 0.07, which is the mean loss rate of N fertilizer from croplands to rivers by runoff and leaching in China. b This value was cited from Johnes (1996) as a mean export coefficient for pig and poultry wastes. c No data. d The value was set according to Johnes (1996), Ding et al. (2010) and Ma et al. (2011), and its unit is kg N/(person·yr); population density in the studied catchment was 3.13 person/ha. e The deposition amount was the measured mean annual deposition rate during 2010 to 2012. f The export coefficient was set to be 0.14 in forest as the mean of the ratios of monthly riverine N export to atmospheric N deposition found in this study; the export coefficients were set as 0.07 for cropland and 0.14 for other land uses.

sources was 12% to 15% in an agricultural watershed in southeastern China. The annual N export load was nearly equal to the measured mean annual riverine N export in the agricultural sub-catchment, suggesting that the parameters used to estimate N export load in this study were appropriate. Using the same parameters, the estimated annual N export load in Jinjing Catchment was 27.76 kg N/ha, with atmospheric deposition contributing to 21.0%, comparable to the counterpart results in the agricultural sub-catchment. The contribution of AD to riverine N export in the agricultural sub-catchment was estimated as the ratio of riverine exported N from AD to the monthly riverine N export. The results showed that monthly atmospheric depositions contributed to 4.2% to 29.0% of the monthly riverine N export (Fig. 5). Low contribution rates usually occurred in months with high precipitation, and vice versa. The correlation analysis showed that there was a significant negative relationship (p < 0.05) between precipitation and the contribution rate of atmospheric N deposition (Fig. 6), indicating that N from fertilizers, livestock and domestic wastes may contributed largely to N runoff loss in the studied agricultural catchment when precipitation was high. One of the reasons for this trend may be that in the high precipitation events, N can be easily lost by runoffs from the ponds for composting livestock and domestic wastes when the ponds were filled with rainwater. Other researchers also found high riverine N export during high precipitation events in the agricultural watersheds. For example, David et al. (1997) found that high flow events led to large exports of N in tiles and in the river in an agricultural watershed in Illinois, USA. Tian et al. (2007) reported that runoffs in rice seasons usually occurred in paddy fields when precipitation was higher than 40 mm.

3. Conclusions Atmospheric N deposition rates were as high as 26.1 to 55.8 kg N/(ha·yr) across different land uses in Jinjing Catchment of Xiangjiang River watershed, while the riverine N exports ranged from 7.2 to 9.6 kg N/(ha·yr) in the forest sub-catchment and 27.4 to 30.3 kg N/(ha·yr) in the agricultural sub-catchment. The correlations between both wet N deposition and riverine N export and precipitation were highly positive (p < 0.05), and so were the correlations between NH+4-N or NO−3-N wet deposition and riverine NH+4-N or NO−3-N exports except for NH+4-N in the agricultural sub-catchment, indicating that N deposition contributed to riverine N export. The calculated mean export coefficient of atmospheric deposited N from land to river in the forest sub-catchment was 14%. Monthly export coefficients showed significant positive correlation with precipitation (p < 0.01), indicating that N deposition contributed largely to N runoff loss in the studied forest. The monthly mean contributions of atmospheric deposition to riverine N export were 18.7% in the agricultural sub-catchment and 21.0% in the Jinjing Catchment, and the monthly contribution rates showed significant negative correlation with precipitation (p < 0.05), indicating that N from other diffuse sources (e.g., fertilizers, livestock and domestic wastes) may contributed largely to N runoff loss in the studied agricultural catchment when precipitation is high. The relative high contribution of N deposition to diffuse N pollution in the catchment suggests that efforts should be done to control anthropogenic reactive N emissions (e.g., from N fertilizer applications, animal production and fossil fuel combustion) to the atmosphere for reduced atmospheric N


deposition as well as diffuse pollution in the hilly red soil regions in southern China.

Acknowledgments This work was supported by the National Basic Research Program (973) of China (No. 2012CB417105), the Key Deployment Program of the Chinese Academy of Sciences (No. KZZD-EW-11), the 100-Talents Program of the Chinese Academy of Sciences for Dr. Yong Li and the National Natural Science Foundation of China (Nos. 41101247 and 41071151).

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Long-term atmospheric wet deposition of dissolved organic nitrogen in a typical red-soil agro-ecosystem, Southeastern China.

Reduced nitrogen dominated nitrogen deposition in the United States, but its contribution to nitrogen deposition in China decreased.

Responsiveness of soil nitrogen fractions and bacterial communities to afforestation in the Loess Hilly Region (LHR) of China.

Potential contribution of anammox to nitrogen loss from paddy soils in Southern China.

Responses of CH4 and N2O fluxes to land-use conversion and fertilization in a typical red soil region of southern China.

Land management: Resolving soil pollution in China.

Impact of nitrogen fertilization on soil-Atmosphere greenhouse gas exchanges in eucalypt plantations with different soil characteristics in southern China.

Effects of Simulated Nitrogen Deposition on Soil Net Nitrogen Mineralization in the Meadow Steppe of Inner Mongolia, China.

Sinks for inorganic nitrogen deposition in forest ecosystems with low and high nitrogen deposition in China.

Influence of vegetation restoration on topsoil organic carbon in a small catchment of the loess hilly region, China.

Annual soil CO2 efflux in a cold temperate forest in northeastern China: effects of winter snowpack and artificial nitrogen deposition.

Spatial distribution of soil organic carbon and total nitrogen based on GIS and geostatistics in a small watershed in a hilly area of northern China.

Effects of fertilization and clipping on carbon, nitrogen storage, and soil microbial activity in a natural grassland in southern China.

Catchment-scale evaluation of pollution potential of urban snow at two residential catchments in southern Finland.

Long-term atmospheric deposition of nitrogen, phosphorus and sulfate in a large oligotrophic lake.

CAN Canopy Addition of Nitrogen Better Illustrate the Effect of Atmospheric Nitrogen Deposition on Forest Ecosystem?

Contribution of Atmospheric Diffusion Conditions to the Recent Improvement in Air Quality in China.

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Nitrogen deposition enhances carbon sequestration by plantations in northern China.

Fractal features of soil particle size distribution under different land-use patterns in the alluvial fans of collapsing gullies in the hilly granitic region of southern China.

Michaelis-Menten kinetics of soil respiration feedbacks to nitrogen deposition and climate change in subtropical forests.

Conditional vulnerability of plant diversity to atmospheric nitrogen deposition across the United States.

Photochemical production of atmospheric carbonyls in a rural area in southern China.

Atmospheric pollution.