Introduction
The global nitrogen (N) cycle has been disturbed by elevated reactive
N
emissions from anthropogenic activities since the mid-19th century (Canfield
et al., 2010; Galloway et al., 2008; Gruber and Galloway, 2008). Accumulated
reactive N in the environment has led to a series of effects on climate
change and ecosystems, e.g., air pollution, stratospheric ozone depletion,
the potential alteration of global temperature, drinking water
contamination, freshwater eutrophication, biodiversity loss, grassland seed
bank depletion, and dead zones in coastal ecosystems (Basto et al., 2015;
Erisman et al., 2011; Erisman et al., 2013; Lan et al., 2015; Pinder et al.,
2012; Shi et al., 2015; Zaehle et al., 2010). To examine the actual amount
of N inputted into ecosystems, several monitoring networks have been
established at national or continent scales, e.g., the National Atmospheric
Deposition Program National Trends Network (NADP/NTN, United States)
(Lehmann et al., 2005), the Canadian Air and Precipitation Monitoring
Network (CAPMoN, Canada) (Zbieranowski and Aherne, 2011), the European
Monitoring and Evaluation Programme (EMEP, Europe) (Fagerli and Aas, 2008),
the Austrian Precipitation Sampling Network (Austria) (Puxbaum et al.,
2002), and the Japanese Acid Deposition Survey (JADS, Japan) (Morino et al.,
2011).
Besides Europe and North America, east Asia has become another high N
deposition region, due to rapid economic growth in recent decades (Dentener
et al., 2006). Across China, inorganic N wet deposition has increased since
the mid-20th century, albeit with inconsistent estimations of the change: 8 kgNha-1yr-1 (from 13.2 kgNha-1yr-1 in the 1980s to
21.1 kgNha-1yr-1 in the 2000s) (X. J. Liu et al., 2013), 2.8 kgNha-1yr-1 (from 11.11 kgNha-1yr-1 in the 1980s to
13.87 kgNha-1yr-1 in the 2000s) (Jia et al., 2014), and 7.4 kgNha-1yr-1 (from 12.64 kgNha-1yr-1 in the 1960s to
20.07 kgNha-1yr-1 in the 2000s) (Lu and Tian, 2014). Enhanced
N
deposition has changed the structure and function of terrestrial, aquatic
and coastal ecosystems in China (Liu et al., 2011). To accurately estimate
the N deposition in China, several monitoring networks have been established
at the regional scale, e.g., in northern China (Pan et al., 2012), in forest
ecosystems along the North–South Transect of Eastern China (NSTEC; based on
the ChinaFLUX network) (Sheng et al., 2013), and in subtropical forest
ecosystems in southern China (Chen and Mulder, 2007). However, there are few
observation sites distributed in western China, particularly in the Tibetan
Plateau (TP), resulting in uncertainty regarding the N deposition for China
as a whole (Jia et al., 2014; X. J. Liu et al., 2013; Lu and Tian, 2014).
The TP covers an area of about 2.57 million km2, occupying
approximately one-fourth of the land area of China (Zhang et al., 2002). Over the
TP, alpine ecosystems are widely distributed and are sensitive to elevated
N
deposition. Multi-level N fertilization experiments have shown that alpine
grassland ecosystems are N limited and have potential capacity to absorb
increased N deposition (Y. W. Liu et al., 2013; Xu et al., 2014). However,
long-term N addition can decrease the species richness of both vegetation
and soil seed banks in alpine meadow ecosystems in the TP (Ma et al., 2014).
Ice core records show that the inorganic N deposition in the TP has
increased during recent decades (Hou et al., 2003; Kang et al., 2002a, b;
Thompson et al., 2000; Zhao et al., 2011; Zheng et al., 2010). This trend is
also apparent in sediment cores of alpine lakes in the western and
southeastern TP (Choudhary et al., 2013; Hu et al., 2014). To recognize the
characteristics of ion deposition in the TP, a number of observations of
precipitation chemistry have been carried out in the eastern TP in recent
years (Jia, 2008; Tang et al., 2000; Zhang et al., 2003; N. N. Zhang et al.,
2012). Nevertheless, in the central and western TP, observation sites are
scarce, indicating that the situation in terms of N deposition across the
entire TP remains unclear.
Map of the inorganic N wet deposition sampling sites in
the TP. The red points indicate the five remote sampling sites of this
study. The black points indicate the sampling sites from previous records.
Southeast Tibet Station is short for Southeast Tibet Observation and
Research Station for the Alpine Environment, Chinese Academy of Sciences;
Nam Co Station is short for Nam Co Monitoring and Research Station for
Multisphere Interactions, Chinese Academy of Sciences; Qomolangma Station is
short for Qomolangma Atmospheric and Environmental Observation and Research
Station, Chinese Academy of Sciences; Ngari Station is short for Ngari
Desert Observation and Research Station; and Muztagh Ata Station is short
for Muztagh Ata Westerly Observation and Research Station.
To quantitatively estimate the inorganic N wet deposition in the TP, we
investigated the precipitation chemistry characteristics at five remote
sites, situated mainly in the central and western TP. The sites are part of
the Tibetan Observation and Research Platform (TORP) network (Ma et al.,
2008). Specifically, our aims were to (1) clarify the characteristics of
inorganic N wet deposition in the central and western TP, and (2) quantitatively
assess the inorganic N wet deposition in the entire TP by
combining site-scale in situ measurements in this and previous studies.
Descriptions of the five precipitation sampling sites in
the TP.
Station name
Station name expanded
Latitude
Longitude
Altitude
Annual mean temperature
Annual precipitation
Vegetation type
References
m a.s.l.
∘C
mmyr-1
Southeast Tibet Station
Southeast Tibet Observation and Research Station for the Alpine Environment, Chinese Academy of Sciences
29∘46′ N
94∘44′ E
3326
5.6
800–1000
Subalpine coniferous forest and temperate deciduous conifer mixed forest
Wang et al. (2010)
Nam Co Station
Nam Co Monitoring and Research Station for Multisphere Interactions, Chinese Academy of Sciences
30∘47′ N
90∘58′ E
4730
-0.6
414.6
Alpine meadow and alpine steppe
Zhang et al. (2011)
Qomolangma Station
Qomolangma Atmospheric and Environmental Observation and Research Station, Chinese Academy of Sciences
28∘13′ N
86∘34′ E
4300
3.9
402.8
Alpine meadow and alpine steppe
Gao et al. (2014) and M. Li et al. (2007)
Ngari Station
Ngari Desert Observation and Research Station
33∘24′ N
79∘43′ E
4264
–
124.6
Desert steppe
This study
Muztagh Ata Station
Muztagh Ata Westerly Observation and Research Station
38∘17′ N
75∘1′ E
3650
–
213.6
Alpine steppe
This study
Materials and methods
Precipitation sampling and chemical analysis
Using the TORP network (Ma et
al., 2008), precipitation chemistry observations were conducted at five
sampling sites: Southeast Tibet Station, Nam Co Station, Qomolangma Station,
Ngari Station, and Muztagh Ata Station (Fig. 1), situated from the eastern
to western TP and covering various climatic zones and vegetation types. A
brief description of the five sites is given in Table 1.
During 2011–2013, we collected precipitation samples at each site, lasting
at least 1 year. Precipitation samples were collected following each
precipitation event, using an inner removable high-density polyethylene
(HDPE) plastic bag in a pre-cleaned HDPE bucket. The HDPE bucket was placed
1.5 m above the ground. We opened the plastic bag at the beginning of the
precipitation event and collected precipitation samples at the end of the
precipitation process. Then, the samples were transferred into pre-cleaned
HDPE bottles (50 mL). Snowfall samples were melted at room temperature
before being transferred into the HDPE bottles. All samples were kept frozen
at the station and during transport until analysis in the laboratory. A
total of 259 precipitation samples were collected, among which eight samples
were abandoned due to breakage during transportation or the samples volume
being less than 10 mL.
We analyzed the chemical composition of all precipitation samplings at the
State Key Laboratory of Environmental Aquatic Chemistry, Research Center for
Eco-Environmental Sciences, Chinese Academy of Sciences. Analyzed ions
included NO3-, Cl-, SO42-, NH4+,
Na+, K+, Ca2+, and Mg2+. All ions were analyzed by the
Dionex ICS-2100 Ion Chromatography System. Samples for cation analysis were
eluted on a Dionex 4 mm CS12A separatory column using 20 mM
methanesulfonic acid solution for an eluent pumped with a flow rate of 1.0 mLmin-1.
Suppression was provided by a Dionex CSRS 300 suppressor in recycle mode. For
anion analysis, an IonPac AS19-HC column, 25 mM NaOH eluent, and ASRS 300
suppresser were used. The analytical detection limit was 2 ngg-1 for
all ions.
Data quality control
Previously documented methods (Rodhe and Granat, 1984; Safai et al., 2004)
were used for quality assurance and quality control purposes. Eight (3.2 %)
samples fell outside the range (m-3δ, m+3δ) and therefore
were excluded. Here, m is the mean value and δ is the standard
deviation. The Pearson correlation between Σanions and Σcations was 0.82 (P<0.001), suggesting credible data quality.
The ratio of total anions to total cations was calculated following Eq. (1):
∑anions∑cations=∑k=1n(NO3-+Cl-+SO42-)∑k=1n(NH4++Na++K++Ca2++Mg2+),
where n is the number of samples, and the unit of ion concentration is µeqL-1. The ratio of Σanions/Σcations was 0.26, indicating that at least one major anion was not
measured (C. Li et al., 2007). Considering that pH was alkaline in both
precipitation and the surface soil layer (Ding et al., 2004; Y. H. Yang et
al., 2012), the unmeasured anion was likely HCO3- (C. Li et al.,
2007).
Statistical analysis
For each site, consecutive samples in one year-round sampling period were
selected to analyze the annual mean values of ions. The sampling time windows of
the samples used at the five sites were as follows: Southeast Tibet Station,
November 2011 to October 2012; Nam Co Station, August 2011 to July 2012;
Qomolangma Station, April 2011 to March 2012; Ngari Station, January 2013 to
December 2013; and Muztagh Ata Station, January 2011 to December 2011. A total
of 168 precipitation samples were selected, among which the number of
samples for Southeast Tibet Station, Nam Co Station, Qomolangma Station,
Ngari Station, and Muztagh Ata Station was 53, 27, 30, 39, and 19,
respectively.
The annual average ion concentration was calculated as the volume-weighted
mean (VWM) following Eq. (2):
C=∑i=1n(Ci×Pi)∑i=1nPi,
where C is the annual average ion concentration (µeqL-1),
Ci is the ion concentration of an individual sample i (µeqL-1),
and Pi is the precipitation amount corresponding to the
sample i (mm).
Wet deposition of atmospheric N was calculated following Eq. (3):
Nwet=0.00014×CN×Pannual,
where Nwet is the annual wet deposition of atmospheric inorganic N
(NH4+–N or NO3-–N, kgNha-1yr-1); CN is
the annual average equivalent concentration of N in precipitation
(NH4+–N or NO3-–N, µeqL-1); Pannual is
annual precipitation (mmyr-1); and 0.00014 is the shift coefficient
for the unit of µeqL-1mmyr-1 to the unit of
kgNha-1yr-1. Here, 1 µeq NH4+–N or
NO3-–N contains 1 µmol N, and the weight of 1 µmol N is
14×10-9 kg. Thus, µeqL-1mmyr-1 =14×10-9 kg N ×103 m-3 ×10-3 myr-1 =14×10-9 kg N
× 104 ha-1yr-1 = 0.00014 kgNha-1yr-1.
Source assessment of ion wet deposition
Enrichment factor
Enrichment factor (EF) has been widely used to examine the source
contributions of major ion wet deposition in previous studies (Cao et al.,
2009; Chabas and Lefevre, 2000; Kulshrestha et al., 1996; Lu et al., 2011;
Okay et al., 2002; Shen et al., 2013; Xiao et al., 2013; Zhang et al.,
2007). Commonly, Na is considered as the best reference element for
seawater, due to its almost purely marine origin (Keene et al., 1986;
Kulshrestha et al., 2003). Another element, Ca, is normally used as a
reference element for continental crust, because Ca is a typical lithophile
element and its composition in soil barely changes (Zhang et al., 2007). In
this study, Na and Ca were used as a reference element for seawater and
continental crust, respectively.
In the TP, multiple lines of evidence demonstrate that Na+ in
precipitation mainly comes from oceans. Balestrini et al. (2014) monitored
the chemical and isotopic compositions of precipitation at the Pyramid
International Laboratory (5050 ma.s.l.) on the southern slope of the
Himalayas, and data analysis suggested that Na+ and Cl- were
derived from the long-range transport of marine aerosols. Ice records in the
central Himalayas show that Cl- / Na+ was positively related with
the monsoon rainfall in northeast India, and there was a teleconnection
between the Na+ and Cl- concentrations and the North Atlantic
Oscillation, indicating that Na+ in the ice core mainly came from
oceans (Wang et al., 2002). Na+ has been used as a marine tracer when
analyzing the source contributions of ion wet deposition in the
northeastern TP (Li et al., 2015), the southeastern TP (B. Liu et al., 2013)
and the southern slope of central Himalayas (Tripathee et al., 2014).
Over the TP, sandy desertification land covers about 3.1×105 km2, accounting for 14 % of the whole plateau, of which moderate
sandy desertification land occupies 55.44 % (Liu et al., 2005). The TP is
regarded as an important dust source region (Fang et al., 2004; Han et al.,
2009, 2008). The TP dust sources contribute 69 % of dust at
the surface and 40 % of dust in the lower troposphere over the TP (Mao et
al., 2013). Moreover, arid regions are widely distributed surrounding the
TP, e.g., central Asia, and the deserts in western China. The dust over the
TP partly comes from the adjacent dust source regions, e.g., the Taklimakan
Desert in western China (Huang et al., 2007; Xia et al., 2008). Atmospheric
dust aerosols over the TP are strongly impacted by local sources and
enriched with Ca (Zhang et al., 2001). These dust aerosols in the atmosphere
can interact with clouds and precipitation (Huang et al., 2014), and deposit
on the surface with precipitation. Thus, Ca2+ is commonly used as a
proxy of dust in ice core studies in the TP (Kang et al., 2002a, 2010; Kaspari et al., 2007; Wang et al., 2008). As a dust proxy,
the Ca2+ record in an ice core from the central TP was significantly
related to regional zonal wind (westerlies) trends and reflected the long-term
control of regional atmospheric circulation strength over atmospheric dust
concentrations (Grigholm et al., 2015). In addition, Ca also has been used
as a reference element for continental crust when assessing sources of ion
wet deposition in precipitation in the northern TP (Li et al., 2015).
In this study, the EF of an ion in precipitation relative to the ion in sea
was estimated using Na as a reference element following Eq. (4):
EFsea=[X/Na+]rain[X/Na+]sea,
where EFsea is the EF of an ion in precipitation relative to the ion in
sea; X is an ion in precipitation; [X/Na+]rain is the ratio
of precipitation composition (µeq X/µeq Na+); and
[X/Na+]sea is the ratio of sea composition (Keene et al., 1986;
Turekian, 1968) (µeq X/µeq Na+).
The EF of an element in precipitation relative to the element in soil was
estimated using Ca as a reference element following Eq. (5):
EFsoil=[X/Ca2+]rain[X/Ca2+]soil,
where EFsoil is the EF of an element in precipitation relative to the
element in soil; X is an ion in precipitation; [X/Ca2+]rain is the
ratio of precipitation composition (µg X/µg Ca2+); and
[X/Ca2+]soil is the ratio of soil composition (Taylor, 1964)
(µg X/µg Ca2+).
To estimate fractions of marine, crustal and anthropogenic sources contributed to
ions in precipitation, we calculated the sources of ionic components in
precipitation using equations from previous studies (Cao et al., 2009; Lu et
al., 2011; Zhang et al., 2007) as follows:
SSF(%)=[X/Na+]sea[X/Na+]rain×100,
CF(%)=[X/Ca2+]soil[X/Ca2+]rain×100,
AF%=100-SSF-CF,
where SSF is sea salt fraction; CF is crust fraction; and AF is
anthropogenic fraction. Note that, if SSF is greater than 1, SSF is
recalculated as the difference between 1 and CF; if CF is greater than
1, CF is recalculated as the difference between 1 and SSF.
Principal component analysis
Principal component analysis has been widely used in precipitation chemical
studies to determine the effect of natural and anthropogenic sources on
chemical composition of precipitation (Balasubramanian et al., 2001; Cao et
al., 2009; Migliavacca et al., 2005; Zhang et al., 2007). In this study,
principal component analysis was also used to examine the various sources of
major ions in precipitation at the five remote sites in the TP.
Varimax-rotated principal component analysis was performed using
“principal” function in package “psych” of R 3.2.0 (R Core Team, 2015;
http://www.R-project.org, last visited 16 October 2015).
Backward trajectory analysis
To identify the long-range transport of water-soluble ions in precipitation,
7-day backward trajectories arriving at the sampling sites for each
individual precipitation event were calculated. Backward trajectories were
calculated using TrajStat (version 1.4.4R4,
http://www.meteothinker.com/products/trajstat_features.html, last visited 16 October 2015),
which is a GIS-based software, including a
trajectory calculation module of HYSPLIT (Hybrid Single Particle Lagrangian
Integrated Trajectory Model; http://www.arl.noaa.gov/ready/hysplit4.html,
last visited 16 October 2015) (Wang et al., 2009). Meteorological
data were input from the Global Data Assimilation System (GDAS) meteorological data
archives of the Air Resource Laboratory, National Oceanic and Atmospheric
Administration (NOAA) (ftp://arlftp.arlhq.noaa.gov/pub/archives/gdas1, last
visited 16 October 2015). All backward trajectories were calculated at
6 h
intervals (00:00, 06:00, 12:00, 18:00 UTC) at each sampling day, with an
arrival height of 500 m above the ground. Then, cluster analysis was
performed using the trajectories during the one year-round sampling period
at each site using TrajStat (version 1.4.4R4).
Seasonal dynamics of ion concentrations (unit:
µeqL-1) and precipitation (unit: mm) at five
remote sites in the TP. The sampling times of the five sites were as
follows: Southeast Tibet Station, November 2011 to October 2012; Nam Co
Station, August 2011 to July 2012; Qomolangma Station, April 2011 to March 2012; Ngari Station, January 2013 to December 2013; Muztagh Ata Station,
January 2011 to December 2011.
Annual average volume-weighted concentration percentages
of measured ions in precipitation (unit: µeqL-1/µeqL-1) at five
remote sites in the TP.
Results
Chemical composition of atmospheric precipitation
Figure 2 shows the seasonal dynamics of ion concentrations in precipitation
at the five remote sites in the TP. Wet deposition of all ions mainly occurs
during summer at all sites. Compared to the sites with relatively higher
precipitation amounts, e.g., Southeast Tibet Station and Nam Co Station, the
sites with relatively lower precipitation amounts had relatively higher ion
concentrations, e.g., Ngari Station and Muztagh Ata Station (Fig. 2, Table 2).
Ca2+ had the highest annual VWM concentration in precipitation at
most sites (except Nam Co Station), with the highest proportion, accounting
for measured ions of 54.6 % at Southeast Tibet Station (Figs. 2 and 3). At
Nam Co Station, NH4+ in precipitation had the highest proportion
accounting for measured ions of 39.5 %, higher than those at the other
sites (ranging from 12.9 % at Southeast Tibet Station to 18.9 % at
Muztagh Ata Station) (Fig. 3). Compared to NH4+, NO3-
had much lower proportion accounting for measured ions in precipitation,
ranging from 0.6 % at Qomolangma Station to 14 % at Nam Co Station (Fig. 3).
The order of the average annual VWM of ion deposition at the five sites
was Ca2+ >NH4+>SO42->Cl->Na+>Mg2+>NO3->K+ (Table 2). All major ion
concentrations in precipitation in the TP were much lower than those in
northern and southern China (Table 2).
Seasonal dynamics of inorganic N wet deposition at five
remote sites in the TP. The sampling time windows of those sites are the same
as in Fig. 2.
Annual mean concentrations of major ions (µeqL-1) in precipitation at five remote sites in the TP
and other sites in China. Unit of precipitation is mmyr-1. VWM
indicates volume-weighted mean.
Area
Sites
Represents
Periods
Precipitation
NH4+
Na+
K+
Mg2+
Ca2+
NO3-
Cl-
SO42-
Data type
References
Tibetan
Southeast Tibet
Remote site
2011–2012
914.6
4.9
3.8
1.0
0.9
20.8
2.2
2.5
2.1
VWM
This study
Plateau
Nam Co
Remote site
2011–2012
382.5
12.7
1.9
0.4
0.9
7.9
4.5
1.1
2.9
VWM
This study
Qomolangma
Remote site
2011–2012
258
25.4
26.0
4.5
1.7
53.4
0.8
27.1
3.2
VWM
This study
Ngari
Remote site
2013
124.6
20.5
12.4
1.8
4.8
50.9
4.8
11.9
11.6
VWM
This study
Muztagh Ata
Remote site
2011
213.6
42.0
10.1
3.0
11.3
119.4
9.9
8.9
17.4
VWM
This study
Waliguan
Remote site
1997
388
45.5
8.7
3.8
12.1
34.0
8.3
6.1
24.0
Mean
Tang et al. (2000)
Wudaoliang
Remote site
Aug 1989
266.5a
27.1
21.7
6.2
–
–
13.2
25.6
29.2
Mean
Yang et al. (1991)
Lhasa
Remote city
1998–2000
250–500
14.3
11.2
5.1
10.9
197.4
6.9
9.7
5.2
Mean
Zhang et al. (2003)
Lijiang
City
1989–2006
900
11.4
2.5
–
7.7
50.2
3.6
11.6
32.6
Mean
N. N. Zhang et al. (2012)
Average
22.6
10.9
3.2
6.3
66.8
6.0
11.6
14.3
Northern
Beijing
City
2001–2005
441
236.0
22.5
13.8
48.4
209.0
106.0
34.9
314.0
VWM
F. Yang et al. (2012)
China
Dalian
City
2007
602
107.8
36.2
6.87
25.29
78.92
51.38
59.83
168.0
VWM
X. Y. Zhang et al. (2012)
Nanjing
City
1992–2003
648–1242
193.2
23.0
12.1
31.7
295.4
39.6
142.6
241.8
VWM
Tu et al.(2005)
Tianshan Mountain
Remote site
1995–1996
–
–
55.7
14.9
15.8
78.0
22.3
40.9
88.1
Mean
Hou (2001)
Southern
Hangzhou
City
2006–2008
1435
79.9
12.2
4.2
7.1
51.9
38.4
13.9
110.0
VWM
Xu et al.(2011)
China
NingBo
City
2010–2011
1374.7
46.2
22.4
7.0
9.3
31.5
38.7
31.0
72.6
VWM
Ding et al. (2012)
Shanghai
City
2005
825.5
80.7
50.1
14.9
29.6
204.0
49.8
58.3
199.6
VWM
K. Huang et al.(2008)
Shenzhen
City
1986–2006
1769
35.2
40.3
7.2
9.7
77.7
22.1
37.9
74.3
Mean
Y. L. Huang et al.(2008)
Guiyang
City
2008–2009
1171
112.8
13.9
9.6
10.5
182.9
7.3
20.7
265.6
VMW
Xiao et al. (2013)
a Precipitation amount data were obtained from Yang et al. (2010).
Annual inorganic N wet deposition (kg N
ha-1 yr-1) at five remote
sites in the TP, as well as other sites in China. Unit of precipitation is
mmyr-1. Inorganic N is the sum of NH4+–N and
NO3-–N.
Area
Sites
Represents
Periods
Precipitation
NH4+–N
NO3-–N
Inorganic N
References
Tibetan
Southeast Tibet
Remote site
2011–2012
914.6
0.63
0.28
0.91
This study
Plateau
Nam Co
Remote site
2011–2012
382.5
0.68
0.24
0.92
This study
Qomolangma
Remote site
2011–2012
258
0.92
0.03
0.94
This study
Ngari
Remote site
2013
124.6
0.36
0.08
0.44
This study
Muztagh Ata
Remote site
2011
213.6
1.25
0.30
1.55
This study
Waliguan
Remote site
1997
388
2.47
0.45
2.92
Tang et al. (2000)
Wudaoliang
Remote site
Aug 1989
266.5a
1.01
0.49
1.50
Yang et al. (1991)
Lhasa
Remote city
1998–2000
250–500b
0.75
0.36
1.11
Zhang et al. (2003)
Naidong
Remote city
2006–2007
451
0.91
0.82
1.72
Jia (2008)
Biru
Remote city
2006–2007
582
1.22
1.86
3.08
Jia (2008)
Jiangda
Remote city
2006–2007
547
1.11
0.80
1.91
Jia (2008)
Lijiang
Remote city
1989–2006
900
1.43
0.46
1.89
N. N. Zhang et al. (2012)
Average
1.06
0.51
1.58
Northern
Beijing
City
2001–2005
441
14.57
6.54
21.12
F. Yang et al. (2012)
China
Dalian
City
2007
602
9.08
4.33
13.41
X. Y. Zhang et al. (2012)
Nanjing
City
1992–2003
648–1242c
25.56
5.24
30.80
Tu et al. (2005)
Beijing
City
2008–2010
572
–
–
27.9
Pan et al. (2012)
Tianjin
City
2008–2010
544
–
–
18.1
Pan et al. (2012)
Baoding
Industrial
2008–2010
513
–
–
23.1
Pan et al. (2012)
Tanggu
Industrial
2008–2010
566
–
–
28.2
Pan et al. (2012)
Tangshan
Industrial
2008–2010
610
–
–
21.6
Pan et al. (2012)
Yangfang
Suburban
2008–2010
404
–
–
20.7
Pan et al. (2012)
Cangzhou
Suburban
2008–2010
605
–
–
22.6
Pan et al. (2012)
Luancheng
Agricultural
2008–2010
517
–
–
22.2
Pan et al. (2012)
Yucheng
Agricultural
2008–2010
566
–
–
24.8
Pan et al. (2012)
Xinglong
Rural
2008–2010
512
–
–
16.3
Pan et al. (2012)
Southern
Tieshanping
Remote site
1999–2004
1228
25.50
9.80
35.30
Chen and Mulder (2007)
China
Luchongguan
Remote site
1999–2004
854
2.40
1.30
3.70
Chen and Mulder (2007)
Leigongshan
Remote site
1999–2004
1714
3.70
2.60
6.30
Chen and Mulder (2007)
Caijiatang
Remote site
1999–2004
1232
21.10
12.70
33.80
Chen and Mulder (2007)
Liuxihe
Remote site
1999–2004
1620
4.30
7.50
11.80
Chen and Mulder (2007)
Hangzhou
City
2006–2008
1435
16.1
7.7
23.77
Xu et al. (2011)
Ningbo
City
2010–2011
1374.7
8.9
7.4
16.34
Ding et al. (2012)
Shanghai
City
2005
825.5
9.3
5.8
15.08
K. Huang et al. (2008)
Shenzhen
City
1986–2006
1769
8.7
5.5
14.19
Y. L. Huang et al. (2008)
Guiyang
City
2008–2009
1171
18.5
1.2
19.69
Xiao et al. (2013)
Notes: Tang et al. (2000), Yang et al. (1991), Zhang et al. (2003), N. N. Zhang et al. (2012), Tu et al. (2005), F. Yang et al. (2012), X. Y. Zhang et al. (2012), Xu et
al. (2011), Ding et al. (2012), K. Huang et al. (2008), Y. L. Huang et al. (2008) and Xiao et al. (2013) reported the concentrations of
NH4+–N and NO3-–N in precipitation but did not
calculate N wet deposition. For these previous studies, we
recalculated the annual inorganic N wet deposition according to the
reported concentrations of NH4+–N and NO3-–N in
precipitation and annual precipitation. a Precipitation amount data
were obtained from Yang et al. (2010). b The mean value of 375 was used to
recalculate inorganic N wet deposition. c The mean value of 945 was used to recalculate inorganic N wet deposition.
Enrichment factors (EFs) relative to seawater and soil for
precipitation constituents of five remote sites in the TP.
Southeast Tibet Station
Nam Co Station
Qomolangma Station
Ngari Station
Muztagh Ata Station
[X/Na+]sea
[X/Ca2+]soil
EFsea
EFsoil
EFsea
EFsoil
EFsea
EFsoil
EFsea
EFsoil
EFsea
EFsoil
Na+
1.0
0.36
1.0
0.48
1.0
0.99
1.0
0.5
1.0
0.17
1.0000
0.5690
NH4+
15 707
350
80 684
2378
11 629
701
19 706
594
49 751
519
0.0001a
0.0006c
K+
11.5
0.18
8.5
0.17
7.83
0.33
6.4
0.13
13.5
0.10
0.0220
0.5040
Mg2+
1.1
0.05
2.0
0.12
0.29
0.03
1.7
0.10
4.9
0.10
0.2270
0.5610
Ca2+
126.2
1.0
95.3
1.0
46.6
1.0
93.1
1.0
269.5
1.0
0.0440
1.0000
Cl-
0.57
67.3
0.50
77.8
0.90
286
0.8
132
0.76
42.2
1.1600
0.0031
NO3-
23 842
154
98 242
840
1237
21.7
15 869
139
40 619
123
0.0000b
0.0021d
SO42-
4.7
13.0
12.7
46.7
1.03
7.76
7.7
29.2
14.3
18.6
0.1210
0.0188e
a Marine N ions were regarded as entire NH4+.
b Marine N ions were regarded as entire NO3-.
c Soil N was regarded as entire range of NH3 compounds.
d Soil N was regarded as entire range of NO3 compounds.
e Soil sulfur was regarded as entire range of SO4 compounds.
Source contributions (%) for major ions in precipitation
of five remote sites in the TP. SSF indicates sea salt fraction; CF
indicates crust fraction; AF indicates anthropogenic
fraction. Boldfaced values indicate the major contribution for each ion at each site.
Southeast Tibet Station
Nam Co Station
Qomolangma Station
Ngari Station
Muztagh Ata Station
SSF
CF
AF
SSF
CF
AF
SSF
CF
AF
SSF
CF
AF
SSF
CF
AF
NH4+
0.0
0.3
99.7
0.0
0.0
100
0.0
0.1
99.8
0.0
0.2
99.8
0.0
0.2
99.8
NO3-
0.0
0.6
99.3
0.0
0.1
99.9
0.1
4.6
95.3
0.0
0.7
99.3
0.0
0.8
99.2
SO42-
21.3
7.7
71.0
7.9
2.1
90.0
87.1
12.9
12.9
3.4
83.7
7.0
5.4
87.6
Ca2+
0.8
99.2
1.0
99.0
2.1
97.9
1.1
98.9
0.4
99.6
K+
8.7
91.3
11.8
88.2
12.8
87.2
15.5
84.5
7.4
92.6
Mg2+
92.9
7.1
49.0
51.0
0
100
59.2
40.8
20.2
79.8
Cl-
98.5
1.5
98.7
1.3
99.7
0.3
99.2
0.8
97.6
2.4
Na+
100
100
100
100
100
Varimax-rotated principal component analysis of major ions in
precipitation at five remote sites in the TP. PC1, PC2 and PC3 indicate the
first, second and thirrd component, respectively. CT means communality. N indicates
the number of precipitation samples at each site. Boldfaced values are the
largest value among the three components for each ion at each site.
Southeast Tibet Station
Nam Co Station
Qomolangma Station
Ngari Station
Muztagh Ata Station
(N=53)
(N=27)
(N=30)
(N=39)
(N=19)
PC1
PC2
PC3
CT
PC1
PC2
PC3
CT
PC1
PC2
PC3
CT
PC1
PC2
PC3
CT
PC1
PC2
PC3
CT
Na+
0.94
0.02
0.24
0.94
0.62
0.73
0.00
0.93
0.91
0.28
0.10
0.92
0.62
0.73
0.06
0.92
0.22
0.96
0.08
0.98
NH4+
0.25
0.22
0.93
0.98
0.36
0.13
0.89
0.94
0.69
-0.15
0.24
0.56
-0.11
0.57
0.75
0.90
-0.19
0.14
0.96
0.98
K+
0.88
0.10
0.35
0.91
0.11
0.93
0.07
0.89
0.84
0.35
0.27
0.89
0.15
0.84
0.40
0.89
0.47
0.79
-0.05
0.85
Mg2+
0.77
0.45
0.29
0.89
0.89
0.19
0.38
0.97
0.39
0.88
0.18
0.97
0.67
0.56
0.09
0.78
0.90
0.38
-0.16
0.99
Ca2+
0.66
0.46
0.21
0.70
0.76
0.11
0.57
0.92
-0.02
0.96
0.03
0.92
0.85
0.17
0.15
0.77
0.85
0.35
-0.25
0.90
Cl-
0.91
0.10
-0.06
0.85
0.05
0.95
0.20
0.95
0.92
0.25
0.11
0.92
0.37
0.83
0.00
0.82
0.25
0.92
0.17
0.94
NO3-
0.03
0.95
0.09
0.91
0.32
0.13
0.92
0.96
0.26
0.16
0.94
0.98
0.36
-0.01
0.84
0.84
0.88
0.12
-0.18
0.83
SO42-
0.24
0.89
0.18
0.89
0.80
0.10
0.52
0.92
0.64
0.64
0.31
0.90
0.91
0.22
0.15
0.90
0.87
0.31
0.11
0.87
Variance (%)
46
27
15
33
30
30
43
30
15
34
33
19
43
35
14
Cumulative (%)
46
73
88
33
63
93
43
74
88
34
66
85
43
78
92
Wet deposition of atmospheric inorganic N
At Southeast Tibet Station, Nam Co Station, Qomolangma Station, Ngari
Station, and Muztagh Ata Station, the NH4+–N wet deposition was
0.63, 0.68, 0.92, 0.36, and 1.25 kgNha-1yr-1, respectively; the
NO3-–N wet deposition was 0.28, 0.24, 0.03, 0.08, and 0.30 kgNha-1yr-1, respectively; and the inorganic N wet deposition was
0.91, 0.92, 0.94, 0.44 and 1.55 kgNha-1yr-1, respectively
(Table 3). Besides the above five sites of the TORP network, previous site-scale
in situ measurements of inorganic N wet deposition at other sites in the TP were
also collected, e.g., at Waliguan (Tang et al., 2000), Wudaoliang (Yang et
al., 1991), Lhasa (Zhang et al., 2003), Naidong (Jia, 2008), Biru (Jia,
2008), Jiangda (Jia, 2008), and Lijiang (N. N. Zhang et al., 2012).
Combining the site-scale in situ measurements in our study and those in previous
studies, the average wet deposition of atmospheric NH4+–N,
NO3-–N, and inorganic N in the TP was estimated to be 1.06, 0.51, and 1.58 kgNha-1yr-1, respectively, and the estimated
NH4+-N:NO3-–N ratio in precipitation in the TP was
approximately 2:1. Both NH4+–N and NO3-–N wet deposition
in the TP were much lower than those in northern and southern China (Table 3).
Seasonal dynamics of inorganic N wet deposition
The inorganic N wet deposition mainly occurred in the form of
NH4+–N during summer at all sites (Fig. 4). Both
concentrations of NH4+ and NO3- did not exhibit any
clear seasonal pattern (Fig. 2). The seasonal dynamics of inorganic N wet
deposition at most stations appeared in the shape of a single peak type (Fig. 4).
The seasonal patterns of inorganic N wet deposition were similar to the
seasonal patterns of precipitation, rather than those of NH4+ or
NO3- concentration (Fig. 2).
Source assessment of wet deposition of inorganic N and other
ions
Enrichment factors
Table 4 shows the EFs of precipitation constituents at the five sites
relative to seawater and soil. If the EF value of an ion in precipitation is
much higher (lower) than 1, the ion is considered to be enriched (diluted)
relative to the reference source. Among the five sites, Cl- had a
relatively lower EFsea value, ranging from 0.50 (Nam Co Station) to
0.90 (Qomolangma Station), but a relatively higher EFsoil value,
ranging from 42.4 (Muztagh Ata Station) to 286 (Qomolangma Station).
Different from Cl-, NH4+ in precipitation was enriched
relative to both marine origin and soil reference source at all sites,
because its EFsea values ranged from 11 629 to 80 684, and its
EFsoil ranged from 350 to 2378. Similar to NH4+,
NO3- also had a relatively high value of both EFsea and
EFsoil at all five sites.
Table 5 shows the source contributions for major ions in precipitation of
the five remote sites in this study. Almost all Cl- and Na+ in
precipitation in the TP appeared to be of marine origin, with SSF value
above 95 % at the five sites. Nearly all Ca2+ in precipitation came
from crust at the five sites, with the CF value being above 90 %. Across
the five sites, anthropogenic sources contributed at least 99 % of
NH4+ in precipitation. NO3- in precipitation was also
mainly influenced by anthropogenic activities, with AF values ranging from
95.3 to 99.9 %.
Principal component analysis
Table 6 shows the first, second and third component of principal component
analysis, which accounted for at least 85 % of the total variance across
the five sites. Na+ and Cl- were mainly explained by the same
component at all sites. Principal component analysis shows that the
variances of Ca2+ and Na+ were represented by different components
at four of five sites (except Southeast Tibet Station) (Table 6). The common
variance of Ca2+, Mg2+ and SO42- as the first component
represents the largest proportion of the total species variation at the three
sites (Nam Co Station, Ngari Station, and Muztagh Ata Station) in the
central and western TP (Table 6). At Qomolangma Station, Na+, Cl-,
K+, SO42- and NH4+ as the first component represents the largest
proportion of the total species variation (Table 6). Except for Qomolangma
Station, at the other four sites, the variances of NH4+ were
mainly represented by the third component (Table 6). At Southeast Tibet Station,
both Ca2+ and Na+ variances were mostly represented by the first
component, but NO3- variances were mainly represented by the second
component (Table 6). At Nam Co Station, Qomolangma Station, and Ngari
Station, NO3- variances were mainly represented by the third
component, which were different from that of both Ca2+ and Na+
(Table 6). However, NO3- variances were mainly represented by the
first component at Muztagh Ata Station (Table 6).
Seven-day backward trajectories at five remote sites in
the TP. Black lines show the backward trajectories calculated at 6 h
intervals (00:00, 06:00, 12:00, 18:00 UTC) on sampling days, with an arrival
height of 500 m above the ground. Red lines show the clustering
trajectories.
Backward trajectory analysis
Figure 5 shows the 7-day backward trajectories of air mass arriving at the
five remote sites at the sampling days. The transport pathways of air masses
were various with the different sites (Fig. 5). The cluster trajectory
results showed that at Muztagh Ata Station nearly all air masses on
sampling days were transported from central Asia and the Middle East (Fig. 5a).
Different from Muztagh Ata Station, almost all air masses at Nam Co Station
were transported from south Asia (Fig. 5d). For Ngari Station, Qomolangma
Station, and Southeast Tibet Station, the air masses on sampling days were
mainly transported from south Asia, with the proportion of 90, 79.8,
and 90.6 %, respectively (Fig. 5b, c, and e). Besides south Asia, central Asia,
Qaidam Basin, and the Middle East were the second source of air masses on
sampling days for Ngari Station, Qomolangma Station, and Southeast Tibet
Station, respectively (Fig. 5b, c, and e).
Discussion
Wet deposition of atmospheric inorganic N in the TP
According to our previous field observations, wet deposition of atmospheric inorganic
N in the western TP was lower than that in the eastern TP. For example, the
rates of inorganic N wet deposition at Ngari Station and Muztagh Ata Station
were 0.44 and 1.55 kgNha-1yr-1, respectively. These inorganic N
wet deposition in the western TP were much lower than those at the sites in
the eastern TP, e.g., Jiangda (1.91 kg N ha-1 yr-1), Lijiang (1.89 kg N ha-1 yr-1) and Waliguan (2.92 kg N ha-1 yr-1)
(Table 3). However, the concentrations of inorganic N in precipitation at
the sites in the western TP were comparable to those at the sites in the
eastern TP. For instance, the annual average concentrations of
NH4+ in precipitation at Ngari Station and Muztagh Ata Station
were 20.5 and 42.0 µeqL-1, respectively, which were even higher
than those at stations in the eastern TP, e.g., Lijiang (11.4 µeqL-1)
and Lhasa (14.3 µeqL-1) (Table 2). Meanwhile,
compared to Lijiang and Lhasa, Ngari Station and Muztagh Ata had lower
annual precipitation rates of 124.6 and 213.6 mmyr-1 (Table 2).
Therefore, compared to the eastern TP, the western TP had relatively lower
inorganic N deposition, probably due to its lower precipitation amount
rather than its comparable inorganic N concentration in precipitation.
Wet deposition of inorganic N for the entire TP was much lower than that in
northern and southern China (Table 3). The average wet deposition of
atmospheric inorganic N (sum of NH4+–N and NO3-–N)
in
the TP was estimated to be 1.58 kgNha-1yr-1. This was much
lower than the inorganic N wet deposition at the cities in both northern and
southern China, e.g., Beijing, Tianjin, Tangshan, Dalian, Nanjing, Hangzhou,
Ningbo, Shanghai, Shenzhen, and Guiyang (Table 3). Moreover, the inorganic N
wet deposition in the TP was also lower than that in the forest ecosystems
of eastern China, e.g., Tieshanping, Luchongguan, Leigongshan, Caijiatang, and Liuxihe (Table 3). Overall, compared to eastern China, the TP had
relatively lower inorganic N wet deposition, probably for the following two
reasons. Firstly, except for Southeast Tibet Station and Lijiang, most N
observation sites in the TP are located in typical arid and semi-arid
regions, with annual precipitation ranging from 124.6 mmyr-1 at Ngari
Station to 582 mmyr-1 at Biru. Compared to this, annual precipitation
rates at sites in eastern China are much higher, particularly in southern
China, where annual precipitation ranges from 825.5 mmyr-1 at Shanghai
to 1769 mmyr-1 at Shenzhen (Table 3). Secondly, the average annual
concentration of inorganic N (NH4+–N and/or NO3-–N) in
precipitation in the TP was much lower than that in eastern China,
especially in cities in northern China (Table 2). This is probably because
the effects of anthropogenic activities in eastern China are much more
intense than those in the TP, which has an average altitude exceeding 4000 m
above sea level and is referred to as “the third pole” (Qiu, 2008;
Yao et al., 2012).
Source assessment of atmospheric inorganic N wet deposition in
the TP
To analyze the source contributions of major ion wet deposition, EF was
applied using Na and Ca as a reference element for seawater and continental
crust, respectively. Here, Na and Ca in precipitation in the TP
were hypothesized as mainly coming from seawater and continental crust, respectively.
This assumption was partly confirmed by the results of principal component
analysis in this study (Table 6). Principal component analysis shows that
the variances of Ca2+ and Na+ were represented by different
components at four of five sites (except Southeast Tibet Station),
indicating a different source of Ca2+ and Na+ in precipitation in
the TP (Table 6). Moreover, Na+ and Cl- were mainly explained by
the same component at all sites. This indicates that Na+ and Cl-
were likely contributed by the same source: sea salt (Table 6). This
assumption was also confirmed by the relatively high Pearson correlation
between Na+ and Cl- at all five sites (Table S1 in the
Supplement). At Southeast Tibet Station, both Ca2+ and
Na+ variances were mostly represented by the first component (Table 6).
This probably because south Asia is also an important source of dust
aerosols in the southeastern TP during the during the monsoon period (Zhao
et al., 2013).
EF analysis results showed that, at all the five sites, both NH4+
and NO3- in precipitation were mainly contributed by anthropogenic
sources (Table 5). This was also confirmed by principal component analysis.
Different with Ca2+ and Na+, NH4+ variances were mainly
represented by the third component at four of five sites (except for Qomolangma
Station) (Table 6). Except for Muztagh Ata Station, at the other four
stations, NO3- variances were also represented by a different
component than that of Ca2+ and Na+ variances (Table 6). This
indicates that the source of inorganic N wet deposition was probably
different with the sources of Ca2+ or Na+ wet deposition.
Meanwhile, at all five sites, Na and Ca mainly came from seawater and
continental crust, respectively. Therefore, inorganic N wet deposition at
the five sites in the TP was mainly influenced by anthropogenic activities.
We applied backward trajectory analysis to identify the long-range transport
of atmospheric inorganic N wet deposition at the five sites in the TP (Fig. 5).
There is large spatial heterogeneity of air mass transport pathways
across the five sites. At Muztagh Ata Station, wet deposition was mainly
transported from central Asia and the Middle East (Fig. 5a). This is probably
because Muztagh Ata Station is located in the northwestern TP, which is
almost completely controlled by westerlies rather than the Indian monsoon (Yao
et al., 2013). Thus, anthropogenic activities in central Asia and the Middle
East are the principal source of the inorganic N wet deposition in the
northwestern TP. Except for Muztagh Ata Station, inorganic N wet deposition
at the other four sites was probably transported by Indian monsoon (Fig. 5b–e). At Ngari Station, 90.0 % of wet deposition was transported from
Nepal and northern India via the Indian monsoon, and 10.0 % of wet deposition
came from central Asia and Qaidam Basin via westerlies (Fig. 5b). At
Qomolangma Station and Nam Co Station, inorganic N wet deposition was mainly
influenced by the anthropogenic activities in northeastern India and
Bangladesh (Fig. 5c and d). At Southeast Tibet Station, 90.6 % of wet
deposition was transported from India, Bangladesh and Myanmar by Indian
monsoon, and the other 9.4 % came from the western TP and the Middle East
(Fig. 5e). Therefore, inorganic N wet deposition at these four stations
principally was influenced by the anthropogenic N emissions in south Asia
(e.g., India). Actually, after China and the United States, India has been the third
largest producer and consumer of fertilizers due to intensification of
agriculture, resulting in high anthropogenic N emissions (Aneja et al.,
2012). For instance, ammonia (NH3) emissions from livestock and
fertilizer applications in India in 2003 was estimated as
1705
and 1697 Ggyr-1 (Gg = 109 g), respectively (Aneja et al., 2012). Moreover, in
India, field burning of crop residue (FBCR) is another critical
anthropogenic activity leading to N emissions. In 2010, 6300 Gg of dry
biomass are estimated to have been subjected to FBCR in India, resulting in
350 Gg
N emissions (Sahai et al., 2011). Besides the Indian monsoon, biomass-burning
emissions in south Asia could be across the Himalayas and transported to the
TP by the mountain–valley wind (Cong et al., 2015).
Comparison of inorganic N wet deposition in the TP with previous
estimations
Long-term data set series of N deposition have been established based on
observations (Lu and Tian, 2007, 2014, 2015) or model simulations (Dentener
et al., 2006). These data sets have been used to estimate global or regional
N deposition (Dentener et al., 2006; Lu and Tian, 2007) and drive ecosystem
models to examine the ecological effects of elevated N deposition (Lu and
Tian, 2013). Thus, reliable N deposition data sets are prerequisites for N
deposition estimation or driving ecosystem models. Here, the estimation of N
wet deposition in the TP based on our field observations is compared with
previous estimations via limited observations or simulations.
Lu and Tian (2007) estimated the inorganic N wet deposition as ranging from
4.16 kgNha-1yr-1 in the Tibet Autonomous Region (in the western TP)
to 4.76 kgNha-1yr-1 in Qinghai Province (in the eastern TP).
Recently, Jia et al. (2014) estimated the inorganic N wet deposition during
the 2000s as ranging from 6.11 kgNha-1yr-1 in the Tibet Autonomous
Region to 7.87 kgNha-1yr-1 in Qinghai Province. Those
estimations were even much higher than the highest record of inorganic N wet
deposition observations in the TP (3.08 kgNha-1yr-1 at Biru
during 2006–2007) (Table 3). In this study, combing in situ measurements at five sites
in this study and seven sites in previous studies (Table 2), the average wet
deposition of atmospheric NH4+–N, NO3-–N, and inorganic
N in the TP were estimated to be 1.06, 0.51, and 1.58 kgNha-1yr-1, respectively. According to our study, both Lu and Tian (2007) and
Jia et al. (2014) highly overestimated inorganic N wet deposition in the TP,
likely for the following two reasons. Firstly, compared to our study,
previous regional-scale estimations used far fewer in situ measurement sites. For
example, there were only four sites in the Tibet Autonomous Region and one site
in Qinghai Province used in the estimation of Jia et al. (2014). Such
limited field observations probably led to large uncertainty in the
conclusions drawn regarding inorganic N wet deposition in the entire TP.
Secondly, the kriging interpolation technique was used in both Lu and Tian
(2007) and Jia et al. (2014) to estimate the spatial pattern of inorganic N
wet deposition in China. However, observation sites are sparsely distributed
in the TP, and the estimation of inorganic N wet deposition in the TP is largely
influenced by N deposition observations in the surrounding regions of much
lower altitude. The average altitude of the TP is above 4000 m, where both
the climate and anthropogenic activities are substantially different with
those in lower-altitude areas. For example, the average inorganic N wet
deposition was 1.58 kgNha-1yr-1, which was much lower than that
in northern and southern China (Table 3). The interpolations at the national
scale in Lu and Tian (2007) and Jia et al. (2014) likely overestimated the
regional inorganic N wet deposition in the TP. In addition, we also
estimated the inorganic N wet deposition for the entire TP using kriging
interpolation, but only based on the site-scale in situ measurements in the TP (12
sites, including 5 sites in this study and 7 sites in previous field
observations), rather than the observations in the surrounding regions of
much lower altitude. The inorganic N wet deposition for the entire TP
estimation based on the kriging interpolation in our study is 1.56 kgNha-1yr-1 (Fig. S1 and spatial data as a NetCDF file in the
Supplement), which is much lower than that in previous
interpolation studies (Lu and Tian, 2007; Jia et al., 2014) but is
comparable with the averaged inorganic N wet deposition among the 12 sites
(1.58 kg N ha-1 yr-1) (Table 2).
Atmospheric chemistry transport models are commonly used to calculate
current and future N deposition. Dentener et al. (2006) used 23 atmospheric
chemistry transport models to assess both global and regional N deposition.
Compared to observation records, Dentener et al. (2006) underestimated
inorganic N wet deposition over the whole of China (Lu and Tian, 2007), but
overestimated it over the TP. According to Dentener et al. (2006), the
NH4+–N, NO3-–N, and inorganic N wet deposition in the TP are
1.97, 0.99, and 2.96 kgNha-1yr-1, respectively – nearly double
that of N deposition estimated in our study. Based on site-scale in situ
measurements, we provide a more accurate regional-scale estimation of
inorganic N wet deposition in the TP, which can be used as background
information in studies focusing on the responses of alpine ecosystems to
elevated N deposition. Besides assessment of N deposition, N deposition
simulated by atmospheric chemistry transport models is usually used to drive
large-scale ecosystem models for integrated ecosystem assessment (Xu-Ri et
al., 2012; Zaehle, 2013). The ecological effects of N addition are probably
influenced not only by the quantity of N deposition but also by the
proportions of each component, e.g., the
NH4+-N:NO3--N
ratio. For example, in African savannas, plants demonstrate N uptake
preference, which is likely influenced by the
NH4+-N:NO3--N ratio in their native habitats (Wang and
Macko, 2011). However, in most current N fertilization experiments, the N
forms of fertilizer are NH4NO3, NH4+–N or
NO3-–N (Liu and Greaver, 2009), with the
NH4+–N : NO3-–N ratio of N wet deposition at experimental
sites not considered. Our work shows that the estimated
NH4+–N : NO3-–N ratio of inorganic N wet deposition in the
TP is approximately 2:1, which is consistent with the modeled estimation of
Dentener et al. (2006), but lower than the NH4+–N : NO3-–N
ratio of 2.5 in forest ecosystems in eastern China (Du et al., 2014). This
NH4+–N : NO3-–N ratio (2:1) is recommended to be
considered when N fertilization experiments are conducted in alpine
ecosystems in the TP.
Uncertainty and recommendations
Combining our in situ measurements at five remote sites and previous site-scale
field observations, the inorganic N wet deposition in the TP was
quantitatively assessed in this study. The assessment is conducive to
accurately estimating N wet deposition for the entire nation of China, and
it provides background information of N wet deposition for the studies focusing
on the alpine ecological effects of elevated N deposition. Despite this,
there are uncertainties in the estimation of N deposition in the TP for the
following reasons. Firstly, total N deposition comprises wet deposition (in
the form of precipitation) and dry deposition (in the form of gases and
particles). Considering the whole of China, dry deposition contributes
30 % to total inorganic N deposition (Lu and Tian, 2007, 2014, 2015). In
northern China, this ratio is much higher, at 60 % (Pan et al., 2012).
However, in this study, we only estimated the inorganic N wet deposition in
the TP, with the situation regarding dry deposition remaining unclear. Thus,
investigation of N dry deposition is critical for assessing total N
deposition in the TP. Secondly, the TP covers an area of about 2.57 million km2,
occupying approximately one-fourth of the land area of China (Zhang et
al., 2002). Precipitation in the TP is influenced by both the Indian monsoon
and westerlies, leading to spatial variation in the origins of N wet
deposition. Therefore, it is necessary to establish N wet deposition
observation sites in different climatic zones. Thirdly, besides spatial
heterogeneity, N deposition in the TP also possesses temporal heterogeneity.
Inorganic N wet deposition in the TP has increased during recent decades, as
recorded in ice cores (Hou et al., 2003; Kang et al., 2002a, b; Thompson et
al., 2000; Zhao et al., 2011; Zheng et al., 2010) and sediment cores of alpine
lakes (Choudhary et al., 2013; Hu et al., 2014). The long-term trend and
interannual variability of inorganic N wet deposition in the TP cannot be
quantitatively characterized by the short-term in situ measurements in this study.
Overall, critical questions remain open regarding the quantitative
understanding of N deposition in the TP. To deepen our understanding of N
deposition in the TP, it is essential to perform long-term in situ measurements of
N wet and dry deposition in various climate zones in the future.
Conclusions
Alpine ecosystems in the TP are sensitive to elevated N deposition, and the
inorganic N deposition has been increasing since the mid-20th century.
However, the amount of inorganic N wet deposition in the TP remains unclear,
due to a paucity of in situ measurement. In this study, using stations in the TORP
network, we conducted in situ measurements of major ion wet deposition at five
remote sites, situated mainly in the central and western TP. Among the five
sites, both NH4+–N and NO3-–N were mainly contributed by
anthropogenic sources. Combining site-scale in situ measurements in our study and
previous studies, the average wet deposition of atmospheric
NH4+–N, NO3-–N, and inorganic N in the TP is estimated
to be 1.06, 0.51, and 1.58 kgNha-1yr-1, respectively.
Considering the entire TP, according to our results, previous regional-scale
assessment has highly overestimated inorganic N wet deposition, either
through simulations with atmospheric chemistry transport models (Dentener et
al., 2006) or interpolations based on limited field observations for the
whole of China (Jia et al., 2014; Lu and Tian, 2007). The
NH4+–N : NO3-–N ratio in precipitation in the TP was found
to be approximately 2:1, which is consistent with model simulations
(Dentener et al., 2006). To clarify the total N deposition in the TP more
clearly, we recommend conducting long-term monitoring of both wet and dry
deposition of N in various climate zones in the future work.