inorganic nitrogen deposition in china's forests: status and characteristics
TRANSCRIPT
Accepted Manuscript
Inorganic nitrogen deposition in China’s forests: status and characteristics
Enzai Du, Prof. Dr. Yuan Jiang, Jingyun Fang, Wim de Vries
PII: S1352-2310(14)00697-9
DOI: 10.1016/j.atmosenv.2014.09.005
Reference: AEA 13236
To appear in: Atmospheric Environment
Received Date: 15 June 2014
Revised Date: 28 August 2014
Accepted Date: 3 September 2014
Please cite this article as: Du, E., Jiang, Y., Fang, J., de Vries, W., Inorganic nitrogen depositionin China’s forests: status and characteristics, Atmospheric Environment (2014), doi: 10.1016/j.atmosenv.2014.09.005.
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Highlights
1. Ammonium and nitrate deposition was assessed in China’s forests
2. Ammonium contributed 2.5 folds of nitrate to nitrogen deposition
3. Canopy captured dry deposition accounted for half of bulk deposition
4. Mean throughfall nitrogen deposition was high at 21.5 kg N ha-1 yr-1
5. Patterns of nitrogen deposition support an urban hotspot hypothesis
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Inorganic nitrogen deposition in China’s forests: status and characteristics 1
2
Enzai Du1,2, Yuan Jiang1*, Jingyun Fang2, Wim de Vries3 3
4
1. State Key Laboratory of Earth Surface Processes and Resource Ecology, and 5
College of Resources Science & Technology, Beijing Normal University, 6
Xinjiekouwai Street 19#, Beijing, 100875, China. 7
2. Department of Ecology, College of Urban and Environmental Sciences, and Key 8
Laboratory for Earth Surface Processes of the Ministry of Education, Peking 9
University, Yiheyuan Road 5#, Beijing, 100871, China. 10
3. Environmental Systems Analysis Group, Wageningen University, PO Box 47, 6700 11
AA Wageningen, The Netherlands 12
13
Corresponding Author: Prof. Dr. Yuan Jiang, State Key Laboratory of Earth Surface 14
Processes and Resource Ecology, and College of Resources Science & Technology, 15
Beijing Normal University, Xinjiekouwai Street 19#, Beijing, 100875, China. Tel: 16
+8610-58806093, Fax: +8610-58809274, Email: [email protected] 17
18
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Abstract 19
Nitrogen (N) deposition in China has been dramatically enhanced by anthropogenic 20
emissions and has aroused great concerns of its impacts on forest ecosystems. This 21
study synthesized data on ammonium (NH4+) and nitrate (NO3
-) contents in bulk 22
precipitation and throughfall from 38 forest stands in published literature to assess the 23
status and characteristics of N deposition to typical forests in China between 1995 and 24
2010. Our results showed that ammonium dominated N deposition in this period, with 25
a mean NH4+-N:NO3
--N ratio of ~ 2.5 in bulk deposition and throughfall. Mean 26
throughfall N deposition in China’s forests was as high as 14.0 kg N ha-1 yr-1 for 27
ammonium, 5.5 kg N ha-1 yr-1 for nitrate and 21.5 kg N ha-1 yr-1 for total inorganic N 28
(TIN), respectively. Mean bulk deposition was 9.4 kg N ha-1 yr-1 for ammonium, 3.9 29
kg N ha-1 yr-1 for nitrate and 14.0 kg N ha-1 yr-1 for TIN, respectively. Canopy 30
captured dry deposition, calculated as the difference between throughfall and bulk 31
deposition, was thus approximately half of the bulk deposition. Spatial patterns of N 32
deposition were in accordance with our urban hotspot hypothesis, showing a strong 33
power-law reduction of ammonium with increasing distance to large cities but only 34
slightly lower nitrate deposition. Our results suggest that high N deposition, especially 35
of ammonium, exceeds critical N loads for large areas of China’s forests. 36
37
Key words: nitrogen deposition; ammonium: nitrate ratio; throughfall; enrichment 38
ratio; dry deposition; urban hotspot hypothesis 39
40
1. Introduction 41
Global anthropogenic nitrogen (N) emissions have dramatically enhanced N 42
deposition since the industrial revolution (Galloway et al., 2004). In China, 43
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acceleration of N emissions has substantially increased N deposition since 1980s and 44
leaded to concerns about the impacts on various ecosystems (Liu et al., 2010; Cui et 45
al., 2013). Enhanced N deposition often causes a stimulating effect on forest growth 46
and related carbon sequestration (Högberg, 2007; De Vries et al., 2009; Thomas et al., 47
2010), but excessive N input may lead to many adverse impacts, including 48
biodiversity loss, soil acidification and nutrient imbalances (Erisman and De Vries, 49
2000; Bobbink et al., 2010; Greaver et al., 2012). Nitrogen deposition includes both 50
inorganic and organic N forms, but in most cases only inorganic N has been measured 51
in China (Liu et al., 2013). Ammonium and nitrate are different in biological and 52
ecological effects. For instance, plants usually have a specific preference in utilizing 53
soil ammonium or nitrate (Kahmen et al., 2008). Ammonium is toxic to higher plants 54
at high concentrations (Gerendás et al., 1997) and can be more detrimental than nitrate 55
in decreasing biodiversity (Brittoa and Kronzucker, 2002; Erisman et al., 2007). 56
Therefore, it is necessary to consider the relative contribution of ammonium and 57
nitrate separately. 58
Recently, the temporal trends and spatial patterns of bulk N deposition in China 59
have been estimated using observed data (Liu et al., 2013; Jia et al., 2014). Average 60
bulk deposition of inorganic N in China has been estimated at 11.1~13.9 kg N ha-1 yr-1 61
during the last two decades (Jia et al., 2014). However, bulk deposition 62
underestimates total atmospheric N inputs because it consists mainly of wet 63
deposition. Comparisons of bulk deposition and wet-only deposition indicate that dry 64
deposition mostly accounts for 2-23 % of bulk N deposition (Kulshrestha et al., 1995; 65
Staelens et al., 2005; Chantara and Chunsuk, 2008). Throughfall is a sum of bulk 66
deposition, canopy captured dry deposition and canopy exchange (Lindberg et al., 67
1986; Lovett et al., 1996; Zimmermann et al., 2008). Nitrogen fluxes in throughfall 68
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can be used as a more precise estimate of total N deposition, although sometimes it 69
likely underestimates it due to canopy uptake (e.g Draaijers et al., 1996; Sparks, 70
2009). 71
Increase in N deposition in China is primarily caused by anthropogenic sources, 72
specifically due to power generation, traffic and agricultural activities (Liu et al., 2010; 73
Jia et al., 2014). This leads to the hypothesis that total N deposition increases near 74
large cities with a lot of anthropogenic activities, while it declines with an increasing 75
distance to large cities (urban hotspot hypothesis). In this study we tested this 76
hypothesis by synthesizing and evaluating data from published literature on 77
ammonium (NH4+) and nitrate (NO3
-) in bulk precipitation and throughfall in China’s 78
forests between 1995 and 2010. More specifically, we addressed the following 79
questions: (1) What are the characteristics for concentrations and fluxes of ammonium, 80
nitrate and total inorganic N (TIN = NH4+ + NO3
-) in bulk precipitation and 81
throughfall in China’s forests? (2) How much does canopy capture dry deposition of 82
ammonium, nitrate and TIN? (3) Do the concentrations and fluxes of ammonium, 83
nitrate and TIN in bulk precipitation and throughfall decrease similarly with 84
increasing distance to large cities? 85
86
2. Data and methods 87
88
2.1 Data sets 89
We collected data from published literatures on ammonium and nitrate concentrations 90
in bulk precipitation and throughfall for typical forests in China, as well as 91
information on site location (latitude and longitude), forest types, annual mean 92
temperature and annual precipitation (see supplementary information). Data were 93
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selected only when ammonium and nitrate in precipitation and throughfall were 94
measured simultaneously. Published concentrations of ammonium, nitrate or TIN for 95
each sampling period were either taken directly from tables or digitized from figures 96
using a GetData Graph Digitizer (Version 2.25, 97
http://www.getdata-graph-digitizer.com). The distance between the sampling site and 98
the nearest large city (nonagricultural population > 0.5 million) was derived using 99
Google Earth for Microsoft Windows (Version 7.1.2.2041, Google Inc., USA). 100
Our database consisted of 38 forest stands in China (Fig. 1, some forest stands 101
could not be distinguished due to close locations), and all observations were 102
conducted in the period between 1995 and 2010 (see supplementary information). 103
Canopy densities, defined as the forest cover as a fraction of the area, were relatively 104
convergent with a range from 0.7 to 0.9. Annual mean temperature ranged from –5 to 105
22 ˚C, and annual precipitation ranged from 500 to 2200 mm. The distance from the 106
sampling sites to the nearest large cities ranged from 5 km to 430 km. 107
2.2 Statistical analysis 108
Volume-weighted mean concentrations (VWC, mg L-1) were calculated for bulk 109
precipitation and throughfall according to equation 1, 110
∑∑ ×
=i
ii
V
V CVWC , (1) 111
where Ci (mg L-1) is the concentration of ammonium, nitrate or TIN for each sampling 112
period, and Vi (mm) is the amount of precipitation or throughfall for each sampling 113
period. Then the fluxes of ammonium, nitrate and TIN (kg N ha-1 yr-1) in the bulk 114
precipitation and throughfall were estimated according to the volume-weighted mean 115
concentration and annual precipitation. Canopy captured dry deposition was 116
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calculated as the difference in fluxes (∆Flux) of ammonium, nitrate and TIN in 117
throughfall and bulk precipitation. 118
We defined an enrichment ratio (ER) to indicate the proportional change in N 119
concentrations in throughfall versus bulk precipitation, according to equation 2, 120
c
cc
BP
BPTFRE
−= , (2) 121
where BPc is the concentration of ammonium, nitrate or TIN in bulk precipitation (mg 122
L-1) and TFc is the concentration of ammonium, nitrate or TIN in throughfall (mg L-1). 123
Values of N (ammonium, nitrate and TIN) concentrations and fluxes were 124
log-transformed to obtain a normal distribution. A Shapiro-Francia W’ test was used 125
to test the normality of the transformed data. A paired sample t-test was used to test 126
the differences of N concentrations, fluxes and NH4+-N : NO3
--N ratios between bulk 127
precipitation and throughfall, and the differences of enrichment ratios between 128
ammonium and nitrate. A linear regression model test was used to explore how 129
natural log-transformed N concentration, natural log-transformed N fluxes, 130
enrichment N ratios and canopy captured dry N deposition change with the natural 131
log-transformed distance between the sampling site and the nearest large city. 132
Applying linear regression while using logarithmic values for N variables versus 133
distance to the nearest large city tested a power-law relationship between these two 134
variables. All statistical analysis was performed using R software (version 3.1.0; R 135
Development Core Team, 2014, http://www.r-project.org/) with a significance level of 136
p < 0.05. 137
138
3. Results 139
140
3.1 Nitrogen concentrations and fluxes in bulk precipitation and throughfall 141
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Concentrations of ammonium, nitrate and TIN, and NH4+-N : NO3
--N ratios in bulk 142
precipitation and throughfall were highly skewed (Fig. 2) and were characterized by a 143
log-normal distribution (Shapiro-Wilk normality test, p > 0.10) except nitrate 144
concentrations in bulk precipitation (p < 0.05). In bulk precipitation, geometric mean 145
concentrations were 0.71 (0.11 ~ 4.02) mg N L-1 for ammonium, 0.29 (0.04 ~ 1.09) 146
mg N L-1 for nitrate and 1.06 (0.15 ~ 5.10) mg N L-1 for TIN, respectively (Figs. 2a, 147
2b & 2c). In throughfall, geometric mean concentrations were 1.06 (0.18 ~ 7.06) mg 148
N L-1 for ammonium, 0.41 (0.02 ~ 2.59) mg N L-1 for nitrate and 1.63 (0.30 ~ 9.08) 149
mg N L-1 for TIN, respectively (Figs. 2e, 2f & 2g). Ammonium dominated TIN with a 150
geometric mean NH4+-N : NO3
--N ratio of 2.42 (0.64 ~ 9.20) in bulk precipitation, and 151
a geometric mean NH4+-N : NO3
--N ratio of 2.55 (0.42 ~ 44.5) in throughfall, 152
respectively (Figs. 2d & 2h). 153
Fluxes of ammonium, nitrate and TIN in bulk precipitation and throughfall were 154
also characterized by a log-normal distribution (Fig. 3; Shapiro-Wilk normality test, 155
p > 0.10). The geometric means of bulk deposition were 9.4 (1.9 ~ 46.7) kg N ha-1 yr-1 156
for ammonium, 3.9 (0.8 ~ 16.2) kg N ha-1 yr-1 for nitrate and 14.0 (2.7 ~ 57.5) kg N 157
ha-1 yr-1 for TIN, respectively (Figs. 3a, 3b & 3c). In throughfall, geometric means of 158
N fluxes were 14.0 (2.3 ~ 95.8) kg N ha-1 yr-1 for ammonium, 5.5 (0.4 ~ 41.8) kg N 159
ha-1 yr-1 for nitrate and 21.5 (3.2 ~ 111.1) kg N ha-1 yr-1 for TIN, respectively (Figs. 3d, 160
3e & 3f). 161
162
3.2 Nitrogen enrichment in throughfall versus bulk precipitation 163
Concentrations of ammonium, nitrate and TIN were significantly enriched (paired 164
samples t-test, df = 37, p < 0.01), and NH4+-N : NO3
--N ratios were slightly increased 165
(paired samples t-test, df = 37, p = 0.05) in throughfall versus bulk precipitation. 166
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Distributions of enrichment ratios of ammonium, nitrate and TIN concentrations as 167
well as NH4+-N : NO3
--N ratios were highly skewed (Figs. 4a - 4c) and were 168
characterized by a log-normal distribution (Shapiro-Wilk normality test, p > 0.10). 169
Enrichment ratios were not significantly different for ammonium (arithmetic mean = 170
0.69, median = 0.53, ranging -0.40 ~ 4.70) and nitrate (arithmetic mean = 0.59, 171
median = 0.35, ranging -0.63 ~ 3.42) (paired sample t-test, df = 37, p = 0.30). The 172
median enrichment ratio of TIN was 0.55 (arithmetic mean = 0.68, ranging -0.22 ~ 173
4.15), indicating that the increase of TIN concentration in throughfall on average 174
accounted for approximately half of that in bulk deposition. 175
Similarly, distributions of canopy captured dry deposition of ammonium, nitrate 176
and TIN were characterized by a log-normal distribution (Fig. 4d - 4f; Shapiro-Wilk 177
normality test, p > 0.10). The medians of canopy captured dry deposition were 5.4 kg 178
N ha-1 yr-1 for ammonium (arithmetic mean = 9.7, ranging -5.8 ~ 79.0), 1.5 kg N ha-1 179
yr-1 for nitrate (arithmetic mean = 3.0, ranging -2.2 ~ 31.0) and 6.9 kg N ha-1 yr-1 for 180
TIN (arithmetic mean = 12.7, ranging -4.1 ~ 84.2), respectively. 181
182
3.3 Spatial characteristic of N concentrations, fluxes and enrichment ratios 183
Concentrations of ammonium and TIN in bulk precipitation and throughfall showed 184
a significant power-law reduction with increasing distance to the nearest large cities 185
(Figs. 5a, 5c, 5e & 5g) but the relationships were not statistical significant for nitrate 186
(Figs. 5b & 5f). The NH4+-N : NO3
--N ratios in bulk precipitation and throughfall both 187
also showed a significant power-law reduction with increasing distance to the nearest 188
large cities (Figs. 5d & 5h). 189
Fluxes of ammonium, nitrate and TIN in the bulk precipitation and throughfall all 190
showed a significant power-law reduction with increasing distance to the nearest large 191
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cities (Fig 6), but nitrate showed a very weak statistical significance (Figs. 6b & 6e). 192
Enrichment ratios of ammonium and TIN showed a significant linear decrease with 193
increasing distance to the nearest large cities (Figs. 7a and 7c), while the relationship 194
was not significant for nitrate (Fig. 7b). Similarly, canopy captured dry deposition of 195
ammonium and TIN decreased significantly with increasing distance to the nearest 196
large cities (Figs. 7d & 7f), whereas the relationship was not significant for nitrate 197
(Fig. 7e). 198
199
4. Discussion 200
201
4.1 Ammonium dominance of inorganic nitrogen 202
Our results showed that ammonium dominated TIN both in bulk precipitation and 203
throughfall in China’s forests. The range in NH4+-N : NO3
--N ratios in bulk 204
precipitation in China’s forests (0.64 ~ 9.20) is comparable to the range reported for 205
809 bulk precipitation measurements in China (Liu et al., 2013). The mean NH4+-N : 206
NO3--N ratio in throughfall (2.55) was slightly higher than in bulk precipitation (2.42). 207
There are two reasons for the dominance of ammonium in N deposition. First, NH3 208
emissions are much higher than NOx (NOx = NO + NO2) emissions in China (Liu et 209
al., 2010 & 2013). Between 1995 and 2010, the national emission ratio of NH3-N: 210
NOx-N was estimated at 2.70±0.09 (Liu et al., 2013), agreeing well with the mean 211
ratio of NH4+-N : NO3
--N in N deposition in our study. Second, wet deposition of 212
ammonium can be accelerated by high sulfur deposition in China due to enormous 213
sulfur dioxide (SO2) emissions (Quan and Zhang, 2008; Lu et al., 2010). The 214
dominance of ammonium indicates a need of regulation policy on the NH3 emissions 215
since ammonium deposition is more detrimental than nitrate in decreasing 216
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biodiversity (Brittoa and Kronzucker, 2002; Erisman et al., 2007). 217
218
4.2 Nitrogen enrichment in throughfall 219
Enrichment ratios generally showed positive values for ammonium, nitrate and TIN, 220
indicating that bulk N deposition substantially underestimate N deposition in forests 221
ecosystems. The average bulk deposition was estimated at 9.4 kg N ha-1 yr-1 for 222
ammonium, 3.9 kg N ha-1 yr-1 for nitrate, and 14.0 kg N ha-1 yr-1 for TIN. Our results 223
are comparable to national bulk deposition of inorganic N in the 2000s in China 224
reported by Jia et al. (2014) ( ~ 14 kg N-1 yr-1), but much lower than the estimate by 225
Liu et al. (2013) (~ 21 kg N-1 yr-1) which is likely an overestimate by using an 226
arithmetic average. Throughfall N deposition was substantially higher and averaged 227
14.0 kg N ha-1 yr-1 for ammonium, 5.5 kg N ha-1 yr-1 for nitrate and 21.5 kg N ha-1 yr-1 228
for TIN, respectively. Overall, bulk deposition is about 50% less than throughfall 229
deposition. 230
Uncertainties remain in the estimates of N deposition by throughfall because N 231
inputs in rainfall may be absorbed by the forest canopy (Brumme et al., 1992; Lovett 232
and Lindberg, 1993; Gessler et al., 2002). In our study, only 5 out of 38 forest stands 233
had throughfall values being lower than bulk deposition, clearly indicating net N 234
uptake by forest canopy. Considering that throughfall is an underestimate of total 235
deposition, our results show that there is a substantial enrichment of inorganic N in 236
throughfall by dry deposition, contributing to the high load of N deposition to China’s 237
forests. 238
A number of observations have suggested that changes of N contents in 239
throughfall versus bulk precipitation may vary with forest type (Crockford and 240
Richardson, 2000; De Schrijver et al., 2007; Gundersen et al., 2009). For instance, De 241
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Schrijver et al. (2007) found higher N content in throughfall of coniferous than 242
broadleaved forest stands at comparable sites, which has been attributed to differences 243
in canopy characteristics. However, in our study we did not observe differences in 244
enrichment ratio for different forest types due to confounding factors, such as soil N 245
availability and amounts of precipitation (Crockford and Khanna, 1997; Polkowska et 246
al., 2005; De Vries et al., 2007). For instance, N fertilization increased release of 247
ammonium and nitrate in throughfall in a radiata pine (Pinus radiata) forest and 248
irrigation treatment further enhanced canopy N leaching (Crockford and Khanna, 249
1997). 250
251
4.3 Urban hotspots of nitrogen deposition 252
Concentrations and fluxes of ammonium in bulk precipitation and throughfall showed 253
a power-law reduction with increasing distance to the nearest large cities. The 254
enrichment ratio and canopy captured dry deposition of ammonium both showed a 255
significant linear decrease with increasing distance to the nearest large cities. Three 256
causes might be responsible for this spatial pattern. First, large cities are NH3 emitting 257
hotspots because waste disposal, energy consumption and suburb agricultural 258
activities (mainly N-fertilizer application and livestock breeding) can lead to high 259
NH3 emissions (Anderson et al., 2003; Gu et al., 2014). Second, atmospheric NH3 has 260
a short residence time, which strongly limits the transport distance (Asman et al., 261
1998; Krupa, 2003). Third, high emissions of urban-derived SO2 can accelerate 262
ammonium deposition in urban areas, while this accelerating effect of SO2 is minor in 263
remote areas (McLeod et al., 1990; Quan and Zhang, 2008). 264
Nitrate fluxes in bulk precipitation and throughfall were higher near urban hotspots 265
with a weak statistical significance, while nitrate concentrations in precipitation and 266
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throughfall did not change with increasing distance to the nearest large cities. In 267
addition, the enrichment ratio and canopy captured dry deposition of nitrate showed 268
no significant decrease with increasing distance to the nearest large cities. This is 269
most likely due to two causes. First, atmospheric NOx can be transported over a long 270
distance in the upper troposphere with a residence time of ~ 4 days (Wenig et al., 271
2003). Second, NOx emissions from traffic and transportation greatly influences the 272
atmospheric concentrations of NOx and hence the nitrate deposition in China (Cai and 273
Xie, 2007). Well-developed road network systems including national highways, 274
provincial roads and county roads have severely fragmented the landscape in China 275
from the eastern plains to the western mountainous regions (Li et al., 2010). 276
Automobile emissions substantially increase NOx concentrations near roadside areas 277
(Redling et al., 2013), resulting in an intermixing effect of road networks on spatial 278
patterns of NOx emissions. The intermixing effects of road networks and atmospheric 279
transport of NOx emissions most likely cause only slightly higher nitrate deposition 280
near urban hotspots. 281
Spatial patterns of TIN deposition were in accordance with the urban hotspot 282
hypothesis, showing a strong power-law reduction of ammonium deposition rather 283
than nitrate deposition with increasing distance to the nearest large cities. The 284
difference in spatial patterns of ammonium and nitrate deposition also leads to a 285
significant power-law reduction in NH4+-N: NO3
--N ratio with increasing distance to 286
the nearest large cities. Consistent with our results, Liu et al. (2008) found high N 287
deposition occurred at the urban area and decreased with distance to the urban center 288
of Guiyang, China. Jia et al. (2014) also illustrated that N deposition at forests sites 289
showed a power-law decrease with increasing distance to the nearest large city and 290
suggested that a critical distance subject to urban source influences is approximately 291
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200 km. However, our results indicated that the power-law decrease of N deposition 292
occurred in a distance range of 5 ~ 430 km and did not show a critical distance of 293
urban source influences, i.e. no change with further distance (Fig. 6a, 6b and 6c). The 294
relationships between N deposition and distance from urban hotspots could be useful 295
in modelling N deposition from regional to global scales. 296
297
4.4 Possible underestimation of total nitrogen deposition 298
Most likely, the N flux in throughfall is a significant underestimation of total N 299
deposition, as it neglects the occurrence of foliar N (e.g. Lovett and Lindberg, 1993; 300
Gessler et al., 2002) and deposition of organic N compounds (e.g. Cornell et al., 2003, 301
2009; Jickels et al., 2013). Foliar uptake of ammonium, nitrate and gaseous N 302
compounds including nitrogen dioxide (NO2) and ammonia (NH3) can be substantial 303
(Lovett and Lindberg, 1993; Tomaszewski et al., 2003; Sievering et al., 2007; 304
Adriaenssens et al., 2012). For instance, Lovett and Lindberg (1993) examined bulk 305
precipitation and canopy throughfall chemistry at twelve sites across Europe and 306
North America in the early 1990s, and estimated that 41% of bulk N deposition is 307
retained by the canopy. Deposition at these sites was however low, ranging between 308
1.4 and 6.8 kg N.ha-1.yr-1 for eleven sites, and only one site had a relative high input 309
of 15.3 kg N.ha-1.yr-1. It is unlikely that N uptake rates will increase linearly at higher 310
levels of N deposition because nitrogen saturation in the canopy might be expected 311
(Eilers et al., 1997). Nevertheless, based on a canopy budget model approach, 312
described by Bredemeier (1988) and extended by Draaijers and Erisman (1995), De 313
Vries et al. (2000) estimated that the median value of the total deposition of inorganic 314
N compounds at 267 forest monitoring plots in Europe was approximately 1.5 times 315
as large as the throughfall deposition. Harrison et al. (2000) estimated that canopy N 316
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uptake may contribute 16 ~ 42 % to the total N demand of trees, depending on the 317
atmospheric N concentration and N status of the tree species. 318
Deposition of dissolved organic nitrogen (DON) compounds is not included in 319
this study due to a lack of observed data. This may also lead to a significant 320
underestimation of total N deposition. Jickells et al (2013) suggested that on a global 321
basis, DON deposition accounts for approximately 25% of total N deposition. 322
Similarly, Zhang et al. (2012) and Du and Liu (2014) estimated that DON is 323
approximately 25% of bulk deposition in China. Assuming that the same ratio holds 324
for dry deposition, this also leads to a ratio of DON versus total deposition at 0.25. 325
In summary, these overviews suggest that total N deposition can be 25 ~ 75% 326
higher than the throughfall flux of inorganic N compounds reported in this study, with 327
fractions becoming lower when the throughfall flux is higher. Considering the range 328
in throughfall of TIN of 3.2 ~ 111 (mean = 21.5) kg N ha-1 yr-1, this implies that the 329
overall range may be somewhere between 5.6 ~ 139 (mean= 32.2) kg N ha-1 yr-1. 330
These values are generally much higher than estimates of critical loads of N 331
deposition in forest ecosystems, which mostly ranged from 10 ~ 20 kg N ha-1 yr-1 332
(Bobbink et al., 2010). The high loads of N deposition in China may thus lead to 333
severe biodiversity loss and even result in N saturation when N availability exceeds 334
the total combination of plant and microbial nutritional demand (Aber et al., 1989; 335
Bobbink et al., 2010). The dominance of ammonium may even lower the levels of 336
critical loads because it exerts more detrimental effects than nitrate (Brittoa and 337
Kronzucker, 2002; Erisman et al., 2007). The high N deposition in China’s forests 338
thus indicates an urgent need to assess and mitigate the negative effects of the N 339
deposition, specifically of ammonium deposition. 340
341
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5. Conclusions 342
Our results showed that ammonium dominated N deposition with a mean NH4+-N : 343
NO3--N ratio of ~ 2.5 in bulk deposition and throughfall in China’s forests. Mean 344
throughfall N deposition in China’s forests was as high as 14.0 kg N ha-1 yr-1 for 345
ammonium, 5.5 kg N ha-1 yr-1 for nitrate and 21.5 kg N ha-1 yr-1 for TIN, respectively. 346
Mean bulk deposition was 9.4 kg N ha-1 yr-1 for ammonium, 3.9 kg N ha-1 yr-1 for 347
nitrate and 14.0 kg N ha-1 yr-1 for TIN, respectively. Mean canopy captured dry 348
deposition was estimated at 5.4 kg N ha-1 yr-1 for ammonium, 1.5 kg N ha-1 yr-1 for 349
nitrate, and 6.9 kg N ha-1 yr-1 for TIN, respectively. Spatial patterns of bulk and 350
throughfall TIN deposition were in accordance with the urban hotspot hypothesis, 351
showing a strong power-law reduction of ammonium rather than nitrate deposition 352
with increasing distance to the nearest large cities. Field observations and experiments 353
should be conducted to assess the impacts of N deposition on the health of China’s 354
forest in context of increasing N deposition. 355
China has recently set goals to reduce NOx emissions by 10% in 2015 against 2010 356
emissions (Twelfth Five Year Plan), while an anticipated increase in meat and dairy 357
consumption and the absence of an NH3 regulation policy may accelerate NH3 358
emissions. Simultaneously, new urban hot-spots of N deposition are emerging due to 359
rapid urbanization in China. This implies that N deposition in China is likely to 360
increase continuously in the next few decades with ammonium deposition becoming 361
even more dominating. Unless measures are taken to reduce ammonium pollution by 362
improving agricultural N use efficiency and managing emissions from livestock 363
breeding, negative effects of excessive N on forest ecosystems may increase in the 364
next decades. 365
366
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Acknowledgement 367
This study was supported by National Natural Science Foundation of China (No. 368
41171067) and National Basic Research Program of China on Global Change (No. 369
2010CB950600). 370
371
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FIGURE LEGENDS 528
Figure 1. Locations of 38 forest stands with bulk precipitation and throughfall 529
observations in this study. Some forest stands could not be distinguished due to close 530
locations. 531
Figure 2. Histograms showing the distributions of ammonium (NH4+), nitrate (NO3
-) 532
and total inorganic N (TIN) concentrations (mg N L-1), and NH4+ : NO3
- ratios in bulk 533
precipitation (BP) and throughfall (TF) for 38 forests in China. AM and GM are 534
abbreviations for arithmetic mean and geometric mean, respectively. 535
Figure 3. Histograms showing the distributions of ammonium (NH4+), nitrate (NO3
-) 536
and total inorganic N (TIN) fluxes (kg N ha-1 yr-1) in bulk precipitation (BP) and 537
throughfall (TF) for 38 forests in China. 538
Figure 4. Histograms showing the distributions of enrichment ratios (ER) and 539
enrichment in fluxes (∆Flux) (kg N ha-1 yr-1) of ammonium (NH4+), nitrate (NO3
-) and 540
total inorganic N (TIN) for 38 forests in China. 541
Figure 5. Changes of ammonium (NH4+), nitrate (NO3
-) and TIN concentrations (mg 542
N L-1), and NH4+:NO3- ratios in the bulk precipitation (BP) and throughfall (TF) 543
with increasing distance to the nearest large cities. Data are transformed using natural 544
logarithm. 545
Figure 6. Changes of ammonium (NH4+), nitrate (NO3
-) and TIN fluxes (kg N ha-1 546
yr-1) in the bulk precipitation (BP) and throughfall (TF) with increasing distance to the 547
nearest large cities. Data are transformed using natural logarithm. 548
Figure 7. Changes of enrichment ratios (ER) and enrichment in fluxes (∆Flux) (kg N 549
ha-1 yr-1) of ammonium (NH4+), nitrate (NO3
-) and total inorganic N (TIN) with 550
increasing distance to the nearest large cities. Values of the distance are transformed 551
using natural logarithm. 552
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Table S1 Information on the forest stands. 1
2
Site Lon (E) Lat (N) AT (˚C) AP (mm) Forest type* Year Reference 1 128.87 47.18 -0.3 676 DBF 2008 Li et al., 2009
2 113.03 28.11 17.2 1411 DBF 2007-2008 Luo and Wen, 2010
3 128.89 47.18 -0.3 676 DCF 2006 Wu et al., 2008
4 121.51 50.83 -5.4 500 DCF 2008 Su, 2009
5 130.71 46.43 2.7 550 DCF 2008 Liang et al., 2010
6 121.57 24.57 18.2 2900 EBF 1995-1996 Lin et al., 2000
7 112.57 23.17 20.9 1929 EBF 2009 Zhang et al., 2010b
8 113.58 23.55 22 1740 EBF 2001-2003 Chen and Mulder, 2007
9 107.02 27.08 13 1043 EBF 2001-2005 Zhang et al., 2007
10 116.92 28.25 17.6 1795 EBF 2009-2010 Wang and Wang, 2011
11 117.55 29.58 16.3 1750 EBF 2009-2010 Zhang, 2010
12 117.75 30.38 16.4 1550 EBF 2008-2009 Zhang et al., 2010a
13 117.35 30.02 16 1626 EBF 2009-2010 Zhang, 2010
14 113.30 23.18 22 1600 EBF 1998-1999 Zhou et al., 2000
15 112.90 27.83 17.4 1549 EBF 2003-2004 Chen et al., 2006
16 101.02 24.53 11.3 1931 EBF 1998-1999 Liu et al., 2002
17 112.88 22.67 21.7 1801 EBF 1996 Liu et al., 2000
18 112.88 22.67 21.7 1801 EBF 1996 Liu et al., 2000
19 118.10 26.55 19.3 1664 ECF 1995-1996 Fan and Hong, 2001
20 118.10 26.55 19.3 1664 ECF 1995-1996 Fan and Hong, 2001
21 104.50 32.91 2.7 805 ECF 2002-2003 Gong et al., 2005
22 116.92 28.25 17.6 1795 ECF 2009-2010 Wang and Wang, 2011
23 104.80 32.97 2.7 805 ECF 2002-2003 Gong et al., 2005
24 112.90 27.83 17.5 1450 ECF 2001 Zhang et al., 2003
25 112.88 22.67 21.7 1801 ECF 1996 Liu et al., 2000
26 113.30 23.18 22 1600 ECF 1998-1999 Zhou et al., 2000
27 116.08 40.06 11.7 586 ECF 2004 Wang et al., 2006
28 128.89 47.18 -0.3 676 ECF 2006 Wu et al., 2008
29 117.34 31.80 15.7 1000 ECF 2009-2010 Zhang, 2010
30 109.83 26.75 16.8 1300 ECF 2000 Tian, 2002
31 113.30 23.18 22 1600 ECF 1998-1999 Zhou et al., 2000
32 113.83 23.70 20.3 2144 EMF 2006-2008 Zhou et al., 2009
33 108.18 26.37 15.7 2210 EMF 2001-2003 Chen and Mulder, 2007
34 106.72 26.63 15.3 1120 EMF 2001-2003 Chen and Mulder, 2007
35 106.43 28.63 13.7 1200 EMF 2004 Chen et al., 2009
36 112.43 27.92 17.5 1250 EMF 2001-2003 Chen and Mulder, 2007
37 104.68 29.63 18.2 1230 EMF 2001-2003 Chen and Mulder, 2007
38 106.37 29.75 13.6 1612 EMF 2009-2010 Guo et al., 2012
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*Note: The abbreviations DBF, DCF, EBF, ECF and EMF indicate the forest types of deciduous 3
broadleaved forest, deciduous coniferous forest, evergreen broadleaved forest, evergreen coniferous forest, 4
and evergreen broadleaved and coniferous mixed forest, respectively. 5
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