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Atmospheric dry and bulk nitrogen deposition to forest environment in the North China Plain
Atmospheric Pollution Research 10 (2019) 1636–1642 HOSTED BY
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Atmospheric dry and bulk nitrogen deposition to forest environment in the North China Plain
T
Yandan Fua,1, Wei Wangb,a,1, Mengjuan Hana, Mireadili Kuerbana, Chen Wanga, Xuejun Liua,∗ a
College of Resources and Environmental Sciences; National Academy of Agriculture Green Development; Key Laboratory of Plant-Soil Interactions of MOE, China Agricultural University, Beijing, 100193, China b Key laboratory of Forest Ecology in Tibet, Ministry of Education; Tibet Agriculture & Animal Husbandry University, Nyingchi, Tibet, 860000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nitrogen Bulk deposition Dry deposition Seasonal variation Forest ecosystem
Atmospheric nitrogen (N) deposition fluxes has dramatically increased in past few decades and has aroused great concerns of its impacts on forest ecosystems. However, complete quantitative information on dry and wet deposition of various reactive nitrogen (Nr) species is very limited for forest environments. Therefore, two-year (2014–2015) measurements of gaseous NH3, NO2, and HNO3 and particulate NH4+ and NO3− (pNH4+ and pNO3-) in air and/or precipitation were conducted in Jigong Mountain National Nature Reserve in the North China Plain (NCP). Our results showed that ambient concentrations of all Nr species were highest in winter and lowest in summer except for NH3; conversely, bulk deposition fluxes of each Nr species generally peaked in summer and were lowest in winter. Annual mean bulk Nr deposition fluxes was 15.5 kg N ha−1 yr−1 (7.9 kg N ha−1 yr−1 for NH4+ and 7.6 kg N ha−1 yr−1 for NO3−), with higher monthly fluxes occurring in spring and summer. Average annual dry Nr deposition fluxes was 44.3 kg N ha−1 yr−1 (30.1 kg N ha−1 yr−1 for reduced N (NH3 plus pNH4+) and 14.2 kg N ha−1 yr−1 for oxidized N (the sum of HNO3, NO2 and pNO3-). Total Nr deposition fluxes (dry plus bulk) reached 59.8 kg N ha−1 yr−1 with more contributions from reduced N than from oxidized N (64% vs 36%). When consider the N critical load (10–20 kg N ha−1 yr−1) in forest ecosystem, potential negative effects may cause by N deposition in the NCP which should be paid more attention.
1. Introduction Atmospheric reactive nitrogen (Nr) emissions dramatically increased in recent decades in China due to growing human activates (Cui et al., 2013). As a result, Nr deposition fluxes were enhanced over China during the same period (Liu et al., 2013); In fact, Nr creation and emission caused by human activities continuously increased all over the world since the industrial revolution (Galloway, 2005; Galloway et al., 2008), and enhanced atmospheric Nr deposition has become one of the main factors of global change (Gruber and Galloway, 2008). It was commonly accepted that plant growth could be influenced by nitrogen (N) input. Högberg (2007) found that tree growth in boreal and temperate forests increased indeed under the situation of extra nitrogen input from anthropogenic sources, and similar results were also found in a large body of studies (Wieder et al., 2015; Tian et al., 2017; Gentilesca et al., 2018; Schulte-Uebbing and de Vries, 2018), while no significant effect also be detected (Lovett et al., 2013). Nevertheless, N
is an essential element for plant growth, and the positive effects of Nr input also existed when the ecosystem is limited by N nutrient. Aiming at evaluate responses of the forest ecosystem to the elevated Nr deposition, lots of experiments were conducted to estimate the Nr deposition threshold by using N addition methods (Lovett et al., 2013; Lovett et al., 2013; Yan et al., 2014). However, the critical load of Nr input to forest ecosystem varied largely. For example, based on field observations, Liu et al. (2011) found that Nr input critical load was 15–30 kg N ha−1 yr−1 for temperate forest and 15–70 kg N ha−1 yr−1 for subtropical nature forest. The critical loads of N deposition in terrestrial ecosystems were mainly within a range of 10–25 kg N ha−1 yr−1 (Dise and Wright, 1995). Given this, numerous researches were conducted to understand the status and characteristics of atmospheric Nr deposition in forest ecosystems (Zhu et al., 2015; Izquieta-Rojano et al., 2016; Du et al., 2014, 2016; Wang et al., 2018). Du et al. (2014) reported that Nr deposition fluxes were in a range of 2.5–72.5 kg N ha−1 yr−1 in Chinese forest ecosystems and large areas of China's
Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control. ∗ Corresponding author. E-mail address: [email protected] (X. Liu). 1 Contributed equally to this work. https://doi.org/10.1016/j.apr.2019.06.004 Received 26 December 2018; Received in revised form 19 May 2019; Accepted 1 June 2019 Available online 04 June 2019 1309-1042/ © 2019 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.
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Nr concentration in different season was calculated as follow:
forests exceeds critical N loads. Hunova et al. (2017) found that the ratio of reduced and oxidized N deposition fluxes increased from 1990 to 2014. However, only bulk Nr deposition fluxes were calculated in those studies. Xu et al. (2015) found that dry Nr deposition fluxes accounted for approximately 50% of total Nr deposition fluxes in China, especially in the NCP (North China Plain). Therefore, we can potentially anticipate higher N deposition to forest ecosystems in the NCP, which may exceed critical loads of N deposition (mainly in a range of 10–20 kg N ha−1 yr−1) in forest ecosystems (Bobbink et al., 2010). Therefore, we established Nr deposition monitoring site in Jigong mountain forest area, which was located in the North China Plain with densely areas agricultural and industrial activities. The objective of this work was to quantify the magnitude and seasonal variability of concentrations and dry and bulk deposition fluxes of major Nr compounds in air and precipitation. The outcomes from this study will provide valuable quantitative information on Nr concentration and deposition in China's forest.
Nc = N/P Nc is NO3− (NH4+) concentrations in a season; N is NO3− (NH4+) deposition fluxes in this season; P is the precipitation amounts in this season. Spring refers to March–May, summer covers June–August, autumn refers to September–November, and winter covers December–February next year. 2.2.2. Atmospheric Nr compounds measurement and dry deposition calculation Dry Nr deposition fluxes were calculated by multiplying the measured Nr concentration by deposition velocity (Vd), which was widely used in quantification of Nr deposition worldwide. In this study, monthly average concentration of each Nr species and corresponding deposition velocity were used to estimate dry Nr deposition fluxes, which were further processed to annual averages. Vd of Nr species were obtained from previous study (Flechard et al., 2011). It should be point out, dry deposition velocities of the Nr species were calculated by Flechard et al. (2011) for forest, semi-natural region, grassland, and cropland in Europe. Those Vd has been used to estimate Nr deposition fluxes for similar land use types in China (Shen et al., 2013). In this study, Nr deposition velocities for forest were used to calculate dry Nr deposition fluxes; the deposition velocities of NH3, NO2, HNO3, pNH4+ and pNO3- were 1.60, 0.16, 3.30, 0.80 and 1.10 cm s−1 in forest ecosystems. DELTA (Denuder for Long-Term Atmospheric Sampling) system was used to collect gaseous NH3, HNO3, and particulate NH4+ and NO3− (pNH4+ and pNO3-) (Xu et al., 2015). The system is based on a set of bore glass denuders through which laminar air is driven by a pump (pumping rate at 0.3–0.4 L min−1) at the end of system. A first set of denuders was coated with citric acid to capture gaseous NH3, and a second set was coated with potassium hydroxide to trap gaseous HNO3. Aerosols pass through the denuders without reacting and then are collected by a couple of filters placed downstream of the denuders. These filters were treated with the acid (13% methanol citrate solution for pNH4+ sampling) and alkaline (5% KOH+10% glycerol methanol solution for pNO3- sampling) solutions as the denuders. The gaseous NO2 sample was collected using a Gradko passive diffusion tube (Gradko International, UK) which consists of three parts: an acrylic tube, two polyethylene caps (gray and white, located at the ends of the diffuser), and a two-layer stainless steel mesh. NO2 was adsorbed on two stainless steel mesh sheets placed on a gray polyethylene cap, and the adsorbent was 30 μL of a 20% triethanolamine solution. The DELTA system and Gradko NO2 tubes were placed at a height of 1.5 m above the ground (Three replication). These samplers were exposed to ambient air for one month (one sample per month). Three onsite blank samples and three laboratory blanks were used for rigorous quality control. After sampling, all samplers were returned to the laboratory, and then stored in a 4 °C refrigerator prior to chemical analysis. As for exposed DELTA samples, the HNO3 denuders and alkalinecoated filters were extracted with 10 mL 0.05% H2O2 in aqueous solution. The NH3 denuders and acid-coated filters were extracted with 10 mL high-purity water; NH4+ and NO3− in the extraction were measured using the AA3 continuous-flow analyzer (as described in section 2.2.1). The stainless steel mesh in the Gradko passive diffusion tube was leached with a mixed solution of 4.2 mL of sulfonamide, phosphoric acid and NEDA (N−1-Naphthylethylene-diamine), and the concentration of NO3− was determined by colorimetry at a wavelength of 542 nm. It should be point out, there were some differences for Nr concentrations between the canopy and the forest floor in a clearing, considering our samples were collected on the forest floor in a clearing, constant correction factors were used in this work to estimate the Nr concentrations on the canopy. We applied a constant correction factor of 1.3 to NH3 and HNO3 concentrations and 1.07 to particulate NH4+
2. Materials and methods 2.1. Site description The monitoring site was located at Jigong Mountain National Nature Reserve (JMNNR, 31.82 oE, 114.07 oN) (Fig. 1), in which there was no large anthropogenic Nr pollution source nearby. Jigong Mountain has typical subtropical evergreen and deciduous broadleaved forests. The meteorology was influenced by East Asian monsoon, with meteorological parameters shown in Table 1. The forest was mainly comprised of Quercus acutissima, Quercus variabilis, Liquidambar formosana, Dalbergia hupeana, and so on. It should be pointed out that JMNNR situated in the NCP, a region with high densely population and intensive agricultural activities. The primary forest ecosystem had been degraded for a long time, additionally, vegetation-restoration and reconstruction of mixed evergreen deciduous broad-leaved forest were conducted and secondary forests could be seen in most areas. In this study, bulk and dry Nr deposition fluxes were estimated for the years 2014 and 2015. 2.2. Sampling and chemical analysis 2.2.1. Precipitation collection and bulk Nr deposition fluxes calculation Precipitation samples were collected using a rain gauge (SDM6, Tianjin Meteorological Instrument Company Ltd. China), which was widely used in the measurements of bulk (wet + part dry) Nr deposition (Xu et al., 2015; Wang et al., 2018). The rain gauge was continuously opened and therefore precipitation samples contained a fraction of gaseous and particulate Nr dry deposited from the atmosphere into the sampler funnel. Precipitation samples was collected immediately after each precipitation event, and then transferred into 100 mL polyethylene bottles. After collection, rain gauge was rinsed with deionized water, and rainfall samples were refrigerated at −20 °C until analysis. Before analysis, rainfall samples were filtered with a 0.45 mm syringe filter (Tengda Inc., Tianjin, China). NH4+ and NO3− were measured using the AA3 continuous-flow analyzer based on Colorimetric method (Bran+Luebbe GmbH, Norderstedt, Germany), both NH4+ and NO3− detection limit was 0.01 mg N L−1. Nr deposition fluxes in different seasons were calculated as follow: i=n
N=
∑ Ni ∗ Pi/100 i=1
N is total bulk deposition fluxes (kg N ha−1), i is the number of precipitation events, Ni is Nr concentrations in i precipitation events (mg N L−1), Pi means precipitation amounts (mm) in i precipitation events, 100 is the unit conversion factor of mg m−2 to kg ha−1. 1637
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Fig. 1. Location and the land use type of Jigong Mountain.
mean concentrations of NH4+ were 0.50, 0.58, 0.90 and 0.32 mg L−1 in spring, summer, autumn and winter, respectively (Fig. 2b); The volumeweighted mean concentrations of NO3− were 0.46, 0.50, 1.25 and 0.44 mg L−1 in spring, summer, autumn and winter, respectively (Fig. 2c); Both NH4+ and NO3− concentrations showed a similar pattern in different seasons, with the highest values in autumn and the lowest in winter. In addition, NH4+ concentrations were higher than NO3− concentrations in spring and summer, by contrast, an opposite behavior was observed in autumn and winter (Fig. 2b and c).
and NO3− concentrations measured in canopy/clearings (Flechard et al., 2011). The blank sample was analyzed in the same way to eliminating errors during the experiment. 3. Results and discussion 3.1. Seasonal variations of precipitation amount and Nr concentrations in precipitation Integrated precipitation amounts were 458, 758, 121 and 44 mm in spring, summer, autumn and winter (Fig. 2a). The volume-weighted Table 1 Meteorological data for Jigong mountain averaged in 1981–2010. Month
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Mean
Sum
Temperature (°C) Humidity (%) Precipitation (mm)
−0.2 67 39.1
2 70 53.3
6.2 72 85.2
12.9 72 94.9
17.8 75 155.4
21.2 81 163.1
23.5 88 299
22.8 88 222.5
18.8 80 94.7
13.9 72 87
8 66 59.9
2.3 63 26.9
12.5 74.5 115
– – 1381
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Fig. 2. Seasonal variations of precipitation and their Nr concentration in Jigong mountain forest ecosystems.
ha−1 yr−1, respectively. Total reduced Nr (NH3 plus pNH4+) was 30.1 kg N ha−1 yr−1, total oxidized Nr (NO2, HNO3 and pNO3-) was 14.2 kg N ha−1 yr−1. In general, total Nr deposition fluxes was reached 59.8 kg N ha−1 yr−1, with a percentage of 74% for dry deposition fluxes and 26% for bulk deposition fluxes; Moreover, total reduced Nr (bulk plus dry) deposition flux was 38.0 kg N ha−1 yr−1, total oxidized Nr deposition flux was 21.8 kg N ha−1 yr−1.
3.2. Seasonal variations of Nr concentrations on the forest floor in a clearing Atmospheric NH3 concentrations were lowest in winter (1.85 μg N m−3), lower than comparable values in other three seasons (3.01–3.76 μg N m−3); by contrast, atmospheric pNH4+ concentrations were highest in winter (5.26 μg N m−3), and were in similar levels in other three seasons (3.09–3.79 μg N m−3). NO2 concentrations were highest in winter (4.77 μg N m−3), followed by spring and autumn (3.47 and 3.41 μg N m−3, respectively). The lowest concentration occurred in summer (2.01 μg N m−3); seasonal mean pNO3- concentrations (1.20–2.43 μg N m−3) showed a similar pattern to that of NO2. Seasonal mean HNO3 concentrations showed a small variation (0.29–0.64 μg N m−3) (Fig. 3).
3.4. Seasonal variations of bulk Nr deposition fluxes Bulk Nr deposition fluxes have seasonal variations and was highest in summer and lowest in winter. Additionally, bulk NH4+ deposition fluxes were 2.3, 4.3, 1.1 and 0.1 kg N ha−1 in spring, summer, autumn and winter; bulk NO3− deposition fluxes were 2.1, 3.8, 1.5 and 0.2 kg N ha−1 in spring, summer, autumn and winter (Fig. 4).
3.3. Reduced and oxidized Nr deposition fluxes 4. Discussion Bulk deposition fluxes of NH4+ and NO3− were 7.9 and 7.6 kg N −1 ha yr−1, accounting for 51% and 49% of the total bulk nitrogen deposition fluxes, respectively. Here, Nr concentrations of different species were converted by constant. After conversion, annual mean concentrations of NH3, NO2, HNO3, pNH4+ and pNO3- were 3.96, 3.41, 0.60, 4.18, 1.85 μg N m−3. Annual deposition fluxes of NH3, NO2, HNO3, pNH4+ and pNO3- were 19.7, 1.70, 6.13, 10.41 and 6.34 kg N
4.1. Seasonal variation of atmospheric Nr deposition fluxes Both Atmospheric Nr concentrations and deposition fluxes had seasonal variations in different land use types, such as farmlands (Xu et al., 2016), urban areas (Chen et al., 2015) and forest area (Liu et al., 2016). Atmospheric Nr concentrations could be influenced by different
Fig. 3. Seasonal variations of different atmospheric Nr species concentrations in Jigong mountain forest ecosystems. 1639
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(Fig. 3a). In other words, both Nr concentrations and deposition fluxes were lowest in winter. The reason might be that precipitation were inputted in the form of snow in winter and scavenging of pollutants from air was not the same as rainfall; Additionally, only few samples were collected in winter because there were rarely precipitation events occurred in winter, as a result, calculation of averaged Nr concentrations in precipitations might be influenced because of insufficient rainwater samples in winter. 4.2. Reduced and oxidized Nr deposition in bulk and dry deposition fluxes The ratio of NH4+ and NO3− concentration in rainwater was widely used to evaluate the regional prevailing Nr pollution source (CalvoFernandez et al., 2017; Wang et al., 2018). A ratio of NH4+ to NO3− concentration greater than 1 was considered as intensive agricultural regions and lower than 1 was considered as industrialized regions (Zhao et al., 2009). The ratio of bulk NH4+ and NO3− concentrations was in the range 0.64–9.20, and mean value was 2.42 in China's forest ecosystems (Du et al., 2014). In this study, the ratio of NH4+ to NO3− concentration in bulk deposition was 1.04 during the entire experiment period, in other word, the contribution of ammonia to total bulk Nr deposition fluxes is approximate to that of nitrate in a year. This result suggests that the contribution of NO3− to total atmospheric bulk Nr in our research area was higher than the averaged in China. Interestingly, seasonal variation of NH4+ and NO3− concentration ratio was obvious, which were above 1 in spring and summer and below 1 in autumn and winter (Fig. 5a). The reason might be that ions in rainfall were influenced by air masses in different directions; Jigong mountain forest ecosystem was influence by East Asian monsoon climate system, means air masses were originated from marine source in southeast direction in summer and terrestrial source in northwest direction in winter. Additionally, rainfall was vertical water input source, ions in rainfall water were not the same as the surrounding ions (Templer et al., 2015).
Fig. 4. Seasonal variations of bulk (a) and dry (b) Nr deposition fluxes in Jigong mountain forest ecosystems.
Nr emission source in different seasons. Agriculture activities were mainly conducted in spring and summer when temperatures were relatively higher. We found that atmospheric NH3 concentration was high in spring and summer (Fig. 2a), mainly resulting from N fertilization and high NH3 volatilization under high temperatures (Xu et al., 2014; Kang et al., 2016; Liang et al., 2016). Unlike NH3, the highest NO2 concentrations were observed in winter (Fig. 2c). Ambient NO2 mainly comes from industrial source (e.g. fossil fuel burning; automobile exhaust; power plant). It was commonly accepted that atmospheric NO2 pollution was the worst in winter in north China because large amount of fossil fuel was consumed for domestic heating (Liu et al., 2018; Chen et al., 2015). In addition, NO2, HNO3, NO3− can be converted to each other in the atmosphere, which to some extent could explain the highest concentrations of HNO3 and NO3− in winter (0.64 and 2.43, respectively). It is interesting that the highest NH3 concentration occurred in summer whereas NH4+ concentrations peaked in winter (Fig. 3). One possible explanation was that atmospheric NH4+ was transported from other areas by air mass because NH4+ could be transported over long distances in atmosphere (Xu et al., 2018). Bulk deposition fluxes were influenced by rainfall amounts and Nr concentrations in precipitations. Lots of studies found that bulk Nr deposition fluxes and rainfall amounts showed a positive correlation and Nr concentrations in rainfall were negative to rainfall amount because of dilution effects (Xu et al., 2015; Wang et al., 2018). Bulk Nr deposition fluxes in Jigong Mountain were agreed with the rainfall amounts, which were highest in summer and lowest in winter, But Nr concentrations in rainfall had different behaviors, which was in the order of Nr concentration was autumn > summer > spring > winter
4.3. Nr deposition fluxes in forest ecosystem and ecological impacts The bulk deposition were estimated at 44.3 kg N ha−1 yr−1 for dry Nr deposition flux, 15.5 kg N ha−1 yr−1for bulk Nr deposition flux, and totaled 59.8 kg N ha−1 yr−1. There was no conflict with the result based on Nationwide Nitrogen Deposition Monitoring Network Nr deposition across China(totaled 2.9–83.3 kg N ha−1 yr−1), additionally, bulk Nr deposition flux was lower than that averaged across China but dry Nr deposition flux was higher than the value averaged in China (average dry was 20.6 kg N ha−1 yr−1and bulk was 19.3 kg N ha−1 yr−1) (Xu et al., 2015), one explanation was that Nr deposition velocities was higher in forest ecosystem than those in other ecosystems (Flechard et al., 2011). However, concentrations of monthly averaged
Fig. 5. The ratio of reduced and oxidized Nr in different season in bulk (a) and dry (b) deposition fluxes. 1640
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atmospheric NH3, HNO3, NO2, NH4+ and NO3− were 1.03, 2.15, 0.24, 0.73 and 0.45 μg N m−3 in European forest ecosystem (NEU inferential network), respectively; analogous value in our research were 3.0, 3.5, 0.5, 4.0 and 1.8 μg N m−3. Obviously, all kinds of atmospheric Nr species were higher in Jigong mountain than those in European forest ecosystem. Furthermore, mean bulk deposition was 9.4 and 3.9 kg N ha−1 yr−1 for NH4+ and NO3− in Chinese forest ecosystems (Du et al., 2014), whereas corresponding values were 7.9 and 7.6 kg N ha−1 yr−1 for NH4+ and NO3− in our research. Extra nitrogen input to forest ecosystems could promote plant growth in natural nitrogen-limited conditions (Högberg, 2007), but negative effects could exist in nitrogen-saturated forest ecosystems (Vangansbeke et al., 2015; Wang et al., 2015), e.g. nitrate leaching, biodiversity reduction, soil acidification (Aber et al., 1998; Magill et al., 2000; Lovett et al., 2013; Ferretti et al., 2014). Dise and Wright (1995) found that nitrogen leaching occurred in forest ecosystems when Nr deposition flux generally > 25 kg N ha−1 yr−1. Total Nr deposition fluxes reached 59.8 kg N ha−1 yr−1 in our research, suggesting that soil nitrogen leaching might exist in Jigong mountain forest ecosystem, as a result, soil acidification may occurred.
NO3− contributed nearly equally to total bulk Nr deposition fluxes (7.9 and 7.6 kg N ha−1 yr−1). Total dry Nr deposition fluxes (sum of NH3, NO2, HNO3, pNH4+ and pNO3-) was 44.3 kg N ha−1 yr−1. HNO3 deposition fluxes were main Nr species in spring, summer and autumn, and pNH4+ was the dominant Nr specie in winter. Annual total Nr deposition flux (bulk plus dry) reached 59.8 kg N ha−1 yr−1 in our study forest. Considering such high atmospheric Nr deposition fluxes, nitrate leaching and soil acidification might exist, and negative effects caused by Nr deposition should be paid more attention. Conflicts of interest The authors declare that they have no conflict of interest. Acknowledgement This work was supported by the National Key R&D Program of China (2017YFC0210101, 2017YFC0210106, DQGG0208), China Postdoctoral Science Foundation (2018M641531), the National Natural Science Foundation of China (41705130, 41425007, 31421092), China Agricultural University-Tibet Agriculture and Animal Husbandry College Young Teachers’ Scientific Research (201806) as well as the National Ten-thousand Talents Program of China (Xuejun Liu).
4.4. Uncertainties of Nr deposition collection and calculation Although both bulk precipitation and throughfall were collected to evaluate the deposition fluxes in forest ecosystems, the differences were existed also. Actually, Nr (e.g.NH4+, NO3−, NO2, NH3) could be uptake by foliar, in other word, forest canopy could catch different kinds of Nr species in ambient environment. Both NO3− and NH4+ in precipitations were intake by canopy in rainfall events, therefore, bulk Nr deposition fluxes were usually higher than that in canopy throughfall. For example, 41% of bulk Nr deposition could be retained by canopy in Europe and North America (Lovett and Lindberg, 1993). It was no doubt that bulk precipitation was more precise than throughfall in Nr deposition flux measurement. However, dissolved organic nitrogen compounds were not include in our research; on a global basis, dissolved organic nitrogen compounds might be account for 25% of total Nr deposition fluxes (Jickells et al., 2013), which could be up to 67% of total Nr deposition flux in forest ecosystem (Zhang et al., 2012). Obviously, bulk Nr deposition fluxes were underestimated in our research. Due to atmospheric N concentration and N status of tree species in different area, the contribution of canopy N uptake to tree total demand was about 16–42% (Harrison et al., 2000). This means forest canopy absorption of Nr was varied largely in forest area and the Nr uptake by forest canopy in our research was not clear. The dry deposition fluxes in our research were calculated by multiplying atmospheric Nr concentration by their deposition velocities, some uncertainty may also exsited due to the fact that dry Vd were directly taken from the study of Flechard et al. (2011) for European forest. Vd can be influenced by several factors (e.g., aerodynamic resistance; friction velocity; leaf mesophyll resistance) which may differ between China and European country. Furthermore, the exchange of NH3 between the surface and the atmosphere can be bi-directional (Flechard et al., 2011), with emissions occurring when the surface potential exceeds the atmospheric concentration, or vice versa. As a result, there might be some difference between the real dry Nr deposition fluxes and those in our measurement.
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5. Conclusion Two-year measurements of atmospheric Nr deposition fluxes were conducted in Jigong Mountain National Nature Reserve in the North China Plain. Both atmospheric Nr concentration and deposition fluxes showed obvious seasonal variations; the highest concentrations appeared in winter for all measured Nr species except for NH3; bulk Nr deposition fluxes totaled 15.5 kg N ha−1 yr−1 and mainly occurred in plant growing seasons (spring and summer). In addition, NH4+ and 1641
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Atmospheric dry and bulk nitrogen deposition to forest environment in the North China Plain
T
Yandan Fua,1, Wei Wangb,a,1, Mengjuan Hana, Mireadili Kuerbana, Chen Wanga, Xuejun Liua,∗ a
College of Resources and Environmental Sciences; National Academy of Agriculture Green Development; Key Laboratory of Plant-Soil Interactions of MOE, China Agricultural University, Beijing, 100193, China b Key laboratory of Forest Ecology in Tibet, Ministry of Education; Tibet Agriculture & Animal Husbandry University, Nyingchi, Tibet, 860000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nitrogen Bulk deposition Dry deposition Seasonal variation Forest ecosystem
Atmospheric nitrogen (N) deposition fluxes has dramatically increased in past few decades and has aroused great concerns of its impacts on forest ecosystems. However, complete quantitative information on dry and wet deposition of various reactive nitrogen (Nr) species is very limited for forest environments. Therefore, two-year (2014–2015) measurements of gaseous NH3, NO2, and HNO3 and particulate NH4+ and NO3− (pNH4+ and pNO3-) in air and/or precipitation were conducted in Jigong Mountain National Nature Reserve in the North China Plain (NCP). Our results showed that ambient concentrations of all Nr species were highest in winter and lowest in summer except for NH3; conversely, bulk deposition fluxes of each Nr species generally peaked in summer and were lowest in winter. Annual mean bulk Nr deposition fluxes was 15.5 kg N ha−1 yr−1 (7.9 kg N ha−1 yr−1 for NH4+ and 7.6 kg N ha−1 yr−1 for NO3−), with higher monthly fluxes occurring in spring and summer. Average annual dry Nr deposition fluxes was 44.3 kg N ha−1 yr−1 (30.1 kg N ha−1 yr−1 for reduced N (NH3 plus pNH4+) and 14.2 kg N ha−1 yr−1 for oxidized N (the sum of HNO3, NO2 and pNO3-). Total Nr deposition fluxes (dry plus bulk) reached 59.8 kg N ha−1 yr−1 with more contributions from reduced N than from oxidized N (64% vs 36%). When consider the N critical load (10–20 kg N ha−1 yr−1) in forest ecosystem, potential negative effects may cause by N deposition in the NCP which should be paid more attention.
1. Introduction Atmospheric reactive nitrogen (Nr) emissions dramatically increased in recent decades in China due to growing human activates (Cui et al., 2013). As a result, Nr deposition fluxes were enhanced over China during the same period (Liu et al., 2013); In fact, Nr creation and emission caused by human activities continuously increased all over the world since the industrial revolution (Galloway, 2005; Galloway et al., 2008), and enhanced atmospheric Nr deposition has become one of the main factors of global change (Gruber and Galloway, 2008). It was commonly accepted that plant growth could be influenced by nitrogen (N) input. Högberg (2007) found that tree growth in boreal and temperate forests increased indeed under the situation of extra nitrogen input from anthropogenic sources, and similar results were also found in a large body of studies (Wieder et al., 2015; Tian et al., 2017; Gentilesca et al., 2018; Schulte-Uebbing and de Vries, 2018), while no significant effect also be detected (Lovett et al., 2013). Nevertheless, N
is an essential element for plant growth, and the positive effects of Nr input also existed when the ecosystem is limited by N nutrient. Aiming at evaluate responses of the forest ecosystem to the elevated Nr deposition, lots of experiments were conducted to estimate the Nr deposition threshold by using N addition methods (Lovett et al., 2013; Lovett et al., 2013; Yan et al., 2014). However, the critical load of Nr input to forest ecosystem varied largely. For example, based on field observations, Liu et al. (2011) found that Nr input critical load was 15–30 kg N ha−1 yr−1 for temperate forest and 15–70 kg N ha−1 yr−1 for subtropical nature forest. The critical loads of N deposition in terrestrial ecosystems were mainly within a range of 10–25 kg N ha−1 yr−1 (Dise and Wright, 1995). Given this, numerous researches were conducted to understand the status and characteristics of atmospheric Nr deposition in forest ecosystems (Zhu et al., 2015; Izquieta-Rojano et al., 2016; Du et al., 2014, 2016; Wang et al., 2018). Du et al. (2014) reported that Nr deposition fluxes were in a range of 2.5–72.5 kg N ha−1 yr−1 in Chinese forest ecosystems and large areas of China's
Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control. ∗ Corresponding author. E-mail address: [email protected] (X. Liu). 1 Contributed equally to this work. https://doi.org/10.1016/j.apr.2019.06.004 Received 26 December 2018; Received in revised form 19 May 2019; Accepted 1 June 2019 Available online 04 June 2019 1309-1042/ © 2019 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.
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Nr concentration in different season was calculated as follow:
forests exceeds critical N loads. Hunova et al. (2017) found that the ratio of reduced and oxidized N deposition fluxes increased from 1990 to 2014. However, only bulk Nr deposition fluxes were calculated in those studies. Xu et al. (2015) found that dry Nr deposition fluxes accounted for approximately 50% of total Nr deposition fluxes in China, especially in the NCP (North China Plain). Therefore, we can potentially anticipate higher N deposition to forest ecosystems in the NCP, which may exceed critical loads of N deposition (mainly in a range of 10–20 kg N ha−1 yr−1) in forest ecosystems (Bobbink et al., 2010). Therefore, we established Nr deposition monitoring site in Jigong mountain forest area, which was located in the North China Plain with densely areas agricultural and industrial activities. The objective of this work was to quantify the magnitude and seasonal variability of concentrations and dry and bulk deposition fluxes of major Nr compounds in air and precipitation. The outcomes from this study will provide valuable quantitative information on Nr concentration and deposition in China's forest.
Nc = N/P Nc is NO3− (NH4+) concentrations in a season; N is NO3− (NH4+) deposition fluxes in this season; P is the precipitation amounts in this season. Spring refers to March–May, summer covers June–August, autumn refers to September–November, and winter covers December–February next year. 2.2.2. Atmospheric Nr compounds measurement and dry deposition calculation Dry Nr deposition fluxes were calculated by multiplying the measured Nr concentration by deposition velocity (Vd), which was widely used in quantification of Nr deposition worldwide. In this study, monthly average concentration of each Nr species and corresponding deposition velocity were used to estimate dry Nr deposition fluxes, which were further processed to annual averages. Vd of Nr species were obtained from previous study (Flechard et al., 2011). It should be point out, dry deposition velocities of the Nr species were calculated by Flechard et al. (2011) for forest, semi-natural region, grassland, and cropland in Europe. Those Vd has been used to estimate Nr deposition fluxes for similar land use types in China (Shen et al., 2013). In this study, Nr deposition velocities for forest were used to calculate dry Nr deposition fluxes; the deposition velocities of NH3, NO2, HNO3, pNH4+ and pNO3- were 1.60, 0.16, 3.30, 0.80 and 1.10 cm s−1 in forest ecosystems. DELTA (Denuder for Long-Term Atmospheric Sampling) system was used to collect gaseous NH3, HNO3, and particulate NH4+ and NO3− (pNH4+ and pNO3-) (Xu et al., 2015). The system is based on a set of bore glass denuders through which laminar air is driven by a pump (pumping rate at 0.3–0.4 L min−1) at the end of system. A first set of denuders was coated with citric acid to capture gaseous NH3, and a second set was coated with potassium hydroxide to trap gaseous HNO3. Aerosols pass through the denuders without reacting and then are collected by a couple of filters placed downstream of the denuders. These filters were treated with the acid (13% methanol citrate solution for pNH4+ sampling) and alkaline (5% KOH+10% glycerol methanol solution for pNO3- sampling) solutions as the denuders. The gaseous NO2 sample was collected using a Gradko passive diffusion tube (Gradko International, UK) which consists of three parts: an acrylic tube, two polyethylene caps (gray and white, located at the ends of the diffuser), and a two-layer stainless steel mesh. NO2 was adsorbed on two stainless steel mesh sheets placed on a gray polyethylene cap, and the adsorbent was 30 μL of a 20% triethanolamine solution. The DELTA system and Gradko NO2 tubes were placed at a height of 1.5 m above the ground (Three replication). These samplers were exposed to ambient air for one month (one sample per month). Three onsite blank samples and three laboratory blanks were used for rigorous quality control. After sampling, all samplers were returned to the laboratory, and then stored in a 4 °C refrigerator prior to chemical analysis. As for exposed DELTA samples, the HNO3 denuders and alkalinecoated filters were extracted with 10 mL 0.05% H2O2 in aqueous solution. The NH3 denuders and acid-coated filters were extracted with 10 mL high-purity water; NH4+ and NO3− in the extraction were measured using the AA3 continuous-flow analyzer (as described in section 2.2.1). The stainless steel mesh in the Gradko passive diffusion tube was leached with a mixed solution of 4.2 mL of sulfonamide, phosphoric acid and NEDA (N−1-Naphthylethylene-diamine), and the concentration of NO3− was determined by colorimetry at a wavelength of 542 nm. It should be point out, there were some differences for Nr concentrations between the canopy and the forest floor in a clearing, considering our samples were collected on the forest floor in a clearing, constant correction factors were used in this work to estimate the Nr concentrations on the canopy. We applied a constant correction factor of 1.3 to NH3 and HNO3 concentrations and 1.07 to particulate NH4+
2. Materials and methods 2.1. Site description The monitoring site was located at Jigong Mountain National Nature Reserve (JMNNR, 31.82 oE, 114.07 oN) (Fig. 1), in which there was no large anthropogenic Nr pollution source nearby. Jigong Mountain has typical subtropical evergreen and deciduous broadleaved forests. The meteorology was influenced by East Asian monsoon, with meteorological parameters shown in Table 1. The forest was mainly comprised of Quercus acutissima, Quercus variabilis, Liquidambar formosana, Dalbergia hupeana, and so on. It should be pointed out that JMNNR situated in the NCP, a region with high densely population and intensive agricultural activities. The primary forest ecosystem had been degraded for a long time, additionally, vegetation-restoration and reconstruction of mixed evergreen deciduous broad-leaved forest were conducted and secondary forests could be seen in most areas. In this study, bulk and dry Nr deposition fluxes were estimated for the years 2014 and 2015. 2.2. Sampling and chemical analysis 2.2.1. Precipitation collection and bulk Nr deposition fluxes calculation Precipitation samples were collected using a rain gauge (SDM6, Tianjin Meteorological Instrument Company Ltd. China), which was widely used in the measurements of bulk (wet + part dry) Nr deposition (Xu et al., 2015; Wang et al., 2018). The rain gauge was continuously opened and therefore precipitation samples contained a fraction of gaseous and particulate Nr dry deposited from the atmosphere into the sampler funnel. Precipitation samples was collected immediately after each precipitation event, and then transferred into 100 mL polyethylene bottles. After collection, rain gauge was rinsed with deionized water, and rainfall samples were refrigerated at −20 °C until analysis. Before analysis, rainfall samples were filtered with a 0.45 mm syringe filter (Tengda Inc., Tianjin, China). NH4+ and NO3− were measured using the AA3 continuous-flow analyzer based on Colorimetric method (Bran+Luebbe GmbH, Norderstedt, Germany), both NH4+ and NO3− detection limit was 0.01 mg N L−1. Nr deposition fluxes in different seasons were calculated as follow: i=n
N=
∑ Ni ∗ Pi/100 i=1
N is total bulk deposition fluxes (kg N ha−1), i is the number of precipitation events, Ni is Nr concentrations in i precipitation events (mg N L−1), Pi means precipitation amounts (mm) in i precipitation events, 100 is the unit conversion factor of mg m−2 to kg ha−1. 1637
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Fig. 1. Location and the land use type of Jigong Mountain.
mean concentrations of NH4+ were 0.50, 0.58, 0.90 and 0.32 mg L−1 in spring, summer, autumn and winter, respectively (Fig. 2b); The volumeweighted mean concentrations of NO3− were 0.46, 0.50, 1.25 and 0.44 mg L−1 in spring, summer, autumn and winter, respectively (Fig. 2c); Both NH4+ and NO3− concentrations showed a similar pattern in different seasons, with the highest values in autumn and the lowest in winter. In addition, NH4+ concentrations were higher than NO3− concentrations in spring and summer, by contrast, an opposite behavior was observed in autumn and winter (Fig. 2b and c).
and NO3− concentrations measured in canopy/clearings (Flechard et al., 2011). The blank sample was analyzed in the same way to eliminating errors during the experiment. 3. Results and discussion 3.1. Seasonal variations of precipitation amount and Nr concentrations in precipitation Integrated precipitation amounts were 458, 758, 121 and 44 mm in spring, summer, autumn and winter (Fig. 2a). The volume-weighted Table 1 Meteorological data for Jigong mountain averaged in 1981–2010. Month
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Mean
Sum
Temperature (°C) Humidity (%) Precipitation (mm)
−0.2 67 39.1
2 70 53.3
6.2 72 85.2
12.9 72 94.9
17.8 75 155.4
21.2 81 163.1
23.5 88 299
22.8 88 222.5
18.8 80 94.7
13.9 72 87
8 66 59.9
2.3 63 26.9
12.5 74.5 115
– – 1381
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Fig. 2. Seasonal variations of precipitation and their Nr concentration in Jigong mountain forest ecosystems.
ha−1 yr−1, respectively. Total reduced Nr (NH3 plus pNH4+) was 30.1 kg N ha−1 yr−1, total oxidized Nr (NO2, HNO3 and pNO3-) was 14.2 kg N ha−1 yr−1. In general, total Nr deposition fluxes was reached 59.8 kg N ha−1 yr−1, with a percentage of 74% for dry deposition fluxes and 26% for bulk deposition fluxes; Moreover, total reduced Nr (bulk plus dry) deposition flux was 38.0 kg N ha−1 yr−1, total oxidized Nr deposition flux was 21.8 kg N ha−1 yr−1.
3.2. Seasonal variations of Nr concentrations on the forest floor in a clearing Atmospheric NH3 concentrations were lowest in winter (1.85 μg N m−3), lower than comparable values in other three seasons (3.01–3.76 μg N m−3); by contrast, atmospheric pNH4+ concentrations were highest in winter (5.26 μg N m−3), and were in similar levels in other three seasons (3.09–3.79 μg N m−3). NO2 concentrations were highest in winter (4.77 μg N m−3), followed by spring and autumn (3.47 and 3.41 μg N m−3, respectively). The lowest concentration occurred in summer (2.01 μg N m−3); seasonal mean pNO3- concentrations (1.20–2.43 μg N m−3) showed a similar pattern to that of NO2. Seasonal mean HNO3 concentrations showed a small variation (0.29–0.64 μg N m−3) (Fig. 3).
3.4. Seasonal variations of bulk Nr deposition fluxes Bulk Nr deposition fluxes have seasonal variations and was highest in summer and lowest in winter. Additionally, bulk NH4+ deposition fluxes were 2.3, 4.3, 1.1 and 0.1 kg N ha−1 in spring, summer, autumn and winter; bulk NO3− deposition fluxes were 2.1, 3.8, 1.5 and 0.2 kg N ha−1 in spring, summer, autumn and winter (Fig. 4).
3.3. Reduced and oxidized Nr deposition fluxes 4. Discussion Bulk deposition fluxes of NH4+ and NO3− were 7.9 and 7.6 kg N −1 ha yr−1, accounting for 51% and 49% of the total bulk nitrogen deposition fluxes, respectively. Here, Nr concentrations of different species were converted by constant. After conversion, annual mean concentrations of NH3, NO2, HNO3, pNH4+ and pNO3- were 3.96, 3.41, 0.60, 4.18, 1.85 μg N m−3. Annual deposition fluxes of NH3, NO2, HNO3, pNH4+ and pNO3- were 19.7, 1.70, 6.13, 10.41 and 6.34 kg N
4.1. Seasonal variation of atmospheric Nr deposition fluxes Both Atmospheric Nr concentrations and deposition fluxes had seasonal variations in different land use types, such as farmlands (Xu et al., 2016), urban areas (Chen et al., 2015) and forest area (Liu et al., 2016). Atmospheric Nr concentrations could be influenced by different
Fig. 3. Seasonal variations of different atmospheric Nr species concentrations in Jigong mountain forest ecosystems. 1639
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(Fig. 3a). In other words, both Nr concentrations and deposition fluxes were lowest in winter. The reason might be that precipitation were inputted in the form of snow in winter and scavenging of pollutants from air was not the same as rainfall; Additionally, only few samples were collected in winter because there were rarely precipitation events occurred in winter, as a result, calculation of averaged Nr concentrations in precipitations might be influenced because of insufficient rainwater samples in winter. 4.2. Reduced and oxidized Nr deposition in bulk and dry deposition fluxes The ratio of NH4+ and NO3− concentration in rainwater was widely used to evaluate the regional prevailing Nr pollution source (CalvoFernandez et al., 2017; Wang et al., 2018). A ratio of NH4+ to NO3− concentration greater than 1 was considered as intensive agricultural regions and lower than 1 was considered as industrialized regions (Zhao et al., 2009). The ratio of bulk NH4+ and NO3− concentrations was in the range 0.64–9.20, and mean value was 2.42 in China's forest ecosystems (Du et al., 2014). In this study, the ratio of NH4+ to NO3− concentration in bulk deposition was 1.04 during the entire experiment period, in other word, the contribution of ammonia to total bulk Nr deposition fluxes is approximate to that of nitrate in a year. This result suggests that the contribution of NO3− to total atmospheric bulk Nr in our research area was higher than the averaged in China. Interestingly, seasonal variation of NH4+ and NO3− concentration ratio was obvious, which were above 1 in spring and summer and below 1 in autumn and winter (Fig. 5a). The reason might be that ions in rainfall were influenced by air masses in different directions; Jigong mountain forest ecosystem was influence by East Asian monsoon climate system, means air masses were originated from marine source in southeast direction in summer and terrestrial source in northwest direction in winter. Additionally, rainfall was vertical water input source, ions in rainfall water were not the same as the surrounding ions (Templer et al., 2015).
Fig. 4. Seasonal variations of bulk (a) and dry (b) Nr deposition fluxes in Jigong mountain forest ecosystems.
Nr emission source in different seasons. Agriculture activities were mainly conducted in spring and summer when temperatures were relatively higher. We found that atmospheric NH3 concentration was high in spring and summer (Fig. 2a), mainly resulting from N fertilization and high NH3 volatilization under high temperatures (Xu et al., 2014; Kang et al., 2016; Liang et al., 2016). Unlike NH3, the highest NO2 concentrations were observed in winter (Fig. 2c). Ambient NO2 mainly comes from industrial source (e.g. fossil fuel burning; automobile exhaust; power plant). It was commonly accepted that atmospheric NO2 pollution was the worst in winter in north China because large amount of fossil fuel was consumed for domestic heating (Liu et al., 2018; Chen et al., 2015). In addition, NO2, HNO3, NO3− can be converted to each other in the atmosphere, which to some extent could explain the highest concentrations of HNO3 and NO3− in winter (0.64 and 2.43, respectively). It is interesting that the highest NH3 concentration occurred in summer whereas NH4+ concentrations peaked in winter (Fig. 3). One possible explanation was that atmospheric NH4+ was transported from other areas by air mass because NH4+ could be transported over long distances in atmosphere (Xu et al., 2018). Bulk deposition fluxes were influenced by rainfall amounts and Nr concentrations in precipitations. Lots of studies found that bulk Nr deposition fluxes and rainfall amounts showed a positive correlation and Nr concentrations in rainfall were negative to rainfall amount because of dilution effects (Xu et al., 2015; Wang et al., 2018). Bulk Nr deposition fluxes in Jigong Mountain were agreed with the rainfall amounts, which were highest in summer and lowest in winter, But Nr concentrations in rainfall had different behaviors, which was in the order of Nr concentration was autumn > summer > spring > winter
4.3. Nr deposition fluxes in forest ecosystem and ecological impacts The bulk deposition were estimated at 44.3 kg N ha−1 yr−1 for dry Nr deposition flux, 15.5 kg N ha−1 yr−1for bulk Nr deposition flux, and totaled 59.8 kg N ha−1 yr−1. There was no conflict with the result based on Nationwide Nitrogen Deposition Monitoring Network Nr deposition across China(totaled 2.9–83.3 kg N ha−1 yr−1), additionally, bulk Nr deposition flux was lower than that averaged across China but dry Nr deposition flux was higher than the value averaged in China (average dry was 20.6 kg N ha−1 yr−1and bulk was 19.3 kg N ha−1 yr−1) (Xu et al., 2015), one explanation was that Nr deposition velocities was higher in forest ecosystem than those in other ecosystems (Flechard et al., 2011). However, concentrations of monthly averaged
Fig. 5. The ratio of reduced and oxidized Nr in different season in bulk (a) and dry (b) deposition fluxes. 1640
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atmospheric NH3, HNO3, NO2, NH4+ and NO3− were 1.03, 2.15, 0.24, 0.73 and 0.45 μg N m−3 in European forest ecosystem (NEU inferential network), respectively; analogous value in our research were 3.0, 3.5, 0.5, 4.0 and 1.8 μg N m−3. Obviously, all kinds of atmospheric Nr species were higher in Jigong mountain than those in European forest ecosystem. Furthermore, mean bulk deposition was 9.4 and 3.9 kg N ha−1 yr−1 for NH4+ and NO3− in Chinese forest ecosystems (Du et al., 2014), whereas corresponding values were 7.9 and 7.6 kg N ha−1 yr−1 for NH4+ and NO3− in our research. Extra nitrogen input to forest ecosystems could promote plant growth in natural nitrogen-limited conditions (Högberg, 2007), but negative effects could exist in nitrogen-saturated forest ecosystems (Vangansbeke et al., 2015; Wang et al., 2015), e.g. nitrate leaching, biodiversity reduction, soil acidification (Aber et al., 1998; Magill et al., 2000; Lovett et al., 2013; Ferretti et al., 2014). Dise and Wright (1995) found that nitrogen leaching occurred in forest ecosystems when Nr deposition flux generally > 25 kg N ha−1 yr−1. Total Nr deposition fluxes reached 59.8 kg N ha−1 yr−1 in our research, suggesting that soil nitrogen leaching might exist in Jigong mountain forest ecosystem, as a result, soil acidification may occurred.
NO3− contributed nearly equally to total bulk Nr deposition fluxes (7.9 and 7.6 kg N ha−1 yr−1). Total dry Nr deposition fluxes (sum of NH3, NO2, HNO3, pNH4+ and pNO3-) was 44.3 kg N ha−1 yr−1. HNO3 deposition fluxes were main Nr species in spring, summer and autumn, and pNH4+ was the dominant Nr specie in winter. Annual total Nr deposition flux (bulk plus dry) reached 59.8 kg N ha−1 yr−1 in our study forest. Considering such high atmospheric Nr deposition fluxes, nitrate leaching and soil acidification might exist, and negative effects caused by Nr deposition should be paid more attention. Conflicts of interest The authors declare that they have no conflict of interest. Acknowledgement This work was supported by the National Key R&D Program of China (2017YFC0210101, 2017YFC0210106, DQGG0208), China Postdoctoral Science Foundation (2018M641531), the National Natural Science Foundation of China (41705130, 41425007, 31421092), China Agricultural University-Tibet Agriculture and Animal Husbandry College Young Teachers’ Scientific Research (201806) as well as the National Ten-thousand Talents Program of China (Xuejun Liu).
4.4. Uncertainties of Nr deposition collection and calculation Although both bulk precipitation and throughfall were collected to evaluate the deposition fluxes in forest ecosystems, the differences were existed also. Actually, Nr (e.g.NH4+, NO3−, NO2, NH3) could be uptake by foliar, in other word, forest canopy could catch different kinds of Nr species in ambient environment. Both NO3− and NH4+ in precipitations were intake by canopy in rainfall events, therefore, bulk Nr deposition fluxes were usually higher than that in canopy throughfall. For example, 41% of bulk Nr deposition could be retained by canopy in Europe and North America (Lovett and Lindberg, 1993). It was no doubt that bulk precipitation was more precise than throughfall in Nr deposition flux measurement. However, dissolved organic nitrogen compounds were not include in our research; on a global basis, dissolved organic nitrogen compounds might be account for 25% of total Nr deposition fluxes (Jickells et al., 2013), which could be up to 67% of total Nr deposition flux in forest ecosystem (Zhang et al., 2012). Obviously, bulk Nr deposition fluxes were underestimated in our research. Due to atmospheric N concentration and N status of tree species in different area, the contribution of canopy N uptake to tree total demand was about 16–42% (Harrison et al., 2000). This means forest canopy absorption of Nr was varied largely in forest area and the Nr uptake by forest canopy in our research was not clear. The dry deposition fluxes in our research were calculated by multiplying atmospheric Nr concentration by their deposition velocities, some uncertainty may also exsited due to the fact that dry Vd were directly taken from the study of Flechard et al. (2011) for European forest. Vd can be influenced by several factors (e.g., aerodynamic resistance; friction velocity; leaf mesophyll resistance) which may differ between China and European country. Furthermore, the exchange of NH3 between the surface and the atmosphere can be bi-directional (Flechard et al., 2011), with emissions occurring when the surface potential exceeds the atmospheric concentration, or vice versa. As a result, there might be some difference between the real dry Nr deposition fluxes and those in our measurement.
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