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Seasonal and annual fluxes of atmospheric nitrogen deposition and riverine nitrogen export in two adjacent contrasting rivers in central Japan facing the Sea of Japan Ryo Sugimoto a,∗ , Tomoko Tsuboi b,1 a b

Research Center for Marine Bioresources, Fukui Prefectural University, 49-8-2 Katsumi, Obama, Fukui 917-0116, Japan Graduate School of Biosciences and Biotechnology, Fukui Prefectural University, 49-8-2 Katsumi, Obama, Fukui 917-0116, Japan

a r t i c l e

i n f o

Article history: Received 16 July 2015 Received in revised form 17 September 2015 Accepted 30 November 2015 Available online xxx Keywords: Atmospheric N deposition Nitrogen saturation Trans-boundary air pollution Riverine N export Coastal ecosystem

a b s t r a c t Study region: Kita and Minami River basins in Japan. Study focus: The coastal watershed in central Japan along the Sea of Japan has suffered large amounts of atmospheric nitrogen (N) deposition from northeastern Asia. However, the quantitative influences of atmospheric N deposition onto forested watersheds in the two basins and riverine N export into coasts remain unclear. To evaluate the current contribution of atmospheric N deposition, N deposition rates from the atmosphere to both basins, and N export rates from the rivers to the sea were quantified. New hydrological insight for the region: Deposition rates of bulk N in each basin exceeded 1000 mg m−2 year−1 , more than 60% of which was supplied from winter to early spring by westerly winds. Annual deposition rates in the two basins did not differ, but annual export rates of inorganic N from the Kita River were significantly higher than those from the Minami River. These results suggest that symptoms of N saturation in the Kita River forested watershed are more serious. Furthermore, recent increasing trends of riverine N concentrations may have caused shifts in the limiting nutrient for coastal primary production from N to phosphorous. We suggest reductions in nitrate exports from forests as a strategy to improve nitrate pollution to both downstream waters and coastal ecosystems; however such efforts would involve intercontinental-scale actions in reducing N emissions. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction Anthropogenic emissions of reactive nitrogen (N) due to fossil fuel combustion and modern agriculture practices have increased at a global scale (Galloway and Cowling, 2002; Gruber and Galloway, 2008). In particular, increases in N emissions in eastern Asia have been dramatic over the last decade (Akimoto 2003; Ohara et al., 2007; Liu et al., 2011). Increased deposition rates of atmospheric N can have impacts on N cycling in forest ecosystems (Aber et al., 1989; Vitousek et al., 1997). Chronic atmospheric N deposition can lead to N saturation of forests, resulting in excess nitrate export from forested

∗ Corresponding author. Fax: +81 770527306. E-mail address: [email protected] (R. Sugimoto). 1 Present address: Marine Biological Research Institute of Japan, 1-7-7 Nishishinagawa, Shinagawa-ku, Tokyo 141-0033, Japan. http://dx.doi.org/10.1016/j.ejrh.2015.11.019 2214-5818/© 2015 The Authors. Published by (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Please cite this article in press as: Sugimoto, R., Tsuboi, T., Seasonal and annual fluxes of atmospheric nitrogen deposition and riverine nitrogen export in two adjacent contrasting rivers in central Japan facing the Sea of Japan. J. Hydrol.: Reg. Stud. (2015), http://dx.doi.org/10.1016/j.ejrh.2015.11.019


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Fig. 1. (A) Study area and altitude. The closed square (Stn. E) indicates the monitoring site for acid rain by the Ministry of the Environment, Government of Japan. (B) Monthly sampling stations in the Kita and Minami River basins, including land use. Closed squares indicate the sampling sites (Stns. R1–R5) for bulk deposition of atmospheric nitrogen. Open squares are the monitoring site for precipitation by the Ministry of Land, Infrastructure and Transport (MLIT) and the Fukui Prefecture Government (FPG). Closed circles show monthly sampling stations of stream and downstream river waters in the Kita River (Stns. KU and KL) and the Minami River (Stns. MU and ML).

watersheds to stream water and ultimately coastal seas (Aber et al., 1989, 1998; Howarth, 1998). Therefore, N emissions caused by energy and food supply can induce water quality issues in local ecosystems. In Japan, wet deposition rates of nitrate have increased during the last few decades by 2–5% year−1 (Morino et al., 2011). Particularly in the watershed along the Sea of Japan, large amounts of atmospheric N deposited onto forest ecosystems during winter, when the dominant direction of the movement of air masses is from the Asian continent toward Japan (Fukuzaki et al., 2001; Kamisako et al., 2008). Several studies have determined that N compounds originating from trans-boundary air masses have caused N saturation in forests and increased nitrate concentrations in stream and river water over the past few decades (Chiwa et al., 2012, 2013; Miyazako et al., 2015). The coastal watershed along Wakasa Bay is one of the sites most affected by trans-boundary wet N deposition in Japan (Ministry of the Environment, 2015). The Kita and Minami Rivers are the adjacent waterways with the forested catchment located in the central part of Wakasa Bay (Fig. 1). Total N (TN) concentrations in the lower part of the rivers have been increasing from the 1980s to the present, while total phosphorous (TP) concentrations have not varied through time (Fig. 2). Because river discharge has not changed over the long term, N fluxes from both rivers into the coastal sea have likely doubled in the last few decades. Efforts have been made to reduce nutrient loads from point sources as well as nonpoint sources such as fertilizer; however, few changes in land use and population densities have occurred within the basins since the 1970s, suggesting that increasing trends of TN concentrations have likely been caused by chronic atmospheric N deposition. However, the quantitative influences of atmospheric N deposition on the forested watersheds and riverine N export into the coastal sea remain unclear. To examine the linkage of atmospheric N input and riverine N export from the basin, a small-basin approach allows for the relatively precise estimation of chemical inputs and outputs (Johnson et al., 2000; Shibata et al., 2001). Although large-scale assessments can mask pronounced local variability among individual sub-basins, some large-scale patterns have indicated that riverine N export can be predicted by atmospheric N deposition rates (Howarth et al., 2002). A few studies have suggested that N-saturated forests may considerably degrade downstream water quality (Howarth et al., 2002; Tabayashi and Yamamuro, 2009; Chiwa et al., 2012). The objective of the present study was to evaluate the contribution of the current level of atmospheric N deposition to N export in two adjacent contrasting rivers. To achieve this objective, atmospheric N deposition within the both basins and N export rates from the rivers to the coastal sea were quantified. 2. Materials and methods 2.1. Study sites The Kita River system drains an area of 215.0 km2 in the central part of Japan, facing the Sea of Japan (Fig. 1B). This river is short (length = 30.3 km) and steep (maximum altitude = 962 m), and mean river discharge is 12.7 m3 s−1 . The overall land use in the drainage basin is mainly forest (80.5%) as well as agricultural lands including paddy fields and farmland (12.1%) and residential areas (3.9%). The Minami River system, which is adjacent to the Kita River, has a drainage basin of 215.4 km2 . This is also short (32.4 km) and steep (maximum altitude = 848 m), with an average discharge of 12.8 m3 s−1 . The overall land use in the drainage basin is mainly forest (93.3%) as well as agricultural lands (2.6%) and residential areas (2.0%). Please cite this article in press as: Sugimoto, R., Tsuboi, T., Seasonal and annual fluxes of atmospheric nitrogen deposition and riverine nitrogen export in two adjacent contrasting rivers in central Japan facing the Sea of Japan. J. Hydrol.: Reg. Stud. (2015), http://dx.doi.org/10.1016/j.ejrh.2015.11.019



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Fig. 2. Yearly variations in mean concentrations of total nitrogen and total phosphorous in the lower part of the Kita and Minami Rivers during normal discharge (Stns. KL and ML, Fig. 1B). Data were obtained from the Ministry of the Environment, Japan.

Both rivers empty into the western part of Obama Bay; together, their watersheds encompass 72% of the bay’s total watershed. Most of the region’s >2000 mm year−1 of annual precipitation occurs during the summer rainy season and the winter snowy season, with relatively little precipitation in between (Sugimoto et al., 2015). This climate results in significantly higher river discharge during summer and winter than at other times of year (Sato et al., 2013). In the past five years, wet deposition rates of nitrate and ammonium along the coast of Wakasa Bay (Stn. E in Fig. 1A) ranged from 500 to 672 mg m−2 year−1 and from 405 to 570 mg m−2 year−1 , respectively, with a low average pH (4.60) at Stn. E (Ministry of the Environment, 2015). 2.2. Monthly precipitation and river surveys To evaluate the total (wet + dry) deposition rate of atmospheric N, bulk samples at open sites were collected monthly from December 2012 to December 2013. One rainfall collector, consisting of a 10 l polypropylene (PE) bottle with a PE funnel (150 mm diameter) was installed at each of the five stations within the two basins (Stns. R1–R5 in Fig. 1B). After sampling, water volume collected in the bottles was measured in the laboratory, and precipitation (mm) was calculated by dividing collected water volume (ml) by the funnel area (cm2 ). Water samples for dissolved nutrients (15 ml polypropylene [PP] bottle) and ammonium concentrations (30 ml PP bottle) were filtered through syringe filters (ADVANTEC, cellulose-acetate membrane, 0.8 ␮m pore size). Samples for TN were directly collected in 50 ml PP bottles. Samples of stream water and downstream river water (Stns. KU, KL, MU, and ML in Fig. 1B) were taken monthly from December 2010 to December 2013. Together with the monthly surveys, flood samples of both downstream rivers were also collected. All water samples for dissolved nutrients (15 ml PP bottle) and ammonium concentrations (30 ml PP bottle) were immediately filtered through syringe filters (ADVANTEC, cellulose-acetate membrane, 0.8 ␮m pore size). At downstream sites, TN samples were collected in 50 ml PP bottles. We compared our measurements of total deposition to wet deposition measured at Stn. E (Fig. 1A) monitored by Japan’s Ministry of the Environment. Moreover, monitoring data for precipitation within the Kita and Minami River basins (Stns. R6–R13 in Fig. 1) were collected from Japan’s Ministry of Land, Infrastructure and Transport (MLIT) and the Fukui Prefecture Government (FPG), respectively. The daily discharge of the Kita and Minami Rivers (Stns. KL and ML) during the observation period was used as the monitoring data by the MLIT and FPG, respectively. 2.3. Chemical analysis Nitrate and nitrite concentrations were measured on an autoanalyzer (TRAACS-800, Bran–Luebbe or QuAAtro 2-HR, BLTEC) using the cadmium reduction method (Wood et al., 1967) and the naphthylenediamine method (Bendschneider and Please cite this article in press as: Sugimoto, R., Tsuboi, T., Seasonal and annual fluxes of atmospheric nitrogen deposition and riverine nitrogen export in two adjacent contrasting rivers in central Japan facing the Sea of Japan. J. Hydrol.: Reg. Stud. (2015), http://dx.doi.org/10.1016/j.ejrh.2015.11.019

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Table 1 Annual precipitation and bulk nitrogen deposition rates in each site. Site

Altitude (m)

Precipitation (mm year−1 )

TN (mg m−2 year−1 )

Nitrate (mg m−2 year−1 )

Ammonium (mg m−2 year−1 )

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 E

77 66 108 18 182 20 54 88 318 32 95 105 360 220

2201 2279 2307 2252 2839 2389 2576 2291 2765 2643 2658 2752 2943 2541

1060 1293 1420 1101 1297 – – – – – – – – –

595 723 711 604 640 – – – – – – – – 601*

343 424 469 426 616 – – – – – – – – 474*

*

Means wet nitrogen deposition.

Robinson, 1952), respectively. Ammonium concentration was measured fluorometrically using the orthophthaldialdehyde (OPA) method (Holmes et al., 1999) on a Trilogy fluorometer equipped with a CDOM/NH4 module (Model 7200-041, Turner Design). We defined dissolved inorganic nitrogen (DIN) as the sum of nitrate, nitrite, and ammonium. Fractions of nitrite in nitrate + nitrite were less than 6%. TN was determined using persulfate oxidation of all forms of N to nitrate (D’Elia and Steudler, 1977) and an autoanalayzer (TRAACS-800, Bran–Luebbe). 3. Results 3.1. Precipitation and nitrogen deposition rates Annual precipitation at Stns. R1–R5 ranged from 2252 to 2839 mm m−2 year−1 , while those at Stns. R6–R13 ranged from 2291 to 2943 mm m−2 year−1 (Table 1). These precipitation rates were positively correlated with the altitude of each sampling station (precipitation = 1.6 × altitude + 2341.1, r = 0.68, p = 0.01). To verify the accuracy of our rain fall collectors, we compared monthly precipitation at Stns. R3 and R8 (Fig. 1B), which resulted in a significant correlation (r = 0.99, p < 0.001, n = 13). Annual atmospheric deposition rates of bulk TN samples at Stns. R1–R5 ranged from 1060 to 1420 mg N m−2 year−1 , with a mean of 1234 mg N m−2 year−1 . No significant relationship was observed between annual TN deposition rates and altitude (r = 0.53, p = 0.35). Around 90% of TN comprised DIN, and nitrate dominated the DIN species (59% of DIN), although ammonium fraction at Stn. R5 was considerably high. The annual wet deposition rates of nitrate and ammonium at Stn. E in 2013 were 601 and 474 mg N m−2 year−1 , respectively, which were within the ranges of bulk nitrate and ammonium deposition rates (Table 1). The nitrate fraction in DIN (56%) was similar to the mean fraction at Stns. R1–R5. Monthly variation in spatially averaged values of precipitation and atmospheric TN deposition are shown in Fig. 3. Precipitation was high in December 2012 (303 mm m−2 month−1 ), after which it declined gradually until May 2013 (65 mm m−2 month−1 ). An abnormal increase (736 mm m−2 month−1 ) in September 2013 was driven by a typhoon. On the other hand, the atmospheric TN deposition rate increased from December 2012 (82.8 mg N m−2 month−1 ) to February 2013 (187.9 mg N m−2 month−1 ), after which it gradually decreased by June 2013 (45.4 mg N m−2 month−1 ). An increase was observed in August 2013 (137.8 mg N m−2 month−1 ). The minimum deposition rate of atmospheric TN was observed in October 2013 (39.5 mg N m−2 month−1 ). 3.2. Concentrations and fluxes of riverine nitrogen In the Kita River, DIN concentrations of the stream water (Stn. KU) and downstream river water (Stn. KL) ranged from 44.3 to 71.6 ␮M with a mean of 55.4 ␮M and from 12.9 to 65.8 ␮M with a mean of 46.5 ␮M, respectively (Fig. 4). DIN concentrations at Stn. KU were significantly higher than those at Stn. KL (p < 0.001, Wilcoxon signed-rank test). Temporal variations in DIN concentrations at Stn. KL were significantly controlled by those at Stn. KU (r2 = 0.39, p < 0.001). On the other hand, DIN at Stns. MU and ML in the Minami River exhibited similar concentrations, ranging from 9.9 to 77.2 ␮M with a mean of 26.5 ␮M and from 7.2 to 53.1 ␮M with a mean of 27.0 ␮M, respectively. DIN concentrations at stations. MU and ML were significantly correlated (r2 = 0.34, p < 0.001). In both rivers, nitrate dominated the DIN species (>99% and >84% of DIN in stream waters and downstream waters, respectively). TN concentrations at Stns. KL and ML ranged from 37.1 to 89.3 ␮M with a mean of 52.8 ␮M and from 17.0 to 62.2 ␮M with a mean of 32.2 ␮M, respectively. Mean concentrations of TN and DIN in the lower part of the Kita River were roughly 1.6 and 1.7 times higher than those in the Minami River, respectively. Please cite this article in press as: Sugimoto, R., Tsuboi, T., Seasonal and annual fluxes of atmospheric nitrogen deposition and riverine nitrogen export in two adjacent contrasting rivers in central Japan facing the Sea of Japan. J. Hydrol.: Reg. Stud. (2015), http://dx.doi.org/10.1016/j.ejrh.2015.11.019



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Fig. 3. Monthly variations in mean values of precipitation and TN deposition rates from December 2012 to December 2013. Error bars are standard deviations from five stations (Stns. R1–R5).

Fig. 4. Monthly variations in dissolved inorganic nitrogen (DIN) concentrations in the stream waters (closed circles) and downstream waters (open circles) of the Kita and Minami Rivers.

Riverine fluxes of TN and DIN in the Kita River (Stn. KL) and Minami River (Stn. ML) on each sampling date were strongly correlated with river discharge (r2 ≥ 0.97; Fig. 5). In the Kita River, the increasing rate of TN was higher than that of DIN, because large amounts of organic N were discharged from the drainage during the flood. In contrast, organic N discharge in the Minami River during the flood was not large. Using the equations in Fig. 5, we calculated the monthly fluxes of TN and DIN in the two rivers. Estimated monthly variation in TN fluxes in the rivers changed seasonally: higher in winter and Please cite this article in press as: Sugimoto, R., Tsuboi, T., Seasonal and annual fluxes of atmospheric nitrogen deposition and riverine nitrogen export in two adjacent contrasting rivers in central Japan facing the Sea of Japan. J. Hydrol.: Reg. Stud. (2015), http://dx.doi.org/10.1016/j.ejrh.2015.11.019


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Fig. 5. Relationships between river discharge and nitrogen fluxes of the Kita and Minami Rivers. Circles and squares are total nitrogen (TN) and dissolved inorganic nitrogen (DIN), respectively. Solid and dashed lines are the best-fit exponential relationships for TN and DIN, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Fig. 6. Monthly variation in river discharge and nitrogen fluxes of the Kita and Minami Rivers from December 2012 to December 2013.

lower in spring to summer (Fig. 6). In September 2013, a rapid increase in river discharge induced high loadings of TN in both rivers. Approximately 52% and 29% of TN fluxes in the Kita and Minami Rivers comprised organic N, respectively. 3.3. Annual water and nitrogen budgets at the basin scale Using the equation obtained from the relationship between annual precipitation and altitude at each station, annual precipitation in the Kita River basin (mean altitude = 285 m, basin area = 215.0 km2 ) and the Minami River basin (mean altitude = 330 m, basin area = 215.4 km2 ) can be assumed to be 608 × 106 and 625 × 106 t basin−1 year−1 , respectively (Table 2). On the other hand, annual export rates of river water from the Kita and Minami Rivers were estimated to be 348 × 106 and 416 × 106 t basin−1 year−1 , respectively. Using the mean value of annual bulk TN deposition rates (1234 mg N m−2 year−1 ), annual bulk deposition rates of atmospheric TN to the Kita and Minami River basins can be assumed to be 265 and 265 t N basin−1 year−1 , respectively. On the other hand, annual TN export rates in the Kita and Minami Rivers were 340 and 197 t N basin−1 year−1 , respectively. The potential rate of atmospheric N retention (riverine TN export/atmospheric TN deposition) in the Minami River basin was estimated to be 74%, whereas the TN export rate through the Kita River exceeded the atmospheric TN deposition rate (128%). Please cite this article in press as: Sugimoto, R., Tsuboi, T., Seasonal and annual fluxes of atmospheric nitrogen deposition and riverine nitrogen export in two adjacent contrasting rivers in central Japan facing the Sea of Japan. J. Hydrol.: Reg. Stud. (2015), http://dx.doi.org/10.1016/j.ejrh.2015.11.019

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Table 2 Annual nitrogen budgets of deposition rates from the atmosphere to both basins and export rates from the rivers to the sea at the basin scale. (A) Input from the atmosphere (t basin−1 year−1 )

(B) Export from the rivers (t basin−1 year−1 )

B/A (%)

Kita River basin Water TN DIN

608 × 106 265 239

348 × 106 340 253

57 128 106

Minami River basin Water TN DIN

625 × 106 266 239

416 × 106 197 169

67 74 71

On the other hand, 239 t N basin−1 year−1 of DIN was supplied from the atmosphere to both basins, but the export rates of DIN of the Kita River (106%) exceeded those of the Minami River basin (71%). 4. Discussion 4.1. Significance of atmospheric nitrogen deposition The atmospheric deposition rates of bulk TN changed seasonally (Fig. 3). In September 2013, heavy precipitation (>700 mm month−1 ) caused by a typhoon occupied around 27% of annual precipitation, and the deposition rate of TN was only 6% of the annual rate. This low TN deposition rate was likely due to the dilution of N concentrations, because half of the monthly precipitation was supplied on only two dates (160.0 mm day−1 on 15 September and 254 mm day−1 on 16 September). In contrast, the TN deposition rate from November to April was more than 60% of the annual TN deposition rate, although approximately 50% of annual precipitation fell during these periods. This is a typical seasonal pattern in the coastal area along the Sea of Japan (Fukuzaki et al., 2001; Kamisako et al., 2008; Miyazako et al., 2015), as the watersheds along the sea are greatly affected by seasonal westerly or northwesterly winds in winter, and these winds transport anthropogenic pollutants from northeastern Asian (Uno et al., 2007). Therefore, trans-boundary air pollution from winter to early spring substantially affects the N dynamics within the Kita and Minami River basins. More than 80% of the Kita and Minami River basins is composed of forest (Fig. 1B), which likely retains deposited N. However, when the availability of DIN exceeds the biological demands of plants and microbes, excess N accumulates in forest ecosystems and is then exported from forest soil to stream water in the form of nitrate (Aber et al., 1989, 1998). In an examination of 65 forested plots and catchments throughout Europe, Dise and Wright (1995) demonstrated that leaching of excess N accumulated in the forest soil occurred at deposition rates >1000 mg N m−2 year−1 . Aber et al. (2003) reported a deposition threshold of 700 mg N m−2 year−1 from the northeastern United States. In Japan, significant nitrate leaching occurs from forests that receive more than 1000 mg m−2 year−1 (Ohrui and Mitchell, 1997). The atmospheric TN deposition in our basins exceeded 1000 mg m−2 year−1 (Table 1), implying that excess N in the form of nitrate is likely to discharge from forest soil to the Kita and Minami Rivers and ultimately flow to the Obama Bay. In the Kita and Minami Rivers, approximately 39% and 34%, respectively, of the temporal variation in DIN concentrations in downstream waters were explained by values in stream water, as the coefficients of determination were significant for each river basin (r2 = 0.39 in the Kita River and r2 = 0.34 in the Minami River, p < 0.001). Other fractions may have been caused by removal and/or alteration within the river and also by mixing with other N sources. DIN concentrations at Stn. KU were significantly higher than those at Stn. KL (Fig. 4A), although the lower part of the Kita River is occupied by paddy field and urban area (Fig. 1B). Longitudinal survey from upstream to downstream of the Kita River revealed that concentrations and nitrogen stable isotope ratios (␦15 N) of nitrate decreased and increased during warm seasons, respectively, while there were no clear relationships in Minami River (Tsuboi, unpublished data). Since the negative relationship between ␦15 N and nitrate concentrations manifests as phenomena such as denitrifcation and nitrate assimilation by algae (Sugimoto et al., 2010, 2014), contribution of other nitrate sources such as fertilizer may be negligible in the basins. 4.2. Potential differences in nitrogen saturation in each basin Interestingly, our basin–scale approach demonstrated that potential retention rates of atmospheric N were markedly different among the basins (Table 2). Although annual rates of atmospheric DIN deposition in the Kita and Minami River basins did not differ, annual export rates of DIN from the Kita River were significantly higher than those from the Minami River, due to the concentration difference. These results suggest that the responses of the forest ecosystem to atmospheric N deposition were substantially different among the basins and that symptoms of N saturation in the Kita River basin forest are likely to be more serious than those in the Minami River basin. Although a clear linkage has been made between atmospheric N deposition and nitrate concentrations and export in surface waters, atmospheric deposition only explains about 30–38% of the spatial variation in surface water nitrate concenPlease cite this article in press as: Sugimoto, R., Tsuboi, T., Seasonal and annual fluxes of atmospheric nitrogen deposition and riverine nitrogen export in two adjacent contrasting rivers in central Japan facing the Sea of Japan. J. Hydrol.: Reg. Stud. (2015), http://dx.doi.org/10.1016/j.ejrh.2015.11.019



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trations (Aber et al., 2003). Other factors – such as climate, forest condition, vegetation type, land-use history, and hydrologic flow path – can also affect the retention or loss of N (Ohrui and Michell, 1997; Dise et al., 2009; Fukushima et al., 2011). Although it is difficult to determine the factors driving differences in N retention in each basin, several of our results imply that the regional differences in factors such as forest condition within each basin may relate to nitrate leaching and/or N retention. During the flood in September 2013, the export rate of organic N from the Kita River was considerably larger than that from the Minami River (Fig. 6). Sugimoto et al. (2013) reported that large amounts of organic materials derived from forest soil, which have lower C:N ratios (∼12), were discharged from the Kita River basin to the sea during the flood. Several studies have revealed that lower C:N ratios in surface soil in forested ecosystems facilitate nitrate leaching to stream water (Dise et al., 1998; Lovett et al., 2002). Moreover, in the Kita River basin, declines in the forest understory due to overbrowsing by Japanese deer Cervus nippon Temminck more severe compared to those in the Minami River basin (Fujiki et al., 2014). Nitrogen uptake by vegetation is also an important factor regulating nitrate leaching (Vitousek and Reiners, 1975). These basin differences could affect not only nitrate concentrations in stream waters (Fig. 4) and potential N retention rates in each basin (Table 2), but also the increasing rate of long-term variation in TN concentrations (Fig. 2). However, further studies are needed to more accurately identify candidate factors. 4.3. Implications for the effect of atmospheric nitrogen on the coastal ecosystem Nitrogen deposition directly onto water surfaces of estuaries and coastal waters can be substantial although difficult to measure (Howarth et al., 2002). If we use a TN deposition rate of 1234 mg m−2 year−1 on land, the direct deposition rate of atmospheric TN onto Obama Bay (area = 58.7 km2 ) is estimated as 72 t N year−1 . This rate is approximately 14% of riverine TN export rates. However, N in Obama Bay was supplied from not only river water but also groundwater and oceanic water (Sugimoto et al., 2015). These authors also determined that DIN fluxes from the ocean and groundwater into Obama Bay are approximately 77% and 265% of riverine DIN flux, respectively; therefore, the contribution of direct TN deposition as an N source for Obama Bay would be negligible compared to other sources. Riverine transport of N from land to coastal seas is a significant process, and accelerated N cycles can alter nutrient conditions in coastal ecosystems (Howarth, 1998; Galloway and Cowling, 2002; Sugimoto et al., 2011). Although eutrophication and hypoxia have not been reported in Obama Bay to date, the key limiting nutrient for primary production in the bay has changed over the last few decades. According to observations from 1993 to 1994, the annual average ratio of DIN to dissolved inorganic phosphorous (N/P) was 10.9 (Hata, unpublished data), suggesting that phytoplankton production was limited by N. However, recent N/P ratios have exceeded the Redfield ratio (N/P = 16), suggesting P-limited phytoplankton production (Sugimoto et al., 2015). The increase in riverine N loading relative to P in the last few decades (Fig. 2) may have caused the shift from N-limitation to P-limitation for primary production. Moreover, the recent increase in anthropogenic emissions of reactive N from northeastern Asia and the subsequent enhanced deposition to the North Pacific Ocean, particularly to the Sea of Japan, have led to a detectable increase in nitrate concentrations (Kim et al., 2014). Considering that the nutrient supply from the Sea of Japan greatly affects nutrient dynamics in Obama Bay, an N-rich nutrient supply from the ocean to Obama Bay as well as from adjacent rivers have caused the shift from N- to P-limitation in the bay. 5. Conclusion Nitrogen saturation in the two basins could be caused in part by the long-range transport of anthropogenic N compounds from northeastern Asia, resulting in the increase in atmospheric N deposition during the last few decades. However, environmental differences in each forest ecosystem may have invoked differences in nitrate export rates from forest soil to stream water. Because the nitrate concentrations of stream waters in each basin have strongly affected concentrations in downstream waters, reductions in nitrate exports from N-saturated forests may serve as a strategy to improve downstream nitrate pollution and to mitigate P-limitation conditions for primary production in Obama Bay. Reactive N emission is expected to further increase by 2050 (Galloway et al., 2004); therefore, intercontinental-scale actions are required to reduce atmospheric N emissions. Conflict of interest The authors have no conflict of interest. Acknowledgments We thank Dr. K. Fukushima of the Tokyo Metropolitan University for his helpful discussion and suggestions. This research was supported in part by JSPS KAKENHI Grant Number 22710014 and was performed with the support of the research project ¨ the Research Institute “Human-Environmental Security in Asia-Pacific Ring of Fire: Water-Energy-Food Nexus (R-08-Init)at for Humanity and Nature (RIHN), Kyoto, Japan. Please cite this article in press as: Sugimoto, R., Tsuboi, T., Seasonal and annual fluxes of atmospheric nitrogen deposition and riverine nitrogen export in two adjacent contrasting rivers in central Japan facing the Sea of Japan. J. Hydrol.: Reg. Stud. (2015), http://dx.doi.org/10.1016/j.ejrh.2015.11.019



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Please cite this article in press as: Sugimoto, R., Tsuboi, T., Seasonal and annual fluxes of atmospheric nitrogen deposition and riverine nitrogen export in two adjacent contrasting rivers in central Japan facing the Sea of Japan. J. Hydrol.: Reg. Stud. (2015), http://dx.doi.org/10.1016/j.ejrh.2015.11.019