Owing to the elevated loading of nitrogen through atmospheric deposition,
some forested ecosystems become nitrogen saturated, from which elevated
levels of nitrate are exported. The average concentration of stream nitrate
eluted from upstream and downstream of the Kasuya Research forested
catchments (FK1 and FK2 catchments) in Japan were more than 90 µM,
implying that these forested catchments were under nitrogen saturation. To
verify that these forested catchments were under the nitrogen saturation, we
determined the export flux of unprocessed atmospheric nitrate relative to
the entire deposition flux (Matm/Datm ratio) in these catchments;
because the Matm/Datm ratio has recently been proposed as a
reliable index to evaluate nitrogen saturation in forested catchments.
Specifically, we determined the temporal variation in the concentrations and
stable isotopic compositions, including Δ17O, of stream nitrate
in the FK catchments for more than 2 years. In addition, for comparison, the
same parameters were also monitored in the Shiiba Research forested
catchment (MY catchment) in Japan during the same period, where the average
stream nitrate concentration was low, less than 10 µM. While showing
the average nitrate concentrations of 109.5, 90.9, and 7.3 µM in FK1,
FK2, and MY, respectively, the catchments showed average Δ17O
values of +2.6 ‰, +1.5 ‰, and +0.6 ‰ in FK1, FK2, and
MY, respectively. Thus, the average concentration of unprocessed atmospheric
nitrate ([NO3-atm]) was estimated to be 10.8, 5.1, and 0.2 µM in FK1, FK2, and MY, respectively, and the Matm/Datm ratio was estimated to be 14.1 %, 6.6 %, and 1.3 % in FK1, FK2, and MY,
respectively. The estimated Matm/Datm ratio in FK1 (14.1 %)
was the highest ever reported from temperate forested catchments monitored
for more than 1 year. Thus, we concluded that nitrogen saturation was
responsible for the enrichment of stream nitrate in the FK catchments,
together with the elevated NO3-atm leaching from the
catchments. While the stream nitrate concentration ([NO3-]) can be
affected by the amount of precipitation, the Matm/Datm ratio is
independent of the amount of precipitation; thus, the Matm/Datm
ratio can be used as a robust index for evaluating nitrogen saturation in
forested catchments.
Introduction
Nitrate is important as a nitrogenous nutrient in the biosphere.
Traditionally, forested ecosystems have been considered as nitrogen limited
(Vitousek and Howarth, 1991). However, owing to the
elevated loading of nitrogen through atmospheric deposition, some forested
ecosystems become nitrogen saturated (Aber et al.,
1989), from which elevated levels of nitrate are exported (Mitchell et al., 1997;
Peterjohn et al., 1996). Such excessive leaching of nitrate from forested
catchments degrades water quality and causes eutrophication in downstream
areas (Galloway
et al., 2003; Paerl and Huisman, 2009). Thus, evaluating the stage of
nitrogen saturation in each forested catchment, including its temporal
variation, is critical for sustainable forest management, especially for
forested ecosystems under high nitrogen deposition.
Both concentration and seasonal variation of stream nitrate have been used
as indexes to evaluate the nitrogen saturation of each forested catchment in
past studies (Aber,
1992; Rose et al., 2015; Stoddard, 1994). A forested stream eluted from
Fernow Experimental Forest USA, for instance, showed an elevated average
nitrate concentration of 60 µM, along with the absence of a seasonal
variation in the stream nitrate concentration, so the forest was classified
into stage 3, the highest stage of nitrogen saturation (Rose et al., 2015).
However, using both the concentration level (high or low) and seasonal
variation (clear or absent) of stream nitrate as indexes to evaluate
nitrogen saturation has limitations, including the following: (1) seasonal
variation of soil nitrate can be buffered by groundwater with long residence
time, so that the seasonal variation is unclear in stream nitrate
concentration in Japan, even in normal forests under the nitrogen saturation
stage of 0 (Mitchell et al., 1997); and (2) the stream nitrate concentration can be enriched or diluted depending on the
volume of rainfall, so the concentration level can be high in low
precipitation area irrespective of the stage of nitrogen saturation.
Nakagawa et al. (2018)
lately proposed that the Matm/Datm ratio, the export flux of
unprocessed atmospheric nitrate (Matm) relative to the deposition flux
of NO3-atm (Datm), can be an alternative, more robust
index for evaluating nitrogen saturation in each forested catchment, because
the Matm/Datm ratio directly reflects the demand for atmospheric
nitrate deposited onto each forested catchment as a whole, and thus reflects
the nitrogen saturation in each forested catchment. That is, we can expect
high Matm/Datm ratios in forested catchments under nitrogen
saturation and low Matm/Datm ratios in forested catchments with
nitrogen deficiency.
To estimate the Matm/Datm ratio accurately and precisely in each
forested catchment, the fraction of unprocessed atmospheric nitrate
(NO3-atm) in the stream needs to be estimated accurately and
precisely. Triple oxygen isotopic compositions of nitrate (Δ17O) have recently been used as a conservative tracer of
NO3-atm deposited onto each forested catchment
(Inoue
et al., 2021; Michalski et al., 2004; Nakagawa et al., 2018; Tsunogai et
al., 2014; Ding et al., 2022), showing distinctively different Δ17O from that of remineralized nitrate (NO3-re)
derived from organic nitrogen through general chemical reactions, including
microbial N mineralization and microbial nitrification. While
NO3-re, the oxygen atoms of which are derived from either
terrestrial O2 or H2O through microbial processing (i.e.,
nitrification), always shows the relation close to the “mass-dependent”
relative relation between 17O/16O ratios and 18O/16O
ratios, NO3-atm displays an anomalous enrichment in 17O
reflecting oxygen atom transfers from atmospheric ozone (O3) during
the conversion of NOx to NO3-atm
(Alexander
et al., 2009; Michalski et al., 2003; Morin et al., 2011; Nelson et al.,
2018). As a result, the Δ17O signature defined by the following
equation (Kaiser et al., 2007) enables us to
distinguish NO3-atm (Δ17O > 0) from
NO3-re (Δ17O = 0):
Δ17O=1+δ17O(1+δ18O)β-1,
where the constant β is 0.5279 (Kaiser
et al., 2007), δ18O =Rsample/Rstandard-1, and R is
the 18O/16O ratio (or the 17O/16O ratio in the case of
δ17O or the 15N/14N ratio in the case of δ15N) of the sample and each standard reference material. In addition,
Δ17O is almost stable during mass-dependent isotope
fractionation processes within terrestrial ecosystems. Therefore, while the
δ15N or δ18O signature of NO3-atm
can be overprinted by the biological processes subsequent to deposition,
Δ17O can be used as a robust tracer of unprocessed
NO3-atm to reflect its accurate mole fraction within total
NO3-, regardless of the progress of the partial metabolism
(partial removal of nitrate through denitrification and assimilation)
subsequent to deposition (Michalski
et al., 2004; Nakagawa et al., 2013, 2018; Tsunogai et al., 2011, 2014,
2018).
Past studies reported that the maximum concentration of stream nitrate was
58.4 µM in the KJ forested catchment in Japan, with the maximum value
of the Matm/Datm ratio as 9.4 % (Nakagawa et al., 2018; Sase
et al., 2022). Whether the index of the Matm/Datm ratio can be
applied to forested catchments, where the leaching of stream nitrate is much
higher than the KJ forested catchment, remained unclarified. Besides, the
advantages of the Matm/Datm ratio within the past indexes of
nitrogen saturation have not been discussed.
Chiwa (2021) has recently
reported the enrichment of nitrate of more than 90 µM on the annual
average in forested streams eluted from the
catchments (FK1 and FK2) in the
Kasuya Research Forest, Kyushu University, Japan (Fig. 1a and b). The
observed enrichment of stream nitrate implied that these forested catchments
were under nitrogen saturation. Thus, in this study, we determined the
Matm/Datm ratio in the FK1 and FK2 forested catchments by
monitoring both the concentration and Δ17O of stream nitrate
for more than 2 years to verify that these forested catchments were under
nitrogen saturation. For comparison, the
catchment (MY catchment) in Shiiba
Research Forest, Kyushu University, Japan (Fig. 1a and c), was also
monitored during the same period, where the average stream nitrate
concentration was low (less than 10 µM). Furthermore, the
Matm/Datm ratios in these forested catchments were compared with
those reported in past studies to verify the reliability of the
Matm/Datm ratio as an index of nitrogen saturation.
A map showing the locations of the study catchments (FK
and MY) in Japan (a), and the maps of FK1, FK2 (b), and MY catchments (c),
shown by orange, yellow, and green areas, respectively, together with the
sampling station A, B, and C, respectively, shown by orange, yellow, and
green circles, respectively. The blue arrows indicate the flow direction of
stream water.
MethodsStudy sites
The FK forested catchments (33∘38′ N, 130∘31′ E) are located in a suburban area about 15 km west of the
Fukuoka metropolitan area (the fourth largest metropolitan area in Japan).
The main plantation in these catchments was Japanese cedar/cypress (Table 1). The MY forested catchment (32∘22′ N, 131∘09′ E) is located in a rural area at the village of Shiiba in
southern Japan's central Kyushu mountain range. This catchment is a mixed
forest consisting of coniferous trees such as Abies firma Sieb. & Zucc. and Tsuga sieboldii Carr., as well as deciduous
broadleaved trees such as Quercus crispula Blume, Fagus crenata Blume, and Acer sieboldianum Miq. Details on the studied forested
catchments have been described in the past studies (Chiwa,
2020, 2021).
Plant information for each forested catchment (Chiwa,
2021).
Overstory vegetation (%)FK1FK2MYArtificial Japanese cedar/cypress plantation744016Other artificial coniferous plantations<1<17Natural trees105475Others1652Sampling
The stream water eluted from the FK1 (14 ha), FK2 (62 ha), and MY (43 ha)
forested catchments was collected about once every month in principle from
November 2019 to December 2021 (Fig. 1). At the FK catchments, stream water was
collected at upstream (station A) and downstream (station B) locations (Fig. 1b). At the MY catchment, stream water was collected at station C (Fig. 1c).
Samples of stream water to determine the concentration and stable isotopic
compositions (δ15N, δ18O, and Δ17O)
of stream nitrate were collected manually in bottles washed with deionized
water before sampling and then rinsed at least twice with the sample before
sampling at each sampling site.
Analysis
All the stream water samples were passed through a membrane filter (pore
size 0.45 µm) within 2 days after sampling and stored in a
refrigerator (4 ∘C) until analysis. The concentrations of nitrate
were measured by ion chromatography (Prominence HIC-SP, Shimadzu, Japan). To
determine the stable isotopic compositions of nitrate in the stream water
samples, nitrate in each sample was chemically converted to N2O using a
method originally developed to determine the 15N/14N and
18O/16O ratios of seawater and freshwater nitrate
(McIlvin and Altabet, 2005) that was later
modified (Konno
et al., 2010; Tsunogai et al., 2011; Yamazaki et al., 2011). In brief, 11 mL
of each sample solution was pipetted into a vial with a septum cap. Then,
0.5 g of spongy cadmium was added, followed by 150 µL of a 1 M NaHCO3 solution. The sample was then shaken for 18–24 h at a rate of 2 cycles s-1. Then, the sample solution (10 mL) was decanted into a
different vial with a septum cap. After purging the solution using
high-purity helium, 0.4 mL of an azide–acetic acid buffer, which had also
been purged using high-purity helium, was added. After 45 min, the solution
was alkalinized by adding 0.2 mL of 6 M NaOH. Then, the stable isotopic
compositions (δ15N, δ18O, and Δ17O)
of the N2O in each vial were determined using the continuous-flow
isotope ratio mass spectrometry (CF-IRMS) system at Nagoya University. The
analytical procedures performed using the CF-IRMS system were the same as
those detailed in previous studies (Hirota
et al., 2010; D. D. Komatsu et al., 2008). The obtained values of δ15N, δ18O, and Δ17O for the N2O derived
from the nitrate in each sample were compared with those derived from our
local laboratory nitrate standards to calibrate the values of the sample
nitrate to an international scale and to correct for both isotope
fractionation during the chemical conversion to N2O and the progress of
oxygen isotope exchange between the nitrate-derived reaction intermediate
and water (ca. 20 %). In this study, we adopted the internal standard
method to calibrate the stable isotopic compositions of sample nitrate.
Specifically, three kinds of the local laboratory nitrate standards were
used in this study, which were named to be GG01 (δ15N =-3.07 ‰, δ18O =+1.10 ‰, and Δ17O = 0 ‰),
HDLW02 (δ15N =+8.94 ‰, δ18O =+24.07 ‰), and NF (Δ17O =+19.16 ‰), which the GG01 and the HDLW02 were
used to determine the δ15N and δ18O of stream
nitrate, and the GG01 and the NF was used to determine the Δ17O
of stream nitrate. The GG01, HDLW02, and NF had been calibrated using the
internationally distributed isotope reference materials (USGS 34 and USGS
35). The oxygen exchange rate between nitrate and water during the chemical
conversion was calculated through Eq. (2):
Oxygenexchangerate(%)=Δ17O(N2O)NF/Δ17O(NO3-)NF,
where the Δ17O(N2O)NF denote the Δ17O
value of N2O that convert from the NF nitrate, and the Δ17O(NO3-)NF denote the Δ17O value of NF
nitrate (Δ17O =+19.16 ‰) (Tsunogai et
al., 2016; Nakagawa et al., 2013, 2018; Ding et al., 2022).
The δ2H and δ18O values of H2O of the stream
water samples were analyzed using the cavity ring-down spectroscopy method
by employing an L2120-i instrument (Picarro Inc., Santa Clara, CA, USA)
equipped with an A0211 vaporizer and autosampler. The errors (standard
errors of the mean) in this method were ±0.5 ‰ for
δ2H and ±0.1 ‰ for δ18O.
Both the VSMOW and standard light Antarctic precipitation (SLAP) were used
to calibrate the values to the international scale. The δ18O
values of H2O were used to calibrate the differences in δ18O of H2O between the samples and those our local laboratory
nitrate standard samples (Tsunogai
et al., 2010, 2011, 2014).
To determine whether the conversion rate from nitrate to N2O was
sufficient, the concentration of nitrate in the samples was determined each
time we analyzed the isotopic composition using CF-IRMS based on the
N2O+ or O2+ outputs. We adopted the δ15N,
δ18O, and Δ17O values only when the concentration
measured via CF-IRMS correlated with the concentration measured via ion
chromatography prior to isotope analysis within a difference of 10 %. We
repeated the analysis of δ15N, δ18O, and Δ17O values for each sample at least three times to attain high
precision. All samples had a nitrate concentration of greater than 3.5 µM, which corresponded to a nitrate quantity greater than 35 nmol in
a 10 mL sample. Thus, all isotope values presented in this study have an
error (standard error of the mean) better than ±0.2 ‰ for δ15N, ±0.3 ‰ for δ18O, and ±0.1 ‰ for Δ17O.
Nitrite (NO2-) in the samples interferes with the final N2O
produced from nitrate, because the chemical method also converts
NO2- to N2O (McIlvin and
Altabet, 2005). Therefore, it is sometimes necessary to remove
NO2- prior to converting nitrate to N2O. In this study,
however, we skipped the processes for removing NO2- because all
the stream samples analyzed for stable isotopic composition had
NO2- concentrations lower than the detection limit (0.05 µM).
Deposition rate of atmospheric nitrate
The annual deposition rate of atmospheric nitrate (Datm; total dry and
wet deposition rate of atmospheric nitrate) in each catchment was estimated
using the annual “bulk” deposition rate of atmospheric nitrate
(Dbulk) calculated in Chiwa (2020) at each catchment by multiplying the
volume-weighted mean concentration of nitrate in the bulk deposition samples
collected every 2 weeks at each catchment for 10 years (from January 2009 to
December 2018) by the annual amount of precipitation. The bulk deposition samples
were those accumulated in a plastic bucket installed in an open site of each
catchment 55 cm above the ground. The distances between the monitoring sites
of bulk deposition in the FK1, FK2, and MY forested catchments and the
stations of stream water sampling (stations A, B, and C) were 3.9, 2.9, and
4.5 km, respectively. The concentrations of nitrate in the bulk deposition
samples were measured by ion chromatography.
The Dbulk determined through this method, however, is less than
Datm (Aikawa et al., 2003) because the dry deposition velocities of
gases and particles on the water surface of the plastic bucket are smaller
than those on the forest (Matsuda, 2008). Thus, we corrected the differences
by using Eq. (3) to estimate Datm from Dbulk:
Datm=Dbulk-Ddry(W)+Ddry(F),
where Ddry(W) and Ddry(F) denote the annual dry deposition rates
onto water and forest, respectively.
The Ddry(W) and Ddry(F) at each catchment were determined using an
inferential method (Endo et al., 2011) through Eqs. (4) and (5),
respectively:
4Ddry(W)=[NO3-atm]gas×Vgas(W)+[NO3-atm]p×Vp(W),5Ddry(F)=[NO3-atm]gas×Vgas(F)+[NO3-atm]p×Vp(F),
where [NO3-atm]gas denotes the concentration of gaseous
nitrate in air; [NO3-atm]p denotes the concentration of
particle nitrate in air; Vgas(W) and Vgas(F) denote the deposition
velocities of gaseous nitrate on the water surface and forest, respectively;
and Vp(W) and Vp(F) denote the deposition velocities of
particulate nitrate on the water surface and forest, respectively. Those
determined by Chiwa (2010) using the annular denuder method from May 2006 to
April 2007 were used for the [NO3-]gas and [NO3-]p
in the FK catchments. Those determined by the National Institute for
Environmental Studies (Environmental Laboratories Association of Japan,
2017) using the filter-pack method at Miyazaki (31∘83′ N,
131∘42′ E) from 2011 to 2017 were used for the
[NO3-]gas and [NO3-]p in the MY catchment. The
Vgas(F), Vgas(W), Vp(F), and Vp(W) of each catchment
were determined by applying the estimation file for dry deposition
(Matsuda, 2008;
http://www.hro.or.jp/list/environmental/research/ies/katsudo/acid_rain/kanseichinchaku/kanseichinchaku.html, last access: 20 December 2022), where Vgas and Vp were
calculated using the meteorological data of wind speed, temperature,
humidity, radiation, cloud amount, and land use. The meteorological data
monitored by the Japan Meteorological Agency at the nearest Fukuoka station
(33∘34′ N, 130∘22′ E) and Miyazaki
station (31∘56′ N, 131∘24′ E) from 2009
to 2021 were used for the FK and MY catchments, respectively. The forested
land use of 100 % was chosen for each area.
Flux of stream water
The flux of stream water (Fstream) in each catchment was not measured
fully in this study. Instead, the water balance in each catchment was used
to estimate Fstream, assuming that the outflux of water from the study
catchments to deep groundwater was negligible:
Fstream=P-E,
where P denotes the annual average precipitation and E denotes the annual
evapotranspiration flux of water in each catchment. In this paper, the
equation obtained by H. Komatsu et al. (2008) was used to
estimate the E of the FK and MY catchments. Details on this equation are
shown below.
H. Komatsu et al. (2008) compiled the annual flux of
evapotranspiration determined in 43 forested catchments in Japan and found
that E shows a positive correlation with the average temperature
(Tavg) of each catchment. Thus, they proposed the modeled relation of E
(mm) = 31.4 Tavg (∘C) + 376 to estimate E in each forested
catchment in Japan, where the standard error of 162.3 mm was included in
the estimated evapotranspiration flux (E). They also confirmed that the
estimated Fstream using the model corresponded well with the observed
Fstream in three forested catchments, with estimated errors of less
than 6 %. As a result, we utilized the water balance method proposed by
H. Komatsu et al. (2008) to quantify the Fstream in each
catchment.
Concentration of unprocessed NO3-atm in each water sample
The Δ17O data of nitrate in each sample were used to estimate
the concentration of NO3-atm ([NO3-atm]) in
each water sample by applying Eq. (7):
[NO3-atm]/[NO3-]=Δ17O/Δ17Oatm,
where [NO3-atm] and [NO3-] denote the
concentrations of NO3-atm and nitrate (total) in each water
sample, respectively, and Δ17Oatm and Δ17O
denote the Δ17O values of NO3-atm and nitrate
(total) in the stream water sample, respectively. In this study, we used
the annual average Δ17O value of NO3-atm
determined at the Sado-Seki monitoring station in Japan (Sado Island; Fig. 1a) from April 2009 to March 2012 (Δ17Oatm=+26.3 ‰; Tsunogai et al., 2016)
for Δ17Oatm in Eq. (7) to estimate
[NO3-atm] in the stream. We allow for an error range of 3 ‰ in Δ17Oatm, where the factor changes
in Δ17Oatm from +26.3 ‰ caused by
both areal and seasonal variations in the Δ17O values of
NO3-atm have been considered (Nakagawa
et al., 2018; Tsunogai et al., 2016; Ding et al., 2022).
The annual export flux of unprocessed NO3-atm per unit area
of the catchment (Matm) was determined by applying Eq. (8):
Matm=[NO3-atm]avg×Fstream,
where [NO3-atm]avg denotes the annual average
[NO3-atm] in each stream. The index of nitrogen saturation
(Matm/Datm ratio) was calculated by dividing Matm with
Datm in each catchment.
Concentration and isotopic compositions of stream nitrate eluted only from the FK2 catchment
The concentration and isotopic compositions (δ15N, δ18O, and Δ17O) of stream nitrate determined at station B
were the mixtures of those eluted from FK1 and FK2 catchments (Fig. 1b).
Assuming that the stream nitrate eluted from FK1 catchment was stable during
the flow path from station A to station B, the concentration of stream
nitrate eluted from the FK2 catchment was determined by applying Eq. (9):
[NO3-]FK2=([NO3-]FK1+FK2⋅FFK1+FK2-[NO3-]FK1⋅FFK1)/FFK2,
where FFK1, FFK2, and FFK1+FK2 denote the flux of stream
water eluted from the FK1, FK2 (only), and FK1+FK2 catchment,
respectively. [NO3-]FK1, [NO3-]FK2, and
[NO3-]FK1+FK2 denote the concentration of stream nitrate
eluted from the FK1, FK2 (only), and FK1+FK2 catchment, respectively. In
this study, the flow rates measured at stations A and B on 15 January 2021 by
using the salt dilution method (Sappa et al., 2015) were used for FFK1
(0.85 L s-1) and FFK1+FK2 (4.75 L s-1), respectively, and the measured
[NO3-] at stations A and B was used for [NO3-]FK1
and [NO3-]FK1+FK2, respectively. Because the relation between
the measured flow rates was comparable with the relation between the
catchment area of FK1 (14 ha) and that of FK1+FK2 (76 ha), we concluded
that the measured flow rates of 0.85 L s-1 and 4.75 L s-1 were reasonable as for
those representing the FFK1 and FFK1+FK2, respectively. According
to the mass balance of water, we can estimate the FFK2 eluted from the
FK2 catchment only to be 3.90 L s-1.
Assuming that the stream nitrate eluted from the FK1 catchment was stable during
the flow path from station A to station B, the δ15N, δ18O, and Δ17O values of stream nitrate eluted from the
FK2 catchment only were determined by applying Eq. (10):
δFK2=(δFK1+FK2⋅[NO3-]FK1+FK2⋅FFK1+FK2-δFK1⋅[NO3-]FK1⋅FFK1)/([NO3-]FK2⋅FFK2),
where δFK1, δFK2, and δFK1+FK2 denote
the δ15N (or δ18O or Δ17O) of stream
nitrate eluted from the FK1, FK2, and FK1+FK2 catchment, respectively. The
δ15N (or δ18O or Δ17O) values of
stream nitrate measured at stations A and B were used for δFK1
and δFK1+FK2, respectively.
ResultsDeposition rate of atmospheric nitrate
The mean annual precipitation (P) from 2009 to 2021 was 1777 and 3981 mm
for FK and MY catchments, respectively (Chiwa, 2020; Masaaki Chiwa, personal
communication, 21 September 2022). The mean annual temperature
(Tavg) was reported to be 15.9 and 10.8 ∘C for
FK and MY catchments, respectively (Chiwa, 2020). Based on these data, the
annual flux of stream water (Fstream) was estimated to be 902.0±162.3 mm at FK catchments and 3266.1±162.3 mm at MY catchment,
respectively, using Eq. (6).
Chiwa (2020) reported the annual
bulk deposition rates of atmospheric nitrate (Dbulk) to be 34.0 mmol m-2 yr-1 at the FK catchments and 24.2 mmol m-2 yr-1 at the
MY catchment. On the other hand, the annual dry deposition rate of
atmospheric nitrate (Ddry) deposited on the forest (Ddry(F)) and
on the water surface (Ddry(W)) were estimated to be 39.9 and 4.1 mmol m-2 yr-1, respectively, at FK
catchments and 18.4 and 2.4 mmol m-2 yr-1, respectively, at MY catchment. As a result, Datm was
estimated to be 69.3 mmol m-2 yr-1 at FK catchments and 40.1 mmol m-2 yr-1 at MY catchment using Eq. (3).
Concentration and isotopic composition of stream nitrate
The concentrations of stream nitrate eluted from the FK1, FK2 (only), and MY
catchments ranged from 97.5 to 121.3 µM, from 65.7 to 148.5 µM, and from 3.5 to 15.3 µM,
respectively, with the average concentrations of 109.5, 90.9, and 7.3 µM, respectively, and the standard deviations (SD)
of 6.3, 18.5, and 3.0 µM, respectively, which
correspond to the coefficients of variation (CV) of 5.7 %, 20.4 %,
and 40.7 %, respectively (Fig. 2a). All catchments showed no clear
seasonal variation during the observation periods. The variation ranges and
the average concentrations of stream nitrate eluted from the three
catchments agreed well with the past observations performed in the same
catchments (Chiwa, 2021).
Temporal variations in concentrations of stream nitrate
(FK1: orange circles; FK2: yellow circles; MY: green circles) (a), together
with those in δ15N (b), δ18O (c), and Δ17O (d) of nitrate, and the concentration of unprocessed
NO3-atm ([NO3-atm]) (e) in the stream water of
the FK1, FK2, and MY forested catchments. Error bars smaller than the sizes
of the symbols are not presented.
The stable isotopic compositions of stream nitrate eluted from the FK1, FK2
(only), and MY catchments ranged from -0.9 ‰ to +1.5 ‰, from -1.4 ‰ to +5.8 ‰, and from -0.8 ‰ to +2.4 ‰, respectively, for δ15N (Fig. 2b), from
+3.9 ‰ to +8.5 ‰, from -2.2 ‰ to +2.8 ‰, and from -5.6 ‰ to +1.7 ‰, respectively, for
δ18O (Fig. 2c), and from +2.0 ‰ to +3.3 ‰, from +0.6 ‰ to +2.2 ‰, and from +0.2 ‰ to +1.0 ‰, respectively, for Δ17O (Fig. 2d), with
no clear seasonal variation during the observation periods. The
concentration-weighted averages for the δ15N, δ18O, and Δ17O values of stream nitrate were +0.2 ‰, +6.4 ‰, and +2.6 ‰, respectively, at FK1, +1.0 ‰,
+0.5 ‰, and +1.5 ‰,
respectively, at FK2, +0.7 ‰, -2.5 ‰, and +0.6 ‰, respectively, at
MY.
Concentration of unprocessed atmospheric nitrate and the
Matm/Datm ratio in each catchment
The concentration of unprocessed atmospheric nitrate
([NO3-atm]) in the streams eluted from the FK1, FK2 (only),
and MY catchments ranged from 8.64 to 14.30 µM, from 2.27 to 10.71 µM, and from 0.03 to 0.46 µM with the average concentration of
10.80 ± 1.30, 5.06 ± 0.67, and 0.16 ± 0.03 µM,
respectively, even though these studied catchments showed little seasonal
variations during the observation periods (Fig. 2e). The annual export flux
of nitrate (Mtotal), the annual export flux of NO3-atm
(Matm), and the Matm/Datm ratio were 98.8 ± 17.8 mmol m-2 yr-1, 9.7 ± 2.1 mmol m-2 yr-1, and 14.1 ± 4.1 % at FK1 catchment, respectively, 82.0 ± 14.8 mmol m-2 yr-1, 4.6 ± 1.0 mmol m-2 yr-1, and 6.6 ± 2.0 % at FK2 catchment, respectively, 23.7 ± 1.2 mmol m-2 yr-1, 0.5 ± 0.1 mmol m-2 yr-1, and 1.3 ± 0.4 % at MY catchment, respectively (Table 2). The uncertainties
of [NO3-atm], Matm, and Matm/Datm ratio in
each catchment were determined from the uncertainties of Δ17O,
Δ17Oatm, Fstream, and Datm according to the
equations of error propagation. The details were described in Appendix A.
Average concentrations of stream nitrate
([NO3-]), the average concentrations of unprocessed
NO3-atm in streams ([NO3-atm]), the
annual export flux of NO3- per unit area of catchments
(Mtotal), the annual export flux of NO3-atm per unit
area of catchments (Matm), the deposition flux of
NO3-atm per unit area of catchment (Datm), and the
Matm/Datm ratios in the study catchments.
Based on the air monitoring data determined at the stations of Fukuoka
(33∘51′ N, 130∘50′ E) and Miyazaki
(31∘83′ N, 131∘42′ E) from 2011 to 2017,
the Environmental Laboratories Association of Japan (2017) reported
Datm to be 57.8 mmol m-2 yr-1 at Fukuoka and 49.1 mmol m-2 yr-1 at Miyazaki. Those values are consistent with the
Datm estimated in this study (69.3 and 40.1 mmol m-2 yr-1
at the FK and MY catchments, respectively), within a difference of
approximately 20 %. Thus, we concluded that the Datm estimated in
this study was reliable within the error margin of 20 % (Table 2).
Because the Datm determined at the FK catchments was the highest among
the forested catchments in Table 3, we further compared the Datm of the
FK catchments with those from the other air monitoring stations in Japan
reported in past studies, along with that of the MY catchment (Table S1 in the Supplement).
While the Datm of the MY catchment corresponded to the average level
among the sites compiled in Table S1, the Datm of the FK catchments
exceeded the average level significantly. In addition, the Datm of the
FK catchments corresponded to one of the highest among the Japanese forested
areas (Table S1). All the catchments in Japan can be suffered from the
long-range transport of air pollutants derived from megacities in the East Asian
region (Chiwa, 2021; Chiwa et al., 2012, 2013). In addition, the shorter
transport distance from the Fukuoka metropolitan area (total population:
1.62 million people; population density: 4715 people km-2) may be mainly
responsible for the Datm higher in FK than in MY, because the FK
catchments are only 15 km west of the Fukuoka metropolitan area.
The annual amount of precipitation (P), the average
concentration of stream nitrate ([NO3-]), the nitrogen
saturation stage, the average concentration of unprocessed
NO3-atm in streams ([NO3-atm]), the
annual export flux of NO3- per unit area of catchment
(Mtotal), the annual export flux of NO3-atm per unit
area of catchment (Matm), the deposition flux of NO3-atm
per unit area of catchment (Datm), and the Matm/Datm ratio in
the FK1, FK2, and MY, along with those in the catchments studied in past
studies using Δ17O of nitrate as a tracer.
a This study.
b Nakagawa et al. (2018), Nakahara et al. (2010).
c Rose et al. (2015).
d Tsunogai et al. (2014).
e Huang et al. (2020).
f N saturation stage estimated in past studies
– means no data.
Excess leaching of unprocessed atmospheric nitrate from FK catchments
The isotopic compositions (δ15N, δ18O, and Δ17O) of stream nitrate eluted from the FK and MY catchments were
typical for those eluted from forested catchments (Hattori
et al., 2019; Huang et al., 2020; Nakagawa et al., 2013, 2018; Riha et al.,
2014; Sabo et al., 2016; Tsunogai et al., 2014, 2016). The striking features
found in the FK catchments were that, in addition to the high
[NO3-] and high Mtotal that had been clarified in a past
study (Chiwa, 2021), both [NO3-atm] and Matm in FK were
higher than those eluted from MY (Table 2). Especially, the average
[NO3-atm] in the stream eluted from the FK1 catchment was the
highest ever reported in forested streams determined through continuous
monitoring for more than 1 year (Bostic
et al., 2021; Bourgeois et al., 2018a, b; Hattori et al., 2019; Huang et
al., 2020; Nakagawa et al., 2018; Rose et al., 2015; Sabo et al., 2016;
Tsunogai et al., 2014, 2016).
The observed high [NO3-atm] in the stream eluted from the FK1
catchment could be caused just by the high [NO3-atm]
deposition in the catchment. Thus, we compiled all past data ever reported
in forested streams through continuous monitoring in Table 3, where the data
of average [NO3-], average [NO3-atm], Matm,
Mtotal, Datm, and Matm/Datm ratio were included for
comparison. The result showed that the Matm/Datm ratio, along with
Matm, was the highest as well in the FK1 catchment among the forested
catchments (Table 3).
Elevated loading of nitrogen through atmospheric deposition was responsible
for the occurrence of nitrogen saturation in forest ecosystems, from which
elevated levels of nitrate are exported (Aber et al.,
1989). Nakagawa et al. (2018) proposed that the Matm/Datm ratio can be an index for
evaluating the nitrogen saturation in each forested catchment, because the
Matm/Datm ratio directly reflects the present demand for
atmospheric nitrate deposited in each forested catchment, and thus reflects
the nitrogen saturation in each forested catchment. The high
Matm/Datm ratios observed in the FK catchments implied that the
demand for atmospheric nitrate was low in the FK catchments and that the
stages of nitrogen saturation at the FK catchments were higher than those at
other forested catchments. That is, the nitrogen saturation at the FK
catchments was responsible for the observed high [NO3-] and high
Mtotal at the FK catchments than at MY and any other catchment ever
studied (Table 3).
The stand age of forests can affect the retention or loss of N
(Fukushima et al., 2011; Ohrui and
Mitchell, 1997). Fukushima et al. (2011) evaluated N uptake rates of
Japanese cedars at different ages (5–89 years old) and demonstrated that the
N uptake rates of Japanese cedars were higher in younger stands (53 kg N ha-1 yr-1 in 16 years old) than in older stands (29 kg N ha-1 yr-1 in 31 years old; 24 kg N ha-1 yr-1 in 42 years old; 34 kg N ha-1 yr-1 in 89 years old). In addition, Yang
and Chiwa (2021) found that the nitrate concentration in the soil water
taken beneath the rooting zone of matured artificial Japanese cedar
plantations (607 ± 59 µM; 64–69 years old) was significantly
higher than that of normal Japanese oak plantations (8.7 ± 8.1 µM; 24 years old). Moreover, by adding ammonium nitrate (50 kg N ha-1 yr-1) to the forest floor directly, Yang and Chiwa (2021) found that
the nitrate concentration in the soil water of the matured artificial
Japanese cedar plantations increased significantly faster than that of the
normal Japanese oak plantations, probably because of the lower N uptake
rates in the matured artificial Japanese cedar plantations. Because most of
the artificial Japanese cedar/cypress plantations in the FK and MY
catchments have reached their maturity
(>50 years; Yang and Chiwa,
2021), the higher proportion of matured artificial Japanese cedar/cypress
plantations in the FK1 catchment (Table 1) was highly responsible for the
observed elevated leaching of nitrate, caused by the reduction in N uptake
rates.
As a result, we concluded that the FK forested catchments were under the
high nitrogen saturation stage, FK1 catchment especially, and the nitrogen
saturation in the FK1 catchment was responsible for the elevated
Mtotal, Matm, [NO3-], [NO3-atm] found in
the stream eluted from the catchment (Fig. 3a, b, c and d).
Annual export flux of nitrate per unit area (Mtotal)
plotted as a function of the Matm/Datm ratio in each forested
catchment (a); the annual export flux of unprocessed atmospheric nitrate per
unit area (Matm) plotted as a function of the Matm/Datm ratio (b); the average concentration of NO3-atm
([NO3-atm]) plotted as a function of the
Matm/Datm ratio (c); the Matm/Datm ratio plotted as a
function of the average concentration of nitrate ([NO3-]) (d); the Mtotal plotted as a function of [NO3-] (e);
the Matm plotted as a function of [NO3-] (f) (FK1:
orange circles; FK2: yellow circles; MY: green circles). Those determined
for the forested catchments in past studies are plotted as well (Qingyuan:
white circle, Huang et al., 2020; KJ, IJ1, and IJ2: white squares, Nakagawa
et al., 2018; Fernow 1, 2, and 3: white diamonds, Rose et al., 2015; Uryu:
white triangle, Tsunogai et al., 2014). The data obtained in the Qingyuan
forested catchment are shown in parentheses and excluded from the
calculation to estimate correlation coefficients (see text for the reason).
The Matm/Datm ratio as an index of nitrogen saturation
Past studies have used the concentration of stream nitrate as one of the
important indexes to evaluate the stage of nitrogen saturation in each
forest (Aber,
1992; Huang et al., 2020; Rose et al., 2015; Stoddard, 1994). The strong
linear relationship (R2=0.76; P<0.0001) between the stream
nitrate concentration and the Matm/Datm ratio, except for the
Qingyuan forested catchment (Fig. 3d), further supported that the
Matm/Datm ratio can be used as an alternative index of nitrogen
saturation, as pointed out in Nakagawa et al. (2018).
The differences in the number of storm and/or snowmelt events could affect
the Matm/Datm ratio as well, because NO3-atm could
be injected into the stream water directly, along with the storm/snowmelt
water (Tsunogai et al., 2014; Ding et al., 2022; Inamdar and Mitchell,
2006). In a recent study, however, we found that storm events have little
impact on the Matm/Datm ratio, based on monitoring temporal
variation of [NO3-atm] in stream water during storm events
(Ding et al., 2022). In addition, the low Matm/Datm ratio found
in Uryu forested catchment (0.7 %; Table 3) implied that the snowmelt has
little impact on the Matm/Datm ratio as well, because 30 % of
the annual mean precipitation was snow in Uryu forested catchment (Tsunogai
et al., 2014).
The differences in the amount of precipitation, temperature, and the flux of
stream water could affect the Matm/Datm ratio as well. As a
result, the annual amount of precipitation, mean temperature, and the annual
mean flux of stream water (Fstream) in the forested catchments were
compiled in Table S2. While the stream nitrate concentration showed a strong
linear relationship (R2=0.76; P<0.0001) with the
Matm/Datm ratio (Fig. 3d), the precipitation, temperature, and
Fstream did not show a significant relationship with the
Matm/Datm ratio (P>0.14; Fig. 4). As a result, we
concluded that the Matm/Datm ratio was mainly controlled by the
progress of nitrogen saturation, rather than the differences in the number
of storm and/or snowmelt events, the amount of precipitation, temperature,
and the flux of stream water.
The Matm/Datm ratio plotted as a function of
the amount of precipitation (a), the Matm/Datm ratio plotted as a
function of the temperature (b), and the Matm/Datm ratio plotted
as a function of flux of stream water (c) (FK1: orange circles; FK2: yellow
circles; MY: green circles). Those determined for the forested catchments in
past studies are plotted as well.
Schematic diagram showing the biogeochemical processing
of nitrate in forested catchments under high precipitation (a) and low
precipitation (b), where NO3-atm (unprocessed atmospheric
nitrate) is represented by pink circles, NO3-re by yellow
circles, the flows of NO3-atm by pink arrows, and those of
NO3-re (remineralized nitrate) by yellow arrows (modified
after Nakagawa et al., 2018). Although the deposition rates of
NO3-atm (Datm) and the biogeochemical reaction rates
between (a) and (b) are the same, we can expect high [NO3-] in (b). On the other hand, the Matm/Datm ratio between (a) and (b) are the same.
The differences in the residence time of water in each catchment could also
impact the Matm/Datm ratio, as the residence time of water in
forested catchments ranges from one month to more than one year (Asano et
al., 2002; Farrick and Branfireun, 2015; Kabeya et al., 2008; Rodgers et
al., 2005; Soulsby et al., 2006; Tetzlaff et al., 2007). While the Matm/Datm ratio
could be higher in catchments with a shorter water residence time, it is difficult to
explain high [NO3-] and high Mtotal eluted from the catchment
by the residence time of water alone, as the
majority of nitrate eluted from the catchment with a high
Matm/Datm ratio was NO3-re produced by microbial
nitrification. The significant correlation between Mtotal and
Matm/Datm ratios (P<0.0001; Fig. 3a) supported nitrogen
saturation as the leading cause of high Mtotal in catchments with a
high Matm/Datm ratio. Additionally, the high loading
of atmospheric nitrogen, the type of plantation, and the old age of
plantation in the FK1 catchment all supported the conclusion that the FK1
catchment was under nitrogen saturation.
The Matm/Datm ratio is a more reliable and robust index than the
stream nitrate concentration, as explained below. The Qingyuan forested
catchment can be classified into the highest nitrogen saturation stage based
only on the highest stream nitrate concentration of 150 µM (Table 3).
However, based on the leaching flux of nitrogen via stream water monitored
by Huang et al. (2020) for 4 years in the Qingyuan forested catchment, along with the deposition flux of
nitrogen, we can obtain the Matm/Datm ratio in the catchment to be
a medium level of 5.8 ± 1.3 %, implying that the nitrogen
saturation stage was not so high (Table 3). Huang et al. (2020) also
concluded that the input of nitrogen exceeded the output in the catchment,
and thus, the catchment was at stage 2 of nitrogen saturation. The
Matm/Datm ratio in the Qingyuan forested catchment with a medium
level among all forested catchments (Fig. 3d) should be a more reliable
index of nitrogen saturation.
Compared with those in the other forested catchments in Table 3, the annual
amount of precipitation (P) has the lowest value of 709 mm in the Qingyuan
forested catchment. The flux of stream water (Fstream) has the lowest
value of 309 mm as well. Thus, we concluded that nitrate was relatively
concentrated in the catchment because of the small precipitation, resulting
in relative enrichment in the concentrations of both nitrate (150 µM)
and unprocessed atmospheric nitrate (8.9 µM) in the stream.
While the concentration of stream nitrate, as an index of nitrogen
saturation traditionally, can be influenced by the amount of precipitation,
as demonstrated in the Qingyuan forested catchment, the Matm/Datm
ratio is independent of the amount of precipitation (Fig. 5). Therefore, the
Matm/Datm ratio can be used as a more robust index for evaluating
nitrogen saturation in each forested catchment.
Conclusions
Both the concentrations and Δ17O of stream nitrate were
determined for more than 2 years in the forested catchments of FK (FK1 and
FK2) and MY to determine the Matm/Datm ratio for each catchment.
The FK catchments exhibited higher Matm/Datm ratio than the MY
catchment and other forested catchments reported in past studies, implying
that the progress of nitrogen saturation in the FK catchments was severe.
Both age and proportion of artificial plantation in the FK catchments were
responsible for the progress of nitrogen saturation. In addition, although
past studies have commonly used the concentration of stream nitrate as an
index to evaluate the progress of nitrogen saturation in forested
catchments, it can be influenced by the amount of precipitation. As a
result, we concluded that the Matm/Datm ratio should be used as a
more reliable index for evaluating the progress of nitrogen saturation
because the Matm/Datm ratio is independent from the amount of
precipitation.
Calculating of uncertainties in the values of
[NO3-atm], Matm, and Matm/Datm ratio
The uncertainty in the values of [NO3-atm] was estimated from
the uncertainties in the Δ17O values of stream nitrate (Δ17O) and NO3-atm (Δ17Oatm) according
to the divisive equation of error propagation (Eq. A1):
σ[NO3-atm]=[NO3-]⋅1Δ17Oatm⋅σΔ17O2+Δ17OΔ17Oatm2⋅σΔ17Oatm2,
where σ[NO3-atm],
σΔ17O, and
σΔ17Oatm
denote the uncertainties in [NO3-atm], Δ17O
values of stream nitrate, and Δ17O values of
NO3-atm, respectively. The standard error of the mean (SE) of
±0.1 ‰ and the areal/seasonal variations of
±3 ‰ was used in calculating σΔ17O and σΔ17Oatm, respectively.
As a result, the uncertainty in [NO3-atm] (σ[NO3-atm]) was
±1.30, ±0.67, and ±0.03µM at FK1, FK2, and MY
catchments, respectively.
The uncertainty in the values of Matm was estimated from the
uncertainties in [NO3-atm] and in Fstream according to
the multiplicative equation of error propagation (Eq. A2):
σMatm=(Fstream⋅σ[NO3-atm])2+([NO3-atm]⋅σFstream)2,
where σMatm, σ[NO3-atm], and
σFstream denote the uncertainties in Matm,
[NO3-atm], and Fstream, respectively.
H. Komatsu et al. (2008) proposed the uncertainty in
Fstream to be ±162.3 mm when using the water balance method in
estimating Fstream. Here, the uncertainty in Matm (σMatm) was ±2.1, ±1.0, and ±0.1 mmol m-2 yr-1 at FK1, FK2, and MY catchments, respectively.
The uncertainty in Matm/Datm ratio was estimated from the
uncertainties in Matm and in Datm according to the divisive
equation of error propagation (Eq. A3):
σMatm/Datmratio=1Datm⋅σMatm2+MatmDatm2⋅σDatm2,
where σMatm/Datmratio, σMatm, and
σDatm denote the uncertainty in
Matm/Datm ratio, Matm, and Datm, respectively. Comparing
the deposition rate of NO3-atm obtained at the other
atmospheric monitoring stations nearby, the uncertainty of 20 % was
adopted for those of Datm in each catchment, which corresponds to the
uncertainty in Datm of ±13.9, ±13.9, ±8.0 mmol m-2 yr-1 at FK1, FK2, and MY catchments, respectively. As a
result, the uncertainty in Matm/Datm ratio was ± 4.1 %,
±2.0 %, and ±0.4 % at FK1, FK2, and MY catchments,
respectively.
Data availability
All the primary data are presented in the Supplement. The other data are
available upon request to the corresponding author (Weitian Ding).
The supplement related to this article is available online at: https://doi.org/10.5194/bg-20-753-2023-supplement.
Author contributions
UT, FN, KS, and MC designed the study. MC and TK performed the field
observations. WD, UT, and FN determined the concentrations and isotopic
compositions of the samples. WD, TS, FN, and UT performed data analysis, and
WD and UT wrote the paper with input from MC, TK, and KS.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We thank the anonymous referees for their valuable remarks on an earlier version of
this paper. We also thank Daisuke Nanki, Takuma Nakamura and Yuko Muramatsu
for their long-term water sampling. Additionally, we are grateful to the
members of the Biogeochemistry Group, Graduate School of Environmental
Studies, Nagoya University, for their valuable support throughout this
study. This work was supported by a Grant-in-Aid for Scientific Research
from the Ministry of Education, Culture, Sports, Science, and Technology of
Japan under grant numbers 22H00561, and 17H00780, the Yanmar Environmental
Sustainability Support Association, and the River Fund of the River
Foundation, Japan. Weitian Ding would like to take this opportunity to thank
the Nagoya University Interdisciplinary Frontier Fellowship supported by
Nagoya University and JST, the establishment of university fellowships
towards the creation of science technology innovation, Grant Number
JPMJFS2120.
Financial support
This research has been supported by the Ministry of Education, Culture, Sports, Science, and Technology of
Japan (grant nos. 22H00561 and 17H00780); the Yanmar Environmental
Sustainability Support Association; the River Fund of the River
Foundation, Japan; and the Nagoya University and JST (grant no. JPMJFS2120).
Review statement
This paper was edited by Perran Cook and reviewed by two anonymous referees.
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