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Abstract

Japanese cedar (Cryptomeria japonica) and Japanese cypress (Chamaecyparis obtusa) plantations account for approximately 30% of the total forested area in Japan. Both are arbuscular mycorrhizal trees that leach more NO₃⁻ in response to nitrogen (N) deposition than do forests of ectomycorrhizal trees. However, little information is available about the size of N exports from these plantations. The aim of this study was to evaluate nonpoint source N exports from a N‐saturated plantation. We collected stream water samples in base‐flow (25 samples) and storm‐flow conditions (20 events) in a watershed (2.98 ha) where Japanese cypress and Japanese cedar were planted in 1969 (41 yr old). The annual NO₃⁻ export was calculated from load–discharge relationships. Atmospheric N deposition was also determined. The stream water contained high NO₃⁻ concentrations (160 and 165 μmol L⁻¹ during base flow and storm flow, respectively), indicating N saturation in the watershed. High bulk atmospheric N deposition (16.5 kg N ha⁻¹ yr⁻¹) could explain the N saturation. There were only small variations in NO₃⁻ concentrations in stream water in response to discharge volume, because of the N saturation of the forest ecosystem. Consequently, there were only small errors in estimating annual NO₃⁻ exports from the studied watershed. The annual NO₃⁻ export was high (36.1 kg N ha⁻¹ yr⁻¹), comparable to values reported for agricultural and urbanized areas. These results suggest that N‐saturated forest plantations can become important nonpoint N sources. Our results also suggest that N exports from forest plantations across Japan should be quantified to evaluate nonpoint source N accurately.

Abbreviations

AM: arbuscular mycorrhizal
ECM: ectomycorrhizal
YEW: Yayama Experimental Watershed

Nonpoint‐source pollution, defined as diffuse pollution, is a major source of pollutants in water (Carpenter et al., 1998; David and Gentry, 2000). Forests are thought to export less nonpoint source nitrogen (N) than do agricultural and urban areas. Forests can mediate downstream water quality (Allan et al., 1997; Wickham et al., 2002; Floyd et al., 2009). In temperate forests, large amounts of N derived from atmospheric N can be retained in the vegetation and soils (Howarth et al., 1996; Norton and Fisher, 2000), where N is usually a limiting nutrient (Vitousek and Howarth, 1991; Fenn et al., 1998; LeBauer and Treseder, 2008). As forests generally occupy a large proportion of the total land area in many countries, including Japan (67%), they play a significant role in maintaining stream water quality (Wickham et al., 2002).

Forests can, however, become important nonpoint N sources, leading to downstream eutrophication. Elevated N deposition has altered N cycling in temperate forests (Vitousek et al., 1997). The global rate of anthropogenic N emissions has dramatically increased since 1960 (Galloway, 2005), especially in Asia (Akimoto, 2003; Ohara et al., 2007). If the amount of N exceeds the biotic demand, then excess N leaks from forest ecosystems (Aber et al., 1989; Vitousek et al., 1997; Fenn et al., 1998), resulting in high nitrate (NO₃⁻) concentrations in stream water. This may cause eutrophication downstream (Chiwa et al., 2012).

Among many kinds of forests, Japanese cedar (Cryptomeria japonica) and Japanese cypress (Chamaecyparis obtusa) plantations can become large nonpoint N sources under elevated atmospheric N deposition in Japan. Midgley and Phillips (2014) reported that arbuscular mycorrhizal (AM) forests leach more NO₃⁻ in response to N deposition than do ectomycorrhizal (ECM) forests. In AM‐dominated stands, there is an inorganic nutrient economy as a result of the rapid mineralization of plant‐derived carbon and nutrients. In contrast, there is an organic nutrient economy in ECM‐dominated stands because of the slow turnover of plant‐derived C and enhanced root–rhizosphere couplings (Phillips et al., 2013). The rapid C mineralization in AM‐dominated stands results from the high chemical quality of AM needle litter, resulting in a lower soil C/N ratio in AM‐associated forests than in ECM‐associated forests (Phillips et al., 2013). The lower C/N ratio of the forest floor increases the risk of NO₃⁻ leaching because of the high rate of nitrification (Gundersen et al., 1998). Japanese cedar and Japanese cypress plantations are AM forests (Yamato and Iwasaki, 2002) that account for 18 and 10% of the total forested area in Japan, respectively.

In addition to mycorrhizal associations, the characteristics of the forest, including stand age, can significantly affect N loss or retention under similar N inputs (Ohrui and Mitchell 1997; Nakahara et al., 2010; Fukushima et al., 2011). The maturation of forests can accelerate NO₃⁻ leaching because N uptake is lower in older stands (Ohrui and Mitchell, 1997; Nakahara et al., 2010). Many Japanese cedar and Japanese cypress plantations were established in the 1950s and 1960s, and are now reaching maturity (>40 yr old) (Forest Agency of Japan, 2009). Poor management of these plantation forests, including inadequate forest thinning, may suppress the rate of tree growth rate because the unmanaged plantations are becoming increasingly dense (Matsushita et al., 2010; Onda et al., 2010). Many plantation forests have been poorly managed in Japan in recent years. Therefore, there is the potential risk that Japanese cedar and Japanese cypress plantations will become large nonpoint N sources under elevated atmospheric N deposition in Japan.

At present, little information is available concerning the amount of N exported from these plantations in Japan (Ohrui and Mitchell, 1997; Chiwa et al., 2010b), even though high concentrations of NO₃⁻ have been detected in headwater streams in plantations located in suburban areas where atmospheric N deposition is high (Shibata et al., 2001; Zhang et al., 2008; Ito et al., 2004; Nakahara et al., 2010; Yoshinaga et al., 2012). Therefore, it is important to understand the size of nonpoint source N exports from forests.

Precise estimates of annual N exports are critical for evaluating nonpoint source N exports from forested watersheds. Storm‐flow sampling is important for estimating annual N exports (Kuroda et al., 1991; Swistock et al., 1997; Chiwa et al., 2010a; Ide et al., 2012). The N exports during storm‐flow periods can make a large contribution to the annual total export if the N concentrations in stream water differ between storm‐ and base‐flow periods. Sum of load (ΣL_total)–sum of discharge (ΣQ_total) relationships can be used to extrapolate unmeasured storm‐flow N exports from storm‐flow data (Ebise, 1984; Chiwa et al., 2010a).

The aim of this study was to evaluate nonpoint source N exports from a N‐saturated plantation of Japanese cedar and Japanese cypress in Japan. The specific objectives were as follows: (i) to analyze inorganic nitrogen concentrations of stream water from the plantation forest and estimate atmospheric N deposition, and (ii) to quantify the N outputs from the plantation forest.

Materials and Methods

Study Site

This study was conducted at the Yayama Experimental Watershed (YEW; 33°31′ N, 130°39′ E; 2.98 ha; 300–400 m asl; Fig. 1), western Japan. The YEW suburban forest is situated 30 km west of Fukuoka, a large metropolitan area in western Japan. Mountain streams flow from the YEW into the Sea of Japan via the Onga River (Fig. 1b). The overstory vegetation in the watershed is dominated by Japanese cedar and Japanese cypress. Detailed information for the plantation is shown in Table 1. The trees were planted in 1969 and were 41 yr old at the start of the study in July 2010. The yield ratio, the ratio of observed volume to maximum volume for stands of equivalent mean dominant height (Ando, 1968; Newton, 1997), was calculated to evaluate the potential growth of the Japanese cedar and Japanese cypress trees. The yield ratio was 0.87 and 0.88 for Japanese cedar and Japanese cypress, respectively, indicating that this plantation already had sufficient stem volume per ground area as the value of 0.8 is considered to be dense enough to indicate a need for thinning (Ando, 1968). This suggests that tree growth is suppressed in the plantation. Cambisol soils overlay granitic bedrock at the study site. The soil C/N ratio in the stand was 15 (standard deviation 1.0; n = 5) for the A layer and 12 (standard deviation is 0.6; n = 4) for the B layer (J. Takahashi J., personal communication, 2014). The mean annual temperature and precipitation at the watershed during the study period (July 2010–June 2013) were 13.4°C and 2351 mm. Figure 2 shows the seasonal variations of temperature and precipitation in the watershed. Details of vegetation properties and meteorological data in the watershed were described by Saito et al. (2013).

**Figure 1**
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Location of the Yayama Experimental Watershed (YEW). Stream water samples were collected approximately 3 m upstream from the weir (white circle) and bulk precipitation samples were collected at the meteorological station (white triangle).

Table 1. Properties of Japanese cedar and Japanese cypress plantations in the Yayama Experimental Watershed.

	Age	No of trees	Tree density	Mean height	Mean DBH†	Mean volume	Yield ratio
	yr	no.	trees ha⁻¹	m	cm	m³ ha⁻¹
Japanese cedar	41	1300	1227	20.8	26.1	635	0.87
Japanese cypress	41	2645	1640	16.2	19.7	433	0.88

† DBH, diameter at breast height.

**Figure 2**
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Monthly variation of precipitation volume and temperature at the Yayama Experimental Watershed. Bars and circles show precipitation and temperature, respectively.

Water Sample Collection and Analysis

We sampled stream water in base‐flow conditions at various times (typically monthly) above a gauging station equipped with combination of a Parshall flume and V notch weir, which was placed at the mouth of the YEW (300 m asl). Base‐flow samplings were conducted 25 times from March 2011 to December 2011. Storm‐flow sampling is important for estimating the annual exports of solutes, including dissolved inorganic N (Swistock et al., 1997; Chiwa et al., 2010a). Therefore, storm‐flow sampling was conducted with an automatic sampler (SIGMA 900, Hach) every 2 to 5 h during a storm‐flow period that included the rising and falling limbs of the hydrograph. Water sampling was triggered by an increase in the water level of the stream after rainfall started, and approximately 500 mL was automatically collected every 2 to 5 h during the storm period. In total, 24 samples were collected in a storm‐flow period, and 6 samples were selected according to changes in discharge for use in chemical analyses. Storm‐flow sampling was conducted 20 times between July 2010 and October 2011. Water levels at the gauging station were continually measured every 10 min using a capacitance probe (WT‐HR 500, Trutrack Ltd.).

Bulk precipitation was collected for 3 yr from July 2010 to June 2013. A polyethylene funnel with a diameter of 120 mm was used to collect bulk precipitation. The funnel and collector were installed in an open area of the meteorological station, which was approximately 300 m from the watershed (Fig. 1c).

The samples were transported to the laboratory within approximately 3 h of being measured at the field site. Aliquots of stream water were filtered through precleaned 0.7‐μm glass‐fiber filters (Whatman, GF/F) and then used for dissolved total N and phosphorus determinations. The filtered samples were also passed through a 0.45‐μm membrane filter (Chromatodisc 25A, GL Science) before measurement of major ions (Cl^–, NO₃^–, SO₄^2–, Na⁺, NH₄⁺, K⁺, Mg²⁺, and Ca²⁺). Major ions were analyzed by ion chromatography (DX‐120, Dionex).

Annual Flux Calculations

Annual exports of NO₃⁻ were calculated as the sum of the exports during base‐flow and storm‐flow conditions. Partitioning of base‐flow and storm‐flow conditions was conducted using the method described by Hewlett and Hibbert (1967). The volume of storm flow in each storm event included both base flow and direct flow. The amount of N exported during base‐flow periods was calculated by the LQ regression method (Ide et al., 2012). The regression is expressed as follows:

$urn:x-wiley:00472425:equation:jeq2jeq2014090401-math-0991$

where L is load (μmol m⁻² s⁻¹), Q is discharge (mm s⁻¹), and a and b are empirical parameters.

The amount of N exported during storms was calculated from the sum of load (ΣL_gross)–sum of discharge (ΣQ_gross) relationship established by Ebise (1984):

$urn:x-wiley:00472425:equation:jeq2jeq2014090401-math-0002$

where a′ and b′ are empirical parameters. Details of the calculation are described in our previous study (Chiwa et al., 2010a).

Annual deposition (flux) of each chemical species via bulk precipitation (kg ha⁻¹ yr⁻¹) was calculated by multiplying volume‐weighted mean concentrations (mg L⁻¹ × 10⁻²) by the annual precipitation depth (mm).

Meteorological Parameters

Precipitation and air temperature were continuously measured in an open area 320 m away from the watershed (Fig. 1c). The precipitation depth (mm) was measured with a tipping‐bucket rain gauge (TK‐1, Takeda Instruments), and air temperature was measured with a humidity and temperature probe (HMP 155, Vaisala).

Results

The average NO₃⁻ concentrations in stream water were 160 and 165 μmol L⁻¹ during base‐flow and storm‐flow conditions, respectively (Table 2). Figure 3 shows the seasonal variations of stream NO₃⁻ concentrations in the watershed. The NO₃⁻ concentration was relatively stable over the experimental period. The bulk atmospheric N deposition in this study area from July 2010 to June 2013 was estimated to be 16.5 kg ha⁻¹ yr⁻¹ (Table 3).

Table 2. Stream water concentrations of NO₃⁻ and NH₄⁺ at the Yayama Experimental Watershed during base‐flow and storm‐flow periods.

	NO₃⁻	NH₄⁺
	μmol L⁻¹
Base flow (n = 25)	160 (17)†	0 (0)
Storm flow (n = 20)	165 (16)	0 (0.6)

† Numbers in parentheses indicate standard deviation.

**Figure 3**
Open in figure viewer

Seasonal variation of NO₃⁻ concentrations in stream water during the study period at the Yayama Experimental Watershed.

Table 3. Atmospheric N (NO₃⁻ + NH₄⁺) deposition at the Yayama Experimental Watershed.

	Precipitation	NO₃⁻	NH₄⁺	DIN† (NO₃⁻ + NH₄⁺)
	mm	kg ha⁻¹ yr⁻¹
July 2010–June 2011	2709	6.6	16.5	23.1
July 2011–June 2012	1993	4.7	7.1	11.8
July 2012–June 2013	2353	8.9	5.7	14.6
Mean		6.8	9.8	16.5

† DIN, dissolved inorganic N.

The NO₃⁻ concentrations did not increase with increasing discharge (Fig. 4a) or with changes in the direct‐flow ratio calculated by dividing direct‐flow by total discharge (direct flow + base flow) (Fig. 4b). There were linear relationships between the logarithmic scales of ΣL_total and ΣQ_total for NO₃⁻ (Fig. 5).

**Figure 4**
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Relationships between (a) discharge and NO₃⁻ concentrations in stream water and between (b) direct‐flow ratio of total (direct‐flow + base‐flow) discharge and NO₃⁻ concentrations.

**Figure 5**
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Relationships between (a) discharge (Q) and export of NO₃⁻ (L) during base‐flow period; and between (b) total discharge (ΣQ_total) and total export of NO₃⁻ (ΣL_total) during storm‐flow period at the Yayama Experimental Watershed.

The annual export of inorganic N (mostly NO₃⁻; Table 2) from the plantation was calculated to be 36.1 kg ha⁻¹ yr⁻¹ (Table 4). This value was higher than those reported for non‐N‐saturated forested watersheds and was comparable to the values reported for agricultural and urban areas (Table 5).

Table 4. Estimates of NO₃⁻ exports at the Yayama Experimental Watershed in 2011.

	Discharge	NO₃⁻
	mm	kg ha⁻¹ yr⁻¹
Base flow	1139	25.4
Storm flow†	472	10.6
Sum	1611	36.1

† Storm flow includes base flow (319 mm [68%]) and direct flow (153 mm [32%]).

Table 5. Comparison of the export of inorganic N from forested, agricultural, and urban areas.

	Site	Input†	Output	Reference
		kg N ha⁻¹ yr⁻¹
Forest	Oregon, USA	0.79–1.24	<1	Vanderbilt et al. (2003)
	Maryland, USA	6.8	1.2	Kaushal et al. (2011)
	Ardennes region (Belgium–Luxembourg)	NA‡	4	Salvia‐Castellvi et al. (2005)
	Kyoto, Japan	5.2	1.4	Ohte et al. (2001a)
	Gunma, Japan	7.6–13.5	9.2–21.1	Ohrui and Mitchell (1997)
	Fukuoka, Japan	9.9–12.6	9.3–12.5	Chiwa et al. (2010b)
	Fukuoka, Japan	16.5	36.1	Current study
Agriculture	Maryland, USA	14	29–42§	Jordan et al. (1997)
	Ardennes region (Belgium–Luxembourg)	NA	27–33	Salvia‐Castellvi et al. (2005)
	Maryland, USA	6.8	29.8	Kaushal et al. (2011)
Urban	Maryland, USA	6.8	6.7	Kaushal et al. (2011)

† Bulk precipitation.
‡ Not analyzed.
§ As total N.

Discussion

Nitrogen Saturation in a Mature Conifer Plantation in Western Japan

The high NO₃⁻ concentrations in stream water throughout the year (Table 2, Fig. 3) indicated that the forested watershed was N saturated. This conclusion was based on a study of the relationship between NO₃⁻ concentrations in stream water and N saturation status in forest ecosystems (Stoddard 1994). The NO₃⁻ concentrations in stream water in this watershed during base‐flow periods (160 μmol L⁻¹) and storm‐flow periods (165 μmol L⁻¹) were higher than those reported for many other forested watersheds in Japan (5–50 μmol L⁻¹ [Shibata et al., 2001], 4–11 μmol L⁻¹ [Zhang et al., 2008]) and the United States (1–2 μmol L⁻¹ [Stottlemyer and Troendle, 1992], < 1 μmol L⁻¹ [Vanderbilt et al., 2003], 5 μmol L⁻¹ [Sickman et al., 2002]) but similar to those reported for urban or suburban forested watersheds in Japan in areas of high atmospheric N deposition (∼100 μmol L⁻¹ [Ohrui and Mitchell, 1997], 76 μmol L⁻¹ [Okochi and Igawa, 2001], 80–90 μmol L⁻¹ [Shibata et al., 2001], 59–66 μmol L⁻¹ [Zhang et al., 2008], 76 μmol L⁻¹ [Ito et al., 2004], 84–129 μmol L⁻¹ [Yoshinaga et al., 2012]).

High atmospheric N deposition at the study site (16.5 kg ha⁻¹ yr⁻¹; Table 3) could be one of the main reasons for the N saturation of the forested watershed. Because the watershed is located 25 km west of the Fukuoka metropolitan area (Fig. 1b), atmospheric N deposition is higher than that in more remote regions. Elevated levels of atmospheric N deposition have been reported in the urban and suburban areas of the Fukuoka metropolitan area (Fig. 1b) located near the YEW (Chiwa et al., 2010b, 2012). Elevated atmospheric N deposition due to long‐range transport of nitrogenous pollutants from the east Asian continent has also been reported (Morino et al., 2011; Chiwa et al., 2012; Chiwa et al., 2013). These pollutants could have enhanced atmospheric N deposition at the experimental site. The amount of atmospheric N deposition at the studied watershed was sufficient to induce significant N leaching from the forested area. The sigmoid relationship between atmospheric N input and output reported by Ohrui and Mitchell (1997) indicates that N inputs above 10 kg N ha⁻¹ yr⁻¹ can induce high rates of N leaching. This relationship also applies to Japanese forests, including that in the YEW (Table 5).

The N saturation in this forested watershed was related to low N retention. The annual NO₃⁻ export was estimated to be 36.1 kg N ha⁻¹ yr⁻¹ (Table 4), while bulk N deposition was 16.5 kg N ha⁻¹ yr⁻¹ (Table 3). However, it remains unclear why the annual exportation of NO₃⁻ was higher than N deposition. Bulk precipitation often considerably underestimates dry deposition (Lindberg et al., 1986; Chiwa et al., 2010b). Therefore, dry deposition should be measured to evaluate precise N budgets in a future study.

Calculation of Annual Nitrogen Exports

We estimated NO₃⁻ exports from the forested watershed. The variations in NO₃⁻ concentrations were much smaller than the variations in discharge volumes (Fig. 4a). This led to high coefficients of determination for the linear relationships between the logarithmic scales of ΣL_total and ΣQ_total and NO₃⁻ (Fig. 5). Hence, the NO₃⁻ exports during storm‐flow periods could be estimated precisely from these relationships.

The smaller variations in NO₃⁻ concentrations compared with discharge volumes could result from the N saturation of the forest ecosystem. In general, the concentrations of NO₃⁻ in stream water are higher in storm‐flow conditions than in base‐flow conditions in forested watersheds (Murdoch and Stoddard, 1993; McHale et al., 2000; Ohte et al., 2001b; Chiwa et al., 2010a). This is because of higher contributions of surface soil water to stream water under storm‐flow conditions (Creed et al., 1996; McHale et al., 2000) and higher contributions of unassimilated atmospherically derived NO₃⁻ (Lajtha et al., 1995; Michalski et al., 2004). In this study, however, the NO₃⁻ concentrations in stream water did not increase in storm‐flow conditions and were comparable to those in base‐flow conditions (Fig. 4a). Also, the NO₃⁻ concentrations in stream water did not increase with changes in the direct‐flow ratio of total discharge (Fig. 4b). The equivalent NO₃⁻ concentrations in stream water in storm‐flow and base‐flow conditions could be because the NO₃⁻ concentration in groundwater was the same as that in surface water as a result of N saturation. Under N‐saturated conditions, NO₃⁻ derived from atmospheric deposition and NO₃⁻ produced in surface soils may pass into the groundwater without being retained in the vegetation or surface layers. Supporting this view, the N concentration in deep groundwater sampled from a 20‐m‐deep well on the valley floor was comparable to that in stream water (H. Haga, personal communication, 2014).

Significance of the N‐saturated Plantation as a Nonpoint Nitrogen Source

The annual export of NO₃⁻ from the studied watershed was comparable to those reported for agricultural and urban areas (Table 5). Therefore, the N‐saturated Japanese cedar and Japanese cypress plantation had become an important nonpoint N source. Consistent with this view, in our previous survey on spatial variations in stream water quality in the Tatara River basin (Fig. 1b), which is located near the YEW, the N‐saturated upland forests dominated by Japanese cedar and Japanese cypress (>50% of the total forest area) contributed sufficient nonpoint source N to cause water eutrophication downstream (Chiwa et al., 2012). In the Shintate Experimental Watershed (8.4 ha; SEW in Fig. 1b), which is located 15 km northwest of YEW, the average NO₃⁻ concentration in stream water collected biweekly from 2009 to 2012 was also high (approximately 120 μmol L⁻¹; Chiwa et al., unpublished data). The forest at the Shintate Experimental Watershed is also dominated by Japanese cedar and Japanese cypress (81% of the total forest area).

Nonpoint source N exports from forests are becoming a larger proportion of total N exports. In Japan, the agricultural area and the number of cattle decreased during 1990 to 2010, implying a reduction in agricultural activity (Chiwa et al., 2013) and a subsequent decrease in agricultural NO₃⁻ exports. In addition, sewage treatment systems are improving in Japan, resulting in lower NO₃⁻ exports from urban areas.

The tree species also strongly affects N cycling, and hence, the species composition of the forest affects the amount of NO₃⁻ lost from forested watersheds. Lovett et al. (2004) reported variations in the N cycling characteristics among five tree species (sugar maple [Acer saccharum Marshall], American beech [Fagus grandifolia Ehrh.], yellow birch [Betula alleghaniensis Britton], eastern hemlock [Tsuga canadensis (L.) Carrière], and red oak [Quercus rubra L.]). Stands of the five species had different soil C‐to‐N ratios. The C‐to‐N ratio is related to the rate of soil N transformation, especially nitrification, which is an important factor regulating NO₃⁻ leaching. Midgley and Phillips (2014) proposed a functional grouping between AM and ECM trees to identify forests that are most susceptible to NO₃⁻ leaching. There are different nutrient economies in AM‐ and ECM‐associated forests (Phillips et al., 2013). Compared with ECM‐associated forests, AM‐associated forests have lower C‐to‐N ratios of litter and soils, leading to increased susceptibility to NO₃⁻ leaching. Because Japanese cedar and Japanese cypress plantations are AM forests (Yamato and Iwasaki, 2002), these plantations should be categorized as being susceptible to NO₃⁻ leaching. Consistent with this view, the soil C‐to‐N ratios in the studied forest were low (15 for A layer; 12 for B layer). Apart from these plantations, forest stands of sugar maple, another AM‐associated species, also have low C‐to‐N ratios in forest soil. Sugar maple plantations are recognized as NO₃⁻ generators because a strong positive relationship has been shown between nitrate concentration in soil water and the proportion of sugar maple trees in a forest stand (Lovett et al., 2004; Mitchell, 2011).

Forest maturation and suppression of tree growth in this study watershed could be another important reason for significant N leaching. The Japanese cypress and Japanese cedar trees in this study watershed were 41 yr old at the start of the study in July 2010, and forest thinning had not been conducted in recent years. Nitrogen uptake by vegetation is an important factor regulating NO₃⁻ leaching (Vitousek and Reiners, 1975). Ohrui and Mitchell (1997) demonstrated that lower N uptake in old stands can lead to lower N retention. They calculated that the difference in N uptake between 24‐ and 75‐yr‐old stands of Japanese cedar and cypress plantations was approximately 30 kg N ha⁻¹ y⁻¹, resulting in higher levels of NO₃⁻ loss from older stands. Fukushima et al. (2011) examined the N dynamics, including N uptake, of Japanese cedar plantation stands of different ages (5, 16, 31, 42, and 89 yr) and found that N uptake was higher in younger stands (16 yr) than in older stands (31, 42, and 89 yr). The difference in N uptake between the younger and older stands was approximately 20 to 30 kg N ha⁻¹ yr⁻¹. For Japanese cedar and Japanese cypress plantations, the growth rate begins to decline when a stand reaches an age of 20 to 40 yr (Nakahara et al., 2010). In Fukuoka prefecture, 68% of the forested area is Japanese cedar and cypress plantations, 77% of which are approaching maturity (>41 yr old) (Fukuoka Prefecture, 2011). The high yield ratio of Japanese cedar and Japanese cypress at this study site (Table 1) indicates that the biomass in this plantation had already nearly reached a maximum level and would not be expected to increase greatly, which may result in a lower N uptake of trees. Therefore, the maturation of Japanese cedar and cypress plantations and suppression of tree growth over recent decades may be an important factor leading to the rapid decrease in N retention rates and significant N leaching from these forests.

Our results suggest that it is necessary to evaluate nonpoint source N exports from Japanese cedar and Japanese cypress plantations across Japan. A large amount of N can leach from Japanese cedar and Japanese cypress plantations under elevated atmospheric N deposition because Japanese cedar and Japanese cypress plantation account for approximately 30% of the total forest area and these plantation forests are now reaching maturity (Forest Agency of Japan, 2009), which decreases N uptake (Ohrui and Mitchell, 1997; Fukushima et al., 2011).

Conclusions

The results of our study show that an N‐saturated Japanese cedar and Japanese cypress plantation can become an important nonpoint N source, contributing to downstream eutrophication. There were high concentrations of NO₃⁻ in stream water (160 and 165 μmol L⁻¹ during base‐flow and storm‐flow conditions, respectively), and N export was estimated to be 36.1 kg N ha⁻¹ yr⁻¹ in the YEW. The forest in this area is dominated by AM‐associated Japanese cedar and Japanese cypress trees, many of which are reaching maturity. The results of this study highlight the importance of quantifying N exports from Japanese cedar and Japanese cypress plantations because these plantations make up approximately 30% of the total forest area in Japan.

Acknowledgments

We thank Ms. Y. Endo, Mr. T. Sano, and K. Nagase (Tottori University) for handling water samples, and Ms. J. Takahashi (Tsukuba University) for analyzing soil physicochemical properties. We thank members of the Eco‐hydrology Laboratory, Kasuya Research Forest (Kyushu University), for help with fieldwork and for helpful discussions. This study was financially supported by the project “Development of Innovative Technologies for Increasing in Watershed Runoff and Improving River Environment by the Management Practice of Devastated Forest Plantation” funded by Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology (JST), and by a Grant‐in‐Aid for Scientific Research (No. 26450198). The cost of publication was supported in part by a research grant for Young Investigators of the Faculty of Agriculture, Kyushu University.

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A Nitrogen‐Saturated Plantation of Cryptomeria japonica and Chamaecyparis obtusa in Japan Is a Large Nonpoint Nitrogen Source