Effects of mountain tea plantations on nutrient cycling at upstream watersheds

Abstract

The expansion of agriculture to rugged mountains can exacerbate negative impacts of agricultural activities on ecosystem function. In this study, we monitored streamwater and rainfall chemistry of mountain watersheds at the Feitsui Reservoir Watershed in northern Taiwan to examine the effects of agriculture on watershed nutrient cycling. We found that the greater the proportion of tea plantation cover, the higher the concentrations of fertilizer-associated ions (NO₃^-, K⁺) in streamwater of the four mountain watersheds examined; on the other hand, the concentrations of the ions that are rich in soils (SO₄^2-, Ca²⁺, Mg²⁺) did not increase with the proportion of tea plantation cover, suggesting that agriculture enriched fertilizer-associated nutrients in streamwater. Of the two watersheds for which rainfall chemistry was available, the one with higher proportion of tea plantation cover had higher concentrations of ions in rainfall and retained less nitrogen in proportion to input compared to the more pristine watershed, suggesting that agriculture can influence atmospheric deposition of nutrients and a system's ability to retain nutrients. As expected, we found that a forested watershed downstream of agricultural activities can dilute the concentrations of NO₃^- in streamwater by more than 70 %, indicating that such a landscape configuration helps mitigate nutrient enrichment in aquatic systems even for watersheds with steep topography. We estimated that tea plantation at our study site contributed approximately 450 kg ha^-1 yr^-1 of NO₃-N via streamwater, an order of magnitude greater than previously reported for agricultural lands around the globe, which can only be matched by areas under intense fertilizer use. Furthermore, we constructed watershed N fluxes to show that excessive leaching of N, and additional loss to the atmosphere via volatilization and denitrification can occur under intense fertilizer use. In summary, this study demonstrated the pervasive impacts of agricultural activities, especially excessive fertilization, on ecosystem nutrient cycling at mountain watersheds.

Hydrol. Earth Syst. Sci., 19, 44934504, 2015 www.hydrol-earth-syst-sci.net/19/4493/2015/ doi:10.5194/hess-19-4493-2015 Author(s) 2015. CC Attribution 3.0 License.

Effects of mountain tea plantations on nutrient cycling at upstream watersheds

T.-C. Lin1, P.-J. L. Shaner1, L.-J. Wang2, Y.-T. Shih3, C.-P. Wang4, G.-H. Huang1, and J.-C. Huang3

1Department of Life Science, National Taiwan Normal University, 11677 Taipei, Taiwan

2Department of Forestry, National Taiwan University, 10617 Taipei, Taiwan

3Department of Geography, National Taiwan University, 10617 Taipei, Taiwan

4Taiwan Forestry Research Institute, Taipei, 10066 Taipei, Taiwan

Correspondence to: J.-C. Huang ([email protected])

Received: 28 March 2015 Published in Hydrol. Earth Syst. Sci. Discuss.: 4 May 2015 Revised: 12 September 2015 Accepted: 27 October 2015 Published: 9 November 2015

Abstract. The expansion of agriculture to rugged mountains can exacerbate negative impacts of agricultural activities on ecosystem function. In this study, we monitored streamwater and rainfall chemistry of mountain watersheds at the Feitsui Reservoir Watershed in northern Taiwan to examine the effects of agriculture on watershed nutrient cycling. We found that the greater the proportion of tea plantation cover, the higher the concentrations of fertilizer-associated ions (NO3,

K+) in streamwater of the four mountain watersheds examined; on the other hand, the concentrations of the ions that are rich in soils (SO24, Ca2+, Mg2+) did not increase with the proportion of tea plantation cover, suggesting that agriculture enriched fertilizer-associated nutrients in streamwater. Of the two watersheds for which rainfall chemistry was available, the one with higher proportion of tea plantation cover had higher concentrations of ions in rainfall and retained less nitrogen in proportion to input compared to the more pristine watershed, suggesting that agriculture can inuence atmospheric deposition of nutrients and a systems ability to retain nutrients. As expected, we found that a forested watershed downstream of agricultural activities can dilute the concentrations of NO3 in streamwater by more than 70 %, indicating that such a landscape conguration helps mitigate nutrient enrichment in aquatic systems even for watersheds with steep topography. We estimated that tea plantation at our study site contributed approximately 450 kg ha1 yr1 of

NO3-N via streamwater, an order of magnitude greater than previously reported for agricultural lands around the globe, which can only be matched by areas under intense fertilizer use. Furthermore, we constructed watershed N uxes to show

that excessive leaching of N, and additional loss to the atmosphere via volatilization and denitrication can occur under intense fertilizer use. In summary, this study demonstrated the pervasive impacts of agricultural activities, especially excessive fertilization, on ecosystem nutrient cycling at mountain watersheds.

1 Introduction

Agricultures expansion is taking place in some of the most rugged mountains in the world, including the Hindu Kush Himalaya (Brown and Shrestha, 2000; Tulachan, 2001), in India, China (Johda et al., 1992) and the Andes (Sarmiento and Frolich, 2002). It is well established that watershed nutrient cycling is tightly linked to land use and that conversion of natural forests to agricultural lands causes nutrient enrichment, especially of N and P, in streamwater (Omernik, 1976; Johnes, 1996; Tilman et al., 2001; Murty et al., 2002; Allan, 2004; Uriarte et al., 2011; Evans et al., 2014). The impacts are likely exacerbated by steep slopes and high precipitation as residence time is reduced and leaching potential increased under such conditions (Brouwer and Powell, 1998; Tokuchi et al., 1999). Thus, mountain agriculture in the tropics and subtropics characterized with high precipitation is likely to have a substantial negative impact on ecosystem function. Yet, empirical studies in tropical or subtropical mountain watersheds are very limited.

Published by Copernicus Publications on behalf of the European Geosciences Union.

4494 T.-C. Lin et al.: Effects of mountain tea plantations on nutrient cycling at upstream watersheds

In addition to nutrient output in streamwater, cultivation and fertilization on agricultural lands could affect atmospheric deposition of nutrients (i.e., nutrient input via wet and dry deposition). Fine particles suspended from exposed lands and volatilized gases such as NH3 from manure are scavenged by precipitation (van Breemen et al., 1982), which can then be deposited back to the watersheds. However, in contrast to the large number of reports on streamwater chemistry, few studies of watershed nutrient cycling have examined the effects of land use on precipitation chemistry.

Proper landscape conguration could potentially mitigate the negative effects of agriculture on watershed nutrient cycling. A study at the Hubbard Brook Experimental Forest demonstrated that watershed-level responses were most sensitive to areas of approximately 1020 ha surrounding the drainage area, where much of the variation in element uxes occurred (Johnson et al., 2000). Such understanding has led to the common practice of establishing riparian buffer zones as a way to remove pollutants and prevent nutrients from entering streamwater (reviewed by Muscutt et al., 1993).Through proper landscape conguration, negative impacts of agriculture on nutrient cycling in mountain watersheds may also be reduced without sacricing socioeconomic benets of agriculture. However, what constitutes a proper landscape conguration is likely to vary with climate and topography.

Here we examined the effects of mountain agriculture, mainly tea plantations, on watershed nutrient cycling at the Feitsui Reservoir Watershed (FRW) in subtropical Taiwan.We rst compared streamwater chemistry across four water-sheds within the FRW, two with substantial agricultural land use and two primarily covered with natural forests. To assess the effects of agriculture on atmospheric deposition of nutrients and its role in watershed nutrient retention, we focused on the pair of watersheds with the highest and lowest tea plantation covers and compared their rainfall chemistry in relation to streamwater chemistry. The FRW is characterized by high rainfall (> 3000 mm; Taipei Feitsui Reservoir Administration), steep slopes (on average 42 %), and heavy use of fertilizers in tea plantations (4252373 kg N ha1 yr1 and 99551 kg P ha1 yr1; Water Resources Agency, 2010; see

Sect. 2 for details). Many studies have demonstrated substantial nutrient efux and sediment production from surrounding tea plantations to the reservoir over the past 2 decades (Chang and Wen, 1997; Lu et al., 1999; Kuo and Lee, 2004;Li and Yeh, 2004; Hsieh and Yang, 2006, 2007; Zehetner et al., 2008; Chiueh et al., 2011; Wu and Kuo, 2012). Yet, to our knowledge none examined both the effects of spatial conguration of agricultural lands on nutrient export and the effects of agriculture on atmospheric deposition. The FRW is rare among (sub)tropical mountain watersheds in that the effects of agriculture on its streamwater quality have been intensively studied. With the addition of this study, we believe that the FRW can serve as a classic case illustrating the effects of agriculture on nutrient cycling in watersheds with rugged topography and high precipitation, which can be very

informative to other less-studied (sub)tropical mountain watersheds.

We hypothesized that agriculture would increase nutrient output in streamwater (H1) as well as atmospheric input of nutrients through rainfall (H2). We also hypothesized that through the disruption of natural vegetation, agriculture would increase nutrient leaching and decrease the retention ratio of essential nutrient elements (H3). Our specic predictions are that

watersheds with higher proportion of tea plantation cover have higher concentrations and uxes of fertilizer-associated ions in the streamwater than forested water-sheds (H1),

watersheds with higher proportion of tea plantation cover have higher concentrations and uxes of fertilizer-associated ions in the rainfall than forested water-sheds (H2),

watersheds with higher proportion of tea plantation cover have a lower nitrogen retention ratio (in proportion to input) than forested watersheds (H3).

In addition, we explored (1) the role of landscape conguration in mitigating agricultural effects by quantifying the dilution effects of a forested watershed downstream from water-sheds with substantial tea plantation cover, and (2) the N and P dynamics associated with tea plantations by quantifying the differences in their uxes between a forested watershed (background values) and a nearby watershed with substantial tea plantation cover.

2 Materials and methods

2.1 Study site

The FRW is located along the Peishi Creek of northern Taiwan, with a drainage area of 303 km2. The elevation of the FRW ranges from 45 to 1127 m, with a mean slope of 42 % (Fig. 1). The underlying geology of the FRW region is mainly argillite and slate with sandstone interbeds, and the soils are mostly Entisols and Inceptisols with high silt contents (Zehetner et al., 2008).

Annual precipitation is high and spatially varied, ranging from 3500 mm in the southwest portion of the FRW to 5100 mm in the northwest during 20012010 (J. C. Huang, unpublished data). The vegetation is primarily composed of secondary-growth, mixed broad-leaf forests dominated by Fagaceae and Lauraceae (Chen, 1993). Approximately 16 % of the FRW is agricultural land with tea plantations covering an area of 1200 ha, or 25 % of all agricultural lands (Chang and Wen, 1997; Chou et al., 2007). In 1986 the FRW was designated as a water resource protection area, followed by the construction of the Feitsui Reservoir in 1987. Today, the reservoir provides drinking water to the six million people

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Table 1. Basic information of the studied watersheds.

A1 A2 F1 F2

Area (km2) 2.92 1.36 86.04 0.67 Slope (%) 39.3 34.8 38.7 48.1

Land use (%)

Natural forest 68.0 75.5 93.5 99.2 Agriculture 22.1 17.1 2.87 0.38 Road 3.61 2.96 0.77 0.00 Building 1.54 1.31 0.35 0.00 Water body 0.69 0.19 1.12 0.00 Others 4.11 2.96 1.44 0.38

600 mL subsample was taken and placed into a PE bottle for transportation back to the laboratory.

2.3 Water chemistry

All samples were transported back to the laboratory within 24 h. Conductivity and pH of the water samples were measured on the same day of collection. The samples were ltered through 0.45 m lter paper. Major cations (Na+, K+,

Ca2+, Mg2+, NH+4) and anions (Cl, SO24, NO3) were analyzed by ion chromatography on ltered samples using

Dionex ICS 1000 and DX 120 (Thermo Fisher Scientic Inc.

Sunnyvale, CA, USA). PO34 was measured using standard vitamin-C molybdenum-blue method with the detection limit of 0.01 M (APHA, 2005). Prior to chemical analysis, samples were stored at 4 C without preservatives.

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Figure 1. Location and land use distribution of the studied watersheds.

in the Taipei metropolitan area. The forests in the FRW have been protected (no cutting, thinning or converting to agricultural use) since 1986. Therefore, current agricultural activities are limited to private lands with a pre-existing agricultural use which still has an impact at the study site.

2.2 Sampling regime

Four watersheds of the FRW (A1, A2, F1, F2; Fig. 1) with varying proportions of tea plantation cover (22 % in A1, 17 % in A2, 2.9 % in F1, 0.4 % in F2; Table 1) were included in this study. Other crops make up only a small proportions of the watersheds (< 1 %), so they are not included in Table 1. Natural forests are the most dominant land cover for all four watersheds (68 % in A1, 76 % in A2, 93 % in F1, 99 % in F2; Table 1), making tea plantation the primary contributor to the differences in landscape across the four watersheds. Weekly samples of streamwater were collected from all four watersheds. In addition, weekly samples of rainwater were collected from the two watersheds with the lowest (F2) and highest proportions of agricultural lands (A1). A1, A2, and F2 are watersheds (< 3 km2) drained by rst-order streams whereas F1 is a much larger watershed (86 km2) drained by a third order stream that drains through A1 and A2 (Fig. 1). We collected weekly rainfall and streamwater samples every Tuesday from September 2012 to August 2014. Rainfall samples were collected using a 20 cm diameter polyethylene (PE) bucket, from which a 600 mL subsample was taken and placed into a PE bottle for transportation back to the laboratory. Streamwater samples were collected by dipping a PE bucket into the stream and, similarly to rainfall sampling, a

4496 T.-C. Lin et al.: Effects of mountain tea plantations on nutrient cycling at upstream watersheds

Data on rainfall and streamow quantity of the watersheds were estimated from the rain gauges and discharge gauges maintained by the Central Weather Bureau and Water Resource Agency of Taiwan, respectively. The distance between a watershed and its nearest rain gauges was 1.08.5 km, and that between a watershed and its nearest discharge gauges was 3.05.0 km. The weekly and monthly rainfall of a watershed was directly assigned to the values registered at the nearest rain gauge (i.e., COA530 for A1 and COA540 for F2;Fig. 1, S1a). The weekly and monthly streamow of a water-shed was estimated by the area ratio method in which the streamow was assigned to the values registered at the nearest discharge gauge (i.e., 1140H099 for A1, A2, and F1, and 1140H097 for F2; Fig. S1b) and then adjusted by the area ratio of the studied watershed relative to the watershed where the discharge gauge was located. The validity of this method has been conrmed for several watersheds in Taiwan (Huang et al., 2012; Lee et al., 2014).

2.4 Element uxes

Weekly element uxes through rainfall and streamow of A1 and F2 were derived by multiplying weekly concentrations by weekly rainfall/streamow. Monthly uxes were accumulated from weekly uxes, and when a weekly sample spanned over 2 months, it was divided into the 2 months in proportion to the rainfall/streamow quantity.

In order to provide a more comprehensive understanding on how mountain agriculture affects watershed nutrient cycling, we constructed and compared N and P uxes for watersheds with the highest (A1) and lowest (F2) tea plantation cover. We made three assumptions in the calculation of watershed nutrient uxes. First, we assumed the input from dry deposition is 28 % of that from precipitation for both watersheds. This value was based on a study using the Na+ ratio method at the Fushan Experimental Forest (Lin et al., 2000), a natural hardwood forest 17 km south of the

FRW. Second, the amount of fertilizer used is assumed to be close to 786 kg N ha1 yr1 and 171 kg P ha1 yr1, the values taken from a case study in which the management practices (e.g., applications of fertilizers and pesticides, time and yield of harvests) were carefully recorded by a farmer in the same region as the current study (Tsai and Tsai, 2008).Although only one farmer was involved in the case study, the values are consistent with those reported by FAO (2002) and very close to the mean values across 10 tea plantations in our study area (743 kg N ha1 yr1 ranging from 425 to 2373 kg N ha1 yr1, and 179 kg P ha1 yr1 ranging from 99 to 550 kg P ha1 yr1; Water Resources Agency, 2010).

Adjusting for the proportion of agricultural lands (22.1,0.38 %), the amounts of fertilizers used in A1 were estimated to be 173.7 kg N ha1 yr1 and 37.8 kg P ha1 yr1, and those in F2 to be 3 kg N ha1 yr1 and 0.6 kg P ha1 yr1.

There was very little change in biomass of tea plantation after 10 years because tea plants are regularly trimmed, with the

litter left in the eld, to maintain the same height optimal for harvest. Thus, our third assumption is that N and P is lost due to the uptake by tea trees being equivalent to the N and P in the harvested tea leaves. The amount of N removed through tea harvest (113 kg ha1 yr1) was taken from the same case study and the amount of P removed (7.35 kg ha1 yr1) was calculated using the median of P : N ratios (0.065) reported for tea trees in Taiwan (Tsai and Tsai, 2008). After adjusting for the proportion of tea plantation cover, A1 was estimated to have 25.0 kg N ha1 yr1 and 1.6 kg P ha1 yr1 removed through harvest, and F2 to have 0.43 kg N ha1 yr1 and 0.03 kg P ha1 yr1 removed through harvest. Using the following mass balance model, we constructed uxes of N and P of the two watersheds:

Ratioret = 1

OUTriv + OUTharv INdep + INfer + INx

. (1)

Here, Ratioret indicates the ratio of input to the watershed that was retained within the watershed. The OUTriv and OUTharv are the riverine export and harvest, respectively. The INdep,

INfer, and INx indicate the atmospheric deposition, fertilizer application, and biological xation. Note that the biologic xation term was not used for P calculation. Since the tea plantation does not use leguminous crop as fertilizers and the biological xation in tropical forest is known to be less than 10 kg N ha1 yr1 (Sullivan et al., 2014), the INx is assumed to be between 0 and 10 kg N ha1 yr1. We did not include the loss through denitrication and volatilization within tea eld in the calculation of N retention ratio because we did not have good estimates. However, the effects of such uncertainties and omissions on estimating N retention ratio were discussed. We did not calculate the retention ratio for P because the majority of P in watersheds was in particulate forms (Smith et al., 1991) that were not analyzed in our study.

2.5 Statistical analysis

We used the general linear model with repeated measurements to compare monthly concentration and ux of ions in streamwater among the four watersheds (F1, F2, A1, A2), followed by Fishers least signicant difference (LSD) post hoc comparisons. NH+4 was excluded from streamwater analysis due to its low concentration. We used a one-tail paired t test to examine if monthly ion concentration (volume weighted from weekly samples) and ux in rainfall were higher at the watershed with higher agricultural land cover (A1) than the more pristine watershed (F2). All statistical analysis was conducted using SPSS 22.0 (IBM Corporation, New York).

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3 Results

3.1 Streamwater chemistry

The concentrations of all analyzed ions in streamwater differed signicantly among the four watersheds (Table 2). A1, the watershed with the highest proportion covered by tea plantations, had signicantly higher concentrations of all ions except H+ than the other three watersheds (Table 2,

Fig. 2). In contrast, F2, the watershed with the lowest proportion covered by tea plantations, had the lowest concentrations of H+, Na+, K+, Cl, and NO3. Furthermore, it is worth noting that F2, the watershed with the steepest slopes, had the second highest concentrations of ions rich in soils and soil solution, including Ca2+, Mg2+, and SO24 (Table 2,

Fig. 2).

Similar to ion concentration, the uxes of all ions differed signicantly among watersheds (Table 2). A1 had the largest uxes of K+, Ca2+, Mg2+, NO3, and SO24 and F2 had the smallest uxes of H+, Na+, K+, Mg2+, Cl, and NO3 (Table 2). PO34 ux was signicantly larger at A1 and A2, which were not so different from each other, than F1 and F2, which were also not so different from each other (Table 2).Although the uxes of Na+ and Cl differed signicantly among A1, A2, and F1, these differences were considerably smaller than the differences between the three watersheds and F2 (Table 2).

3.2 Rainfall chemistry

Five of the 10 measured ions had signicant (p < 0.05) or marginally signicant (p < 0.1) higher concentrations in A1 than in F2 (H+, Na+, Cl, NO3, p < 0.05; NH+4, p = 0.067;

Table 3, Fig. 3). Furthermore, 7 of the 10 measured ions had signicant or marginally signicant higher uxes in A1 than in F2 (H+, Ca2+, Cl, p < 0.05; Na+, Mg2+, NH+4, NO3, p < 0.1; Table 3).

3.3 N and P uxes

Because the proportion of agricultural cover was very low at F2 (i.e., 0.38 %) and the resulting fertilizer input and harvest output were small and already accounted for (Table 4), we treated F2 as a background and attributed the differences between A1 and F2 to agricultural activities. We estimated stream N and P outputs from the tea plantation at A1 to be approximately 105.7 and 1.6 kg ha1 yr1, respectively (Table 4). Scaling up from 22 % of tea plantation cover to 100 %, the stream N and P outputs from A1 could reach as high as 450 and 7.3 kg ha1 yr1, respectively.

From our mass balance construction of element uxes, N input exceeded output at both watersheds (Table 4, Fig. 4).At A1, 35 % of the N input (69 kg ha1 yr1) to the watershed was retained (Table 4, Fig. 4). At F2, 72 % of the

N input (15 kg N ha1 yr1) was retained (Table 4, Fig. 4).

For P, the output through streamow (2.6 kg ha1 yr1) was smaller than the input through atmospheric deposition(3.6 kg ha1 yr1) at F2. At A1, the output of P through streamow and harvest (5.8 kg ha1 yr1) was greater than the input through atmospheric deposition (4.6 kg ha1 yr1), but when fertilization was taken into account, the total output of PO34-P was trivial relative to the total P input(42.4 kg ha1 yr1) (Table 4).

4 Discussion

4.1 Streamwater chemistry

The watershed with the highest proportion of tea plantation cover (A1) had the highest concentrations and uxes of most ions in streamwater, suggesting the role of agriculture on increasing nutrient output. Furthermore, the fact that the output of fertilizer-associated ions (NO3 and K+) matched the proportion of tea plantation cover across the four water-sheds (i.e., the rank of the proportion of tea plantation cover from high to low: A1, A2, F1, and F2; rank of ion concentration and ux from high to low: A1, A2, F1, and F2) strongly supports the effects of agriculture on streamwater chemistry (H1).

However, streamwater chemistry is affected by complex processes beyond a single factor of land use. For example, P is also an important component of fertilizers but, unlike NO3 and K+, the concentration of PO34 at F2 was not signicantly different from that at A1 and A2, and all were signicantly higher than F1. Erosion is known to enhance leaching loss of PO34 (Gaynor and Findlay, 1995; Turtola and

Jaakkola, 1995; Liu et al., 2006; Chang et al., 2008; Lee et al., 2013). The greater erosion and leaching associated with the steeper slopes of F2 may have matched the effect of fertilization and led F2 to have a PO34 concentration as high as

A1 and A2. To further illustrate this topographic effect, we compared streamwater chemistry between the two forested watersheds (F1 and F2), removing the potential confounding effect of land use. Indeed, the steeper F2 (48 %) had a higher PO34 concentration than the less steep F1 (39 %) (Fig. 2,

Table 2), despite that F2 has a higher proportion of natural forest cover. Soil erosion is arguably the greatest concern to most P mitigation programs because the concentration of P on the surface of soil particles is often orders of magnitude greater than that in a soil solution (Sharpley et al., 2002; Kleinman et al., 2011). Therefore, it is not surprising that topography may be a more important driver for riverine P than land use at our study site. The enhanced erosion/leaching associated with the steeper slope at F2 may also explain why F2 had the second highest concentration of SO24, Ca2+, and

Mg2+, the ions that are abundant in soils.

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4498 T.-C. Lin et al.: Effects of mountain tea plantations on nutrient cycling at upstream watersheds

Figure 2. Monthly ion concentration (volume-weighted from weekly samples) of streamwater of watersheds A1, A2, F1, and F2.

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Figure 3. Monthly ion concentration (volume-weighted from weekly samples) of rainfall of watersheds A1 and F2.

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4500 T.-C. Lin et al.: Effects of mountain tea plantations on nutrient cycling at upstream watersheds

Table 3. Mean (1 SE) monthly ion concentration (volume-

weighted from weekly samples) and ux of rainfall.

Ion Concentration (eq L1) Flux (meq m2 mo1)

A1 F2 A1 F2

H+ 39 6.7 31 5.4 12 3.9 7.9 1.5

Na+ 107 24 84 18 30 8.5 23 6.5( )

K+ 8.0 1.3 7.8 1.3 2.2 0.45 1.9 0.32

Ca2+ 21 3.2 19 4.4 5.7 1.0 4.2 0.61

Mg2+ 30 5.8 26 5.6 8.2 2.1 6.5 1.7( )

NH+

4 19 2.9 15 2.7( ) 5.1 1.3 3.8 0.67( )

Cl 140 30 100 22 38 11 28 8.2

NO

3 24 3.9 18 3.0 7.0 2.0 4.7 0.90( )

SO2

4 58 8.6 53 7.7 15 3.6 13 2.4

PO3

4 0.96 0.03 0.63 0.03 0.75 0.30 0.51 0.12

A1 and F2 denote the two watersheds; an asterisk indicates a signicant difference between the two watershed (p < 0.05); an asterisk inside a parenthesis ( ) indicates a marginally signicant difference between the two watersheds (p < 0.1).

A1,A2,F1,andF2denotethefourwatersheds;diff:posthoccomparisonsamongthefourwatershedswithdifferentlettersindicatingstatisticaldifferences(p<0.05).

H+ 0.96 0.006 1.22 0.006 0.91 0.007 0.76 0.004 a, b, a, c 0.030 0.001 0.038 0.001 0.036 0.001 0.016 0.004 a, b, ab, c

Na+ 266 4.88 254 3.65 233 4.45 231 4.10 a, b, c, c 76.4 1.74 73.0 1.70 80.1 1.68 46.7 0.90 a, b, ab, c

K+ 282 0.87 213 6.27 125 0.49 108 3.63 a, b, c, d 8.24 0.20 6.14 0.14 4.27 0.50 2.19 0.36 a, b, c, d

Ca 2+ 306 7.49 193 5.41 170 7.34 273 8.04 a, b, c, d 87.0 1.92 54.1 1.17 55.8 1.02 54.4 1.00 a, b, b, b

Mg 2+ 255 5.10 188 4.25 148 4.72 206 5.78 a, b, c, d 72.5 1.62 52.8 1.15 49.2 0.94 41.0 0.74 a, b, b, c

Cl 199 4.00 182 3.06 178 4.76 145 2.55 a, b, b, c 59.2 1.51 53.2 1.34 62.8 1.49 29.8 0.64 a, b, a, c

NO

3 209 5.31 158 2.80 28.3 0.76 16.1 0.95 a, b, c, d 62.9 1.63 46.8 1.19 10.2 0.25 3.32 0.078 a, b, c, d

SO 2

4 212 6.29 123 3.96 116 3.96 183 6.45 a, b, c, d 59.2 1.30 33.9 0.74 39.1 0.78 35.7 0.66 a, b, b, b

PO 2

4 1.50 0.182 1.38 0.174 0.72 0.114 1.29 0.026 a, b, b, a 1.14 0.0030 1.08 0.0054 0.69 0.028 0.69 0.0030 a, a, b, b

A1A2F1F2diffA1A2F1F2diff

IonConcentration(eqL 1 ) Flux (meq m 2 mo 1 )

Table2.Mean( 1 SE standard error) monthly ion concentration (volume-weighted from weekly samples) and ux of streamow.

Figure 4. Schematic diagram of N uxes of watersheds A1 and F2.

A1 represents a watershed with 22 % agricultural lands and 68 % forests (a); F2 represents a watershed with 0.38 % agricultural lands and 99 % forests (b) (unit: kg N ha1 yr1).

4.2 Rainfall chemistry

We conrmed that agricultural activities can inuence water-shed nutrient cycling via atmospheric deposition in our study site (H2). We found higher concentrations and uxes of NO3 and NH+4 in rainfall at A1, a watershed with 22 % of tea plantation cover, compared to F2, the watershed almost entirely covered by natural forests. Ammonium sulfate, urea and calcium ammonium nitrate [5Ca(NO3)2 NH4NO3 10H2O],

which contain a high quantity of NO3 and NH+4 are commonly used N fertilizers in Taiwan (Huang, 1994). Therefore, in tea plantations at FRW, substantial suspension and volatilization of ammonium sulfate, urea, and calcium ammonium nitrate likely contributed to the high concentrations and uxes of NO3 and NH+4 in rainfall at A1. On the other hand, the concentrations of PO34 and K+ in rainfall were not higher at A1 compared to F2, which may be explained

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Table 4. Inputs and outputs of nitrogen and phosphorus of water-sheds A1 and F2. See text for the assumptions made in the calculations of dry deposition, fertilization, and harvest.

Nitrogen Phosphorus (kg ha1 yr1) (kg ha1 yr1)

A1 F2 A1 F2

Input

Wet deposition 20.4 14.3 3.6 2.8 Dry deposition 5.7 4.0 1.0 0.8 Fertilization 173.7 3.0 37.8 0.6 Total 199.8 21.3 42.4 4.2

Output

Harvest 25.0 0.4 1.6 0.0 Stream output 105.7 5.6 4.2 2.6

Total 130.7 6.0 5.8 2.6

For stream output, only dissolved inorganic forms are considered.

by the low mobility of PO34 and smaller quantity of P and K in fertilizers.

Once in the atmosphere, aerosols/chemicals can be transported to other locations but most of them will be deposited in nearby ecosystems. In central Taiwan, the high NH+4 concentration in precipitation in a high elevation forest (2000 m)

was attributed to mountain agriculture that occurred 10 km away (Ding et al., 2011). With the predicted expansion of agriculture to the mountains both in Taiwan and many other regions (Johda et al., 1992; Brown and Shrestha, 2000; Tulachan, 2001), even pristine ecosystems will not be free from the impacts (e.g., acidication and eutrophication associated with H+ and NO3) of agricultural activities.

Because Taiwan is a small island, sea salt aerosols are important components of rainfall (Lin et al., 2000). The distance to the coast, specically, has been used to explain the variation of Na+ and Cl concentrations in precipitation among four sites in central Taiwan (Ding et al., 2011). The higher concentrations and uxes of Na+ and Cl, and to a lesser degree Mg2+, at A1 than at F2 likely reected such oceanic inuences. The watersheds receive winter rains, along with sea salt aerosols, from the north/northeast coasts (northeast monsoon). While A1 is located on the windward side, F2 is on the leeward side. Therefore, a substantial proportion of the sea salt aerosols may have been intercepted before they can reach F2. Although summer rains move from the opposite direction, the watersheds are relatively far from the west/southwest coasts (> 60 km), making summer rains less important to the input of sea salt aerosols to the watersheds.

In contrast to Na+ and Cl, the differences in topo-graphic position and distance to the ocean between A1 and F2 seemed to have a limited effect on SO24 deposition. Many studies reported signicant contributions of long-range-transported S and N from eastern China to Taiwan via

the northeast monsoon (Lin et al., 2005; Junker et al., 2009).

Because A1 is on the windward side of the northeast monsoon, it may experience a higher input of pollutants from long-range transport than F2, which is on the leeward side.The lack of signicant differences in SO24 between the two watersheds suggest that the two watersheds are too close to show differential inuences of pollutants that are transported from sources several hundred kilometers away.

4.3 Landscape conguration and streamwater chemistry

The large differences in NO3 concentration and ux between F1 and A1, A2 highlight the role of landscape conguration on streamwater chemistry. Both A1 and A2 are subwater-sheds of F1; however, the inuence of tea plantation on A1 and A2 largely dissipated as water entered into forested F1.Specically, the concentration of NO3 was 70 % lower at

F1 than at A1 and A2. Comparing to the difference in concentration and ux of NO3 between F1 and F2 (< 30 %), that between F1 and A1, A2 is striking (> 300 %; Fig. 2).

Thus, by constraining agricultural activities away from the main stream and maintaining natural cover of its watershed, the impact of agriculture on nutrient enrichment could be reduced. Our result conrmed the importance of landscape conguration on streamwater chemistry (Dillon and Molot, 1997; Johnson et al., 1997; Palmer et al., 2004).

4.4 N and P output from agriculture

The per-hectare output of N from tea plantations reported here (450 kg ha1 yr1) is extraordinary high compared to those reported for many agricultural watersheds around the globe. For example, a study from the Baltimore Ecosystem Study reported an annual output of NO3-

N at 1320 kg ha1 yr1 for a 7.8 ha watershed that is completely covered by agricultural lands and has gentle slopes (Groffman et al., 2004). For the four watersheds that were 3040 % covered by row crops and received fertilization at 5070 kg N ha1 yr1 in the southeastern coastal plain of the US, nutrient output through streamow was < 6 kg N ha1 yr1 (Lowrance et al., 1985). In the Great Barrier Reef, Australia, total output via streamow was approximately 5 kg ha1 yr1 for NO3-N from a watershed with 29 % of the land covered by pasture and 14 % by crop lands (Hunter and Walton, 2008).

High N output from agricultural lands is probably common in Taiwan and other regions under intensive fertilizer use. It has been reported that over-fertilization is common in Japan, Korea, and Taiwan, and despite an estimated 23 63 % over-fertilization the use of fertilizers is still increasing in the region (Ahmed, 1996). In the Danshui River of northeastern Taiwan, the output of dissolved inorganic N ranged from 3 kg ha1 yr1 in relatively pristine headwaters covered mostly by natural forests to 100 kg ha1 yr1 in a pop-

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4502 T.-C. Lin et al.: Effects of mountain tea plantations on nutrient cycling at upstream watersheds

ulated estuary (Lee et al., 2014; Shih et al., 2015). In humid southeastern China, N output from a watershed with17.5 % of agricultural lands, steep slopes (the watershed has a mean slope of 21 % and the site is located in the hilly upstream region), and very heavy application of N fertilizers (3001000 kg ha1 yr1) reached 73 kg ha1 yr1 (Chen et al., 2008), approximately the same magnitude as those reported here. Our study clearly demonstrated that high application of fertilizers in regions with high rainfall and steep slopes could lead to an extremely high output of N and, therefore, eutrophication risk for downstream watersheds.The misconception that heavy fertilization leads to high economic prot has resulted in the popular practice of heavy fertilization in tea plantations, commonly at a level similar to or higher than that in our study site (740 kg N ha1 yr1).

For example, conventional N fertilization in tea plantations is approximately 1100 kg ha1 yr1 in Japan, which is more than twice the suggested amount with the same tea yield (Oh et al., 2006).

In contrast to N, most of the P fertilizer was retained within the watershed or transported in particulate form so that dissolved P only accounts for a small proportion of the input.In most agricultural watersheds, the majority (> 90 %) of P leaves the watersheds in particulate form (Smith et al., 1991), and the loss in dissolved form (i.e., PO34) through runoff is relatively minor (Brady and Weil, 1999). Thus, while the dissolved form of P could respond to land use changes, a complete P budget at watershed scale still requires reliable estimates on the particulate P.

4.5 Watershed N uxes

The 72 % N retention at F2 is likely an underestimate because the input from biological N xation (BNF) was not included in the calculation. Based on a recent synthesis (Sullivan et al., 2014), BNF in tropical forests is not as high as previously reported and, on average, is slightly less than 10 kg ha1 yr1 for secondary forests. Thus, adding BNF to N input could increase the N retention ratio at F2 (assuming a BNF of 10 kg ha1 yr1, the N retention ratio at F2 would increase from 72 to 81 %). The high N retention ratio of F2 suggests that the secondary natural forest is probably still growing. In contrast, because N fertilizers were applied at rates that are 1 order of magnitude greater than BNF at A1, and high N fertilization is known to negatively affect BNF (Sanginga et al., 1989; Fuentes-Ramrez et al., 1999), adding BNF to nutrient input has little effect on the N retention ratio at A1 (assuming a BNF of 10 kg ha1 yr1, the N retention ratio at A1 would increase from 35 to 37 %).

In addition to BNF, the calculation of the N retention ratio did not take into account the loss through volatilization and denitrication. Because it rains frequently at the FRW, soil moisture is likely high throughout the year and, consequently, N loss through denitrication could be substantial. In addition, because fertilizers are applied in solid form,

volatilization of NH3 could also be high. Thus, if both denitrication and volatilization are taken into account, the N retention ratio at A1 is even lower. The return of N back to the atmosphere through denitrication and volatilization helps explain the higher atmospheric N deposition at A1 than at F2. The low retention ratio and the resulting high leaching loss of N at A1 impose a major threat to the streamwater quality that could lead to reservoir eutrophication.

Surprisingly, from our construction of the N uxes, the loss of N through the annual harvest (25 kg ha1 yr1) at

A1 approximately equals the annual atmospheric deposition (26 kg ha1 yr1), of which only a small portion should have come from fertilizers (atmospheric N deposition at F2 is only 8 kg lower than at A1, suggesting that less than 8 kg of atmospheric N deposition could potentially come from fertilizers). In other words, to maintain the current harvest, not much N fertilization is actually required, and most of the 173.7 kg N ha1 yr1 from fertilization is simply lost through hydrological process (i.e., leaching) to the streams and the

Feitsui Reservoir and/or returned to the atmosphere, both of which could have negative environmental impacts. Our construction of the element uxes clearly showed that the N fertilizers are applied at rates that are neither ecologically nor economically sound, and such excessive fertilization may cause fundamental changes in watershed nutrient cycling (Fig. 4).

5 Conclusions

Agricultural and forested watersheds in tropical/subtropical mountains could have distinct patterns of nutrient cycling.Even a moderate proportion of tea plantation cover (17 22 %) in mountain watersheds, when in combination with steep slopes and high precipitation, could lead to much higher ion concentrations in both streamwater (nutrient output) and rainwater (nutrient input) and much lower N retention ratios at watershed scale. Thus, mountain watersheds may be particularly vulnerable to agricultural expansion.

Topographic control is important in nutrient leaching from mountain watersheds, particularly for ions that are rich in soils, such as SO24, Ca2+, and Mg2+.

Proper spatial conguration of agricultural lands in mountain watersheds can mitigate the impact of agriculture on NO3 output by 70 %, thus reducing the risk of eutrophication for streams and lakes.

The contribution of tea plantations to the N output in streamwater for one of the studied watersheds (i.e., A1) is estimated at approximately 450 kg N ha1 yr1. This level of fertilization exceeds previous reports around the globe and can only be matched in magnitude by one study in China where fertilizers were excessively applied.

The conservative construction of the N uxes for the watersheds indicates over-fertilization at one of the studied watersheds (i.e., A1), which likely resulted in leaching of N and

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T.-C. Lin et al.: Effects of mountain tea plantations on nutrient cycling at upstream watersheds 4503

additional loss of N to the atmosphere via volatilization and denitrication.

Acknowledgements. This research was supported by grants from the National Science Council of Taiwan (101-2116-M-003-003-, 102-2116-M-003-007-). We thank Craig Martin for proofreading this manuscript.

Edited by: M. Hrachowitz

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