Soil organic carbon pools in alpine to nival zones along an altitudinal gradient (4400–5300 m) on the Tibetan Plateau

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Polar Science 2 (2008) 277e285 http://ees.elsevier.com/polar/

Soil organic carbon pools in alpine to nival zones along an altitudinal gradient (4400e5300 m) on the Tibetan Plateau Toshiyuki Ohtsuka a,*, Mitsuru Hirota b, Xianzhou Zhang c, Ayako Shimono d, Yukiko Senga e, Minguan Du f, Seiichiro Yonemura f, Shigeto Kawashima f, Yanhong Tang d a Institute for Basin Ecosystem Study, Gifu University, 1-1 Yanagito, Gifu, Gifu 501-1193, Japan Sugadaira Montane Research Center, University of Tsukuba, 1278-294 Sugadaira, Ueda, Nagano 386-2204, Japan c Institute of Geographical Sciences and Natural Resources Research, Daduanlu Jia 10, Zhaoyang, Beijing 100101, China d Environmental Biology Division, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan e Faculty of Geo-Environmental Science, Rissho University, 1700 Magechi, Kumagaya, Saitama 360-0194, Japan f Agro-Meteorology Division, National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba, Ibaraki 305-8604, Japan b

Received 13 June 2008; revised 12 August 2008; accepted 26 August 2008 Available online 25 September 2008

Abstract To accurately estimate soil organic carbon (SOC) storage in upper alpine to nival zones on the Tibetan Plateau, we inventoried SOC pools in 0e0.3 m profiles along an altitudinal gradient (4400e5300 m asl). We also studied vegetation properties and decomposition activity along the gradient to provide insight into the mechanisms of SOC storage. The vegetation cover and belowground root biomass showed a gradual increased with altitude, reaching a peak in the upper alpine zone at 4800e4950 m before decreasing in the nival zone at 5200e5300 m. Decomposition activity was invariant along the altitudinal gradient except in the nival zone. SOC pools at lower sites were relatively small (2.6 kg C m2 at 4400 m), but increased sharply with altitude, reaching a peak in the upper alpine zone (4950 m; 13.7 kg C m2) before decreasing (1.0 kg C m2 at 5300 m) with altitude in the nival zone. SOC pool varied greatly within individual alpine meadows by a factor of five or more, as did bulk density, partly due to the effect of grazing. Inventory data for both carbon density and bulk density along altitudinal gradients in alpine ecosystems are of crucial importance in estimating global tundra SOC storage. Ó 2008 Elsevier B.V. and NIPR. All rights reserved. Keywords: Soil organic carbon; Alpine ecosystems; Tibetan Plateau; Altitudinal gradient

* Corresponding author. Present address: Institute for Basin Ecosystem Study, Gifu University, 1-1 Yanagito, Gifu, Gifu 501-1193, Japan. Tel./ fax: þ81 58 293 2065. E-mail addresses: [email protected] (T. Ohtsuka), [email protected] (M. Hirota), [email protected] (X. Zhang), [email protected] (A. Shimono), [email protected] (Y. Senga), [email protected] (M. Du), [email protected] (S. Yonemura), [email protected] (S. Kawashima), [email protected] (Y. Tang). 1873-9652/$ - see front matter Ó 2008 Elsevier B.V. and NIPR. All rights reserved. doi:10.1016/j.polar.2008.08.003

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1. Introduction Arctic and alpine ecosystems contain extensive soil organic carbon (SOC) reserves that could provide a substantial positive feedback to climate warming (Grogan and Chapin, 2000; McKane et al., 1997; Oechel et al., 1993). The effects of warming on net carbon balance, especially SOC, in alpine ecosystems may be of crucial importance in understanding future global warming, even though only 3% of the global land area is covered by alpine vegetation (Diaz et al., 2003). The Tibetan Plateau, the ‘‘third pole’’ of the Earth, contains an extensive SOC pool (33.52 Pg C in the 0e0.75 m profile) (Wang et al., 2002), and is the part of the alpine terrestrial ecosystem most sensitive to global climate change (Luo et al., 2002; Zhang et al., 2007). In mountainous areas, SOC pools change along altitudinal gradients: SOC per unit land area generally increases with altitude, reaching a peak in the montane forest zone, and sometimes extending into the lower alpine zone (Ko¨rner, 2003; Vitousek et al., 1992). Altitudinal gradients (mainly gradients in temperature) possibly affect the rate at which carbon accumulates in soil, as related to plant productivity and decomposition rates in alpine ecosystems. In the alpine zone, nutrient limitation constrains the number of higher plant species and their coverage (Haselwandter et al., 1983; Rehder, 1976); thus, the SOC pool might be as low as zero above the nival zone. However, little is known about the distribution of SOC pools along an altitudinal gradient up to the nival zone in temperate alpine areas, compared with nutrient dynamics (Go¨kc¸eog˘lu and Rehder, 1977; Rehder and Scha¨fer, 1978). Accurate and verifiable estimates of the size of the soil carbon pool obtained via standard sampling methods, which give bulk density values in combination with measures of organic matter content in soils, are needed to better predict variations in the global SOC pool. The major factors controlling SOC pools are temperature, precipitation, soil texture, geomorphology, nutrient status, and disturbance (Bird et al., 2001). Predictable variations in SOC pools should therefore be analyzed in comparative studies along environmental gradients such as latitude, altitude, and topographical position, and between disturbed and undisturbed areas (Vitousek et al., 1992). Here, we report soil properties and SOC pools in the 0e30 cm profile along an altitudinal gradient (4400e 5300 m; all elevations in the text are given in meters above sea level) in an upper alpine meadow to the nival zone, located close to the town of Damxung, on the

Tibetan Plateau, with the aim of accurately estimating SOC storage in alpine soils. We also studied vegetation properties and decomposition potential along the gradient to gain an insight into the mechanism of SOC storage in relation to temperature gradient in the upper alpine zone. 2. Materials and methods 2.1. Study area and sampling sites The study was carried out in alpine grassland on the Tibetan Plateau, at an average altitude of more than 4000 m, close to Damxung (91 080 E, 30 290 N, 4288 m). The study area lies within the alpine Kobresia meadow zone (ca. 3200e5300 m), which is dominated by Kobresia pygmaea and Kobresia humilis according to soil water availability (Wu, 1980). A southeastfacing slope on the Nyainqentanglha Mountains, near Damxung, was selected for surveying (Fig. 1). Eight sampling sites were established on the slope along an altitudinal gradient (i.e., at 4400, 4500, 4650, 4800, 4950, 5100, 5200, and 5300 m) from the alpine Kobresia meadow to the nival zone. Soils on the Tibetan Plateau are poorly developed, and soil profile differentiation is relatively weak and the soil layer is thin, being 0.2e1.0 m thick over 80% of the region’s grasslands (Wang et al., 2002). This region, the source of the Yangtze River, is the most important area in the region for the grazing of Tibetan sheep and yak (Zhou et al., 2005). In Damxung, herders set up a base camp for summer pasturing at around 4400 m, and almost all of the meadows in the area are grazed by livestock, except for areas protected by fences during the summer to permit winter pasturing. Data from the meteorological station at Damxung obtained from 1960 to 2000 show a mean annual air temperature of 1.5  C, with a mean minimum monthly temperature of 9.6  C in January and a mean maximum of 10.8  C in July. Annual rainfall is 474 mm. Meteorological measurements have been made at the study site every 30 min since August 2005 using a meteorological tower equipped with sensors and a data logger (HOBO weather station, Onset Computer, Bourne, MA, USA). The tower was used to monitor temporal changes in air temperature and soil temperature at 5 cm depth with permanently installed instruments (air temperature, S-THA-M006; soil temperature, S-TMB-M006). Air and soil temperature in the middle of August decreased gradually with increasing altitude on the study slope (4400e5300 m).

T. Ohtsuka et al. / Polar Science 2 (2008) 277e285

a

E80°

50°

40°

90°

100°

110°

b

120°

279

Overview of the study site (Aug.2005)

Tibetan Plateau

30°

Damxung N20° 500 (km)

c

5500

Altitude (m a.s.l.)

0

5300

5300 m 5200 m 5100 m 4950 m

5100

4800 m

4900

4650 m

4700

4500 m 4400 m

4500 4300

0

1

2

3

4

5

Distance from the summit (km) Fig. 1. (a) Location of the study site on the Tibetan Plateau, close to Damxung (91 080 E, 30 290 N, 4288 m). (b) Photograph of the southeastfacing slope surveyed within the Nyainqentanglha Mountains. (c) Eight sampling sites were selected on the slope along an altitudinal gradient from 4400 to 5300 m.

The lapse rate in air and soil temperature during summer was ca. 0.6  C per 100 m (Fig. 2). 2.2. Soil sampling and analysis Three plots of 1  1 m were set at each sampling site to investigate floristic composition and soil properties in early August 2005. Vegetation cover (%) of all higher

a

plants was measured in each plot. Electric conductivity (EC) was measured in the surface soil (0e5 cm) using a hand-held EC probe (Daiki-691A, Daiki Rika Kogyo Co. Ltd., Saitama, Japan). EC is a measure of the ability to conduct an electric current, which indicates the total amount of soluble mineral nutrient in soil. Soil samples were taken near each plot. A 100-mL cylindrical soil corer (B ¼ 5 cm, height ¼ 5 cm) was used to collect

b

40

Temperature (°C)

30

20

10

0

-10 4000

4500

5000

5500

4000

4500

5000

5500

Altitude (m) Fig. 2. Variations in air temperature (a) and soil temperature at 5 cm depth (b) along the altitudinal gradient from 4400 to 5300 m. Data are daily mean values measured from 5 to 17 August 2005. Vertical bars indicate the maximum and minimum daily temperature.

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samples of topsoil beneath the litter (0e5 cm) and at three successive depths (5e10, 15e20, and 25e30 cm) with two replicates. One core served for the measurement of bulk density (g cm3) and volumetric water content after oven-drying at 105  C for more than 24 h. The other served for the measurement of soil pH and gravimetric soil carbon and nitrogen contents. Small subsamples of fresh soil were used to measure soil pH (soil:water ratio of 1:2.5, w/w); the remainder was airdried in the laboratory. Roots in the air-dried sample were picked out by hand for estimation of belowground plant biomass, although it was difficult to distinguish live root biomass from dead root necromass. The airdried soils were passed through a 2-mm mesh sieve, and the sieved soils and remaining gravel were weighed to measure gravel content (%; gravel weight/weight of the air-dried soils  100). Total soil carbon and nitrogen contents (g C or g N g1 sieved soil) were measured with a CN-analyzer (Sumigraph NC-800, Sumika Chemical Analysis Service Ltd., Tokyo, Japan) using the air-dried sieved soils. Inorganic N (NH4-N, NO3-N, and NO2-N) was extracted with 1.5 N KCl solution using air-dried sieved soils, and gravimetric soil N contents (mg N g1 sieved soil) were measured colorimetrically (AACS-II, Bran þ Luebbe Ltd., Northampton, UK). 2.3. Calculation of volumetric soil carbon and nitrogen contents The standing stocks of soil inorganic N and total C and N were estimated in each 5-cm slice. The volumetric soil mass of total carbon (TC: kg C m2) was evaluated from the products of gravimetric carbon content and soil bulk density in each layer as follows:     TC0e5 kg C m2 ¼ 5  104  BD0e5  ð1  GC0e5 =100Þ  CC0e5  103 TC030 ¼

i¼ð2530Þ X

TCi

i¼ð05Þ

where TC0e5 is volumetric total carbon content (kg C m2) in soil at depths of 0e5 cm, BD0e5 is bulk density (g cm3) at 0e5 cm, GC0e5 is the gravel content of the total soil mass (%) at 0e5 cm, and CC0e5 is gravimetric carbon content (g C g1 sieved soil) at 0e 5 cm. Soil carbon and nitrogen contents, and belowground biomass in the unsampled layers (i.e., 10e15 and 20e 25 cm), were interpolated as the average of the upper and lower depths; i.e., CC10e15 ¼ (CC5e10 þ CC15e20)/2. The volumetric mass of inorganic N and total N was calculated using the same procedure.

2.4. Decomposition potential by mass loss of cotton strip Decomposition of cellulose in cotton strips was used as a proxy in evaluating the relative rates of decomposition of organic matter in the field. Cellulose comprises about 70% of the organic carbon compounds in plant tissue (Mendelssohn et al., 1999); its decay rate is therefore a key factor in organic matter decomposition. Decomposition potential using cotton strips has been evaluated previously from the difference in tensile strength before and after burial in soil or sediment (e.g., Bridgham et al., 1991; Harrison et al., 1988; Mendelssohn et al., 1999). In this study, we calculated potential total decomposition (PD) from mass loss of a sample cotton strip, as follows: PDð%Þ ¼ 100ðWi  Wiþt Þ=Wi where PD is the relative potential total decomposition throughout the experimental period, Wi is the initial dry mass of the cotton strip, and Wiþt is the final dry mass. In this study, t was ca. 1 year, from 10 and 11 August 2005 to 12 and 13 August 2006. Prior to setting strips of standard cotton material (Shirley Institute, Didsbury, Manchester, UK) in the field, all the strips were ovendried at 70  C for 2 days and weighed (Wi). In each plot at each of the eight sampling sites, three strips (total of nine strips per elevation) of 10  30 cm were carefully inserted vertically into a slit in the soil substrate with braided rigid plastic (5 mm mesh size) using a sharp spade on 10 and 11 August 2005. To evaluate the impact of a series of processes (from inserting to retrieving the strips) on PD, we measured mass change before and immediately after inserting a strip at each sampling site. The mass change of each strip, however, was quite small, within 0.5% of the initial weight of the strip. After we retrieved all the strips on 12 and 13 August 2006 (except for those at 4400 m, which were not found), they were gently washed in running water, oven-dried at 70  C for 2 days, and weighed (Wiþt). 2.5. Statistical analyses Statistical analyses were carried out using the StatView version 5.0 software package (SAS Institute). Statistical differences in the mean value of carbon pools at each sampling site along the altitudinal gradient were evaluated using one-way ANOVA, with statistical significance determined at the P < 0.05 level. Scheffe´’s test was used for pairwise comparisons of the mean value to follow-up significant main effects determined by ANOVA.

T. Ohtsuka et al. / Polar Science 2 (2008) 277e285

3. Results 3.1. Biomass pool Table 1 lists the vegetation cover and dominant species at each sampling site along the altitudinal gradient. K. pygmaea and K. humilis dominated the alpine meadow zone; however, the vegetation cover was low at the lower sites, where Artemisia desertorum and Stipa aliena were dominant. The vegetation cover gradually increased with altitude to a peak of more than 80% in the upper alpine zone at around 4800e 4950 m. Patches of plants (e.g., Arenaria kansuensis and Poa calliopsis) were found at 5300 m in the nival zone, but the vegetation cover was 0%. Belowground root biomass showed the same pattern as that for vegetation cover: low biomass at lower altitudes and a peak in the upper alpine zone at around 4900 m (Fig. 3). The sites at 4800 and 4950 m had significantly higher root biomass than the other sites. Root biomass was distributed mainly in the upper 10 cm of the soils. 3.2. Soil properties Chemical and physical soil properties in the alpine Kobresia meadow varied with altitude (Table 2). The bulk density of the surface soil (0e5 cm) at the sites between 4800 and 5100 m was significantly lower than Table 1 Vegetation cover (mean  SE, n ¼ 3) and main dominant species along the altitudinal gradient from 4400 to 5300 m. Altitude (m)

Vegetation cover (%  SE)

5300

0  0a

5200

41.7  18.6ab

5100

78.3  6.7b

4950

85.0  2.9b

4800

88.3  4.4b

4650

63.3  14.5b

4500

40.0  13.2ab

4400

13.3  4.4a

Dominant species Arenaria kansuensis, Poa calliopsis, Sedum celatum Kobresia pygmaea, Poa tibetica, Potentilla sp. Kobresia pygmaea, Potentilla sp., Saussurea ceterach Kobresia pygmaea, Carex atrofusca, Potentilla sp. Kobresia pygmaea, Kobresia humilis, Cyananthus incanusa Kobresia humilis, Kobresia schoenoides, Chamaesium thalictrifolium Artemisia desertorum, Stipa aliena, Anaphalis sp. Artemisia desertorum, Stipa aliena, Anaphalis sp.

Superscript letters indicate the result of one-way ANOVA; means with the same letters were not significantly different by Scheffe´’s test (P ¼ 0.05).

281

that at the other sites. The surface soil at sites below 5100 m contained only a minor gravel content, but sites in the nival zone contained a much higher content. The soil pH was approximately neutral at the lower sites, much higher than that recorded at the higher sites. The EC of the surface soil was invariant with altitude, although an anomalous value was obtained at 5300 m. The NH4-N content of the soil profile showed peaks at 4950 and 5100 m, significantly higher than the values recorded at lower and higher sites. The NO3-N þ NO2-N contents of the soil were low compared with the NH4-N content. There were no significant differences in NO3-N þ NO2-N content with altitude, except at 4650 m. The C/N ratio was lowest at 4400 m, gradually increased with altitude up to 5100 m, then decreased at 5300 m. SOC pools at 4800, 4950, and 5100 m were significantly greater than at the other sites (Fig. 4). SOC ranged from a clear peak of 13.7  1.93 kg C m2 at 4950 m to 1.03  0.16 kg C m2 at 5300 m. SOC pools in the lower alpine zone were small: 2.60  0.27 kg C m2 at 4400 m and 2.97  0.41 kg C m2 at 4500 m. SOC pools at each soil depth differed slightly with altitude (Fig. 4). SOC pools in the surface soil (0e5 cm) did not differ significantly with altitude, except between the lowest and highest altitudes and the middle range. SOC pools at 5e10 and 15e20 cm showed a clear peak at 4950 m. In this case, SOC accumulated in the deeper soil in addition to the surface soil. 3.3. Decomposition potential The decomposition potential differed significantly among the eight sites, and we divided the sites into two groups on this basis (Fig. 5): in the nival zone, the decomposition potential was relatively low (6.1  2.2% at 5300 m and 8.0  4.3% at 5200 m); at the other five sites (alpine meadow zone), it was relatively high, peaking at 4800 m (29.4  11.2%). The decomposition potential showed a non-significant decreasing trend from 5100 to 4500 m. 4. Discussion 4.1. Soil organic carbon (SOC) storage in the upper alpine zone The regional patterns of SOC storage in alpine ecosystems are strongly affected by the local altitudinal gradient, though there exists minimal inventory data of SOC storage along altitudinal gradients in the upper alpine zone (Ko¨rner, 2003). The SOC pool in an

T. Ohtsuka et al. / Polar Science 2 (2008) 277e285

282 Soil depth: 0–5 cm

5400 a

a

a

5200

Altitude (m)

Soil depth: 5–10 cm a a

a 5000

a

a b

4800

a a a

a a

4400 0

1

2

3

4

0

1

2

3

4

Soil depth: 25–30 cm

Soil depth: 0–30 cm

5400

a

a a a

a a a

5200

a

a

5000

b

a

a

4800

b

a

a

4600

Soil depth: 15–20 cm

a

a

a a

a a

0

Root biomass (kg DW

1

2

3

4

0

a a

a

4600

1

2

3

4

4400

a a 0

m-2)

2

4

6

Root biomass (kg DW m-2)

Fig. 3. Profiles of belowground root biomass in each soil layer along the altitudinal gradient from 4400 to 5300 m (mean  SE, n ¼ 3). Superscript letters indicate the results of one-way ANOVA: means with the same letters are not significantly different (Scheffe´’s test, P ¼ 0.05).

ecosystem is determined by the balance between the litter input into the soil and carbon loss due to respiration by microbial activity (Post and Kwon, 2000; Schlesinger, 1990), as well as loss due to soil erosion and leaching (Guggenberger and Zech, 1994; Partson et al., 1993). Within an alpine meadow consisting of similar dominant plant types along an altitudinal gradient, it is possible that litter input to the soil decreases with declining plant productivity (or plant biomass) (Luo et al., 2002). The decomposition of SOC is also expected to decrease with altitude (Couˆteaux et al., 2002; Nakatsubo et al., 1997), and the effects of temperature on the decomposition rate constant have been shown to be log-linear (Uchida et al., 2000; Vitousek et al., 1994). The SOC pools should be greatest at relatively high elevations under steady-state conditions (Post et al., 1982), as the low temperatures would affect decomposition to a greater degree than production; however, in the present study neither vegetation cover nor

decomposition potential were simple log-linear functions of temperature. Indeed, the nival zone had the lowest decomposition rate (Fig. 5) and root biomass (Fig. 3), within sparse vegetation cover (Table 1). In contrast, the apparent vegetation decrease on the lower parts of the studied slope (Table 1) resulted not from temperature, but from other factors such as overgrazing by domestic animals. Moreover, decomposition potential of the cotton strips (PD) did not differ with altitude (Fig. 5). Decomposition potential was significantly related to vegetation cover (VC) in our sites (PD ¼ 0.26VC þ 6.02, r2 ¼ 0.73, P ¼ 0.014). In this case, overgrazing might have affected the decomposition potential of the cotton strips by reducing the soil water content present under conditions of sparse vegetation cover. The degradation of Chinese grassland ecosystems due to recent rapid increases in pasturing has been recognized elsewhere (Wang et al., 1998; Zhou et al., 2005). Wang et al. (2002) demonstrated that the mean

Table 2 Chemical and physical properties of soils along the altitudinal gradient from 4400 to 5300 m (mean values, with SE in parentheses, n ¼ 3). Altitude (m)

Bulk densitya (g cm3)

5300 5200 5100 4950 4800 4650 4500 4400

1.30 0.97 0.53 0.50 0.85 1.06 1.05 1.31

(0.03)a (0.13)a (0.06)b (0.01)b (0.03)b (0.04)a (0.10)a (0.05)a

Gravel contenta (%)

pHa

(4.6)a (11.7)a (5.6)b (0.1)b (1.4)c (1.7)b (1.6)c (3.5)c

6.4 6.1 6.2 6.0 6.4 6.4 7.0 6.9

39.0 44.0 5.9 0.2 10.1 5.9 13.4 15.6

(0.1)a (0.1)a (0.2)a (0.2)a (0.2)a (0.1)a (0.2)b (0.1)b

ECa (ms cm1)

b NHþ 4 -N (kg N ha1)

0.4 (0.2)a ND 3.4 (1.1)b 4.2 (0.0)b 3.6 (0.1)b 3.5 (0.1)b 3.5 (0.2)b ND

7.1 10.9 42.2 42.3 29.7 23.9 9.7 10.7

(1.8)a (8.2)a (18.2)b (22.3)b (5.6)ab (4.5)ab (1.2)a (1.4)a

 b NO 3 þ NO2 -N 1 (kg N ha )

0.5 1.5 2.2 1.8 2.1 5.4 2.3 9.6

(0.1)a (1.7)a (0.5)a (0.7)a (0.6)a (0.8)b (0.9)a (7.1)ab

Total Cb (t C ha1) 10.3 36.6 91.2 136.8 92.1 61.1 29.7 26.0

(1.6)a (18.2)a (8.5)b (19.3)b (11.3)b (7.9)a (4.1)a (2.7)a

Total Nb (t N ha1) 0.9 2.1 5.8 8.2 6.3 5.2 2.6 2.5

(0.1)a (1.0)a (0.8)b (1.1)b (0.7)b (0.6)b (0.3)a (0.1)a

C/N ratio 11.3 15.6 16.0 16.6 14.5 11.8 11.6 10.2

(0.4)a (1.9)b (1.0)b (0.1)b (0.8)b (0.2)a (0.1)a (0.6)a

Superscript letters indicate the result of one-way ANOVA; means with the same letters were not significantly different among the sites by Scheffe´’s test (P ¼ 0.05). a Data for surface soil (0e5 cm depth). b Data for volumetric soil mass of 0e0.3 m soil profiles.

T. Ohtsuka et al. / Polar Science 2 (2008) 277e285 Soil depth: 0–5 cm

Soil depth: 5–10 cm

Soil depth: 15–20 cm

283

Soil depth: 25–30 cm

Soil depth: 0–30 cm 5400

5400

Altitude (m)

a

5200

a

ab

a a

a

b

5000

b

4800

a

4400 0

4

0

6

2

4

6

0

b

2

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0

b a

4600

a a

SOC pool (kg C m-2)

b

4800

a

a a

a a

ab

2

ab

a

5000

a

ab

a

5200 b

b

b

ab

4600

a b

b b

a

a a a

a a

4400

2

4

6

0

4

8

12 16

SOC pool (kg C m-2)

Fig. 4. Profiles of soil organic carbon (SOC) pools in each soil layer along the altitudinal gradient from 4400 to 5300 m (mean  SE, n ¼ 3). Superscript letters indicate the results of one-way ANOVA: means with the same letters are not significantly different (Scheffe´’s test, P ¼ 0.05).

usage of natural grasslands on the Tibetan Plateau now exceeds 90%, and estimated that degraded grasslands now make up 18.4% of the total grassland area. Although we did not quantify the grazing intensity in this study, the alpine meadow in the lower part was heavily grazed, especially around 4400 m, where the base camp for summer pasturing was located. SOC storage is expected to be relatively low in the lower alpine meadow on our study slope (Fig. 4) because overgrazing is likely to result in reduced litter input and to a lesser degree a reduction in decomposition activity. The negative effect of grazing on SOC pools, via soil leaching and erosion caused by low vegetation cover, is expected to be of significant magnitude. 5400 a a

5200

Altitude (m)

b 5000

b b

4800 b 4600 b 4400 0

10

20

30

40

50

Decomposition potential (%) Fig. 5. Profile of decomposition potential (mean  SE, n ¼ 9), estimated using weight loss of cotton strips, along the altitudinal gradient from 4400 to 5300 m. Superscript letters indicate the results of one-way ANOVA: means with the same letters are not significantly different (Scheffe´’s test, P ¼ 0.05).

4.2. Estimation of global-scale SOC storage In considering the global pattern of soil carbon storage, Arctic and alpine ecosystems contain extensive SOC reserves. Schlesinger (1977) suggested that tundra and alpine soils contain 173 Pg C of detritus pools, representing 11.9% of the total world detritus. Post et al. (1982) also studied global-scale SOC distribution among Holdridge life zones along latitudinal and altitudinal gradients, finding that the soil carbon density generally increases with decreasing temperature for a given level of precipitation. The authors estimated that tundra ecosystems contain 191.8 Pg C of detritus pools, representing 13.7% of the total world detritus. The SOC storage of the Tibetan Plateau is also of great importance globally, and is estimated to be 33.5 Pg C (Wang et al., 2002). To estimate the global-scale SOC pool, we used the carbon density of existing regional-scale inventory data. The mean carbon density of tundra soils is ca. 22 kg C m2 (Post et al., 1982; Schlesinger, 1977), one of the highest values among the world’s biomes. Actual calculations of SOC pools in the temperate wet alpine zone reveal that those under closed vegetation are somewhat variable, ranging from 4 to 53 kg C m2 (Table 3). Given that all natural ecosystems are heterogeneous at a local scale, the question arises as to whether estimates of tundra SOC density based on a limited number of samples represent the average of all tundra ecosystems at a global scale. We inventoried SOC pools in 0e0.3 m profiles along an altitudinal gradient from the alpine meadow zone to the nival zone (4400e5300 m) on the Tibetan Plateau. Upon our study slope, SOC pools at lower sites were relatively small (2.6 kg C m2 at 4400 m), but increased sharply with altitude, reaching a peak in the upper alpine zone (13.7 kg C m2 at 4950 m) before decreasing with

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Table 3 Estimates of soil carbon pools per unit area at medium to upper alpine altitudes of temperate-zone mountains. Total pool (kg C m2)

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2.9 1.5 53.1

Wang et al. (2002)

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4e22

Ko¨rner (2003)

the ‘‘typical site effect’’ for inventory data is expected to result in overestimates of global-scale SOC storage in tundra areas. To estimate SOC storage upon the Tibetan Plateau, inventory data along an altitudinal gradient is of crucial importance. The rapid increase in pasturing on the Tibetan Plateau makes it necessary to quantitatively measure the effects of grazing on alpine ecosystems. Post et al. (1982) stated that a significant source of error is present in estimates of SOC pools, especially in tundra ecosystems, because of a steep gradient in soil carbon densities across tundra life zones. Inventory data for both carbon density and bulk density along altitudinal gradients in alpine ecosystems is needed for precise estimates of global-scale tundra SOC storage.

Swiss Alps Sedge turf (2470 m)

9

Ko¨rner (2003)

Acknowledgments

Tirol Alps Heath (2180 m)

18

Ko¨rner (2003)

Niwot Ridge Dry meadow

6e7

Seastedt (2001)

Mauna Loa (>2000 m) Old and wet site Old and dry site

0.6e0.8 0.9e2.2

Vitousek et al. (1992)

We are grateful to the members of the Lhasa Plateau Ecological Research Station for their kind cooperation during our field survey and laboratory work. We also appreciate the laboratory assistance of Dr S. Nohara, National Institute for Environmental Studies, Japan. This study was supported by the Global Environmental Research Fund from the Ministry of Environment (S1), Japan, and the project titled ‘‘Early Detection and Prediction of Climate Warming Based on the LongTerm Monitoring of Alpine Ecosystems on the Tibetan Plateau’’.

Location

Nyainqentanglha Mts. (4400e5300 m) Alpine meadow 2.6e13.7 Nival zone 1.0e3.7

This study

altitude in the nival zone (4950 m) to 1.0 kg C m2. SOC pools varied markedly within the same alpine meadow, by a factor of five or more (2.6e13.7 kg C m2), even for the same slope (4400e5100 m). This finding was also true for bulk density data, as soil bulk density was sometimes estimated using a regression analysis in the case that data were not reported. Post et al. (1982) reported that bulk density has been determined for a wide range of vegetation types, but varies little between profiles when the same vegetation type is considered. However, the authors found that the bulk density of surface soil also varied with altitude, by a factor of two or more (0.50e1.31 g cm3), partly due to the effects of grazing. Moreover, field inventory studies of carbon density at a regional scale tend to be designed with the intention of sampling sites that are under undisturbed mature vegetation (climax). Wang et al. (2002) studied carbon storage down to 0.75 m depth beneath various types of grasslands across the Tibetan Plateau, reporting large SOC pools (29.0 kg C m2) compared with those in our study, although we only analyzed down to 0.3 m depth. Our data reveal that difficulty involved in selecting a ‘‘typical’’ site within the alpine meadow zone; thus,

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