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Plant impact on C[O.sub.2] consumption by silicate weathering: the role of bamboo.

Introduction

Consumption of C[O.sub.2] by silicate weathering has exerted a major control on atmospheric C[O.sub.2] over geologic time (Berner et al., 1983; Berner, 1992; Moulton et al., 2000; Kurtz et al., 2002; Derry et al., 2005). More and more studies suggest that land plants (especially vascular plants) may accelerate rock weathering and thus the removal of C[O.sub.2] from the atmosphere via water recycling, organic acid production, root-induced acidification and reduction of the rhizosphere, and root uptake of inorganic ions, etc. (Berner et al., 1983; Berner, 1992; Alexandre et al., 1997; Kelly et al., 1998; Moulton et al., 2000; Hinsinger et al., 2001; Derry et al., 2005; Johnson-Maynard et al., 2005).

Bamboos belong to the Gramineae family of plants and are widely distributed in subtropical and temperate zones (Bystriakova et al., 2003; Li et al., 2006). Recent studies showed that bamboos are a significant organic silicon pool and play an important role in biogeochemical cycles of silicon and other nutrients because of their special growth habit and biogeochemical characteristics, i.e., rapid biomass accumulation, the extremely high biomass of fine roots, and the extremely high silicon content (Christanty et al., 1997; Mailly et al., 1997; Li et al., 2006; Ding et al., 2008). As silicon (Si) in bamboos originate from silicate weathering and is removed quickly through frequent harvest of bamboo products (i.e., bamboo shoots and bamboo), we suggest that bamboo growth may accelerate silicate weathering, and thus, C[O.sub.2] consumption via the uptake of Si and other mineral nutrients.

The study investigated stream water geochemistry, using mass balance of major ions to quantify C[O.sub.2] consumption by silicate weathering under bamboo and other vegetation conditions and to learn about the possibility of bamboo-enhanced silicate weathering as a biogeochemical remediation strategy of atmospheric C[O.sub.2].

Materials and Methods

The study sites are located in counties of Anji and Lin'an, Northwest Zhejiang Province, China (30.7[degrees]N/119.7[degrees]E). The geology of the study sites consists predominantly of Paleozoic granitic intrusions with detrital sedimentary rocks and low grade metamorphosed sediments. The climate in the region is subtropical, controlled by East Asian monsoons. The annual average temperature in the region ranges from 15-17[degrees]C and the annual rainfall is between 1,500 and 1,800 mm (average 1,645 mm). The average runoff depth is 912 mm. The hydrographs of the rivers are dominated by summer rain with ~40% of total discharge of the rivers occurring between June and August. Evergreen broadleaved and Moso bamboo (Phyllostachys pubescens) forests dominate the natural areas in this region.

Six types of watersheds were selected in this study. "A" represents upper pure granitic watersheds covered with pure Moso bamboo forest. "B" represents upper granite-dominated watersheds covered with pure Moso bamboo forest. "C" represents upper granite-dominated watersheds covered with bamboo- broadleaf-coniferous mixed forest. "D" represents upper granite-dominated watersheds covered with broadleaf- coniferous mixed forest without or with rare bamboos. "E" represents down granite- dominated watersheds covered with bamboo- broadleaf-coniferous mixed forest. "F" represents upper pure granitic watersheds covered with broadleaf forest (Fig. 1). A total of 74 stream water samples were taken from the above watersheds during spring and summer seasons in 2010.

Water pH was measured at the sampling sites with a portable pH meter. HC[O.sub.3.sup.-] was titrated by HC1 on the spot. Immediately after collection, all the water samples were filtered through 0.22 [micro]m membrane filters (Millipore) and a small portion of these samples was stored for measuring anions, while another portion was acidified with ultra-purified hydrochloric acid to pH<2 after collection for measuring cations (Han & Liu, 2004). All of the samples were stored in darkness. Major cations ([K.sup.+], [Na.sup.+], [Ca.sup.2+], [Mg.sup.2+]) were determined by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). Anions ([Cl.sup.-], S[O.sub.4.sup.2-], N[O.sub.3.sup.-]) were measured by ion chromatography.

[FIGURE 1 OMITTED]

Results and Discussion

The TDS concentration ranged from 20 to 70 mg/L, average 33 mg/L (Fig. la). Generally, it was higher in the spring season than in the summer season. The TDS concentration in upper-reach stream water samples from near pure granitic watersheds (A and F) and from granite-dominated watersheds covered with broadleaf-coniferous mixed forest without or with rare bamboos was generally lower than other types of stream water samples (B and E). The results indicated that the stream water was not obviously polluted.

Si[O.sub.2] concentration ranged from 15 to 160 p, [micro]mol/L, average 82 [micro]mol/L (Fig. 1b). The seasonal variation of Si[O.sub.2] concentration was fairly small. Si[O.sub.2] concentrations (80~150 [micro]mol/L, average 105 [micro]mol/L) in the upper-reach stream water samples from watersheds covered with pure bamboo forest (A and B) were higher than that (15~85 [micro]mol/L, average 60 [micro]mol/L) from other watersheds.

The good correlations between elemental ratios such as Ca/Na vs. Mg/Na ([r.sup.2] =0.98) and Ca/Na vs. HC[O.sub.3]/Na ([r.sup.2] =0.98) (Fig. 2a, b) reflected mostly mixing between silicates and carbonates. The upper-reach stream water samples from nearby pure granitic watersheds covered with pure Moso bamboo forest reflected that mostly weathering of granites and their elemental ratios were used to calculate concentrations of Ca, Mg, Na and K from silicate weathering ([Ca.sub.Sil], [Mg.sub.Sil], [Na.sub.Sil], and [K.sub.Sil)].

The molar ratio of Si/([Na.sub.Sil]+[K.sub.Sil]) in stream water reflecting the intensity of silicate weathering ranged from 0.6 to 4 with an average of 2.4 (Fig. 3a). Generally, it was lower in the spring season than in the summer season. The ratios of Si/([Na.sub.sil]+[K.sub.sil]) in stream water draining from bamboo watersheds (2.0~4.0, average 2.9) were higher than that from other forest watersheds (0.7~2.7, average 2.2) indicating that the weathering intensity of silicates in bamboo watersheds was higher than that in other watersheds.

Atmospheric C[O.sub.2] consumption rate (C[O.sub.2] flux=(2[Ca.sub.sil]+ 2[Mg.sub.sil]+[Na.sub.sil]+[K.sub.sil]) x runoff) calculated from data of cations and runoff ranged from 1.5 to 3.4 [l0.sup.5] mol/ [km.sub.2]/a (average 2.2 [10.sup.5] mol/[km.sup.2]/a) (Fig. 3b). It was lower in the spring season than in the summer season. The consumption rates of C[O.sub.2] by silicate weathering in bamboo watersheds (1.8~3.4 [l0.sup.5] mol [km.sup.-2] [a.sup.-l], average 2.5 [10.sup.5] mol/[km.sup.2/a) were generally higher than that of other forest watersheds (1.5~2.6 [10.sup.5] mol/[km.sup.2]/a, average 2.0 [10.sup.5] mol/[km.sup.2]/a).

Bamboos are globally distributed in regions of similar petrologic (e.g., granite, volcanic tuff and rhyolite) and climatic (e.g., subtropical climate) conditions. Our data revealed that bamboos can enhance silicate weathering intensity and consumption rate of C[O.sub.2] by silicate weathering as bamboos belong to the Gramineae plant family and generally need much more Si than other trees. Therefore, bamboo-enhanced silicate weathering revealed in this study is a potential biogeochemical remediation approach to address atmospheric C[O.sub.2] at a regional (e.g., eastern Asia) to global scale. However, further work such as the effects of bamboo on weathering of other silicate rocks, effects of physic-chemical parameter changes (e.g., monthly or seasonal changes of temperature and runoff) in bamboo watersheds on silicate weathering rate, mechanism of bamboo enhanced- silicate weathering, biogeochemical cycle of silicon and other cations in bamboo ecosystems should be conducted before the strategy can be applied to sequester globally significant amounts of C[O.sub.2] under realistic conditions.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Conclusions

1. The Si[O.sub.2] concentration and Si/([Na.sub.silicate]+[K.sub.silicate]) ratio in stream water draining from bamboo watersheds were higher than those from other forest watersheds, indicating that the silicate weathering intensity in bamboo watersheds was higher than that in other forest watersheds.

2. The consumption rates of C[O.sub.2] by silicate weathering in bamboo watersheds (1.8~3.4 [10.sub.5] mol/[km.sup.2]/a, average 2.5 [10.sup.5] mol/[km.sup.2]/a) were generally higher than that in other forest watersheds (1.5~2.6 [10.sup.5] mol/[km.sup.2]/a, average 2.0 [10.sup.5] mol/[km.sup.2]/a).

3. Our data revealed that bamboos can enhance silicate weathering intensity and consumption rate of C[O.sub.2] by silicate weathering as bamboos belong to the Gramineae plant family and generally need more Si than trees, and that bamboo-enhanced silicate weathering is a potential biogeochemical remediation approach for atmospheric C[O.sub.2].

Acknowledgements We are grateful for support from Zhejiang Provincial Natural Science Foundation Program (Grant No. Y5080110 and Z5080203); Opening Project of State Key Laboratory of Environmental Geochemistry (SKLEG9011); Research Development Program of Zhejiang A & F University (2351000741 and 2451012029).

DOI 10.1007/s12229-011-9077-9

Published online: 10 June 2011

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Zhaoliang Song (1,2) * Songlai Zhao (1) * Youzhen Zhang (3) * Guoliang Hu (3) * Zhihong Cao (1,4,5,6,7) * Minghung Wong (4,5)

(1) Zhejiang Provincial Key Laboratory of Carbon Cycling in Forest Ecosystems and Carbon Sequestration, Zhejiang A & F University, Lin'an 311300, China

(2) State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China

(3) Lin'an Institute of Forestry Sciences, Lin'an 311300, China

(4) Joint Laboratory of Soil and Environment, Hong Kong Baptist University, Kowloon Tong, Hong Kong

(5) Institute of Soil Sciences, Chinese Academy of Sciences, Nanjing 210008, China

(6) School of Environment and Resources, Zhejiang A & F University, No. 88 North Huancheng Road, Lin'an, Zhejiang 311300, China

(7) Author for Correspondence; e-mail: zhcao@issas.ac.cn
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Author:Song, Zhaoliang; Zhao, Songlai; Zhang, Youzhen; Hu, Guoliang; Cao, Zhihong; Wong, Minghung
Publication:The Botanical Review
Article Type:Report
Geographic Code:9CHIN
Date:Sep 1, 2011
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