Dynamics of carbon accumulation during the fast growth period of bamboo plant.
Global climate change has inspired scientific and political communities to have an increasing interest in the study of carbon storage and carbon balance (Houghton, 2001; Marland & Marland, 2003; Nath et al., 2009). Forest ecosystems are major reserves for terrestrial carbon, and they play an important role in alleviating atmospheric C[O.sub.2] increase and associated carbon balance (Malhi et al., 1999). In the forest ecosystem, however, the ability of carbon sequestration in different stands are quite different, in which Moso bamboo is characterized by high carbon sequestration ability with its fast growth rate and special reproduction manner. Previous results showed the amount of carbon fixation in bamboo forest was 2~4 times of those in Chinese fir, tropical montane rainforest and masson pine forests (Zhou et al., 2006).
There are 4.99 million hectares (Mha) of bamboo stands in China, among which 3.37 Mha, accounting for 70% of the total, is Phyllostachys pubescens (Chen et al., 2009). Because of the special way of asexual reproduction, a large number of bamboos are grown every year. Therefore, as the forest area declines sharply in the world today, the Moso bamboo area has been increasing at a rate of about 3% per year, which means bamboo forest has become an ever-increasing carbon stock (Wang et al., 2008).
Since bamboo has great potential for carbon sequestration, much attention has been paid to carbon balance and recycling of bamboo forests. However, most of the previous studies have dealt with carbon accumulation based on the whole bamboo forest ecosystem (Chen et al., 2009). Therefore, our understanding of how bamboo fixes carbon is scarce. Usually, the shape of bamboo is formed within 3 months after a new shoot has sprouted, and its fastest growth rate could reach 100 cm/d. After that, the bamboo height and diameter barely changes whilst the nutrients can be further accumulated along with secondary cell wall development. It was reported that before the emergence of new shoots, almost all of the dry biomass and carbon came from bamboo culm (Jiang, 2002). Therefore, this fast growth period determines its ability to accumulate carbon. The objective of this study is to investigate the dynamics of the biomass and carbon accumulation during fast growth period of bamboo culm.
Materials and Methods
Field experiments were carried out on Moso bamboo in Qingshan town (30[degress]13' 41" N, 119[degress]47' 27" E) near Lin'an city, Zhejiang Province. The area has a monsoonal subtropical climate with four distinct seasons. The mean annual rain fall is 1,442 mm and the mean annual temperature is 15.9[degress]C with a maximum and minimum temperature of 41.7[degress]C (July) and -13.3[degress]C (January), averaging 1,774 daylight hours and 235 frost-free days. Red soil is the major soil type in this area with a typical pH of 5.94. The content of soil organic matter, total N, available-N, available P, available K were 24.20 g/kg, 1.06 g/kg, 60.67 mg/kg, 13.32 mg/kg, 30.41 mg/kg respectively. The substrate is a quartziferous sandstone. The research area of this study consisted of a pure bamboo forest with a total area of 23.7 ha. The density of bamboos was 3,100 ~ 3,300 plant/[hm.sup.2], with a breast high diameter mostly between 10 and 11 cm. A composite fertilizer (N: [P.sub.2][O.sub.5]: [K.sub.2]O=15: 15: 15) was applied at 1,500 kg/[hm.sup.2] in mid-September every year.
The bamboo field was divided into five 225 [m.sup.2] plots (each 15 m x 15 m). Each newborn bamboo in the plots was labeled and its growing days was recorded from the time of bamboo shoot emergence from the ground. The day after shoot emergence (DASE) was designated as day 0. On day 14, 20, 29, 38, 56 and 88, one healthy labeled bamboo from each plot was randomly selected and cut just above the ground. The whole sampling period was divided into three stages, as shown in Table 1. After measuring the height and removing the shells, the stem was evenly divided into three sections (upper, middle, and lower) for further analysis.
Plant samples were oven-dried to constant weight at 80[degrees]C, and ground to pass a 30-mesh (0.5 mm) screen. Wet oxidation method was used to determine the carbon concentration. To analyze for N, P, K, Ca, Mg, dry plant samples were wet-ashed with [H.sub.2]S[O.supb.4]. Water extracts from plant digestion were used for N determination by micro-Kjeldahl method, and P, K, Ca, and Mg measured by ICP OES. All methods described above followed Lu (1999).
Extraction of crude cell wall materials and subsequent fractionation of cell wall compositions were carried out according to Zhong and Laiuchli (1993) with minor modifications. Pectin was extracted twice by boiling water for 1 h each and pooling the supernatants. The pellet was subjected to triple extraction with 24% KOH containing 0.1% NaB[H.sub.4] for a total time of 24 h and pooling the supernatants the residue was considered as cellulose fraction. Uronic acid content in each supernatant was assayed according to Blumenkrantz and Asboe-Hansen (1973) using galacturonic acid (Sigma) as standard.
Results and Discussion
Characterization of Culm Elongation
Moso bamboo is a fast-growing renewable biomass that is widely distributed in Asia (Shimokawa et al., 2009). The growth rate of Moso bamboo is markedly higher than other trees (Laclau, 2003; Stoffberg et al., 2010). In this study, we found that the growth of Moso bamboo exhibited differential growth rates at different stages although the overall growth rate was still high. The appearance of the culm was very slender and the growth of the new shoot was initially slow. For example, after 20 days of a newborn bamboo shoot emergence, the average height was 38.9 cm, with an average daily elongation of only 2.0 cm (Fig. 1). When the shells began to detach, the growth of bamboo started to speed up, until the DASE of 29, whereby the average height of culm was 378 cm. During stage II, from the DASE of 29 to 56, the average height, along with the emergence of new branches, of bamboo culm increased from 378 cm to 1,378 cm, with an elongation rate reaching 37.4 cm per day. At DASE of 88, almost all of the shells detached except for a few on the bottom, and during this stage, the elongation rate of bamboo culm declined to 3.7 cm per day. This study has demonstrated that the bamboo experienced a 'slow-fast-slow' process with maximum growth of Moso bamboo occurring at DASE of 29 to 55, thereby ceasing its fast growth within no more than 2 months.
[FIGURE 1 OMITTED]
Characterization of Biomass Accumulation
The major biomass accumulation occurred along with the fast elongation of bamboo culm. When the shells detached completely at DASE of 88, the dry weight of bamboo was 440-fold greater than that at DASE of 14 (Fig. 2). The contributions of the three stages to total dry weight accumulation were 14%, 55%, and 31% respectively, indicating that the increase in biomass took place mainly at stage 2. As far as different parts are concerned, the dry weight in the lower part exceeded 50% of the total bamboo weight. This was in accordance with the fact that both the diameter and wall thickness decreased acropetally in bamboo culm (Jiang, 2002). Therefore, the biomass accumulation of Moso bamboo had also experienced a 'slow-fast-slow' process, indicating that the fast growth was accompanied by rapid biomass accumulation. However, the causal relationship between fast growth and fast biomass accumulation has yet to be investigated.
The Carbon Fixation is Associated with Biomass Accumulation
It is generally recognized that the increase of biomass is closely related to carbon accumulation in some plant species (Zheng et al., 2008; Peri & Lasagno, 2010). In order to examine whether the accumulation of biomass is related to carbon fixation, we investigated the relationship between carbon accumulation and biomass accumulation on the basis of growth stages and different parts. As shown in Fig. 3a, carbon accumulation in different parts of bamboo culms did increase with age, For example, the carbon mass increased from 6.7 g at DASE of 14 to 3953.4 g at DASE of 88, exhibiting a 590-fold increase in carbon mass during the experimental stage. As far as different parts are concerned, the lower part accumulated more than 50% of total carbon which was 1.6 and 4.5-fold greater than middle and upper parts, respectively. Therefore, a significant positive correlation could be established between the carbon accumulation and the dry weight accumulation (Fig. 3b), indicating that the fast growth of bamboo was associated with its extraordinary ability to fix carbon. It is well known that the high capacity of carbon fixation in plants is due to the high photosynthetic efficiency (Gratani et al., 2008). However, during this study period, the contribution of photosynthesis to the carbon demand is likely to be partial since the leaves were not yet fully expanded. Therefore, the source of carbon demanded during this special period merits further study.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Mineral Nutrients is not Associated with Biomass Accumulation
Nutritional status always affects plant growth, especially during the young seedling stage (Day et al., 2006). In order to examine whether mineral nutrients are also related to fast biomass accumulation during the Moso bamboo fast growth stage, we analyzed N, P, K, Ca, and Mg concentrations in different parts and at different stages. As shown in Fig. 4, typically, the concentrations of minerals decreased with age in all three parts, probably due to the dilution effect from the plant's rapid growth. However, the significant decrease of mineral concentrations in the upper parts did not occur until the burgeon of new branches, i.e. at DASE of 38 (Table 1). This could be explained by the fact that the upper parts grew slowly before this stage. Therefore, we could not establish a relationship between mineral nutrients accumulation and biomass accumulation. Wu et al. (2009) and Shanmughavel and Francis (1996) also demonstrated that N, P, K, Ca and Mg contents in the stem were intensively reduced within 6 weeks after the emergence of young bamboo shoot. This indicates that the accumulation of N, P, K, Ca and Mg was not responsible for its fast growth and carbon sequestration in Moso bamboo.
[FIGURE 4 OMITTED]
Partitioning of Fixed Carbon to Cell Wall
In order to follow the carbon distribution, we examined the cell wall content on the basis of dry weight. We found that more than 90% of dry weight was cell wall. This was not unexpected since the cell wall is a major pool of carbohydrate. To further understand the final destination of the fixed carbon in cell walls, we compared the contents of pectin, hemicellulose, and cellulose in cell walls. During the stage of Moso bamboo shoot emergence to the detachment of all the shells, the cellulose content was stable at various parts of the culm (Fig. 5c), while hemicellulose content increased by about 1 fold (Fig. 5b) and the pectin content rapidly decreased to a low level within stage II and remained constant thereafter (Fig. 5a). It has been reported that the carbon in cell walls of bamboo exists mainly in the form of fiber polysaccharides. Its content increases along with the expansion, elongation and thickening process of cell walls (Liese & Weiner, 1996; Lybeer & Koch, 2005). These results provided evidence pointing towards the role of carbon accumulation in cell walls, by ensuring a continuous carbon fixation to the cellulose and hemicellulose, the fixed structural components of bamboo cell walls, which in turn facilitated the rapid carbon assimilation in the source by contributing to the biomass accumulation.
We corroborated a 'slow-fast-slow' growth feature within the first 3 months of Moso bamboo development. It was also found that the accumulation of biomass was well correlated with its growth with carbon fixation playing a major role. The fixed carbon was distributed to cell walls and incorporated into the cellulose and hemicelluolse to meet the demand of fast elongation. However, the changes of other mineral nutrients did not appear to be associated with fast growth. The reason for the high capacity of carbon fixation in newborn Moso bamboo is still unclear. There are at least two main aspects worthy of attention. On the one hand, the carbon demanded by Moso bamboo during the primary stage could come from the underground parts, and on the other hand, whether or not the Moso bamboo culm possesses photosynthetic ability. Is so, its photosynthetic efficiency, the output and process of photosynthetic product, and other related issues deserve further examination.
[FIGURE 5 OMITTED]
Acknowledgements The authors wish to acknowledge the funding support from the Natural Science Foundation of China (No.30972356, No.30771715), Zhejiang A & F University (2005FR003), and the Type B Creative Group Grant of Zhejiang A & F University.
Published online: 11 June 2011
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Yong Xu (1,2) * Minghung Wong (3) * Jianli Yang (1) * Zhengqian Ye (2) * Peikun Jiang (2) * Shaojan Zheng (1,6)
(1) College of Life Sciences, Zhejiang University, Hangzhou 310058, China
(2) Zhejiang Provincial Key Laboratory of Carbon Cycling in Forest Ecosystems and Carbon Sequestration, School of Environmental Science and Technology, Zhejiang A & F University, Lin'an 311300, China
(3) Croucher Institute for Environmental Sciences, and Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, People's Republic of China
(4) Author for Correspondence; e-mail: firstname.lastname@example.org
Table 1 Different Stages, Stage Periods, Characteristics of Growth Period, and Sampling Date Stage Stage Sampling date Characterization of period(d) at DASE (d) growth period I 0-28 14 bamboo shoot 20 emergence to first shell detached II 29-55 29 first shell detached 38 to branch emergence III 56-88 56 branch emergence to 89 all shells detached
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|Author:||Xu, Yong; Wong, Minghung; Yang, Jianli; Ye, Zhengqian; Jiang, Peikun; Zheng, Shaojan|
|Publication:||The Botanical Review|
|Date:||Sep 1, 2011|
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