Evaluation of carbon accumulation in the Kandelia obovata Sheue, Liu & Yong plantation in Northern Vietnam

JOURNAL OF SCIENCE OF HNUE Chemical and Biological Sci., 2012, Vol. 57, No. 8, pp. 43-55 This paper is available online at EVALUATION OF CARBON ACCUMULATION IN THE Kandelia obovata SHEUE, LIU & YONG PLANTATION IN NORTHERN VIETNAM Mai Sy Tuan Faculty of Biology, Ha Noi National Universuty of Education Abstract. This study was conducted to evaluate carbon accumulation in a mangrove plantation containing one species of Mangrove tree (Kandelia obovata Sheue, Liu & Yong) in Giao Thuy District, Nam Dinh Province, Vietnam. Sixteen sample plots were established and surveyed from December 2008 to May 2010 in order to determine the amount of carbon present in the trees and forest soil. The amount of CO2 released from the forest soil was estimated once a month using a KIMOTO-HS7 gas absorption machine. The amount of carbon accumulation was calculated from estimated variables. The results show that the amount of carbon accumulated in trees and forest soil is higher than that of emitted CO2. The annual amount of carbon that is accumulated by K. obovata plantations is equivalent to the amount of CO2 emitted, and this amount increases as the forest ages, reaching the highest value in the 9 year old forest (27.138 ton/ha/year) in this present study (a CO2 equivalent of 99.596 ton/ha/year (39.35%). The least amount of CO2 emitted was observed in the 1-year old forest (2.207 ton/ha/year), a CO2 equivalent of 8.099 ton/ha/year (3.20%). Therefore, a K. obovata plantation can accumulate a large amount of carbon and function as a carbon sink and reduce the amount of CO2 in the atmosphere. This study presents scientific data that could be used to justify the implementation of mangrove plantation projects under CDM and PES programs along the Vietnamese coastline that could protect the environment, inhibit climate change, improve living standards, alleviate hunger and reduce poverty among local populations. Keywords: Mangrove plantation, Kandelia obovata, carbon accumulation, Northern Vietnam. Received September 10, 2012. Accepted November 2, 2012. Contact Mai Sy Tuan, e-mail address: tuanmaisy@yahoo.com 43 Mai Sy Tuan 1. Introduction Mangrove forests were found along the coastlines in most countries in the tropics and subtropics and many still remain or have been rather recently planted. Tropical forests in general are a disproportionately important component in the global carbon cycle and are thought to represent 30 - 40% of the terrestrial net primary production and storage of carbon [4, 10]. Although, mangrove forests represent only a small fraction of total tropical forest area, the amount of carbon accumulated in the biomass of mangrove forest trees and/or sediments is significant. In addition to this accumulation, the mangrove ecosystem also includes a carbon release process that involves the aqueous environment and particularly the sediment due to the decomposition of microorganisms. Although a large number of studies have been done which have examined carbon accumulation in natural mangrove ecosystems around the world [9], there is little information available about carbon accumulation in man-made mangrove plantations. Vietnam’s 3260-km long coastline and dense river system with abundant alluvial effluent has the potential to support a substantial area of mangrove trees. Eleven years ago, in 2001, the Forest Inventory and Planning Institute carried out a national forest inventory [5] and at that time it was estimated that there were 156,608 ha of mangrove trees in Vietnam, about two thirds of that being plantation forest trees. This current study has been carried out in order to evaluate carbon accumulation in mangrove plantations in northern Vietnam, the specific objectives being: 1) to evaluate the efficacy of mangrove plantations as carbon sinks, 2) to propose an effective methodology for measuring carbon accumulation, 3) to contribute to the means of valuating carbon credits based on the Kyoto protocol approved in 2002 regarding Clean Development Mechanism (CDM) and Payment for Environmental Services (PES) carbon options and 4) to help policy makers and managers formulate an integrated management plan regarding mangrove plantation areas that would result in increased carbon accumulation. 2. Content 2.1. Materials and methods 2.1.1. Study site This study was conducted from December 2008 to May 2010 in plantations of K. obovata Sheue, Liu & Yong mangrove trees (other mangrove/associated species were not found). The trees are now 1, 5, 6, 8, 9, and 12 years old and were planted in the Giao Lac Commune, Giao Thuy District, Nam Dinh Province, in Northern Vietnam. Giao Lac Commune is situated between 20◦13’- 20◦15’ latitude and 106◦15’- 106◦30’ longitude. The commune is bordered by the Giao An Commune to the north, the Giao Xuan Commune to the south and the sea to the east (Figure 1). 44 Evaluation of carbon accumulation in the Kandelia obovata Sheue, Liu & Yong plantation... Figure 1. Study site in the coast of northern Vietnam Giao Lac Commune is in the Xuan Thuy Ramsar Site which is a buffer zone. At this time in the Giao Lac commune, approximately 407.7 ha have been planted in mangrove trees. The mangrove ecosystem here receives a large amount of alluvial material from the Red River and the mangrove habitat soil is basically mud mixed with clay and sand. In this site, plantations of 8, 9 and 12 years old mangrove trees are situated in relatively high elevation areas with a long non-tidal inundation time (4,776 hours/year) and have tree density of 17,900, 18,200 and 18,050 trees/ha respectively. Mangrove plantations in mid elevation areas (4358 hours/year) are 5 and 6 years old with tree density of 17,300 and 17,500 trees/ha and the plantations in the lower elevation areas (3,960 hours/year) are 1 year old with the tree density of 15,400 trees/ha. According to the Vietnam National Hydro-meteorological Service and Marine Hydrological Center, the tide is diurnal at this site. Low tide is 0.1 m and high tide is 3.9 m. The mean annual temperature is 23 - 24◦C. Mean rainfall ranges from 1,056 to 1,470 mm/year, maximum rainfall is normally in July and August (227 - 315 mm/month) and the least amount of rain falls in January (9 mm/month). Average humidity is about 82%. 2.1.2. Data collection Sixteen sample plots (10 m × 10 m) of trees of 6 age classes were surveyed; for each age class (1, 5, 6, 8, 9 years) we measured the diameter of the trees growing in 3 randomly selected plots but in the 12-year old plantation we measured the trees in only 1 plot and was left out of our analysis due to its small area. In each sample plot, 3 mangrove trees, of the big, medium and small size, were cut and categorized in regards to roots, 45 Mai Sy Tuan stems, branches, leaves and reproductive organs. The tree parts were weighted at that time. Samples obtained from these parts were then dried at 80◦C and the dry weight was recorded. Thirty-six soil samples were obtained from plantations of each age of tree (1, 5, 6, 8, 9 years), collected in a 20 cm× 20 cm × 20 cm frame ranging in depth from surface soil to 100 cm in depth at low tide. The samples were brought to the Environment and Soil Analysis Laboratory for treatment and analysis. 2.1.3. Determining the amount of carbon in the trees The amount of organic carbon (%) in the trees was determined using the L.O.I (loss on ignition) method. Basing on the accumulated carbon, the amount of CO2 absorbed by photosynthesis and converted to mangrove tree biomass is 3.67 times the total accumulated carbon (ton/ha). Therefore, 3.67 can be used as a conversion constant to convert organic carbon to CO2, this being determined from the molecular weight of CO2 and carbon. 2.1.4. Determining the amount of carbon in the soil The amount of organic carbon in the soil was determined using the Walkley-Black method [13]. This method makes use of the following equation. 3C + 2K2Cr2O7 + 8H2SO4 → 3CO2 + 2K2SO4 + 2Cr2(SO4)3 + 8H2O The redundant amount of K2Cr2O7 was titrated by 0.5N FeSO4. 2.1.5. Determining the amount of CO2 emitted from soil The gas sampling method was used to measure the amount of CO2 emitted from the soil. The gas absorption machine, a KIMOTO-HS7, was placed in a 1 m3 box tightly covered in transparent white nylon that had an absorption speed of 1 litre/minute (Fig. 2). Figure 2. KIMOTO-HS7 equipment and sampling of gas emitted from soil in 1-year old forest The amount of carbon dioxide was determined referring to the absorption rate of barite. The formula used is as follows. 46 Evaluation of carbon accumulation in the Kandelia obovata Sheue, Liu & Yong plantation... CO2 + Ba(OH)2 → BaCO3 + H2O Air with CO2 was added to a given amount of barium hydroxide and the redundant amount of barium hydroxide was titrated by acid oxalic. The colored indicator is 0.1% phenolftalein. Ba(OH)2 + HOOC-COOH→ Ba(COO)2 + 2H2O The added amount of barium hydroxide is calculated based on the amount of redundant barium hydroxide. The amount of CO2 in the air can then be calculated. In this study, the amount of CO2 being emitted from forest soil was estimated once a month at low tide. 2.2. Results and discussion 2.2.1. Carbon accumulation in K. obovata plantation forest biomass * Amount of carbon accumulated in K. obovata forest Amount of carbon accumulated in K. obovata plantation forest trees increased with the rising age of the trees (Table 1) and is in direct proportion with tree biomass. Table 1. Amount of carbon (C) accumulation (ton/ha) (n = 36) Forest age Planting year Density (trees/ha) Parts of tree Carbon accumulated in forest trees Stems Roots Leaves 1 2007 15400 0.039 ± 0.226 0.954 ± 0.728 0.022 ± 0.557 1.015 5 2003 17300 22.134 ± 1.253 2.856 ± 2.142 2.244 ± 1.785 27.234 6 2002 17500 22.855 ± 1.873 3.895 ± 0.784 2.327 ± 2.614 29.077 8 2000 17900 29.929 ± 2.658 6.800 ± 2.162 3.276 ± 2.143 40.005 9 1999 18200 32.614 ± 1.475 12.793 ± 2.769 2.621 ± 1.856 48.028 Among mangrove forest trees which are 1, 5, 6, 8 and nine years old, the greatest amount of carbon accumulated in 9-year old K. obovata plantation trees (48.02 ton C/ha), followed by 8-year old plantation trees (40.005 ton/ha), 6-year old plantation trees (29.077 ton/ha), 5-year old plantation trees (27.234 ton/ha), and 1-year old plantation trees (1.015 ton/ha). On each tree in the 1-year old plantation was found to be growing from 14 to 50 leaves; therefore, the ability of these trees to fixate CO2 for synthesis of organic carbon 47 Mai Sy Tuan is low. In addition, the 1-year old plantation of this study is situated in a relatively low elevation area which is inundated by sea water for 10 - 14 hours/day. This would also lower the trees’ photosynthetic ability. Despite the low amount of accumulated carbon in newly planted mangrove trees and their poor growth performance, their survival rate is greater than 70%, showing that K. obovata trees are quite resilient with regards to relatively high concentrations of salinity in its environment. * Absorption of CO2 in K. obovata plantations For the study on absorption of CO2 in mangrove plantation forests, it is essential to determine the forest biomass. From an estimated total forest biomass, we can determine the amount of carbon accumulated in each tree and then calculate the amount of CO2 absorbed during photosynthesis. The amount of CO2 absorbed in forest trees was found to be highest in the 9-year old forest (176.263 ton/ha), followed by the 8-year old forest (146.818 ton/ha), 6-year old forest (106.713 ton/ha), 5-year old forest (99.949 ton/ha), and the lowest in the 1-year old forest (3.725 ton/ha) (Table 2). Table 2. Amount of CO2 absorbed by K. obovata forest (ton/ha) Forest age Density Biomass Accumulated carbon Absorbed CO2 (trees/ha) (ton dw/ha) (ton C/ha) (ton CO2/ha) 1 15400 2.15 1.02 3.72 5 17300 51.21 27.23 99.95 6 17500 57.58 29.08 106.71 8 17900 72.32 40.00 146.82 9 18200 82.26 48.03 176.26 The annual average carbon accumulation of mangrove trees in a 1-year old plantation forest is 0.839 ton/ha/year, in a 5-year old forest 7.31 tons of C /ha/year, in a 6-year old forest 8.03 tons of C/ha/year, in an 8-year old forest 13.42 tons of C/ha/year and in a 9-year old forest its 15.09 tons of C/ha/year, which is the equivalent of 3.08, 26.82, 29.49, 49.26 and 55.38 tons of CO2/ha/year, respectively (Table 3). Table 3. Annual average amount of carbon accumulation of K. obovata forest (ton/ha/year) Forest age Annual increase in biomass % carbon in biomass Annual carbon accumulation rate Annual CO2 absorption rate (ton dw/ha/year) (ton C/ha/year) (ton CO2/ha/year) 1 1.69 49.53 0.84 3.08 5 14.54 50.25 7.31 26.82 6 15.78 50.89 8.03 29.49 8 25.96 51.70 13.42 49.26 9 29.23 51.61 15.09 55.38 It can be said that the absorption of CO2 by mangrove trees in general and by K.obovata trees in particular is fairly large. However, to evaluate the role of plantation 48 Evaluation of carbon accumulation in the Kandelia obovata Sheue, Liu & Yong plantation... mangrove forests in greenhouse gas reduction, we need to consider the whole process of carbon accumulation in the soil and the emission of CO2 through soil respiration in the forest ecosystem. Through the process of CO2 absorption (photosynthesis), carbon accumulates in forest trees, and when forest leaves and branches fall to the ground, carbon is returned to the forest soil. Carbon returns to the air in the form of CO2 through soil respiration and decomposition. Thus, only if CO2 absorption exceeds CO2 emission should the planting of mangrove forests under CDM and PES carbon options be considered effective and feasible. 2.2.2. Carbon accumulation in forest soil The amount of carbon in forest soil varies at different depths in the soil; the amount of carbon is higher at a surface layer and becomes less at increasing depth in the soil (Table 4). It was found that most of the carbon accumulates at a depth of 0 - 40 cm. Table 4. Amount of carbon (ton C/ha) accumulation at different soil depths (n = 36) Soil depth Forest age (years) 1 5 6 8 9 0 cm 14.04 ± 0.26 16.49 ± 1.95 15.55 ± 1.06 18.62 ± 0.30 18.68 ± 0.50 20 cm 13.46 ± 0.26 14.74 ± 0.69 18.48 ± 3.94 17.86 ± 0.32 17.46 ± 0.06 40 cm 12.76 ± 1.22 13.66 ± 3.29 13.26 ± 0.22 15.52 ± 1.68 16.82 ± 0.30 60 cm 11.77 ± 1.73 11.00 ± 1.91 10.26 ± 1.03 12.96 ± 0.70 15.33 ± 0.71 80 cm 9.00 ± 1.11 8.43 ± 0.64 10.65 ± 2.35 11.45 ± 0.19 13.54 ± 0.67 100 cm 7.35 ± 0.76 7.99 ± 0.16 8.62 ± 0.34 9.73 ± 0.25 10.35 ± 0.64 Total 68.373 72.397 76.820 86.140 92.183 Table 4 shows carbon accumulation in forest soil. The amount of carbon accumulated at depths of 0 - 40 cm is greater than the amount of carbon accumulated at depths of 40 - 100 cm. The amount of carbon accumulation at a depth of 0 - 20 cm in 8 and 9-year old forests is less than that accumulated in the soil of 5 and 6-year old forests. An explanation for this could be that the 5 and 6-year-old forests are located a mediumelevations and are inundated by tidal water for a longer period of time. Increased tidal inundation means increased amount of organic matter in sea sediment and river alluvia. The amount of carbon in forest soil decreased corresponding to increased soil depth. The research results are in accordance with those of Fujimoto and et al. [6] whose study presented the amount of carbon accumulated in Ca Mau and Can Gio forest soils in southern Vietnam. The author stated that most of the carbon accumulated in soil is at a depth of 0 - 60 cm with the amount of carbon decreasing at deeper soil layers [12]. Carbon accumulation in mangrove forest soil rises with increasing forest age [12]. 49 Mai Sy Tuan In this study, the amount of carbon accumulated at depths of 0 - 100 cm in the soil of K. obovata plantations ranges from 68.373 to 92.183 ton/ha. The highest value is found in 9-year old forest at 92.183 ton/ha, followed by an 8-year old forest at 86.140 ton/ha, a 6-year old forest 76.820 ton/ha, a 5-year old forest at 72.397 ton/ha, and the lowest is seen in 1-year old forest at 68.373 ton/ha (Figure 3). Figure 3. Carbon dynamics in K. obovata plantations in Giao Thuy District, northern Vietnam Carbon accumulates in the soil of mature forests at a higher rate than in the soil of newly planted forests. Soil in a 9-year old forest accumulates 12.41 ton of carbon/ha/year, in an 8-year old forest 4.82 ton/ha/year, in a 6-year old forest 3.57 ton/ha/year, in a 5-year old forest 3.02 ton/ha/year, and in a 1-year old forest 1.46 ton/ha/year. Although the rate of carbon accumulation in a 1-year old forest is not high, it can be seen that forestation is significant factor in carbon accumulation, making a significant contribution to the reduction of greenhouse gas. Carbon accumulation in soil increases with forest age and biomass increases with the age of forest trees, especially root biomass. It was found that forest tree roots contribute more to total organic material in soil than leaf litter. However, when there is a large amount of carbon accumulated in mangrove soil, decomposition of organic matter in the soil (mainly the roots) proceeds very slowly. Albright L. J. [1] states that 90% of mangrove tree leaf litter decomposes within 7 months. At the same time, 50 - 88% of root tissue is intact after one year and when roots are buried in the soil, decompositionis even slower. Studies on root decomposition in boreal peat bogs have also shown that under normal conditions, roots decompose more slowly than above-ground components [14]. Leaf litter breaks down very rapidly or is carried away by tidal water. In contrast, roots decompose slowly and accumulate for a long time and therefore roots play a crucial role in carbon accumulation in mangrove soil [11]. 50 Evaluation of carbon accumulation in the Kandelia obovata Sheue, Liu & Yong plantation... 2.2.3. Emission of CO2 from mangrove soil The amount of CO2 emitted from mangrove soil rises with increasing forest age (Table 5). The amount of CO2 emitted from a 1-year old plantation is 0.338 ton CO2/ha/year, from a 5-year old plantation its 0.51 ton CO2 /ha/year, from a 6-year old plantation 0.59 ton CO2/ha/year, from an 8-year old plantation 0.89 ton CO2/ha/year, from a 9-year old plantation 1.32 ton CO2/ha/year. Table 5. The amount of CO2 emitted from the soil at different forest ages (ton CO2/ha/year) Forest age (year) 1 5 6 8 9 12 Amount of CO2 emitted from soil (ton/ha/year) 0.34 0.51 0.59 0.89 1.32 1.51 In this study, CO2 emissions were found to increase with increasing forest age due to 1) decreased bed elevation; 2) litterfall (leaves, stems and branches) and organic matter brought in by tides and deposited on forest floors, an important factor influencing emission of CO. It was found that litterfall increases as forests age and the older the forest plantation the more the litterfall there will be. A high amount of organic matter in forest soil means potential increased decomposition and therefore an increase in CO2 emissions and 3) older plantations have more root biomass than younger plantations and CO2 emission occurs due to root respiration. The decomposition of dead roots also causes an increased amount of CO2 to be emitted from the soil. Our findings are consistent with those of Alongi [2] about the contribution of mangrove forests to global carbon cycle and greenhouse gas emission. The rate of CO2 emission from mangrove soil varies from 2 to 373 mmol/m2/day or 0.088 to 16.412 g/m2/day, this wide variation in rate being dependent on both the amount of organic matter and soil temperature. 2.2.4. Carbon balance in forest plantations of different ages Through photosynthesis, trees absorb and