8-014 (O) The Joint International Conference on Sustainable Energy and Environment (SEE) 1-3 December 2004, Hua Hin, Thailand

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1 Carbon Stocks and CO 2 Exchange in Tropical Soils under Different Land Use Natthaphol Lichaikul 1, Amnat Chidthaisong 1,, Narumon W. Harvey 1 and Chonrak Wachrinrat 2 : Font: 11 pt, Not Italic, Complex Script Font: 11 pt, Not Italic : Distributed 1 The Joint Graduate School of Energy and Environment, King Mongkut s University of Technology, Bangkok 10140, Thailand 2 Faculty of Forestry, Kasetsart University, Bangkok 10900, Thailand Abstract: Carbon dioxide is the most important greenhouse gas. Its concentration in the atmosphere has increased since 200 years ago and this leads to the current concerns of enhanced greenhouse effect and global warming. The main sources of CO 2 are burning of fossil fuels and deforestation. The main sink is absorption by forests and soils. Land-use changes, especially forest conversion to agricultural lands, lead to reduction in ecosystem carbon storage and thus increase carbon release to the atmosphere. This loss of carbon is mainly due to removal of plant biomass and cultivation practices that subsequently lead to gradual reduction in soil organic carbon. However, the amount of carbon loss caused by changes in land-use depends on various factors, and there are only a few estimates of such soil carbon loss in Thailand. The objective of this study is to study the effects of land use on soil carbon storage in Thailand. We compare carbon storage in three different land uses; a natural forest (SK), a reforestation planted with 16-year old Acacia mangium (AC) and an agricultural area continuously planted with Zea mays for more than 16 years (CF). CO 2 exchange over the soil surface was measured monthly during March-December 2003 using a close chamber method. Soil samples from 0-50 cm depth were collected and analyzed for total soil carbon. Soil texture at SK site is Clay (clay content = 51%) while both AC (clay content = 30%) and CF (clay content = 21%) sites are Sandy Clay Loam soils. The ph range of SK soil was compared with for AC soil and for CF soil. Carbon content was usually highest in SK soil, ranging from 15 mg C g soil -1 in subsoil to 30 mg C g soil -1 in the top 10-cm soil, followed by soil at CF site (10-15 mg C g soil -1 ) and CF soil (<10 mg C g soil -1 ), respectively. On the area-base, carbon storage in the upper 50 cm-layer of soils under different land use was estimated at 118, 66 and 60 ton C ha -1 in SK, AC and CF soils, respectively. Adding the amount of standing biomass will significantly increase carbon storage in forest system but will not make much difference in agricultural system. Due to large spatial and temporal variations, total CO 2 emission over 8-month measurements was not significantly different among these three sites ( g C m -2 ). The results indicate that carbon storage differs significantly according to different land use. Natural forest soil stores more carbon than other soils (about twofold), especially in the surface layer. Conversion of forest to agriculture (for more than 16 years), in addition to loss of carbon stored in standing biomass, leads to about 50% loss of soil carbon in the upper 50 cm. On the other hand, reforestation for 16 years only increases soil carbon by 10% compared with a continuous cultivated soil (maize). Keywords: Carbon Storage, Land Use, Natural Forest,, Corn Cultivation. 1. INTRODUCTION Increase in atmospheric carbon dioxide (CO 2 ) concentration is one of the main environmental issues concerned nowadays. This is because CO 2 is the primary cause of the global warming. The main sources of the emitted CO 2 are combustion of fossil fuels and land use change such as deforestation. On the other hand, uptake by terrestrial ecosystems through photosynthesis by plants is the major sink for carbon. Land use changes, for example, by conversion of natural forest to agricultural lands, therefore lead to the loss of carbon from plants and soils in the ecosystem to the atmosphere [1]. Carbon translocation from the atmosphere to soil occurs due to photosynthetic plant growth and the decomposition of carbon-containing material and is subsequently incorporated into the soils. Because forest is the main terrestrial ecosystem that stores or fixes the atmospheric CO 2, it continues to accumulate carbon as plant grows and carbon is then preserved in the forest floor as plant dies. The most remarkable change in land use in the past decades is deforestation in the tropics. It is estimated that deforestation rates in the tropic are as rapid as 2% yr -1 [2]. This results in, in addition to the release of carbon from energy consumption, the increase in CO 2 concentration in the atmosphere. To understand and finally to find the appropriate options to stabilize or reduce the atmospheric CO 2 concentration, the carbon dynamic must be understood. Much of our recent understanding of the carbon budget has been resulted from the investigations in the temperate. Since deforestation has been mainly occurred in the tropics, knowledge on the effects of Corresponding author: amnat_c@jgsee.kmutt.ac.th deforestation on carbon dynamics need to be established. In this study, we reported the preliminary results on estimating soil carbon in different land use types including natural forest, reforestation and agricultural areas in Thailand. 2. MATERIAL AND METHODS 2.1 Study sites CO 2 flux measurements and soil samples were taken from three adjacent sampling sites located in Wang Nham Keaw District, Nakorn Ratchasrima province in Northeast Thailand. The forest site (SK) was a natural dry evergreen forest located in the Sakerat Environmental Research Station. This Research Station is located at 390 m (from MSL) with an average annual temperature of 26 C and average annual rainfall of 1,260 mm. The dominant tree species were Hopea ferrea, Pterocarpus marcrocarpus, Xylia xylocarpar, Dalbergia cochinchinensis, Lagerstroemia duppereana, and Shorea henryana. The reforestation site (AC) located about 5 km away from the SK site was planted with fast-growing, nitrogen-fixing tree species Acacia mangium in The third sampling site was a cornfield (CF), situated adjacent to the reforestation area (2 km away). It was deforested more than 40 years ago and maize has been continuously cultivated at this site during the last 16 years since Soil preparation for corn plantation at CF site started in the mid of June, 2003 and seeds of corn were sown on July 10, On this date, chemical fertilizer ( , N-P 2 O 5 -K 2 O) was applied at the rate of 50 kg ha Soil sampling Soil cores with a diameter of 5 cm were used to collect soil provided : Line spacing: Multiple 0.5 li : Indent: First line: 0.4 cm, Tabs: Not at cm : Line spacing: Multiple 0.75 li : Line spacing: : Indent: First line: 0.4 cm : Indent: First line: 0.4 cm to date Sampling : Font: Not Italic, Complex Script Font: Not Italic : Font: Not Italic, Complex Script Font: Not Italic : Line spacing: Author for correspondence, Position: Horizontal: Center, Relative to: Margin 839

2 samples. Twenties-one cores were sampled at each location. Three replicates per soil layer, seven soil layers were collected (0-5, 5-10, 10-15, 15-20, 20-25, 30-35, and cm). In the laboratory, soil samples were air-dried, sieved through 2-mm mesh and stored at room temperature until analysis. Undisturbed soil samples were taken separately to measure density. 2.3 Soil CO 2 flux measurement Field measurements were conducted at three sites from January 2003 to February CO 2 flux was measured by the closed chamber method. Chambers were made of a acrylic glass with the dimension of 30 cm x 30 cm x 15 cm (width x length x height) and chamber base; 30x30x10 cm 3. The internal volume of the chamber was approximately 13,500 cm 3. The chamber base was made of stainless steel and custom constructed with an upper trough that exactly fit the base of the chamber. They were inserted into the sampling location at cm deep into the soil and were remained in the same proximal and distal locations during the entire study period. Gas samples from the chamber headspace were taken at 0, 5, 10, and 15 min with a gas syringe through a top three-way stopcock type R (ethyleneoxide gas sterilized) from the. Together with gas collection, other environment parameters such as soil and air temperature were also recorded. After sampling, the gas samples were kept in a cool box to maintain the stable temperature. Upon returning to the lab, the concentration of CO 2 was determined by GC as soon as possible (Shimadzu GC-14A). The GC operating conditions were: FID temperature: 300 C, Injection temperature; 120 C, Column temperature; 100 C, Carrier gas; Helium (99.99% purity), Carrier gas flow rate; 65 ml/min, Column; Unibead C packed column 2.4 Soil analysis Soil texture were determined by using hydrometer method (Bouyoucous Particle Size Analysis) on air-dried soils that had been passed through a 2 mm soil sieve to remove small rock, roots, pebbles, and debris followed by wet sieving to separate sand function. Sand, silt, and clay were expressed as a percentage of oven-dry weight [3]. Determinations of bulk density were done by measurement of volume and weight [4]. 3. RESULTS AND DISCUSSION 3.1 Soil properties Soil bulk density: Bulk density of a natural forest, reforestation and corn cultivation are shown in Fig. 1. In the natural forest, reforestation and corn cultivation soil, the values ranged from 1.46 to 2.01, 1.56 to 2.04 and 1.62 to 1.94 g cm -3, respectively. The bulk density tended to increase when the soil depth increased. Except at cm depth, soil at corn cultivation site exhibited a higher bulk density than both natural forest and reforestation soils. Relatively higher bulk density in agricultural than forest soils is expected, since cultivation practices can lead to soil compaction. From the field observation, there was also higher amount of gravel in the soil profile of natural forest (SK) and reforested area (AC), especially in the deeper profile layer than in CF soil. In the soil surface, bulk density in the natural forest and reforestation soil is significantly lower than other soils and layers. This is possibly due to greater amount of litters as compared to subsoil. Bulk density (g/cm 3 ) Soil depth (cm) Natural forest Corn cultivation Fig. 1 Soil bulk density along profile and in different land use types. Soil Texture: Different land use types result in different soil texture (Table 1). Clay content was highest in natural forest soil. Silt particle was highest in reforestation soil and highest abundance of sand particle was found in cornfield soil. It is also noted that, as the same as other properties such as density and ph, soil from reforestation site exhibits the characteristic falling between soil from natural forest and agriculture area (corn). This indicates that soil at reforestation site is in transition period changing from formerly human disturbed condition toward forest-like soil. Clay content in every horizon at 0-50 cm was higher in natural forest soils than reforestation and corn cultivation soils. Among them, the soils at cm and cm in natural forest showed the highest clay content throughout the profile (60.08 %, data not shown). On the other than, in all soil layers, sand particle percentage was highest in cornfield soil. Phopinit and Limtrakul [5] reported that change of soil texture compositions after reforested 10 years was found. Percentage of sand particle increased after 10 years of continuous agricultural practices such plowing that mixes the soil from sublayers and the soil surface. In addition, leaching of clay particles from surface to subsoil by rainfall is one of possible reasons making the surface soil rich in sand particle. Soil ph: Soil ph in all types of land uses examined was lowest (most acidic) in surface layer and the ph increased towards the subsoil (data not shown). Soils in the corn cultivation showed ph value higher than in natural forest and corn cultivation. The ph value of CF soils ranged from and highest when compared with three study sites. Puriyakorn [6] found that ph value was changed along the soil depth in deforested area, abandoned area and agricultural land. ph changes associated with changing from forest to agriculture is possibly attributed to liming and burning. In natural forest (DEF) and reforestation (A. mangium plantation), the surface soils were highly acidic with ph of about 3.86 and Because of surface soils are always covered by the vegetation all the year, and therefore, affected by the organic matter supplied from the vegetation as litterfall. This leads to the acidification of the surface soils. Another reason is that organic matter consists of carboxyl group, phenolic group and amino group. Protonation of these functional groups releases hydrogen ion (exchangeable H + ) into the soils. Thus, high organic matter usually associates with relatively acidic ph.... [1] : Distributed... [2]... [3]... [4] Rate of ere... [5], with and, measured under headspace of the... [6]... [7] is Analysis... [8]... [9]... [10]... [11] is According to,... [12], p was higher in The termination... of bulk [13] due to site preparation... [14] L... [15]... [16]... [17] from were in soil, was higher... [18] 840

3 The results obtained in this study are consistent with the finding of Jongsuksuntigool [7] that A. mangium plantation (aged 6 years old) soil at 0-30 cm was very strongly acidic due to A. mangium consumed more base element such as potassium, calcium, and magnesium for fast growing and resulting in abundance of H + ions left in the soils. Table 1 Soil texture Site Natural forest Soil Depth Soil texture (cm) 0-5 Sandy clay 5-10 Clay Clay Clay Clay Clay Clay 0-5 Sandy clay loam 5-10 Sandy clay loam Sandy clay loam Sandy clay loam Sandy clay loam Clay Clay Agriculture 0-5 Sandy loam 5-10 Sandy loam Sandy clay loam Sandy clay loam Sandy clay loam Sandy clay loam Sandy clay loam 3.2 Total soil carbon and nitrogen Total soil carbon and nitrogen content is shown in Fig. 2. In all land use types, highest carbon content was found in the topsoil. Carbon content was usually highest in SK soil, ranging from 15 mg C g soil -1 in subsoil to 30 mg C g soil -1 in the top 10-cm soil, followed by soil at CF site (10-15 mg C g soil -1 ) and CF soil (<10 mg C g soil -1 ), respectively. Similar tendencies were also found in nitrogen content, but about 10 times in magnitude less than carbon content. Multiplying this carbon and nitrogen content with bulk density in each soil layer, carbon and nitrogen storage in each land use types was estimated. Total carbon stock for 50-cm soil depth was 118, 66, and 57 ton C ha -1 in natural forest, reforestation and cornfield soils, respectively. Approximately 51, 46 and 51% of this total carbon is stored in the top 20 cm of soil at forest, reforestation and cornfield, respectively. This calculation results indicate that natural forest, besides storing large amount of carbon in plant biomass, stores more carbon than in other land use types. Conversion of forest to agriculture soils, therefore, results in a substantial loss of soil carbon (about 40 ton C ha -1 in this case). On the other hand, reforestation to Acacia mangium for 16 years does not significantly result in increasing soil carbon storage (only 10 ton C ha -1 higher than in CF soil). Thus, the main carbon sequestration at ecosystem level is due mainly to an increase in standing biomass. It is noted that since in AC soil it is expected that carbon input through litter fall is higher than in agriculture soil, thus, it its carbon content should be higher than in agricultural soil. However, such relatively higher input does not lead to significant increase in soil carbon stock. This is possibly due to higher rate of carbon decomposition under A. mangium plantation than under other land use types (see Section 3.3). Total nitrogen storage in natural forest, reforestation and agriculture soil is 10, 6.5 and 5 ton N ha -1, respectively. Similar to carbon storage, about 50% of total nitrogen in the soil profile was found in the top 20 cm soil layer. Throughout the soil profile, the C/N ratio narrowly varied between 10 and 11. Simple regression analysis of soil carbon and nitrogen content and other soil properties reveals that both soil and carbon content do not correlate with any soil physical or chemical properties. However, soil carbon content significantly correlated with nitrogen content (Soil C content = 12.2 {soil N content} 1.14, r 2 = , n = 21, p<0.0001). This is expected since these two shares the same input sources. However, it may also indicate that higher N input into the system will increase the amount of carbon storage and se questration. Total soil C ( mg C / g soil Total soil N ( mg N / g soil Soil depth (cm) Natural forest Corn cultivation Natural forest Corn cultivation Soil depth (cm) Fig. 2 Soil carbon and nitrogen concentration under different land use types and in different soil layers. 3.3 Net CO 2 flux from soil surface Net CO 2 flux is given in Fig. 3. In all land use types, relatively high CO 2 emission was found during the rainy months (July October), indicating that soil moisture is one of the limiting factors. Integrating CO 2 fluxes over eighth months monitoring period yields net CO 2 emission of 3781, 3991 and 2748 g CO 2 m -2 in natural forest, reforestation and agricultural soils, respectively. Thus, relatively higher CO 2 flux was found in soil under reforestation than in other land : Font: 11 pt, Not Italic, Complex Script Font: 11 pt, Not Italic : Distributed : Font: Bold, Complex Script Font: Bold : Left, Indent: First line: 0 cm d ( ) : Line spacing: lose : Line spacing: : Position: Horizontal: Center, Relative to: Margin 841

4 use types. It is noted that lowest CO 2 emission was found in agricultural soil, possibly due to relative small of amount of biomass input under corn cultivation when compared with forest sites. 4. CONCLUSIONS Results from the present study show that soil carbon stock is significantly different depending on land use types. The highest carbon stock was found in forest soil (118 ton C ha -1 ), followed by reforestation (66 ton C ha -1 ) and agricultural soils (55 ton C ha -1 ), respectively. More than 50% of total soil carbon and nitrogen was found in the upper 20 cm. Thus, conversion of forest to agriculture leads to a substantial loss of carbon storage in soil. Such loss presumably occurs in the top layer of soil profile. On the other hand, different land use types do not significantly result in different net CO 2 flux from the soil surface. However, relatively higher net CO 2 flux in reforestation than in other land use types was found. The main carbon sequestration in reforestation soil, thus, is due to increase in plant biomass Natual forest Agriculutre (maize) analysis: Part 1: Physical and Mineralogical Methods. Monograph Number 9 (Second Edition). ASA, Madison, WI. [5] Phopinit, S. and Limtrakul, K. (1999) Change of soil proporties after 10 years forest plantation. Silvicultural Research Report 1990, Division of Silviculture Research, Office of Forest Technique Advisor, Royal Forest Department, Thailand. pp [6] Puriyakorn, B. (1982) Changes of soil properties in the natural forest by different land use patterns at Sakearat, Pakthongchai, Nakornratchasrima. M. Sc. Dissertation, Kasetsart University, Bangkok, Thailand. : Font: 11 pt, Not Italic, Complex Script Font: 11 pt, Not Italic : Distributed [5] And K. : Font: Italic, Complex Script Font: Italic : Line spacing: Multiple 0.5 li [6] : Font: Italic, Complex Script Font: Italic e d o [7] 180 CO 2 flex (mg CO 2 m -2 h -1 ) May Jun Jul Aug Oct Nov Jan Feb Months in 2003 Fig. 3 Seasonal variations in CO 2 flux in different land use types. REFERENCES [1] Bonan, G. B. (2002) Ecological climatology: concepts and application. The Press Syndicate of the University of Cambridge., Cambridge, United Kingdom. [2] Keller, M., Reiners, W.A. (1994) Soil-atmosphere exchange of nitrous oxide, nitric oxide, and methane under secondary succession of pasture to forest in the Atlantic lowlands of Costa Rica. Global Biogeochem. Cycl. 8: [3] Gee, G. W. and Bauder, J.W. (1982) Particle-size analysis. p In: A. Klute (ed.) Methods of soil analysis: Part 1. Physical and mineralogical methods. ASA Monograph Number 9. [4] Blake, G. R. and Hartge, K.H. (1986) Bulk density. p In: A. Klute et al. (ed.) Methods of soil : Font: Bold, Complex Script Font: Bold [1]... [20]... [21] : Bullets and Numbering... [22] [2] [3] J. W., [4] K. H.... [19]... [23]... [24]... [25]... [26] 842

5 Page 839: [1] Suwajchai 16/11/47 ๑๖/๑๑/๔๗ ๑๕:๕๔ น. Font: 11 pt, Not Italic, Complex Script Font: 11 pt, Not Italic Page 840: [2] Deleted Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๑๑ น. Page 840: [3] Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๐๙ น. Line spacing: Page 840: [4] Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๐๙ น. Page 840: [5] Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๑๐ น., Tabs: Not at 0.53 cm Page 840: [6] Deleted Administrator 29/10/47 ๒๙/๑๐/๔๗ ๑๕:๔๕ น. headspace of the chamber Page 840: [7] Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๑๑ น. Page 840: [8] Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๑๐ น. Font: Not Italic, Complex Script Font: Not Italic Page 840: [9] Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๑๐ น. Font: Not Italic, Complex Script Font: Not Italic Page 840: [10] Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๑๐ น. Line spacing: Page 840: [11] Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๑๐ น. Page 840: [12] Deleted Administrator 29/10/47 ๒๙/๑๐/๔๗ ๑๕:๔๗ น. Page 840: [13] Deleted Administrator 29/10/47 ๒๙/๑๐/๔๗ ๑๕:๔๗ น. The termination of bulk density consists of drying (105 C) at less than 48 hours and weighing a soil sample, the volume of which is known (core method) or must be determined. Page 840: [14] Deleted Administrator 29/10/47 ๒๙/๑๐/๔๗ ๑๕:๕๒ น. due to site preparation by Page 840: [15] Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๑๑ น. Line spacing: Multiple 0.75 li Page 840: [16] Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๑๑ น. Line spacing: Page 840: [17] Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๑๒ น. Page 839: [18] Suwajchai 16/11/47 ๑๖/๑๑/๔๗ ๑๕:๕๕ น. Position: Horizontal: Center, Relative to: Margin Page 842: [19] Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๑๓ น. Line spacing: Multiple 0.5 li Page 842: [20] Suwajchai 03/11/47 ๐๓/๑๑/๔๗ ๑๔:๓๘ น. Page 842: [21] Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๑๕ น. Indent: Left: cm, Hanging: 0.53 cm, Tabs: 0.35 cm, List tab + Not at 1.38 cm Page 842: [22] Change Suwajchai 29/10/47 ๒๙/๑๐/๔๗ ๒๐:๑๓ น. Bullets and Numbering

6 Page 842: [23] Suwajchai 03/11/47 ๐๓/๑๑/๔๗ ๑๔:๓๘ น. Page 842: [24] Suwajchai 03/11/47 ๐๓/๑๑/๔๗ ๑๔:๓๗ น. Page 842: [25] Suwajchai 03/11/47 ๐๓/๑๑/๔๗ ๑๔:๓๗ น. Page 839: [26] Suwajchai 16/11/47 ๑๖/๑๑/๔๗ ๑๕:๕๕ น. Position: Horizontal: Center, Relative to: Margin