Effect of groundwater level on soil respiration in tropical peat swamp forests

Transcription

1 Full Paper J. Agric. Meteorol. (): 11 13, 1 Effect of groundwater level on soil respiration in tropical peat swamp forests Siti SUNDARI*, Takashi HIRANO**,, Hiroyuki YAMADA**, Kitso KUSIN***, and Suwido LIMIN*** *Graduate School of Agriculture, Hokkaido University, Sapporo, -59, Japan **Research Faculty of Agriculture, Hokkaido University, Sapporo, -59, Japan ***CIMTROP, University of Palangkaraya, Palangkaraya, 7311, Indonesia Abstract Tropical peatlands store soil carbon constituting up to 15-19% of global peat carbon. That huge carbon pool is presently being disturbed on a large scale by land development and management, and has consequently become vulnerable. Peat degradation occurs most rapidly and massively in Indonesia s peatlands because of fires, drainage and deforestation of swamp forests. Peat burning releases carbon dioxide (CO ) intensively but occasionally, whereas drainage increases CO emissions steadily through accelerated aerobic peat decomposition. Under such circumstances, tropical peatlands might become a huge source of carbon emissions to the atmosphere. Nevertheless, the effects of drainage on the carbon balance of tropical peatland ecosystems are not well understood; more field data must be accumulated. Therefore, we measured soil respiration (RS), which is a major source of CO efflux, continuously for more than one year using automated chamber systems, with consideration of microtopography, at two sites in Central Kalimantan, Indonesia: undrained and drained peat swamp forests. The RS was determined mainly by local hydrology. In the undrained forest, RS decreased sharply under flooded conditions because of anoxia. In contrast, in the drained forest, with its lower groundwater level (GWL), RS showed a quadratic relationship with GWL and gradually increased as GWL decreased when GWL was lower than about -. m, which was caused chiefly by the enhancement of peat decomposition. These relationships indicate that lowering GWL by drainage increased RS, whereas annual RS was larger in the undrained forest (137 gc m - y -1 ) than in the drained forest (15 gc m - y -1 ) in 5. The difference in annual RS was probably attributable to higher forest productivity in the undrained forest. Key words: Drainage, Groundwater level, Peat decomposition, Soil respiration, Tropical peat swamp forest. 1. Introduction Tropical peatlands, which are widely distributed worldwide over km and which store up to. Pg of soil carbon, account for 15-19% of global peat carbon (Page et al., 11). In Southeast Asia, peatlands cover almost 1% of the land area, mainly in coastal and sub-coastal lowlands (Hooijer et al., 1). Tropical peat has formed under swamp forests Received; November, 11. Accepted; January 5, 1. Corresponding Author: hirano@env.agr.hokudai.ac.jp over millennia (Sorensen, 1993; Page et al., 1999; Dommain et al., 11). However, peatland ecosystems have been devastated by logging, land development and management since the 197 s (Sorensen, 1993; Rieley and Muhamad, ). This huge carbon pool is presently being disturbed on a large scale, and consequently it has become vulnerable (Canadell et al., 7). Peat degradation occurs most rapidly and extensively in Indonesia, where 7% of tropical peatlands are located (Page et al., 11). There, peat is degraded by fires, drainage and deforestation of swamp forests (Page et al., ; van der Werf et al., ; Couwenberg et al., 1; Hooijer et al., 1; Murdi

2 J. Agric. Meteorol. (), 1 yarso et al., 1; Hergoualcʼh and Verchot, 11). Peat burning releases carbon dioxide (CO ) intensively but occasionally, whereas drainage raises CO emissions steadily through accelerated aerobic decomposition of peat. Therefore, under such anthropogenic pressure, tropical peatlands present the threat of switching from a role as a global carbon sink to a role as a huge carbon source to the atmosphere. Moreover Li et al. (7) has reported a prediction, obtained using several climate models, that the groundwater level (GWL) will fall more during the dry season over Southeast Asia in the late 1st century. According to that prediction, even when direct CO emissions from peatland fires are excluded, tropical peatlands will become a huge carbon source in the near future if the present socioeconomic circumstances prevail. It is crucial to assess the present carbon balance of tropical peatland ecosystems and the response of that balance to environmental disturbances. That information can then be used to understand the function of tropical peatlands within global carbon cycles. To date, little knowledge based on field data has been accumulated. Several field studies of soil CO emissions or soil respiration (RS) in tropical peat swamp forests in Micronesia (Chimner and Ewel, ), Malaysia (Melling et al., 5) and Indonesia (Inubushi et al., 3; Furukawa et al., 5; Hadi et al., 5; Jauhiainen et al., 5; Ali et al., ; Jauhiainen et al., ) have specifically examined the carbon balance in relation to greenhouse gas emissions. They measured RS manually using static or dynamic chambers and subsequently analyzed the effects of environmental factors such as GWL, soil moisture and temperature on RS to determine the RS relationship with GWL because local hydrology is recognized as an important abiotic factor affecting peat carbon dynamics (Limpens et al., ; Page et al., 9). Methane (CH ) emissions should also be quantified to assess the carbon balance of peatlands, whereas the contribution of CH efflux to soil carbon emissions is limited in tropical peatlands, even if its global warming potential (GWP) is taken into account (Jauhiainen et al., 5; Hirano et al., 9). It is expected that lowering GWL through land-use change, by drainage, will accelerate aerobic peat decomposition and increase CO emissions, although the effects of drainage remain ambiguous partly because of sporadic measurements to date. Clearly, intensive studies using comprehensive sequential field data are necessary. We therefore measured RS continuously for more than one year using automated chamber systems with consideration of the microtopography at two sites near Palangkaraya, Central Kalimantan, Indonesia, representing undrained and drained peat swamp forests. Using field data, we investigated the environmental effects on RS, determined the relationship between RS and GWL, assessed annual RS and discussed the effects on RS of drainage caused by canal excavation.. Materials and Methods.1 Study site This study was conducted at two tropical peat swamp forests in the upper catchment of the Sebangau River, about km southeast from Palangkaraya, the capital city of Central Kalimantan province, Indonesia. In Central Kalimantan, a large peatland area was deforested and drained during the late 199s, mainly to develop farmlands according to a national project: the Mega Rice Project (MRP). Although the project was terminated in 1999, it left vast tracts of devastated peatlands. The two forest sites were located near each other, at a distance of about 15 km, with a river between them. One site was an undrained forest (UF) in the Setia Alam area. The other was a drained forest (DF) in the Kalampangan area (Hirano et al., 9). The UF site (.3ºS, 113.9ºE), called NDF in our previous study (Hirano et al., 9), had been logged selectively until the late 199s. It was designated a National Park in. It retains a relatively intact peat swamp forest. Although no large canal has been excavated in this area, a network of small canals that had been built for illegal logging remains, influencing the forest hydrology (Page et al., 9). Dominant tree species included Tetramerista glabra, Calophyllum sp., Shorea sp., Combretocarpus rotundatus, Palaquium sp., Buchanania sessilifolia, Syzygium sp., Dactylocladus stenostachys, Dyera costulata, Ilex cymosa, Tristaniopsis obovata and Dyospyros sp. (Tuah et al., 3). Rich shrubbery, including young trees of the dominant species, grows in the understory. The canopy height was about 3 m. The plant area index was 5. m m - at 1.3 m height in June, as measured using a plant canopy analyzer (LAI; Li-Cor Inc., USA). The soil surface was covered with thick tree debris, mainly comprising leaf litter. Few herbaceous plants existed on the soil surface. The forest floor was uneven, - 1 -

3 S. Sundari et al.:soil respiration in tropical peat forests with hummocks and hollows. s, which are formed on dense tree roots, are surrounded by hollows. Hollows are networks of open soil surfaces between hummocks. surfaces are typically -3 cm higher than hollow surfaces (Jauhiainen et al., 5). The peat depth was -3 m. A tower was built at the site in for flux measurement. The DF site (.35ºS, 11.1ºE) is in a forest that still remains in Block C of the MRP. The forest had also been selectively logged until the end of the 199s. Dominant tree species and the microtopography of the forest floor resembled those at the UF site. The canopy height was about m. The plant area index was 5. m m - in June. Peat depth was about m. A large canal (5 m wide m deep) that was excavated in 199 and 1997 has functioned to facilitate drainage of the forest (Page et al., 9). The site was located about m from the canal. In June-August 5, the canal was blocked at several points by small dams to facilitate hydrological restoration of the devastated ex-mrp area (Jauhiainen et al., ). In 1, a tower was built for flux measurement (Hirano et al., 7, 9). Annual values of precipitation and air temperature (mean±standard deviation (SD)) for the nine years of -1 were 5±59 mm y -1 and.±.3, respectively, as measured on the flux tower located at the DF site. Interannual variation in precipitation was large; the maximum (375 mm y -1 ) and minimum (15 mm y -1 ) were recorded, respectively, in a La Niña year (1) and an El Niño year (). The dry season generally begins in May or June and lasts through October (Hirano et al., 7). However, air temperatures showed no seasonal or interannual variation. Monthly mean air temperatures ranged within 1 of 5.9 in July and. in May.. Soil respiration Soil CO effluxes or soil respiration rates (RS), consisting of microbial and root respirations, were measured continuously using automated chamber systems at both sites (Hirano et al., 9). The system consisted of six chambers, a programmable data logger (CR1X; Campbell Scientific Inc., Logan, UT, USA), an infrared CO analyzer (LI; Li-Cor Inc., Lincoln, NE, USA), an air pump and some electric parts to switch airflow between the analyzer and chambers. The chamber was made of an opaque PVC cylinder, cm high, with a 5 cm internal diameter and an opaque PVC disc as a lid, which was opened and closed automatically using a motor according to a program. The chambers were inserted 1 cm into the ground to prevent horizontal advection in porous surface soil. The chamber insertion cut fine roots in the surface soil and probably affected RS (Wang et al., 5), especially in the early period after insertion. The ground-covering area and effective volume of each chamber were, respectively,.91 m and.17 m 3. An air vent with a diameter of 1 mm was set in each chamber to stabilize the air pressure in the chamber headspace. Each chamber was closed for four minutes, one after another. It took minutes to cycle through a round of six chambers. Then the system was stopped for subsequent six minutes to save electricity, which was supplied by a solar panel system with rechargeable batteries. Halfhourly RS was calculated from the rate of increase of CO concentration in the chamber headspace for the last three minutes of each closing. The CO concentration was measured every five seconds. Its rate of increase was calculated from 3 measurements over three minutes by linear fitting using the least-squares method. For quality control, an RS measurement was excluded if the coefficient of correlation (r) of the fitting was less than., over which the linearity was significant at a 1% level. The r values were greater than.95 for most measurements, except when the system malfunctioned. The CO analyzer was calibrated every three months using standard gas with two CO concentrations. Two and four chambers were installed respectively on hollows (U1 and U) and hummocks (U3-U) at the UF site in July. In February 5, the hollows were inundated and a chamber sucked water into the analyzer. Consequently, the chamber was moved from a hollow (U1) to a hummock (U7). In addition, another chamber on a hollow (U) was moved to a hummock (U) in December 5 to prevent inundation. The inundation reduced the effective volume of the chambers on hollows. The volume used to calculate RS was corrected using the groundwater level (GWL). On the other hand, all six chambers were initially installed on hummocks in January at the DF site. The chambers were moved to other points in October. Two were in hollows (D1 and D) and the others were on hummocks (D3-D). In addition, a chamber was moved from a point (D) to another point

4 J. Agric. Meteorol. (), 1 (D7) on the same hummock in June 5. During January-October in, the chamber system malfunctioned frequently, mainly because of air leakage. All chambers were installed within m of each other at both sites. Young plants growing in the chambers, which were not common, were clipped periodically. The volumetric soil water content (SWC) was measured, respectively, between and cm deep using a time-domain-reflectometry (TDR) sensor (CSI15; Campbell Scientific Inc.) on a hummock at both sites and in a hollow only at the UF site. The TDR output was calibrated using the oven-drying method. The soil temperature at 5 cm depth was measured at three points using thermocouple thermometers at both sites. Signals from the sensors were measured every 3 seconds; their half-hourly means were recorded using the data logger. The GWL was measured every 3 minutes using a water level logger (DL/N; Sensor Technik Sirnach AG, Switzerland or DCX- VG; Keller AG, Switzerland). The zero position of GWL was set at the level of a hollow surface. These underground sensors were installed within 3 m from the chamber system at both sites. In addition, precipitation was measured on the tower at the DF site using a tipping-bucket rain gauge (Hirano et al., 7). The RS showed diurnal variations, which would be related to the biological processes of trees (Hirano et al., 9). For analyses, we used daily mean values of RS at each point to exclude such diurnal variations. A daily mean value was calculated if half-hourly data were more than 3 (three quarters) on a day after the quality control. Consequently, daily mean RS data were prepared for July -April and October -April, respectively, for the UF and DF sites. 3. Results 3.1 Seasonal variations in environmental factors We specifically describe our analyses of the dataset for 5, for which 5-3 days and days of RS data were available for the UF and DF sites, respectively (Table 1). The annual sum of precipitation was mm y -1 in 5, which was almost equivalent to the mean of 5 mm y -1 obtained over nine years, whereas the seasonal pattern of precipitation differed in 5 from the mean pattern. On average, the dry season lasted for the three months of July-September, judging from a common criterion, in which the dry season is defined as the period during which monthly precipitation is less than 1 mm (e.g. Malhi and Wright, ). In 5, however, the dry season lasted for only one month in August (Fig. 1a). That short dry season was probably caused by a La Niña event lasting for several months from the last quarter of the year. The seasonality of GWL was similar at the UF and DF sites (Fig. 1b). In 5, GWL peaked in late February and continued to decrease through late September, when it increased rapidly with the onset of the rainy season (Fig. 1a). At the UF site, the minimum GWL was -.3 m in 5, which was higher than the mean value (±SD) of -.7±.31 m for the four years of 5- (Table ). The maximum GWL was.7 m, which was similar to the mean value of.±. m. At the DF site, the minimum and maximum GWLs were -1. and -. m, respectively, in 5, which were almost equal to the means (- 1.±.37, -.1±.3 m), respectively, for six years between 3 and. The SWC varied with seasonal variations in precipitation and GWL (Fig. 1c). At the UF site, SWC at hollows was saturated under flooded conditions (GWL > m) and decreased to lower than.3 m 3 m -3 at the end of the dry season. However, SWC at a hummock increased under flooded conditions, although SWC was almost constant around m 3 m -3 when GWL was below m. At a hummock at the DF site, seasonal variation was small: within m 3 m -3. The inter-site difference in SWC at hummocks under non-flooded conditions was mainly attributable to soil inhomogeneity. Soil temperature showed poor seasonality; its daily mean values varied between and (Fig. 1d). Except during the latter half of 5, soil temperatures were higher by.5-1. at the DF site. 3. Seasonal variations in soil respiration At the UF site, RS at hollows showed a large seasonal variation, following GWL variation (Fig. a). The RS decreased sharply below 1 µmol m - s -1 under flooded conditions (GWL> m). When GWL moved underground in late May 5, RS increased suddenly to 1 µmol m - s -1 and maintained a high level above µmol m - s -1 for about three weeks. We measured RS at two points in hollows until mid-february 5, although RS was measured only at U from March 5. The RS at U varied in parallel with that at U1. Their mean values were almost equal until February 5; - 1 -

5 S. Sundari et al.:soil respiration in tropical peat forests Table 1. Annual soil respiration (RS) and the number of available data at each point in 5. The RS was spatially averaged in each site using the areal ratio of hollows and hummocks at 5:5. Site Microtpography Point Available data Annual soil respiration (days) (gc m - y -1 ) UF Hollow Hollow U1 U Average U3 U U5 U U7 Average Spatial average 137 DF Hollow Hollow D1 D Average D3 D D5 D D7 Average Spatial average 15 Precipitation (mm d -1 ) SWC (m 3 m -3 ) 1 a DOY (-).... c UF_hollow 5 UF_hummock DF_hummock Day of year (-).5 5 b 5. Soil temperature ( C) UF DF DOY (-) 9 7 d 5 5 UF DF Day of year (-) Fig. 1. Seasonal variations in precipitation (a), groundwater level (GWL) (b), soil water content (SWC) at - cm depth (c) and soil temperature at 5 cm depth (d) on a daily basis for July -April

6 J. Agric. Meteorol. (), 1 Table. Annual soil respiration (RS) estimated from groundwater level (GWL) using the fitted equation of Fig.. a) UF site Year Groundwater level (m) Soil respiration Minimum Maximum Average (gc m - y -1 ) 5 7 Average b) DF site Year Groundwater level (m) Soil respiration Minimum Maximum Average (gc m - y -1 ) Average RS (µmol m - s -1 ) RS (µmol m - s -1 ) 1 a U1 U DOY (-) 1 c D1 D RS (µmol m - s -1 ) RS (µmol m - s -1 ) 1 b U3 U U5 U U7 U DOY (-) 1 d D3 D D5 D D Day of year (-) Day of year (-) Fig.. Seasonal variation in daily mean soil respiration (RS) at hollows in the UF site (a), hummocks in the UF site (b), hollows in the DF site (c) and hummocks in the DF site (d) for July -April. they were 3. and 3.1 µmol m - s -1, respectively, at U1 and U. Seasonal variations differed among points at UF hummocks (Fig. b), probably because relative heights from the hollow surface differed. Inter-site differences in RS were clear during March-May 5 and during February. At the DF site, seasonal variation in RS was small at hollows (Fig. c). At hummocks, seasonal variations in RS differed among points (Fig. d). Inter-site variation was relatively large in

7 S. Sundari et al.:soil respiration in tropical peat forests RS (µmol m - s -1 ) 1 U1 (Hollow ) RS (µmol m - s -1 ) 1 U (Hollow) - DOY, 5 DOY7-, 5 DOY3, 5 - RS (µmol m - s -1 ) 1 U3 () RS (µmol m - s -1 ) RS (µmol m - s -1 ) U () U7 () RS (µmol m - s -1 ) U5 () r = RS (µmol m - s -1 ) U () r =. Fig. 3. Daily soil respiration (RS) and groundwater level (GWL) for the UF site. Fitted curves are drawn. The coefficient of determination (r ) is shown if a single polynomial curve is fitted. RS (µmol m - s -1 ) D1 (Hollow ) D (Hollow ) D3 () r =.3 r =.9 r =.1 RS (µmol m - s -1 ) RS (µmol m - s -1 ) RS (µmol m - s -1 ) D () r = RS (µmol m - s -1 ) D5 () r =.1 RS (µmol m - s -1 ) D () r =.1 RS (µmol m - s -1 ) D7 () r =.1 Fig.. Daily soil respiration (RS) and groundwater level (GWL) for the DF site. A quadratic curve is fitted. The coefficient of determination (r ) is shown

8 J. Agric. Meteorol. (), Relationship between soil respiration and groundwater level Local hydrology is known to strongly influence peat decomposition (e.g. Hirano et al., 9; Page et al., 9). Here, the relationships between RS and GWL are shown. The relationship depended on GWL at the UF site (Fig. 3). For hollows (U1 and U), different relationships were found for the high, middle and low GWL regions. The RS decreased slightly with GWL in the high region (GWL>.3-. m). Mean RS in this region was about.7 µmol m - s -1 at both points. In the middle region (GWL>-.1 to -. m), RS decreased linearly with GWL. In contrast, in the low region (GWL<-.1 to -. m), RS was almost independent of GWL or varied quadratically with GWL. At U, the relationship showed a hysteresis. The RS was greater when GWL was decreasing than when it was increasing, particularly in the middle GWL region. For hummocks, RS decreased linearly with GWL in the high region at U3, U and U7 (Fig. 3). The GWL, at which such decreases occurred, was.,.11 and.5 m for U3, U and U7, respectively, which depended on their relative heights. In the middle and low GWL regions, a quadratic or linear relationship was found between RS and GWL. Although RS decreased in the high-gwl region also at U5 and U, the entire GWL region from -. to. m was fitted significantly by cubic curves (p<.1) (Fig. 3). At the DF site, the relationship between RS and GWL for the entire GWL region from -1. to. m for both hollows and hummocks was fitted significantly by quadratic curves (p<.1) (Fig. ). The GWL accounted for 11-1% of RS variations. The minimum of each quadratic curve occurred at GWL between -1. and -.9 m. Mean values of the minimum RS were.5 µmol m - s -1 at -.7 m and 3. µmol m - s -1 at -.9 m, respectively, for hollows and hummocks. Daily mean RS had a significant exponential relationship with soil temperature at the DF site as well as half-hourly RS (Hirano et al., 9). The Q 1, which is an increase factor of respiration with a temperature rise of 1, was 1.9 and 1.7 for hollows and hummocks, respectively. At the UF site, however, no significant relationship was found, because RS was more sensitive to GWL. Soil temperature and SWC did not significantly explain variations in the RS residual between measurements and estimates from GWL at either site. 3. Variations in spatially averaged soil respiration To show the seasonal pattern of RS in 5 clearly, daily values were averaged for each type of microtopography at each site after filling data gaps at each point using GWL with the fitted equations. At the UF site (Fig. 5a), RS at hollows showed a distinct seasonal variation with large decreases under flooded conditions. At hummocks, RS showed a limited variation with a sharp decrease when GWL was higher than about.5 m. However, RS at the DF site remained almost constant at around µmol m - s -1 at hummocks and - µmol m - s -1 with small fluctuations at hollows, which was probably caused by small replications of two chambers (Fig. 5b). The spatially averaged RS was calculated for both sites according to the areal ratio of 5:5 between hollows and hum- RS (µmol m - s -1 ) RS (µmol m - s -1 ) RS (µmol m - s -1 ) 7 a 5 3 Hollow b Hollow c 5 3 UF site 1 DF site Day of year in 5 Fig. 5. Seasonal variation in daily mean soil respiration (RS): averages for each microtopography at the UF site (a) and at the DF site (b), and spatial averages for the UF and DF sites (c) in 5. Spatially averaged RS was calculated using the areal ratio of hollows and hummocks as 5:

9 S. Sundari et al.:soil respiration in tropical peat forests RS (gc m - d -1 ) RS (gc m - d -1 ) a b r =.5 r = r =. Fig.. Spatially averaged soil respiration (RS) and groundwater level (GWL) on a daily basis for the UF site (a) and DF site (b). Spatially averaged RS was calculated using the areal ratio of hollows and hummocks as 5:5. For the UF site, data were separated at GWL of -.1 m. For the DF site, a quadratic curve was fitted. The coefficient of determination (r ) is shown. mocks on the forest floor (Jauhiainen et al., 5) (Fig. 5c). Seasonal variation in RS was found only at the UF site. The annual mean values and SDs of RS were 3.5±1. and 3.9±. µmol m - s -1, respectively, for UF and DF sites. The coefficient of variation at the UF site was four times as large as that of the DF site. Figure shows the relationship between the spatially averaged RS and GWL. At the UF site, the relationship was approximated significantly (p<.1) as a line (y= x; r =.9) or a quadratic curve (y =. +.5x +.11x ; r =.5) when GWL was, respectively, higher or lower than -.1 m. Overall, the two equations accounted for 9% of the total RS variation with RMSE of.39 gc m - d -1. For the DF site, the relationship was approximated significantly (p<.1) by a quadratic curve (y=.3+.1x+1.9x ; r =.) and RMSE of.13 gc m - d -1. The minimum RS of 3.17 gc m - d -1 was calculated at GWL of -.71 m using the quadratic equation. 3.5 Annual sums of soil respiration Annual RS was summed at each point for 5 (Table 1). Missing data were estimated at each point from GWL using the fitted equations (Figs. 3-). The quantities of missing data were 5-3 and 7-35 days, respectively, for the UF and DF sites (Table 1). For the UF site, annual RS was estimated at gc m - y -1 with 11±1 gc m - y -1 (mean±sd) for hollows from two points and at gc m - y -1 with 17±19 gc m - y -1 for hummocks from five points. At the DF site, annual RS was 9-19 gc m - y -1 with 1±119 gc m - y -1 for hollows from two points and gc m - y -1 with 1±3 gc m - y -1 for hummocks from five points. For annual RS at hollows and hummocks between the two sites, no significant difference was found. Spatially averaged annual RS values were estimated at 137 and 15 gc m - y -1 for the UF and DF sites, respectively, under the assumption that hollows and hummocks were distributed equally in each area (Jauhiainen et al., 5). The RS was estimated on a daily basis from GWL data using models (Fig. ) for the years of 5- for the UF site and for 3- for the DF site. The annual sums of RS are presented in Table along with GWL statistics. The annual RS values estimated in 5 were 131 and 1 gc m - y -1 for the UF and DF sites, respectively, which approximated the respective measured RS values of 137 and 15 gc m - y -1 within % (Table 1). This agreement indicates that the models are useful. Of the four years of 5-, GWL decreased the most in because of the prolonged dry season caused by an El Niño event. The annual minimum GWL was lower than that in 5 by more than.5 m. The minimum GWL was the highest in 7 and, especially at the DF site. However, variations in the annual maximum GWL were only within the range of.1 m. Of the four years, annual RS at the UF site fluctuated between 11 gc m - y -1 in 7 and 155 gc m - y -1 in with a mean of 1353±11 gc m - y -1. At the DF site, variations in annual RS were small, between 13 gc m - y -1 in and 13 gc m - y -1 in with a mean of 155±3 gc m - y -1 ; the mean for six years during 3- was almost identical at 159±7 gc m - y -1. Variations in annual RS were greater at the UF site than at the DF site. No significant difference was found in annual RS between two sites using a paired t-test (p =.15)

10 J. Agric. Meteorol. (), 1. Discussion.1 Factors influencing soil respiration The RS was chiefly affected by GWL at both the UF and DF sites, although their relationships differed in shape because of their different GWL ranges. At the UF site (Fig. 3), RS was distinctly lower in hollows with increasing GWL when GWL was closer to the surface than -. or -.1 m. The RS was also lower at hummocks with increasing GWL when GWL was higher than about.1 m. These clearly lower levels resulted from decreased RS in surface soil under anoxic conditions created by water saturation. They underscore the large contribution of surface soil, including dense fine roots, above -. m or.1 m to RS (Hirano et al., 9). In hollows, RS was almost constant at about.7 µmol m - s -1 at GWL higher than.1 m, which indicates that a certain amount of CO continues to flow out of the hollow surface even under flooded conditions. The CO efflux from the water surface resulted from the decomposition of soil organic matter and root respiration even under water-saturated conditions (Rydin and Jeglum, ), and is equivalent to that measured by Jauhiainen et al. (5) under flooded conditions. However, RS increased at several points as GWL lowered to below -. to -. m. At a hollow (U), RS showed a hysteresis with GWL. The RS was greater when GWL decreased after flooding than when it was increasing. Particularly, RS showed a large peak with 1 µmol m - s -1 when GWL dropped to just below the hollow surface (Figs. 1 and ). Ascending soil water can bring up labile underground organic matter to the hollow surface, and much of the organic matter might have been left on the ground when the water descended underground. The labile substrate can decompose easily under moist and oxic conditions on the ground, which led to the RS peak immediately after the flooding subsided. Jauhiainen et al. (5) presented a similar relationship with a peak at GWL of around -.3 m for hollows in the same forest. At hummocks, RS also had a distinct peak at GWL of around.1 m at U3, although no clear hysteresis was shown. This peak is attributable to the enhancement of fresh litter decomposition in the wet season (Goulden et al., ) (Figs. 1 and ). At the DF site (Fig. ), however, the relationship at each point was fitted significantly to a quadratic curve, irrespective of microtopography. The RS showed a tendency of increase at high and low GWLs, showing a minimum level at GWL of -.5 to -1. m. In the high- GWL region in the wet season, microbial respiration or peat decomposition in the deeper soil profile was lower under anoxic conditions, whereas RS in surface soil and litter decomposition were enhanced under moist conditions. The latter was probably larger than the former, which explains the higher RS. In contrast, in the low-gwl region in the dry season, the peat decomposition in deeper soil profile would have increased more under unsaturated conditions than the RS decrease in surface soil and litter decomposition attributable to desiccation (Hirano et al., 9). Soil temperature is a key environmental factor determining RS. Nevertheless, no clear relationship was found between RS and temperature on a daily basis at the UF site, although Hirano et al. (9) showed exponential relationships at these sites using halfhourly data. Moreover, temperature variation was insufficient to account for RS residual between measurements and estimates from GWL using equations that had been fitted to the data of each site. The obscure temperature effect was caused mainly by the small amplitude of the daily mean soil temperature:< 3 (Fig. 1). The SWC is another important factor for RS, particularly in the tropics (Ohashi et al., ; Adachi et al., 9). The surface soil was porous and had low water-holding capacity. Therefore, seasonal variations in SWC of the uppermost.-m-thick layer were small under unsaturated conditions (Fig. 1). In addition, a linear relationship was found between SWC and GWL under partially saturated conditions. Therefore, the effect of SWC on RS was obscured by that of GWL; SWC could not additionally explain RS variations. Root respiration is controlled by canopy processes through metabolism of recently fixed carbohydrates (Ryan and Law, 5; Kuzyakov and Gavrichkova, 1). We analyzed the relationship between the SR residual and gross primary production (GPP) measured using the eddy covariance technique at the DF site (Hirano et al., 7, 9). Even when GPP data were lagged by -5 days in consideration for translocation (Kuzyakov and Gavrichkova, 1), no significant relationship was found. Results show that GPP gave no additional explanatory value to elucidate RS variations.. Effect of drainage on soil respiration A large canal, excavated in the late 199s, has drained the DF site. For that reason, GWL at the DF

11 S. Sundari et al.:soil respiration in tropical peat forests site was considerably lower than that at the UF site. The differences in annual minimum, maximum and mean GWLs between the two sites were.,.3 and.3 m, respectively, on average for 5- (Table ). Jauhiainen et al. () showed annual RS of about gc m - y -1 in the drained forest, as measured using a static chamber method for a year immediately before dam construction in June 5. The annual RS was about twice as large as that (11 gc m - y -1 ) in the same forest as the UF site (Jauhiainen et al., 5), as estimated using GWL data for The estimate was smaller than our result of 137 gc m - y -1 at the UF site (Table 1), and the difference of gc m - y -1 is partly attributable to the GWL difference between the two periods. Annual RS of about gc m - y -1 in the drained forest (Jauhiainen et al., ) differed from our result of 15 gc m - y -1 (Table 1). That great difference cannot be explained solely by the GWL difference because Jauhiainen et al. () reported that elevation of the minimum GWL by.55 m through canal blocking gave no effect on annual RS. Their site was closer to the canal. For that reason, it was possible for it to be disturbed much more than our DF site. For our study site, we applied automated chamber systems. Those chambers, which had been installed for a long period, might have hindered litter fall on the ground and led to some underestimation of RS. Although we assumed greater soil respiration at the DF site because of the enhancement of peat decomposition under aerobic conditions by drainage, the annual RS in 5 was less at the DF site than at the UF site by 1 gc m - y -1 (Table 1). Annual values of RS estimated from GWL for 5- showed no difference between the two sites (p=.15) (Table ). In addition, annual RE measured by the eddy covariance technique was less at the DF site (Hirano et al., 9) than at the UF site (unpublished data) by about gc m - y -1 in 5. These facts do not support the inference that RS is increased by drainage, although peat decomposition is expected, in theoretical terms, to be greater at the DF site. Forest productivity probably explains this discrepancy. Annual GPP was greater at the UF site (unpublished data) than at the DF site (Hirano et al., 9) by about 3 gc m - y -1. Root respiration and litter production are also expected to be less at the DF site according to the difference in GPP. Results show that the DF site had lower RS than the UF site. As portrayed in Fig. a, RS decreased sharply along with increasing GWL when GWL was closer to the surface than -.1 m. This fact indicates that CO emissions from surface soil, in which dense fine roots exist, and litter were inhibited to a great degree under water-saturated conditions. Additionally, that fact indicates a large contribution of surface soil and litter to RS and to soil CO efflux (Hirano et al., 9). Therefore, flooding duration is a key determinant of annual RS at the UF site. When GWL is underground, lowered GWL slows litter decomposition because of desiccation of the litter (Sotta et al., ), but it facilitates the decomposition of deeper peat through enhanced aeration. Conversely elevated GWL increases litter decomposition but decreases deeper peat decomposition. In fact, RS can be regarded as including two components that are oppositely affected by GWL. Therefore, the lowering of GWL does not necessarily increase RS (Laiho, ; Page et al., 9; Muhr et al., 11). However, given equal forest productivity, the annual RS is expected to be greater at the DF site because GWL did not rise aboveground and decreased usually below -. to -1. m (Table ). Furthermore, at the DF site, GWL between -. and -1. m was likely to have been the lowest GWL before the canal excavation, whereas the canal lowered it to -1.5 to m. Although peat in the profile below -. or -1. m has already decomposed well (Page et al., ), Fig. b suggests that the additional decrease in GWL enhanced peat decomposition in the deeper profile..3 Comparison with other tropical forests Annual RS was compared among several forest sites in the tropics. Periodic measurements with static or dynamic chambers were temporally interpolated to obtain annual RS using empirical models at the other sites. Within a tropical peat swamp forest, our results of 137 and 15 gc m - y -1 for the UF and DF sites were larger than that of a secondary forest with high GWL of -.1 to.3 m in Micronesia (1 gc m - y -1 ; Chimner and Ewel, ) and that of a reclaimed secondary forest in South Kalimantan province, Indonesia (9 gc m - y -1 ; Hadi et al., 5). Our results are compatible with that obtained in a secondary forest in South Kalimantan (1 gc m - y -1 ; Inubushi et al., 3). However, a natural forest in Sarawak, Malaysian Borneo showed much greater RS of 13 gc m - y -1 (Melling et al., 5). In comparison with upland tropical forest sites, our result was much less than

12 J. Agric. Meteorol. (), 1 those of Asian tropical forests: 19 gc m - y -1 in a tropical rain forest in Sarawak (Ohashi et al., ) and 5 gc m - y -1 in a tropical seasonal forest in northern Thailand (Hashimoto et al., ). However, annual RS of gc m - y -1 in Amazonian forest (Chambers et al., ; Sotta et al., ; Malhi et al., 9) was similar to our results. 5. Conclusions Distinct relationships were found between soil respiration (RS) and groundwater level (GWL) from continuous datasets obtained during periods of longer than one year. The RS decreased sharply as GWL increased under flooded conditions in the undrained forest. In the drained forest, RS gradually increased as GWL decreased when GWL was lower than -.7 to -. m, to which GWL seldom lowers in the undrained forest. These relationships indicate that GWL-lowering by drainage increased RS or soil CO efflux through peat decomposition, although we were unable to quantify the effect of drainage because of the two sites differing forest productivity. Acknowledgements This work was supported by JSPS Core University Program, JSPS KAKENHI (Nos , 131 and 1551), JSPS A3 Foresight Program (CarboEastAsia) and the JST-JICA Project (Wild Fire and Carbon Management in Peat-Forest in Indonesia). The authors acknowledge Yuji Kodama and Shun-ichi Nakatsubo for chamber preparation, and Takashi Inoue for providing GWL data for the DF site. References Adachi, M., Ishida, A., Bunyavejchewin, S., Okuda, T., and Koizumi, H., 9: Spatial and temporal variation in soil respiration in a seasonally dry tropical forest, Thailand. J. Trop. Ecol., 5, Ali, M., Taylor, D., and Inubushi, K., : Effects of environmental variations on CO efflux from a tropical peatland in eastern Sumatra. Wetlands,, 1-1. Canadell, J. G., Pataki, D. E., Gifford, R., Houghton, R. A., Luo, Y., Raupach, M. R., Smith, P., and Steffen, W., 7: Saturation of the terrestrial carbon sink. In Terrestrial ecosystems in a changing world (ed. by Canadell, J. G., Pataki, D. E., and Pitelka, L. F.). Springer-Verlag, Berlin Heidelberg, pp Chambers, J. Q., Tribuzy, E. S., Toledo, L. C., Crispim, B. F., Higuchi, N., dos Santos, J., Araujo, A. C., Kruijt, B., Nobre, A. D., and Trumbore, S. E., : Respiration from a tropical forest ecosystem: Partitioning of sources and low carbon use efficiency. Ecol. Appl., 1, S7-S. Chimner R. A., and Ewel, K. C., : Differences in carbon fluxes between forested and cultivated micronesian tropical peatlands. Wetl. Ecol. Manag., 1, Couwenberg, J., Dommain, R., and Joosten, H., 1: Greenhouse gas fluxes from tropical peatlands in south-east Asia. Global Change Biol., 1, Dommain, R., Couwenberg, J., and Joosten, H., 11: Development and carbon sequestration of tropical peat domes in south-east Asia: links to post-glacial sea-level changes and Holocene climate variability. Quat. Sci. Rev., 3, Furukawa, Y., Inubushi, K., Ali, M., Itang, A. M., and Tsuruta, H., 5: Effect of changing groundwater levels caused by land-use changes on greenhouse gas fluxes from tropical peat lands. Nutr. Cycl. Agroecosyst., 71, Goulden, M. L., Miller, S. D., da Rocha, H. R., Menton, M. C., de Freitas, H. C., Figueira, A. M. E. S., and de Sousa, C. A. D., : Diel and seasonal patterns of tropical forest CO exchange. Ecol. Appl., 1, S-S5. Hadi, A., Inubushi, K., Furukawa, Y., Purnomo, E., Rasmadi, M., and Tsuruta, H., 5: Greenhouse gas emissions from tropical peatlands of Kalimantan, Indonesia. Nutr. Cycl. Agroecosyst., 71, 73-. Hashimoto, S., Tanaka, N., Suzuki, M., Inoue, A., Takizawa, H., Kosaka, I., Tanaka, K., Tantasirin, C., and Tangtham, N., : Soil respiration and soil CO concentration in a tropical forest, Thailand. J. For. Res., 9, Hergoualc h, K., and Verchot, L. V., 11: Stocks and fluxes of carbon associated with land use change in Southeast Asian tropical peatlands: A review. Global Biogeochem. Cycles, 5, doi:1.19/9gb 371. Hirano, T., Jauhiainen, J., Inoue, T., and Takahashi, H., 9: Controls on the carbon balance of tropical peatlands. Ecosystems, 1, Hirano, T., Segah, H., Harada, T., Limin, S., June, T., Hirata, R., and Osaki, M., 7: Carbon dioxide balance of a tropical peat swamp forest in

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