Methane oxidation in Japanese forest soils
Shigehiro Ishizuka*, Tadashi Sakata, Kazuhiro Ishizuka
Forest Environment Division, Forestry and Forest Products Research Institute, P.O. Box 16, Tsukuba Norin Kenkyu Danchi-nai, Ibaraki 305-8687, Japan
Accepted 17 October 1999
Abstract
To evaluate the CH4 uptake rate in Japanese deciduous and coniferous evergreen forest soils, the CH4 ¯ux and CH4
concentration in soil gas were measured at seven sites in central Japan. The CH4 uptake potential was calculated from
incubation of soil cores. The CH4¯uxes at all sites were negative (uptake by the soils) at every sampling time. The CH4uptake
rate was very high (7.6 mg CH4 mÿ2 dÿ1) in one deciduous forest soil. Fluxes were highly correlated to the air temperature
except a coniferous forest site. The most active layer of CH4uptake in each plot diered with site: subsurface (10±15 cm) at two
coniferous forest sites and topsoil (0±5 cm) at the other ®ve sites. The potential of the subsurface layer to oxidize CH4made a
substantial contribution to soil CH4uptake mechanisms, especially when the topsoil had a low ability to oxidize CH4. Methane
uptake rates were nine times higher than those of previous studies. The soil CH4 uptake rate on a global scale may be
underestimated.72000 Elsevier Science Ltd. All rights reserved.
Keywords:Methane ¯ux; Forest soils; Methane oxidation; Global methane uptake rate
1. Introduction
Methane (CH4) is a greenhouse gas with strong
absorption bands in the infrared region, and currently contributes 15% to global warming (Rohde, 1990). Methane concentration in the atmosphere is approxi-mately 1.71 ml lÿ1 in 1992 and was increasing at an annual rate of 0.4% between 1990 and 1992 (Prather et al., 1995). Methane evolves from rice paddy ®elds (Seiler et al., 1984b; Holzapfel-Pschorn and Seiler, 1986; Yagi and Minami, 1990), swamps (Harriss et al., 1982; Bartlett et al., 1988) and marshes (Bartlett, 1985; Sebacher et al., 1986). The uptake of CH4by soils was
®rst reported by Harriss et al. (1982) in peat soils in a relatively dry season. Since then the uptake of CH4
has been measured in forest soils (Seiler et al., 1984a;
Keller, 1986; Whalen et al., 1992; Adamsen and King, 1993; Singh et al., 1997), grassland soils (Whalen and Reeburgh, 1990; Mosier et al., 1993, 1997; Tate and Striegl, 1993) and deserts (Striegl et al., 1992). Many studies suggest that soil moisture has an important role in CH4uptake (Steudler et al., 1989; Mosier et al.,
1991; Castro et al., 1994; Czepiel et al., 1995). DoÈrr et al. (1992) mapped the CH4 uptake rate on a global
scale by soil texture class. Their model was based on the assumption that CH4 uptake rate depends on gas
transport, which was determined by soil texture. They estimated that global CH4 uptake was in the range of
9.0±55.9 Tg CH4 yrÿ1 (28.7 Tg CH4 yrÿ1 as the best
estimate in the report). They divided soil into eight textured classes, which were combinations of coarse, medium and ®ne according to a FAO soil map classi®-cation and adopted the uptake rate 1.43 for coarse, 0.42 for medium and 0.19 mg CH4 mÿ2 dÿ1 for
®ne-textured soils. Koschorreck and Conrad (1993) obtained ¯ux data comparable with these uptake rates. Although most studies have been made in the USA
0038-0717/00/$ - see front matter72000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 0 7 1 7 ( 9 9 ) 0 0 2 0 0 - X
www.elsevier.com/locate/soilbio
* Corresponding author. Present address: Hokkaido Research Cen-ter, Forestry and Forest Products Research Institute, 7 Hitsujigaoka, Sapporo, 062-8516, Japan. Tel.: 4131; fax: +81-11-851-4167.
and Europe, data from other regions of the world are needed to more precisely estimate global CH4uptake.
In Japan, forest soils mostly contain volcanic ash, and the textures are usually sandy clay loam, clay loam or loam (®ne or medium according to the FAO classi®cation). We measured the CH4 concentration in
the soil gas, CH4 ¯uxes and incubated soil cores to
evaluate the CH4 uptake rate by Japanese temperate
deciduous and coniferous forest soils.
2. Materials and methods
2.1. Sampling sites
Four sampling sites (seven sampling plots) were selected for ®eld measurement. Ogawa Forest Reserve (6856'N, 140835'E, altitude 620±670 m) was in Kitai-baraki City, IKitai-baraki Prefecture, about 150 km NNE from Tokyo. The mean annual temperature is 12.48C; and the annual precipitation is about 1200 mm. Masaki et al. (1992) describe other details for this site. At this site we established three plots, one at the valley head (OFD1) and one on the upper part of a slope (OFD2). The vegetation of these plots was an old-growth deciduous forest dominated by oak (Quercus serrata) and beech trees (Fagus japonica and Fagus crenata). The third plot (OFC) was in Japanese cedar forest (Cryptomeria japonica, 44 yr old) 2 km west from OFD1 and OFD2. The second sampling site was at Hitachi Ohta Experimental Site (36834'N, 140835'E,
altitude 280±340 m) in Hitachi Ohta City, Ibaraki Pre-fecture, about 120 km NNE from Tokyo. The mean annual temperature is 13.78C and the annual precipi-tation is about 1500 mm. Tsuboyama et al. (1994) describe details of this site. Two plots were established in this site: one (HEC) was planted with cypress ( Cha-maecyparis obtusa) and Japanese cedar and the other (HED) was deciduous forest dominated by oak trees (Q. serrata). The third sampling site was at Tsukuba Research Site (TRC, 36820'N, 140818'E, altitude 300± 360 m), in Ibaraki Prefecture, about 70 km NNE from Tokyo, planted with Japanese cedar. The mean annual temperature is 14.18C and the annual precipitation is about 1400 mm (Ohnuki and Yoshinaga, 1995). The fourth sampling site was at Kaba Research Site (KRD, 36820'N, 140818'E, altitude 470 m), which was deciduous forest located near TRC. In summary the vegetation at OFC, TRC and HEC was coniferous for-est, and at OFD1, OFD2, HED and KRD, deciduous forest.
Table 1 shows the general soil properties of these plots. All soils contain volcanic ash, so bulk densities were relatively low (range: 0.30±0.58 Mg mÿ3). The soils at OFD1, OFC, TRC and KRD were strongly aected by volcanic ash, with high carbon contents (125±230 mg gÿ1 soil) and weak or strong andic soil properties. The soil pH ranged from 4.2 to 5.0. The soils of OFD1 and TRC were wet and had a high water-holding capacity. The ratio of air volume of OFD1 and HEC was >0.1 m3 mÿ3 soil volume and was lower than those of the other sites.
Table 1
General properties of surface soilsa
OFD1 OFD2 OFC HEC HED TRC KRD
Soil type Andisol Inceptisol Andisol Inceptisol Inceptisol Inceptisol Inceptisol Vegetation deciduous deciduous coniferous coniferous deciduous coniferous deciduous
pH (H2O) 4.8 5.0 4.5 4.2 4.5 4.4 4.6
Water content (g gÿ1) 1.69 0.75 1.04 0.98 0.50 1.47 0.94
Bulk density (Mg mÿ3) 0.33 0.51 0.35 0.58 0.63 0.30 0.34
Air volume (m3mÿ3)
0±5 cm 0.27 0.42 0.51 0.29 0.40 0.40 0.57
10±15 cm 0.21 0.39 0.35 0.28 0.30 0.33 0.46
20±25 cm 0.14 0.31 0.33 0.24 0.22 0.25 0.38
2.2. Soil sampling and chemical analysis
Soil for chemical analysis was sampled from 0±5, 10±15 and 20±25 cm depth and fresh soils were sieved (2 mm) in the laboratory. The concentrations of NH4
+
-N and -NO3
ÿ
-N in the surface soils were determined on soil extracts by steam-distillation methods (Mulvaney, 1996). After NH4+-N and NO3ÿ-N determination, the
soil was stored at 58C in a refrigerator until analyzed. The total soil carbon and nitrogen contents were measured with a CN analyzer (Yanaco, MT-600). Soil water content was measured by drying the soil samples at 1058C for 24 h.
2.3. Methane ¯ux
The CH4 ¯ux was measured by the closed chamber
method. Cylindrical stainless steel chambers (40 cm dia and 15 cm height) were inserted into the soil to a depth of approximately 5 cm and were left throughout the study. Measurement began at least 7 d later after setting the chambers in order to eliminate the disturb-ance of soil. When sampling, the static chamber was covered with stainless steel lids with sampling ports and air bags to equilibrate air pressure in the chamber. A 30-ml headspace of air was sampled at 0, 15 and 30 min using syringes equipped with three-way cocks and silicon rubber septa. The leakage of these sampling syringes was negligible (<1% dÿ1). The gas was ana-lyzed in the laboratory within 24 h after sampling by gas chromatography (see below). The CH4 ¯ux was
calculated by the method of Hutchinson and Mosier (1981), which is a nonlinear model based on the theory of molecular diusion in soils. In 1995 the CH4¯uxes
were measured at OFD1, OFD2, TRC and HEC, and in 1997 they were measured at OFD1, OFD2, OFC, HED and KRD. The data are shown as means of tri-plicate measurements, except for OFD2 in 1995 (only one chamber).
2.4. Soil gas
Soil gas was sampled using a stainless steel tube (3.0 mm outer dia, 1.0 mm inner dia, 0.5 m length and with a side port to sample the air), equipped with a silicon tube and a rubber septum to remove a sample. When sampling, the tube was inserted to 10 cm depth and 20 ml soil gas was taken from the septum. Next, the tube was inserted to 20 cm and a sample taken, and the same procedure continued at intervals of 10± 50 cm depth.
The ¯ux in the soil was calculated by Fick's law
q ÿDdC
dz,
where q is the ¯ux (kg mÿ2sÿ1), D is the binary mol-ecular diusion coecient for CH4in the soil (m2sÿ1),
C is the gas concentration in the soil (kg mÿ3), and z
is the distance (m). D was correlated with the volume of the gas phase. We adopted the equation of Sakata et al. (1996) because it was the only equation calcu-lated by the data of Japanese forest soils, which con-tains volcanic ash materials and might be dierent from other countries. The equation is as follows.
D=D0 0:602eÿ0:061,
whereeis the ratio of gas phase volume to soil volume
(m3mÿ3) and D0 is the diusion coecient of CH4 in
air (m2 sÿ1). D0 is unique to the gas and we used the
value of 1.96 10ÿ5 for CH4 (at 98C; Marrero and
Mason, 1972). The ratio of gas phase volume-to-soil volume was calculated from the water content when no data was available from the three-phase analysis. The CH4 absorption rate was calculated using the
nearest two ¯uxes. For example, the CH4 uptake rate
at 10 cm was calculated using the remainder of the ¯ux of 5 and 15 cm (see De Jong and Schappert, 1972).
2.5. Incubation analysis
Intact soil cores were collected from the plots OFD1, OFD2, TRC and HEC, using 100-ml soil cores (5 cm internal dia 5.1 cm height stainless cylinder, Daiki Co.) from the same depth as the soil samples taken for chemical analysis. The samples were placed in a 500-ml glass jar with a butyl rubber stopper and a silicon septum. The headspace was ®lled with ambient air (approximately 1.8 ml CH4 lÿ1) and the sample
incubated for up to 24 h at 258C. The air in the head-space was sampled successively using a 1-ml gas-tight syringe with a stopcock. Because the CH4
concen-tration in the headspace decreased according to ®rst-order kinetics, the uptake rates were calculated from the exponential regression of the time series of CH4
concentration. All data are shown as means of the results of triplicate cores. The values obtained from the core incubation method gave the potential for CH4
uptake because the soil at deeper layers was usually exposed to the soil gas and had a low CH4
concen-tration, unless macropores were present.
2.6. Methane analysis
The CH4 concentration was determined using a gas
respectively. Gas samples, 5 ml, were used for CH4
¯ux and soil gas analyses and 1-ml samples for incu-bation analysis. A standard calibration was made using standard gases of 1.25 ml CH4 lÿ1 and 2.00 ml
CH4l ÿ1
(Sumitomo Seika Chemicals Co.).
3. Results
3.1. Methane ¯ux
Methane ¯ux was negative and CH4 was absorbed
by every soil and at each season (Fig. 1). The highest ¯ux was 10.7 mg CH4mÿ2dÿ1in August at KRD and
the lowest was 0.7 mg CH4 mÿ2 dÿ1 in November at
OFD1. The averages of the ¯ux (mg CH4 mÿ2 dÿ1)
were 2.5 (OFD1, n12), 4.9 (OFD2, n10), 4.9 (OFC, n4), 1.8 (HEC, n4), 3.3 (HED, n6), 1.8 (TRC, n6 and 7.6 (KRD,n5). Methane ¯ux was higher in summer than in winter and was negatively correlated with air temperature, except at HEC. The averages of CH4¯uxes in the summer in 1995 (2.7 mg
CH4mÿ2dÿ1; OFD1 and 4.8 mg CH4mÿ2dÿ1; OFD2)
were nearly equal to those in 1997 (2.9 mg CH4 mÿ2
dÿ1; OFD1 and 5.2 mg CH4 mÿ2 dÿ1; OFD2). The
average of the CH4 ¯ux of coniferous forest soils
(OFC, HEC and TRC) was 2.8 mg CH4mÿ2 dÿ1 and
was smaller than that of deciduous forest soil (OFD1, OFD2, HED and KRD, 4.6 mg CH4mÿ2dÿ1).
3.2. Soil gas
CH4 concentrations in the atmosphere were 1.832
0.06 ml lÿ1 n41and decreased with soil depth to a concentration of 0.2020.02 ml lÿ1 n4 at 0.5 m depth (Fig. 2). In Fig. 2, the triangles show the CH4
concentration at the date with the highest CH4uptake
rate (HU) and the triangles (inverted) show the CH4
concentration at the date with the lowest CH4 uptake
rate (LU). At HEC, OFC, and KRD, the CH4
concen-trations were almost equal (0.2 ml lÿ1) below 0.2 m depth. This result suggests that at these sites CH4
uptake was not observed below 0.2 m because of the very small CH4 concentration. At LU, the CH4
con-centrations at 0.2 m depth at TRC and HED were 0.93 and 1.36 ml lÿ1, respectively, and they decreased to 0.52 and 0.82 ml lÿ1 at 0.3 m depth, respectively. This result suggested that CH4 was absorbed by the
soil below 0.2 m.
Fig. 3. Depth pro®le of methane uptake rate of four soils.-, CH4 uptake potential rate obtained by incubation analyses, Q, CH4 uptake rate calculated by Fick's equation. The vertical lines indicate 1 S.D.not determined.
3.3. Incubation analysis and gas diusion calculation
Neither organic layer nor autoclaved soils consumed CH4, indicating that the CH4 ¯ux was due to
metha-notrophic bacteria in mineral soils. Soil core samples from almost all samples consumed CH4 as soon as
they were put into the jar. The CH4 uptake rate
decreased as the concentration of CH4 in the
head-space decreased. The uptake rates of methane in the uppermost layer of soil core samples from OFD1 and OFD2 were larger than those of the subsurface layer, and the largest consumption was observed with the 10±15-cm samples from TRC and HEC (Fig. 3). The uptake rates calculated from the CH4concentration of
soil gas gave similar results to ®eld measurements. The highest uptake rates of CH4were generally observed in
the uppermost layers of soil core samples (OFD1, OFD2, KRD, HED and OFC; Figs. 3 and 4, black box). At TRC, the consumption of CH4was largest at
20±30 cm soil depths. The order of CH4 uptake rate
obtained from incubation analysis was OFD2 > OFD1 > HEC > TRC, which was comparable with the order for CH4¯ux data.
4. Discussion
4.1. Methane ¯ux estimation
In this study, the rate of soil CH4consumption
ran-ged from 1.8 to 7.6 mg CH4 mÿ2 dÿ1 and the mean
CH4 uptake rate was 3.8 mg CH4 mÿ2 dÿ1. These
¯uxes were comparable to some reports (Steudler et al., 1989; Adamsen and King, 1993; Castro et al., 1995; Goldman et al., 1995) where relatively high
values had been demonstrated (Table 2). According to the soil texture class of the FAO classi®cation, all soils in our study were medium texture. The soil CH4 ¯ux
was ninefold larger than the uptake rates (0.42 mg CH4 m
ÿ2
dÿ1) of the medium soils estimated by DoÈrr et al. (1992). Our results suggest that DoÈrr et al. (1992) may have underestimated the global CH4 uptake rate,
and that more information is needed on the global scale variations of CH4uptake.
At KRD, the average ¯ux was 7.6 mg CH4mÿ2dÿ1
and the highest uptake rate was 10.7 mg CH4mÿ2dÿ1
in August 1997. This was one of the highest ever reported for forest soils. This soil had a high porosity (the air volume was 57% of the whole soil volume) and the plot had good drainage properties because it was in the middle of a long slope (318). These soil properties minimized diusion limitation of CH4 from
the air into the soil and maintained good aeration properties. In addition to good aeration, other factors may have contributed to the unusually large ¯ux, because the ¯ux was much larger than DoÈrr et al. (1992) estimated for coarse-textured soils. The factors aecting CH4uptake rate in the ®eld have been widely
reported, including inorganic nitrogen (Steudler et al., 1989; Adamsen and King, 1993) and soil temperature (Prieme and Christensen, 1997). Little attention has been given to site-to-site dierences. The ¯uxes reported by Singh et al. (1997) were very high. They discussed the relationship between ¯ux and water con-tent, but did not explain why the uptake rates were higher than those reported elsewhere. The population of methanotrophs is likely to be an important factor aecting CH4 uptake rate, but little information on
this exists. Further research is needed to clarify the re-lationship between ¯ux and the population of metha-notrophs.
4.2. Depth distribution of methane consumption
The CH4 concentration in the soil was lower than
that in ambient air and it decreased with depth, suggesting that CH4 was absorbed at every layer and
that CH4 production was negligible in these forest
soils. Many studies have reported dierences in soil depth pro®les of CH4 consumption. Some showed
maximum uptake rates occurred in topsoils (Whalen and Reeburgh, 1990; Koschorreck and Conrad, 1993), whilst others indicated that uptake rates are highest in subsurface soils, including 10±20 cm (Whalen et al., 1992), 6±10 cm (Adamsen and King, 1993; Prieme and Christensen, 1997 (the interface of organic and mineral soils)), and 3±6 cm depth (Czepiel et al., 1995). Our in-cubation experiment and the calculation from soil gas concentrations in the ®eld, suggest that the layer con-suming the most CH4 diered among the sites. Our
results at HEC and TRC showed that the maximum
CH4 uptake rate was in subsurface soil (10±15 cm
depth), while the maximum CH4 uptake rate was
observed in the topsoils at OFD1, OFD2, OFC, HED and KRD. Adamsen and King (1993) suggested that the subsurface maximum uptake is associated with the mineral soil horizon, but our results do not agree with theirs. Schnell and King (1994) and Prieme and Chris-tensen (1997) suggested that inorganic N possibly inhi-bits the CH4 uptake rate. The inorganic N content of
the soil we used was higher than that of the soils they used, and the CH4 uptake rate was not related to the
inorganic N contents. It is possible that the depth properties of CH4uptake depend on other soil
charac-teristics that aect the activity of methanotrophs. On several occasions the CH4 concentration at 20
cm depth was higher than 0.5 ml lÿ1. At these times the CH4 uptake rate of the soil between 0 and 20 cm
depth was smaller than usual. If the soil below 20 cm had a low ability to absorb CH4, the CH4
concen-tration at 40 cm was higher than usual, for example, 0.7 ml lÿ1 at 40 cm at LU in HED. The gradient of CH4concentration between the air and surface soil gas
aected the ¯ux of CH4, and the CH4concentration of
surface soil gas was aected by the capacity of the sub-surface soil to oxidize CH4, especially when the uptake
rate of surface soil was low. This suggested that the potential of the subsurface layer to oxidize CH4made
a substantial contribution to soil CH4uptake
mechan-isms, especially when the uptake rate of the surface soil was unusually small, as in winter and at dawn.
4.3. Comparison between coniferous and deciduous forest
The mean ¯uxes of deciduous forests were lower than for coniferous forests. Many studies have obtained similar results, which indicate that soils in deciduous forests absorb more CH4than soils in
coni-ferous forests (Steudler et al., 1989; Born et al., 1990; Castro et al., 1995; Dobbie et al., 1996a). Heyer (1977) provided a clue to this mechanism, suggesting that methanotroph isolates from acid soils of coniferous woods and heath were rare. Further research is needed to con®rm this suggestion.
Table 2
CH4uptake rate of the forest soils in the world (by chamber method)
Region Country Vegetation Uptake rate (mg CH4mÿ2dÿ1 Reference
range (seasonal) average (annual)
North America USA Pinus 3.2±4.2a 3.5b Steudler et al., 1989
Quercus, Acer 3.5±5.3a 4.2b Steudler et al., 1989
USA Tsuga, Pinus, Prunusetc. 0±2.8 1.65 Crill, 1991
USA Populus 0.55c ND Whalen et al., 1992
Betula 0.22c ND Whalen et al., 1992
Picea 0.62 and 0.55c ND Whalen et al., 1992
Canada Picea, Ledum, Betula ND 0.27±1.57 Adamsen and King, 1993
Pinus, Quercus ND 2.7 Adamsen and King, 1993
USA spruce and ®r 0.64±2.6b 0.64±1.7b Castro et al., 1993
USA Pinus 3.2±7.0b ND Castro et al., 1994
USA Pinus 0±7.4 2.9 Castro et al., 1995
hardwood 0.8±6.4 4.5 Castro et al., 1995
USA Quercus 2.1±7a 3.8±5.4 Goldman et al., 1995
Central America Costa Rica Laetia, Pentaclethra 0.3±2.3a 1.20±1.26 Keller and Reiners, 1994
Europe Germany ? 0±1.8b 0.49b Koschorreck and Conrad, 1993
deciduous forest 0±5.9a,b 2.2b Born et al., 1990
spruce forest ND 0.25b Born et al., 1990
Scotland Acer, Fraxinus 0.19±3.30 1.4 Dobbie et al., 1996a
Denmark Fagus, Piceaetc 0.27±1.06 0.7 Dobbie et al., 1996a
Poland birch, alder, oak, pine, etc. 0.84±1.23c 1.0 Dobbie et al., 1996a
UK Acer, Fraxinus, Fagus 2.19±2.97 ND Dobbie and Smith, 1996b
Denmark Picea, Quercus ND 0.64±1.7b Prieme and Christensen, 1997
Middle Asia India Ziziphus, Shorea, Acacia 6.2±17.0 8.6±13.7 Singh et al., 1997
East Asia Japan Cryptomeria 0.81±5.59 1.8, 4.9 this study
Chamaecyparis 1.62±1.93 1.8 this study
Quercus, Fagus, Acer 0.69±10.7 2.5±7.6 this study
a
Value read from graphs. b
Recalculated. c
5. Conclusion
The CH4 uptake rates at seven sites in Japanese
deciduous and coniferous evergreen forest soils were highly correlated with air temperature except at one site (HEC). The CH4uptake rate was higher than that
of previous studies. In one deciduous forest soil (KRD), the CH4 uptake rate (7.6 mg CH4 mÿ2 dÿ1)
was one of the highest ever reported for forest soils. We conclude that CH4uptake rates by soil on a global
scale may be underestimated.
Acknowledgements
We thank Dr. Masamichi Takahashi for giving im-portant advice about this paper.
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