Gas Diffusion Coefficient of Undisturbed Peat Soils
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Short Paper Soil Sci. Plant Nut!:, 61 (3). 43 1-435.2005 43 1
Gas Diffusion Coefficient of Undisturbed Peat Soils
Ippei Iiyama' and Shuichi Hasegawa
Soil Amelioration Laboratory. Research Group of Regional Environment, Graduate School of Agriculture, Hokkaido University, Sapporo, Hokkaido. 060-8589 Japan
Received November 4,2004; accepted in revised form March 3,2005
Determination of the gas diffusion coefficient D, of peat soils is essential to understand the mechanisms of soil gas transport in peatlands, which have been one of major potential sources of gaseous carbons. In the present study, we aimed at determining the D, of peat soils for various values of the ahfilled porosity a and we tested the validity of the Three- Porosity Model (Moldrup et al. 2004) and the Millington-Quirk model (1961) for predicting the relative gas diffusivity, the ratio of D, to Do, the gas diffusion coefficient in free air. Undisturbed peat soil cores were sampled from aerobic layers in the Bibai mire, Hokkaido, Japan. The MQ model reproduced the measured D,/D, curves better than the TPM. The TPM, a predictive model for undisturbed mineral soils, overestimated the D,/Do values for peat soils, implying that in the peat soils the pore pathways were more tortuous than those in the mineral soils. Since the changes in the D,/D, ratios with the a values of a well-decom- posed black peat soil tended to be more remarkable than those of other high-moor peat soils, the existence of a positive feedback mechanism was assumed, such that peat soil decomposition itself would increase the soil gas diffusivity and promote soil respiration.
Key Words: &filled porosity, gas diffusion coefficient, peat soils, relative gas diffusivity.
Peatlands in the northern hemisphere have been con- sidered to be large carbon pools amounting to 455 Pg-C (Gorham 199 l), and accounting for approximately one- third of the total world pool of soil carbon (1,395 Pg-C) estimated by Post et al. (1982). Peatlands are ecosys- tems vulnerable to influences from human activities or climatic changes and can become one of the major sources of carbon in global carbon cycling.
Since soil carbon is emitted from peatlands mainly through the gaseous phase as methane and carbon diox- ide, understanding of the transport of these gases in peat soils is essential for predicting the amount and the rate of carbon losses from peatlands.
The movement of gases in soils occurs mainly by dif- fusion, and modeling of the soil gas movement requires information about the soil gas diffusion coefficient D,, which depends on the soil air-filled porosity a. In early studies, the shape of the curves of the D,(a) functions was examined using simplifying assumptions for describing the structure of porous media (Millington 1959; Millington and Quirk 1961). These D,(a) models
I Present address: Water Quality and Solute Dynamics Group, Department of Environmental Chemistry and Biochemistry, National Institute for Agro-Environmental Sciences, Kannondai 3-1-3, Tsukuba, Tbaraki, 305-8604 Japan
have been tested mainly using repacked soils. And in recent studies, predictive models of D,(a) functions for undisturbed soils, including Japanese volcanic ash soils (Moldrup et al. 2003), have been improved by consider- ing the soil water characteristic curves (Moldrup et al. 1999,2000,2004). However, presently, the gas diffusion coefficient for peat soils has not been fully documented, which makes it difficult to predict the amount and the rate of gas transport in peat soils.
In the present study, we determined the soil gas diffu- sivity DJD, (Do = gas diffusion coefficient in free air) of undisturbed peat soil samples, and characterized the changes in the D,/D, with a values for peat soils by comparing the measured data with existing D,(a) mod- els.
Materials and methods Study field and soil profile at the sampling
point. The Bibai mire (43"19'N,14lo48'E), Hokkaido, Japan was used as a study field. This mire, originally a typical ombrotrophic bog, had been preserved by the National Agricultural Research Center for Hokkaido Region with a size of approximately 50 ha. The sur- rounding area of the mire had been completely reclaimed as crop fields for more than 40 years and a decrease in the groundwater table level took place in the
432 I. IIYAMA and S. HASEGAWA
Bulk density(0) (Mg m-3) Carbon content(A) (kg kg-I)
0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4 0.5 0
.$30 5 3 40
1 1 1 ,
0 0.5 1.0 1.5 2.0 2.5 0 10 20 30 40 50 60 Particle density(.) (Mg ) C N A )
peripheral area of the mire, threatening indigenous plant species like sphagnum and sedge. We collected undis- turbed peat soil samples from the area within a distance of 10 m from a drainage ditch. Peat layers at the sam- pling point had been subjected to aerobic conditions, resulting in the decomposition of peat soil near the sur- face. Figure l(a) shows the profiles of the bulk density and soil particle density, while Fig. l(b) shows the pro- files of the soil carbon content and C/N ratio. Soil car- bon content and C/N ratio were determined using an automatic C-N analyzer (SUMIGRAPH NC- 1O00, Sumika Chemical Analysis Service, Ltd., Tokyo, Japan) and a gas chromatograph (GC-gA, Shimadzu Corp., Kyoto, Japan).
Undisturbed peat soil samples. Duplicate undisturbed peat cores for determining the soil gas dif- fusivity D,/D, were sampled from each layer using a sharpened knife and 100cm3 samplers with an inner diameter and height of 50 and 51 mm, respectively. Samples were taken at 12.5, 22.5, 32.5, 42.5, 52.5 and 62.5 cm depths.
To change the air-filled porosity of the peat samples, we applied the hanging water column method (Dane and Hopmans 2002). Firstly, the undisturbed samples were saturated for 24 h in a constant temperature room at 25C. Thereafter, the samples were dehydrated by low- ering the matric potential step-by-step from - 5 to - 100 cm. For each moisture equilibrium, the samples were weighed to determine the moisture content and the air-filled porosity a, and the DJDO values were deter- mined.
Soil gas difusivity determination. We deter- mined DJD,, values based on N2-air binary diffusion phenomena through a soil sample in a single chamber apparatus, according to the methods of Taylor (1949),
Fig. 1. Soil profile at the sampling point in the Bibai mire showing (a) bulk density ( n = 3), soil particle density (n = 2), (b) carbon content ( n = 2) and C/ N ratio ( n = 2 ) . Solid lines denote the average values.
Currie (1960) and Osozawa (1987). The tracer gas was 0, and we measured the changes in its concentration inside the diffusion chamber with a Galvanic cell sensor (OS-3S-D, New Cosmos Electric Co., Ltd., Osaka, Japan). When we found that the space between a peat soil sample and a core wall started to increase due to drying and shrinking processes of the sample at low matric potentials, we filled the space with a sealing compound (NEO SEAL B-3, Nitto Chemical Industry Co., Ltd., Osaka, Japan) to prevent the gas from bypass- ing the soil matrix during the measurements of the breakthrough curves of 0,.
For the determination of D, in the single chamber method, it is preferable to consider the diffusion behav- ior of the tracer gas inside the chamber. For example, El-Farhan et al. (1996) reported that the determination of D, from the solution of Currie (1960) may overesti- mate the true value if the air inside the chamber is not mixed well. When the diffusion behavior inside the chamber is considered, the governing equation describ- ing the measurement system becomes
a2c ax2 (0 S x S L , ) ac = D I
where C is the concentration of the tracer gas in the gas- eous phase, t is the elapsed time, x is the distance at the flow direction of the gas, D, is the N,-air binary diffu- sion coefficient in free air, L, is the length of the soil core (=5.1 cm) and La is the length of the diffusion chamber (= 10.3 cm). Do (cm2 s-I) at an absolute tem- perature T, Do., , was calculated by the following equa- tion (Satou 1970):
Gas Diffusion Coefficient of Peat Soils 433
where Do.,., = 0.178 (Satou 1970). In the present study, the atmospheric pressure p was assumed to be the same as pwc. The initial and the boundary conditions of the measurement system are
C = C i , O s x ~ L , , t = O
c = c,, L, s x s L, + La, t = 0 c = ci, x = 0, t>O
~- - 0, x = L, + La, t>O ac ax (4)
where Ci is the gas concentration in free air and C, is the gas concentration inside the chamber at r = O . We determined the D, values by fitting finite element solu- tions of the initial-boundary problem in Eqs. (l), (3) and (4) to the measured breakthrough curves.
Comparison of the DB(a)/.. , functions of peat soils with existing models. In order to characterize the changes in the DJD, with the a values for peat soils, we used the Three-Porosity Model (TPM) proposed by Moldrup et al. (2004). The TPM is a predictive model that showed reliable predictions of the D,(a)/D, in undisturbed soils for a wide range of soil types and total porosities (Moldrup et al. 2004).
The TPM is described by the following monomial power law function:
D,/Do = @' (a/@)' for O< a 5 @ (5) where @ is the soil total porosity (m3 m-3) and X is a tortuosity-connectivity parameter. To determine X, the
o.20 -1 decomposed (22.5cm)
TPM adopted the empirical relationship given by (Mold- rup et al. 2000)
D,,,/D, = 2aIw3 + 0 . 0 4 ~ ~ ~ (6) where DFl, is D, at a matric potential of - 100 cm and a, , is the corresponding air-filled porosity. Eq. (6) described the measured data sets well (coefficient of regression R2 = 0.97) for 126