Relevance of soil physical properties for the microbial oxidation of methane in landfill covers

Soil Biology & Biochemistry 43 (2011) 1759e1767

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Relevance of soil physical properties for the microbial oxidation of methane in landfill covers Julia Gebert*, Alexander Groengroeft, Eva-Maria Pfeiffer University of Hamburg, Allende-Platz 2, 20146 Hamburg, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 February 2010 Received in revised form 22 June 2010 Accepted 9 July 2010 Available online 23 July 2010

The microbial oxidation of methane in landfill cover soils offers great potential to reduce methane emissions from landfills. High methane degradation rates can only be accomplished if the supply of atmospheric oxygen to the methanotrophic community is adequate. Thus, if environmental variables such as pH or nutrient status are not limiting, system performance is suggested to be governed by the share of pores available for gas transport. Diffusion tests as well as column studies were conducted to investigate the effect of air-filled porosity and degree of compaction on diffusivity and methane oxidation efficiency. Results show that the effective diffusion coefficient governing oxygen migration through soil is exponentially related to air-filled porosity space and can be significantly decreased by compaction. Discontinuity and tortuosity of the pore system strongly impeded diffusive migration at air-filled porosities below 10%. In the column study, soil gas composition and methane oxidation rates correlated with both the degree of compaction and the magnitude of advective bottom flux. Low aeration and hence low methane oxidation rates prevailed at high compaction rates and/or high bottom fluxes whereas high rates could be maintained at lower fluxes and/or low compaction rates. At a low degree of compaction (75% of the Proctor density), fluxes of 3.5 g CH4 m
2 h
1 could be fully oxidized at all times by a sandy loam, the capacity limit of which was not reached during the experiment. Our studies suggest that soils intended for use as methane-oxidizing biocovers are to maintain an air-filled porosity of at least 14 vol.%. At low and medium degree of compaction, this is provided by sands, loamy sands, sandy loams and some of the coarsely textured loams. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Greenhouse gas emissions Methane oxidation Landfill cover Diffusion Compaction

1. Introduction Methane is a large potential contributor to climate change with a global warming potential (GWP100) of 25 (IPCC, 2007). Making up 28% of the total anthropogenic methane emissions, solid waste disposal on land constitutes the second largest anthropogenic source of methane in Europe, following agriculture (EEA, 2009), with an estimated annual release of 3.7 Gg for the EU-15 or 5.0 Gg for the EU-27. Related to overall anthropogenic greenhouse gas emissions, i.e. including CO2 release from fossil fuels, the waste sector globally represents the fourth largest source with an annual release of 0.5 Tg CO2 equivalent in 2007 (UNFCCC, 2009). Subsequently, understanding the environmental controls of methane cycling has received increased attention.

* Corresponding author. Tel. þ49 40 42838 6595; fax. þ49 40 42838 2024. E-mail addresses: [email protected] (J. Gebert), a.groengroeft@ifb. uni-hamburg.de (A. Groengroeft), [email protected] (E.-M. Pfeiffer). 0038-0717/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2010.07.004

The potential of microbial methane oxidation to mitigate landfill methane emissions at the interface between the landfill body and the atmosphere has been confirmed in a number of laboratory and field scale studies (e.g. Streese and Stegmann, 2003; Barlaz et al., 2004; Scheutz et al., 2004; Gebert and Gröngröft, 2006; Haubrichs and Widmann, 2006; Powelson et al., 2006; Zeiss, 2006). Landfill cover soils as well as methane-oxidizing biofilters have been shown to harbour high counts of methanotrophic bacteria (e.g. Gebert et al., 2003; Ait-Benichou et al., 2009) and their methane oxidation potential is known to reach exceptionally high values (e.g. Börjesson et al., 1998; De Visscher et al., 2001; Scheutz and Kjeldsen, 2004). The methanotrophic community in landfillrelated environments has been shown to be dominated by type I (e.g. Stralis-Pavese et al., 2004; Jugnia et al., 2009) as well as by type II organisms (e.g. Héry et al., 2008; Gebert et al., 2009). Recently, the IPCC Working Group III assessment report (Bogner et al., 2007) has listed biocovers (i.e. soil covers optimized for the microbial oxidation of methane) and biofilters as key mitigation technologies and practices to mitigate landfill greenhouse gas emissions. A review of the available technologies is given by Huber-

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Humer et al. (2008). The methane oxidation efficiency of a soil is, amongst other factors, regulated by the combination of physical and chemical cover material properties, landfill gas source strength and climate. While several studies have indicated an influence of environmental variables on the composition of the methanotrophic community (Graham et al., 1993; Amaral and Knowles, 1995; Wise et al., 1999; Henckel et al., 2000; Stralis-Pavese et al., 2004; Gebert et al., 2008; Jugnia et al., 2009), the evidence on the link between community structure and methane removal rates is less clear. Gebert et al. (2009) showed that the methane oxidation potential of close to 60 samples retrieved from the soil covers of five old landfills was best explained by the soil total nitrogen content and variables that enhance the accumulation of organic matter and hence of nitrogen such as field capacity. However, no link between community structure, assessed by cultivation-independent diagnostic microarray and terminal restriction length polymorphism (TRLP), and methane oxidation potential could be established, indicating that the drivers for the differentiation of the community composition and for the actual methane turnover rates (activity) are not the same. Texture and compaction determine the pore size distribution effective for both water retention and gas transport, thus determining the rate at which methane and atmospheric oxygen become available to the methane-oxidizing micro-organisms. Soil compaction not only decreases total porosity but may also change the pore size distribution by mainly affecting wide coarse pores (>50 mm in diameter) which predominantly control gas transport. Secondary macropores formed by the processes of soil aggregation, rootage or animal burrows can greatly increase the diffusivity and permeability inherent in the primary soil properties, providing pathways for preferential gas (Tuli et al., 2005; Allaire et al., 2008) and water (Gebhardt et al., 2009) flow. The saturation with water strongly affects diffusivity (e.g. Cabral et al., 2004). Water increases the tortuosity of the pore system by discontinuing diffusion pathways through creation of water menisci (Moldrup et al., 2001). Also, gaseous diffusion in water is four orders of magnitude lower than in air. The effective mitigation of emissions thus requires the selection of materials that retain sufficient water free pore space available for the diffusive migration of oxygen into the landfill cover soil under the prevailing climatic conditions and the level of compaction resulting from the common practice of soil cover construction. In order to derive design criteria for soil physical properties that enable the construction of an effective methane-oxidizing biocover, the following experiments were carried out: Firstly, gas diffusivity was determined for a series of undisturbed soil cores retrieved from the existing cover of five old landfills in Germany. Furthermore, the effect of compaction on diffusivity was evaluated. Lastly, a column experiment simulating a landfill cover soil was carried out to assess the influence of compaction and magnitude of landfill gas flux on the distribution of atmospheric components in the soil as well as on the system’s methane oxidation efficiency. 2. Material and methods 2.1. Diffusion tests On the five investigated landfills excavations of the respective cover soil were carried out at three locations per site. In addition to disturbed samples for the analysis of soil chemical and biological properties (for data see Gebert et al., 2009), five undisturbed soil cores (100 cm3) were retrieved from each of the three to five identifiable individual soil layers, yielding a total of approximately 300 cores. Of these, 120 were selected that represented the range of observed values for bulk density and air capacity. The soil cores were adjusted to field capacity water content by first saturating

them on a sand bath followed by de-watering in a pressure-plate apparatus using a pressure head of 6 kPa (DIN ISO 11274, 2001). The effect of compaction on gas diffusivity was investigated by preparing artificial cores using disturbed cover soil (sandy loam) from a Dutch landfill (operated by NV Afvalzorg, for properties see section 2.2). The soil was packed into triplicate 100 cm3 steel soil cores at 75%, 85% and 95% of the Proctor (i.e. maximum attainable) density that had been determined according to DIN 18127 (see Table 1). The air-filled pore volume of undisturbed and re-packed samples was determined by pycnometry. Subsequently, the soil cores were placed in a diffusion chamber similar to that described by Rolston (1986) that was purged with N2 (Fig. 1a). Diffusive reentry of atmospheric O2 into the chamber via the soil core (Fig. 1b) was monitored by GC-TCD analysis (Agilent JAS2). The effective diffusion coefficient was calculated by relating the change in O2 concentration in the chamber, i.e. the diffusive influx of O2, to the gradient of O2 concentration between the chamber and the atmosphere. Fick’s first law was transformed to calculate the effective diffusion coefficient (Deff) as follows:

Deff ¼

JO2  dx dc

(1)

where Deff ¼ effective diffusion coefficient of the soil [m2 s
1]; JO2 ¼ diffusive O2 flux [mol m
2 s
1]; dx ¼ distance over which diffusion occurs, i.e. height of cylinder [m]; dc ¼ concentration gradient [mol m
3]. The final value of Deff for each soil core was calculated as an average of five to six individual measurements.

2.2. Column study 2.2.1. Soil properties The soil used in the column study was collected from the cover of a closed landfill in the Netherlands. As the landfill had no top liner, landfill gas could migrate freely from the waste body through the soil so that prior to the experiment the soil had already developed a methanotrophic potential. Table 1 lists selected soil chemical and physical properties. According to the texture analysis the bulk soil (fraction < 2 mm) was classified as sandy loam (FAO, 2006). 2.2.2. Experimental setup Three columns were constructed from PVC pipes (DIN EN 14011) with a length of 1070 mm, an inner diameter of 190 mm and a wall thickness of 4.9 mm. The columns were equipped with an

Table 1 Selected properties of the soil used in the column study. Parameter

Method

Unit

Value

Loss on ignition Total organic carbon Total inorganic carbon pH Electric conductivity Soluble cations P K Mg Nmin Clay Silt Sand Proctor density Optimal water content

DIN DIN DIN DIN DIN

mass-% mass-% mass-% e mS/m

3.8 1.4 7.9 6.9 42.2

mg/100 g mg/100 g mg/100 g mg/100 g mass-% mass-% mass-% g/cm3 mass-%

2.4 5.5 9.3 0.19 14.0 20.3 64.8 1.67 18.0

18128 ISO 10694 18129 ISO 10390 ISO 11265

VDLUFA VDLUFA VDLUFA VDLUFA DIN 18123 DIN 18123 DIN 18123 DIN 18127 DIN 18127

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Fig. 1. Setup of diffusion test, not to scale. (a) ¼ step 1, purging the chamber with N2; (b) ¼ step 2, monitoring of the diffusive re-entry of O2 by means of gas chromatography.

inlet for synthetic landfill gas at the bottom and an inlet for air and a clean gas outlet at the top. Vertically, gas sampling points were mounted at intervals of 10 cm, consisting of a needle (outer diameter ¼ 0.9 mm) penetrating a tightly sealed butyl-rubber stopper and intersecting 10 cm of the soil. The needles were closed with a disposable 1 ml syringe that was also used for sampling the soil gas. Each column was packed with a gas distribution layer of coarse gravel of 17 cm in thickness. This was topped by 80 cm of soil, the water content of which had been adjusted to the average water content at 30 kPa suction for the given texture in accordance with Table 75 of the German Soil Survey Guidelines (Bodenkundliche Kartieranleitung by Ad-Hoc-Arbeitsgruppe Boden, 2005). Construction and compaction of the soil were performed at intervals of 10 cm, interface effects between layers were minimized by scraping off the top cm of each layer before placement of the subsequent layer. Three levels of compaction were tested: 75%, 85% and 95% of the previously determined Proctor density. Soil physical properties are shown in Table 2, the experimental setup is illustrated in Fig. 2. At the bottom, the columns were continuously charged with moisturized gas composed of 40 vol.% CO2 and 60 vol.% CH4, reflecting the typical composition of landfill gas (Scheutz et al., 2009). At the top, moisturized air was pumped through the column headspace at an excess rate compared to the inlet flux. Inlet and outlet flow rates were controlled with needle valves and recorded using rotameters operating in the range of 0e19 ml/min (inlet) and 0e150 ml/min (outlet), purchased from ANALYT-MTC

Messtechnik GmbH. Laboratory temperature ranged around 19e20  C throughout the experiment. At a minimum of two weeks duration each, inlet flow rates were applied in three different test ranges: 1.47 g CH4 m
2 h
1, 2.4 g CH4 m
2 h
1 and 3.53 g CH4 m
2 h
1. 2.2.3. Gas profiles The vertical distribution of CH4, CO2, O2 and N2 at nine levels of depth was determined weekly using a gas chromatograph (Agilent JAS2) equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD). Separation of CH4, CO2, O2 and N2 was achieved by two Inventory #AB002 capillary columns (30.0 m  530 mm  3.00 mm). Oven initial, FID and TCD temperatures were 40  C, 300  C and 250  C, respectively. Injection volume was 500 ml. 1000 ml of sample was collected from the respective column depth and immediately analyzed. 2.2.4. Calculation of CH4 oxidation efficiency Both inlet and outlet fluxes were recorded five times per week and combined with the CH4 and CO2 concentrations in the headspace (analyzed with the GC-FID/TCD mentioned above) to

Table 2 Dimensions of columns; water content, solids and pore volumes, air capacity and water content of column materials (not considering the gas distribution layer). Inner diameter [cm]

19

Base area [m2]

0.02835

Compaction [% Proctor] Height (incl. gravel) [cm] Height (soil) [cm] Weight (soil) [kg ww] Bulk density [g dw cm
3] Water content [kg] Solids volume [l] Total pore volume [l] Gas volume [l] Total pore volume [vol. %] Water content [vol. %] Gas volume [vol. %]

95 97 80 41.34 1.59 5.75 13.43 9.0 3.28 40.13 25.65 14.48

ww ¼ wet weight, dw ¼ dry weight.

85 97 80 36.99 1.42 5.15 12.02 10.42 5.33 46.43 22.95 23.48

75 97 80 32.64 1.25 4.54 10.60 11.83 7.37 52.74 20.25 32.49 Fig. 2. Schematic of column setup, not to scale.

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calculate CH4 and CO2 inlet and outlet fluxes. CH4 oxidation efficiency was calculated according to Eq. (2):

CH4 ox i ¼

ðfluxin
fluxout i Þ  100 fluxin

(2)

where CH4ox_i ¼% of CH4 inlet flux oxidized at time i (efficiency); fluxin ¼ CH4 flux into the column [ml/min] at time i; fluxout_i ¼ CH4 flux out of the column [ml/min] at time i. Due to the time required for the transport of gas through the soil, the outlet flux at time i cannot be related directly to the inlet flux at time i. For this reason a floating average of bottom flux values over a period of three days was used for the term fluxin in the above equation. 3. Results 3.1. Soil gas diffusivity The samples investigated show a wide range of air-filled pore volume (pores > 50 mm in diameter) and water content at field capacity (5e40 vol.%), indicating significant differences in the soil physical properties. The values for bulk density varied between 0.7 and 1.9 g/cm3 and were linearly and positively correlated to values for total porosity (data shown in the appendix of Gebert et al., 2009). As seen in Fig. 3, the effective diffusion coefficient increases with increasing air-filled porosity, with a factor of 30 between 5 and 40 vol.% (fraction 0.05e0.4) porosity. The relationship was non-linear and best described by the exponential growth function

Deff ¼ 1:319  10
7  eðFa =0:116Þ
1:477  10
7

(3)

E ¼

Deff D0  Fa

(4)

where Deff ¼ effective diffusion coefficient of the soil [m2/s]; D0 ¼ diffusion coefficient of oxygen in air [m2/s]; Fa ¼ air-filled porosity [v/v]. Values for E range between 0 and 1. Both tortuosity and discontinuity of the available pore space reduce the diffusion coefficient effective in soils compared to the diffusion coefficient of oxygen diffusion in air (D0, 2.08  10
5 m2/s). In order to assess the influence of soil properties on the variability of the soil gas diffusivity, the measured values for Deff were subtracted from the calculated values by the ideal exponential function shown in Fig. 3. These residuals were then subjected to correlation analyses with the following soil properties: contents of sand, silt and clay, bulk density, organic carbon, and the effectivity factor. No significant correlation could be found for the relationship between the residuals and either soil physical property or organic carbon. However, there was a strong negative relationship between residuals and the effectivity factor E (Fig. 4). It is seen that improved effectivity of the pore system, i.e. higher connectivity and/or lower tortuosity of pores in relation to the available pore volume, increases the measured value of Deff in relation to the calculated value. Gas diffusivity is strongly impacted by the level of soil compaction, as shown in Fig. 5 for a sandy loam consisting of 14% clay, 20% silt, and 65% sand: increased compaction of the material results in a strong decrease of gas diffusivity, which varied by a factor of six for the range investigated. The impoverishment in gas diffusivity with increasing levels of compaction was related to a decreasing share of coarse pores available for gas transport, and to an increasing share of fine pores which under field conditions in middle Europe are usually permanently saturated with water.

2

where Deff ¼ effective diffusion coefficient of the soil [m /s]; Fa ¼ air-filled pore space [v/v]. The data further show that there was a significant variability of the diffusion coefficient at the same air-filled porosity. Also, diffusivity seems to be strongly limited below approximately 10 vol.% of air-filled porosity. Perdok et al. (2002) introduced the effectivity factor (E) of the soil pore system to describe the combined soil-specific effect of pore tortuosity and pore space discontinuity on the gas diffusivity. It is calculated as follows (Eq. (4)):

Fig. 3. Relationship between air-filled porosity and soil gas diffusivity in undisturbed samples from landfill cover soils. Deff ¼ effective diffusion coefficient. D0 ¼ diffusion coefficient for oxygen in air. Each symbol represents the average of five individual measurements on one sample. Line ¼ exponential fit. n ¼ 120.

3.2. Column studies on methane oxidation Our experiments show that the methane oxidation efficiency varies with both the extent of inlet flux and the degree of compaction (Fig. 6). At the highest degree of compaction (95% of the Proctor density), the CH4 oxidation efficiency declined strongly with increasing inlet flux. It can be seen that an average of 74%

Fig. 4. Magnitude of residuals related to the effectivity of the pore system. Deff_diffusion coefficient for O2 as calculated from the exponential function shown in Fig. 3; Deff ¼ measured effective diffusion coefficient. n ¼ 120.

calc ¼ effective

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The concentration of N2 was used as an indicator for assessing the extent of aeration. In contrast to O2, N2 is neither produced nor consumed in the methane oxidation process and can thus be regarded as an inert tracer for the presence of atmospheric components. Gas profiles depicted in Fig. 7 for the lowest average inlet flux show that the extent of N2 influx and hence also the influx of O2 was highest in the column with the lowest degree of compaction (corresponding to the highest air-filled porosity, compare Table 2) and vice versa. An upward increase in the ratio of CO2 to CH4 can be observed only in the highest compacted column (see also Table 4). At a medium level of compaction and even more so at the lowest level, the balance was clearly dominated by CO2 throughout the entire

Fig. 5. Relationship between the degree of compaction and soil gas diffusivity in repacked soil samples. Deff ¼ effective diffusion coefficient. Line ¼ linear fit. Each symbol represents the average of five individual measurements conducted on each sample.

efficiency could be maintained at an average inlet flux of 1.47 g m
2 h
1. However, the system’s performance strongly decreased to an average of 32% after the inlet flux was increased to 2.4 g m
2 h
1 and further to only 20% efficiency when the inlet CH4 flux was further increased to 3.53 g m
2 h
1. At 85% of the Proctor density, the increase of flux from 1.47 to 2.4 g m
2 h
1 did not reduce the oxidation efficiency that fluctuated around 94%. Performance only decreased to an average 62% after increasing the bottom flux to the highest level of 3.53 g CH4 m
2 h
1. The least compacted sample (75% of the Proctor density) maintained very high methane oxidation efficiencies of close to 100% over the entire flux range. In cases of low air-filled porosity such as in the sample compacted to 95% of the Proctor density and/or high advective bottom flux, the methane oxidation process is restricted to the upper few centimetres or even millimetres of the soil. This can also be derived from the low ratios of CO2 to CH4 at the shallowest depth level (5 cm) of the more intensely compacted columns, indicating very little methane oxidation up to this depth (Table 4). The effect is noted to occur particularly at the higher flux levels.

Fig. 6. CH4 oxidation efficiency for all columns at three flux ranges. Error bars ¼ standard deviation. n ¼ 10 for each flux range.

Fig. 7. Profiles of CH4, CO2, O2, and N2 at low (A), medium (B) and high (C) degrees of compaction (DPR) and an inlet flux of 1.47 g CH4 m
2 h
1. Symbols ¼ averages and error bars ¼ mean range of values for three measurements over a period of three weeks. The ratio of CO2 to CH4 is given in Table 4.

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depth, indicating microbial methane consumption down to the base of the column. At higher bottom fluxes (gas profiles not shown), diffusive aeration ceased altogether at the compaction to 95% of the Proctor density. This is also reflected by the ratio of CO2 to CH4 which remained close to that of the feed gas (60% CH4, 40% CO2, ratio ¼ 0.67) throughout the entire profile (see the two rightmost columns in Table 4). In the less compacted columns aeration was maintained at higher bottom fluxes. Nevertheless, soil gas composition and especially the ratio of CO2 to CH4 showed that also in these columns the extent of air ingress decreased with increasing bottom flux. 4. Discussion 4.1. Relationship between soil physical properties and soil diffusivity Our data show that diffusivity is strongly correlated with the airfilled porosity (Fig. 3) and hence, at field capacity, with the share of the wide coarse pores (>50 mm in diameter), which for a given soil is determined by its texture and degree of compaction. It is obvious that materials providing a larger air-filled porosity warrant higher rates of diffusive oxygen supply to a methane-oxidizing bacterial community in landfill covers. The extent of air-filled porosity that can be maintained even at higher levels of saturation generally increases with increasing coarseness, i.e. the sand fraction in the soil. Both the observed magnitude and the exponential nature of the relationship between porosity and diffusivity confirm reports by Richter and Grossgebauer (1978), Richter et al. (1991), Moldrup et al. (2000), and Allaire et al. (2008). The non-linearity implies a less than proportional diffusivity at lower porosities and has been assigned to the soil-specific effects of tortuosity and discontinuity of the pore system, both impoverishing the rate of gas transport. Moldrup et al. (2000) show the relationship between effective diffusivity and the air-filled porosity, determined after de-watering the soil using a pressure head of 10 kPa, equalling a pore diameter of >30 mm. Our data set was determined for samples at field capacity corresponding to a pressure head of 6 kPa, corresponding to pore diameters of >50 mm. Both datasets therefore refer to the fraction of coarse pores and are very well comparable. The data presented by Moldrup et al. (2000) best followed by the function

Deff ¼ 2  F3a þ 0:04Fa D0

(5)

where Deff ¼ effective diffusion coefficient of the soil [m2/s]; D0 ¼ diffusion coefficient of oxygen in air [m2/s]; Fa ¼ air-filled pore space [v/v]. The fit of our data set to this function is very similar to that of the exponential function given in Eq. (3). However, the average magnitude of residuals (subtracting measured values from calculated values) is slightly lower for the exponential function given in Eq. (3) at a factor of 0.9. As air-filled porosity increases, the continuity of the pore system increases and the tortuosity decreases (Moldrup et al., 2001). Perdok et al. (2002) demonstrated that in three differently textured soils the effectivity factor E, describing the combined effect of tortuosity and discontinuity, was near zero for air-filled porosities between 5 and 10 vol.% and increased linearly towards 25 vol.% airfilled porosity. For the investigated data set diffusivity was indeed very low at air-filled porosities below 10 vol.%, indicating a strong limitation of diffusion by the discontinuity and/or tortuosity of the pore system at low levels of air-filled porosity. When relating the differences between measured and calculated values of Deff to the effectivity factor E, a negative correlation was found (Fig. 4). This

implies that the variability of Deff at a given air-filled porosity is mainly caused by differences in the degree of connectivity and tortuosity of these pores which is to be expected for field samples retrieved from different locations and consisting of different materials. In contrast, variability of Deff was far less in the packed samples or which a homogenised soil material was used that did not contain stones and was not influenced by processes of aggregation or rootage (Fig. 5). Compressive stress may significantly alter the pore size distribution of the soil and hence can strongly impact its water retention and gas permeability characteristics. The extent of these effects depends on the susceptibility towards compaction and is more pronounced in fine-textured materials than in soils dominated by the sand fraction. This, for example, has been demonstrated by Gebhardt et al. (2009) who submitted differently textured soils to various degrees of compressive stress and thereafter measured pore size distribution, air and water permeability. The authors showed that the volumetric distribution of the solid, liquid and gaseous phase and, consequently, of the air and water permeability, remained mostly unaltered only in sandy, i.e. only slightly compressible soils, while the share of the gaseous phase was dramatically reduced when compressing more finely textured soils. In our case, the pore size distribution of the investigated sandy loam showed a clear response to the degree of compaction (Table 3), with a significant decrease in the share of wide coarse pores and an increase in the share of fine pores as a result of increased compaction and thus, increased bulk density. Correspondingly, a strong impact of the level of compaction on the effective diffusion coefficient was observed (Fig. 5). Soil-conserving construction practice adapted to the susceptibility towards compaction thus is of high importance for maintaining maximum levels of air permeability to support microbial transformations dependent on substrate supply in the gaseous phase. 4.2. Relationship between soil diffusivity, soil gas profiles and methane oxidation The relevance of the extent of air-filled porosity and therefore, of the gas diffusivity, for the process of microbial methane oxidation, was well illustrated by the composition of the soil gas phase and the corresponding methane oxidation rates in a column experiment simulating a landfill cover setup. The capacity limit of a loamy sand compacted to a bulk density of only 1.25 g/cm3 (75% of the Proctor density), resulting in an effective diffusion coefficient of 2.29  10
6 m2/s (Fig. 5), was not reached during the experiment; CH4 loads of close to 4 g CH4 m
2 h
1 were completely oxidized. This was in line with the observed high extent of aeration of the soil (Fig. 7), warranted by its high gas diffusivity. Oxidation rates declined with increased bulk density and decreased diffusivity, but also with increased bottom flux. At the interface of anaerobic to aerobic environments, such as in a landfill cover soil, methane and oxygen fluxes will usually be oriented in opposing directions. This means that diffusion has to warrant adequate ingress of oxygen against an advective bottom flux and that therefore especially at higher magnitudes of advective flux the soil’s diffusivity is of Table 3 Pore size distribution in relation to degree of compaction for a loamy sand. Pore size classes with equivalent pore diameters

75% Proctor, 1.25 g/cm3

85% Proctor, 1.42 g/cm3

95% Proctor, 1.59 g/cm3

Fine pores < 2 mm Medium pores 2e10 mm Narrow coarse pores 10e50 mm Wide coarse pores > 50 mm

17.0 8.0 10.3 19.1

19.3 8.3 11.9 8.1

21.5 8.7 8.7 2.3

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Table 4 Average of the ratio CO2 to CH4 at the different depths in the gas profiles at three different inlet fluxes. The values in the first column (inlet flux of 1.47 g CH4 m
2 h
1) correspond to the gas profiles shown in Fig. 7. Depth [cm]

5 15 25 35 45 55 65 75 90

Column at 75% Proctor

Column at 85% Proctor

Column at 95% Proctor

1.47 g m
2 h
1

2.4 g m
2 h
1

3.53 g m
2 h
1

1.47 g m
2 h
1

2.4 g m
2 h
1

3.53 g m
2 h
1

1.47 g m
2 h
1

2.4 g m
2 h
1

3.53 g m
2 h
1

728 359 83 20 6.30 4.68 3.27 2.51 2.18

13205 3178 31 6.37 3.43 2.76 2.17 1.80 1.75

0.67 0.65 0.65 0.67 0.67 0.69 0.66 0.70 0.65

14 6.93 1.77 1.45 1.27 1.12 1.03 0.95 0.93

35 2.35 1.24 1.06 0.97 0.93 0.84 0.79 0.78

0.85 0.76 0.70 0.69 0.68 0.67 0.68 0.67 0.68

2.85 1.48 0.86 0.68 0.66 0.66 0.65 0.64 0.63

0.67 0.66 0.66 0.64 0.65 0.65 0.65 0.64 0.65

0.67 0.65 0.65 0.67 0.67 0.69 0.66 0.70 0.65

increased importance. Gas profile data indicate that the performance of all three columns was not governed by the magnitude of the methane oxidation potential of the methanotrophic community as such but by the differing extent of diffusive oxygen ingress as a result of the variations in pore space available for gaseous transport which in turn depended on the degree of compaction (Fig. 7). This is in line with the fact that both the air-filled porosity and the diffusivity were highest in the least compacted material and lowest at the highest compaction level (Tables 2 and 3, Fig. 5). Correspondingly, concentrations of methane in the soil gas phase increase with increasing compaction and the vertical position of strong increase in the ratio of CO2 to CH4, indicative of the methane oxidation process, moves further upward. The change in the ratio of CO2 to CH4 can only be used as an indicator for the methane oxidation process if there is no other significant source for CO2, such as soil respiration. The respiration rate of the investigated sandy loam was determined in a batch experiment with loose, well aerated, uncompacted soil (data not shown). CO2 production from respiration amounted to only 6% of the CO2 production from methane oxidation found in the column with the highest extent of aeration (i.e. best comparable to a well aerated batch experiment). We therefore concluded the use of the CO2 CH4 ratio to be a valid measure for the methane oxidation activity in the given setup. Overall, the observed oxidation rates compare well to the range found by other authors who investigated the methane oxidation capacity of comparably textured landfill cover soils in similar column experiments (2.3e9.6 g CH4 m
2 h
1; De Visscher et al., 1999; Hilger et al., 2000; Scheutz and Kjeldsen, 2003). However, neither of these studies addressed the role of soil compaction or airfilled porosity. The highest rate found by De Visscher et al. (1999) for a sandy loam was achieved in a column packed to a bulk density of only 1.03 g/cm3. The low density presumably allowed for a very high degree of aeration and hence, a high methane oxidation rate, but is unrealistic to be achieved under conditions of real construction practice. In field experiments, significantly higher oxidation rates have been reported: up to 80 g m
2 h
1 were achieved by a highly porous biofilter material (Gebert and Gröngröft, 2006). Powelson et al. (2006) found oxidation rates up to 22 g m
2 h
1 for a compost-based biofilter without reaching the capacity limit and Cabral et al. (2010) estimated removal rates of up to 60 g m
2 h
1 from stable isotope probing in a biofilter consisting of a sandecompost mixture. 4.3. Application of diffusivity data for the design of methane-oxidizing landfill covers The diffusivity required for the oxidation of a particular methane load can be estimated using the relationship between airfilled porosity and diffusivity (Fig. 3). Stegmann et al. (2007)

envisage a methane flux of 0.33 g CH4 m
2 h
1 as the acceptable upper limit load for the release of a landfill from aftercare based on the assumption that this load can be fully oxidized during all seasons in a cover soil. This equals 0.022 mol CH4 m
2 h
1, corresponding to a total landfill gas flux of 0.037 mol m
2 h
1 at an assumed CH4 concentration of 60 vol.%. The stoichiometry of methane oxidation requires a flux of 0.044 mol O2 m
2 h
1 to fully oxidize this load. According to Fick’s first law, this magnitude of diffusive oxygen flux is provided by an effective diffusion coefficient of 2.64  10
7 m2 s
1, assuming an average oxidation horizon of 20 cm thickness. According to the exponential fit shown in Fig. 3, this would be warranted by a material with an air-filled porosity of 14.4 vol.%. Soils meeting this criterion under conditions of field capacity and a degree of compaction (up to 1.4 g cm
3) are restricted to the group of sands, inclusive of loamy sands, sandy loams and some of the coarsely textured loams (classification according to FAO, 2006). This corresponds to a combined maximum of 50% silt and 25% clay that must not be exceeded. At higher degrees of compaction, e.g. up to 1.6 g cm
3, the proportion of silt and clay needs to be significantly lower, 40% and 17%, respectively. However, the above simple approach to determine the air-filled porosity required to treat a given load of methane needs to be refined for future application, based on assumptions concerning the desired depth, thickness and activity-differentiation of the oxidation horizon and under the consideration of possible advective influx of oxygen resulting from the underpressure induced by the methane oxidation process. 5. Conclusions Soil diffusivity is linked to both the air-filled porosity and the tortuosity and connectivity of the pore system. Below a threshold of w10 vol.% of air-filled porosity, diffusivity was found to be very low as the effectiveness of the pore system for gas transport is low due to impoverished pore continuity. Compaction mostly affected the fraction of wide coarse pores (>50 mm in diameter, also referred to as the soil’s air capacity) which always remain drained and are thus available for gas transport even at elevated moisture levels. Subsequently, diffusivity decreased with increasing levels of compaction. Correspondingly, both the rate and the depth of the diffusive ingress of atmospheric oxygen strongly decreased with increasing degrees of compaction and in addition, with increasing bottom flux. This was inversely linked to the methane oxidation efficiency. We conclude that provided the soil chemical properties such as pH, nutrient status, and salinity, which basically depend on the mineralogy of the parent material, do not restrict methanotrophic activity, the methane oxidation capacity of a landfill cover soil is strongly linked to its physical properties. Conditions that limit oxygen supply, adverse to methanotrophy, are more likely to arise

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from finely textured materials with an inherent low share of coarse pores and/or compacted soils. The experimentally obtained relationship between diffusivity and porosity can be used as design criterion for the selection of materials suitable for landfill cover soil engineering, provided that the magnitude of methane fluxes to be treated are known. Construction practice should be adapted to avoid the unnecessary compaction of soils intended for use as methane-oxidizing covers.

Acknowledgements The column study was conducted in cooperation with Heijo Scharff from NV Afvalzorg, The Netherlands. Analyses of the soil properties as presented in Table 1 were carried out by melchior þ wittpohl Ingenieurgesellschaft. Further, the authors wish to thank Volker Kleinschmidt for his invaluable technical support.

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