Introduction

Methane (CH4) is produced in every environment that contains biodegradable organic matter in anaerobic conditions. Examples are wetland soils and sediments, the digestive tract of many animals and landfills. Microbial CH4 oxidation (methanotrophy) occurs in any ecosystem that contains an oxic zone permeated by CH4 from a source of sufficient strength to support microbial life. A typical example is a rice field, where the aerenchyma of rice plants brings oxygen to the roots, thus forming a thin methano-trophic zone in the rhizosphere. A second example is a landfill cover soil, which absorbs part of the CH4 produced in the landfill waste mass (Fig. 12.1).

This chapter refers to CH4 sinks in human-made environments. The primary focus is on landfill cover soils, which are the clearest examples of such artificial CH4 sinks. Rice fields, which are more natural environments similar to swamps, are covered, but to a lesser extent, as less quantitative information is available on these ecosystems. Due to the strong interaction between CH4 production and CH4 oxidation in rice fields, it is difficult to distinguish the two processes. However, in both these ecosystems the process is mediated by the same microorganisms, so many observa tions compiled here are applicable to rice fields as well.

CH4 production in landfills is a complex process involving different types of microorganisms that are responsible for hydrolysis of complex organic molecules into smaller ones, fermentation into H2, CO2 and formic acid (HCOOH), and CH4 production from either HCOOH or CO2. The first global estimate of CH4 production from landfills was made by Bingemer and Crutzen (1987). They arrived at a global emission of 50 ± 20 Tg CH4/year, with the assumption that 1 kg of degradable carbon in waste leads to 0.5 kg of CH4 production. Recent estimates indicate lower emissions. Bogner and Matthews (2003), for instance, indicated that 20.7 Tg CH4/year was a more realistic emission estimate for 1996. An estimated 3.8 Tg CH4/year is recovered from landfills by combustion or flaring. In their estimate, Bogner and Matthews assumed that 10% of the CH4 was oxidized biologically before entering the atmosphere. This leads to an in situ CH4 production estimate of 26.8 Tg CH4/year, of which 2.3 Tg CH4/ year is oxidized biologically.

However, estimates of oxidation efficiencies of landfill cover soils vary widely. Depending on climate and operation conditions, efficiency estimates range from 10% to 100%. Table 12.1 shows an overview of

CH4 vent CH4 recovered (e.g. fuel or electricity generation)

CO2 and CH4 emission

Gas extraction

Anoxic zone: CH4 generation

CH4 migration

Fig. 12.1. Conceptual methane (CH4) balance in a landfill.

Table 12.1. Efficiency estimates of CH4 oxidation in landfill cover soils.

Source

Efficiency (%)

Landfill status

Whalen et al.

50

All landfills in

(1990)

USA

Jones and

10-40a

8-9 years

Nedwell

closed

(1993)

Oonk and

10-20b

Active

Boom (1995)

Boeckx et al.

40-100

>5 years closed

(1996)

Czepiel et al.

10

Active

(1996a)

Kjeldsen et al.

up to 80

4 years closed

(1997)

Bergamaschi

53

Active

et al. (1998)

Liptay et al.

30c

Not specified

(1998)

Chanton et al.

12

Active

(1999)

Chanton and

20d

5 years closed

Liptay (2000)

Borjesson et al.

0/41-50e

<1 year closed

(2001)

0/60-94e

17 years closed

De Visscher

30

Model result

(2001)

Barlaz et al.

21; 55f

Active

(2004)

Conservative estimate. bEstimate depends on stoichiometry. cIn summer.

dAverage of two cover soils: mulch/topsoil (26%) and clay soil (14%); each are annual mean estimates. eWinter/summer.

fSoil cover and biocover, respectively. Measurements do not include winter.

Conservative estimate. bEstimate depends on stoichiometry. cIn summer.

dAverage of two cover soils: mulch/topsoil (26%) and clay soil (14%); each are annual mean estimates. eWinter/summer.

fSoil cover and biocover, respectively. Measurements do not include winter.

efficiency estimates found in the literature. If an estimate of 30% oxidation is applied to the data of Bogner and Matthews (2003), a global biological oxidation of 6.9 Tg CH4/ year and a residual emission of 16.1 Tg CH4/ year are obtained. Clearly, biological oxidation is an important factor affecting net CH4 emissions from landfills, and could hence be the main artificial CH4 sink.

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