Production, oxidation, emission and consumption of methane by soils: A review
Introduction
Methane is the main hydrocarbon present in the atmosphere, with an average concentration of 1.7 ppm. Variations between the northern and southern hemispheres average 0.14 ppm and exhibit seasonal variations of about 0.03 ppm 〚55〛.
Despite a short residence time in the atmosphere (about 10 years), the CH4 ability to absorb infrared radiation makes it 20 to 30 times more efficient than CO2 as a greenhouse gas 〚17〛, 〚169〛. Methane is chemically very reactive and is therefore involved in changes in the chemical composition of the atmosphere 〚34〛. In particular, it reacts with hydroxyl radicals in the troposphere, reducing its oxidative power and ability to eliminate pollutants such as chloro-fluoro carbons (CFCs), and leading to the production of other greenhouse gases (ozone, CO, CO2). In the stratosphere, such reactions produce water vapour, which is involved in the destruction of the stratospheric ozone layer, the natural barrier against detrimental solar radiations. Methane is considered the second or the third greenhouse gas after CO2 and CFCs 〚121〛, 〚138〛.
Annual CH4 emission, estimated from the analysis of air trapped in polar ice, were 180 Tg·year–1 during the 15th century (1 Tg = 1012 g) and 200 Tg·year–1 at the beginning of the 18th century 〚102〛. The recent estimates of the International Panel for Climate Changes (IPCC) 〚88〛 are around 300 Tg in 2000, and between 400 and 600 Tg in 2010.
Atmospheric CH4 is mainly (70–80 %) of biological origin. It is produced in anoxic environments, including submerged soils, by methanogenic bacteria during the anaerobic digestion of organic matter. Methane is mainly eliminated in the troposphere through oxidation by OH• radicals, according to the reaction: CH4 + OH• → CH3• + H2O. In the stratosphere, CH4 also reacts with chlorine (originating from CFCs) according to the reaction: CH4 + Cl• → HCl + CH3•. Methane is also eliminated in soils by microbial oxidation, which takes place in the aerobic zone of methanogenic soils (methanotrophy) and in upland soils, which oxidise atmospheric methane. Soils most efficient in methanotrophy are generally those from sites that are often submerged or water-saturated and where a significant methanogenic activity develops at intervals 〚146〛. Ricefield soils, peat soils 〚204〛 and soils from landfills 〚238〛 often exhibit very high potential methanotrophic activities but in such environments, where anaerobiosis predominate, the balance between CH4 production and oxidation is usually positive.
An environment is a CH4 source when the balance between production by methanogenic bacteria and consumption by methanotrophic bacteria is positive, leading to CH4 emission. When the balance is negative, the environment is a CH4 sink.
Natural CH4 sources are considered responsible for about 30 % of total emissions. Wetland soils (swamps, bogs, etc.) are the main natural source with an estimated emission of 100–200 Tg·year–1. Other sources are oceans, some forest soils, termites and wild ruminants (table I). About 70 % of CH4 emissions are of human origin. Domesticated ruminants (65–100 Tg·year–1) and ricefields (25–150 Tg·year–1) are responsible for 15–40 % of total emissions, therefore agriculture is the main anthropic source of CH4.
Because of their economical importance and high potential as CH4 source, ricefields have been the most studied methanogenic ecosystems. They are also the most suitable model to study CH4 emission because methanogenesis and methanotrophy are very active and all modes of CH4 transfer occur in ricefields. Assuming an annual emission of 50 Tg CH4 by ricefields, the production of 1 kg rice corresponds to the emission of 100 g CH4.
As the sources of atmospheric CH4 are closely related to human activities, it is theoretically possible to control them. According to Thompson et al. 〚208〛, the global temperature increase could be reduced by 25 % if CH4 emissions could be stabilised.
Temperate and tropical oxic soils that are continuously emerged and exposed to atmospheric concentrations of CH4 are CH4 sinks. They usually exhibit low levels of atmospheric CH4 oxidation but, because of the large areas they cover, they are estimated to consume about 10 % of the atmospheric CH4 (table I). Among upland soils, forest soils are probably the most efficient CH4 sink. Atmospheric CH4 oxidation also occurs in extreme environments such as deserts and glaciers, in the floodwater of submerged soils and in river waters.
According to IPCC estimates 〚87〛, natural and cultivated submerged soils (landfills not included) contribute about 55 % of the CH4 emitted into the atmosphere, corresponding to 175 Tg·year–1, while upland soils are responsible for 6 % of the CH4 consumption, corresponding to 30 Tg·year–1 (table I). Soils are therefore a major actor of the global CH4 cycle. New trends in atmospheric CH4 studies deal with modelling the retroactive effect of global warming and atmospheric CO2 increase on CH4 emissions by terrestrial environments 〚138〛 with a special focus on ricefields 〚157〛, 〚183〛.
The role of soils as source and sink has been discussed before 1997 in general reviews 〚37〛, 〚145〛, 〚211〛, or in reviews dealing with specific environments such as ricefields 〚147〛, 〚230〛, forests and temperate cultivated soils 〚203〛. Since then, in relation with the increasing scientific and political interest in greenhouse gases, numerous papers have been published on this topic. This review summarises current knowledge with emphasis on recent developments.
Section snippets
Methanogenesis
The complete mineralisation of organic matter in anaerobic environments where sulphate and nitrate concentrations are low occurs through methanogenic fermentation, which produces CH4 and CO2 according to the reaction: C6H12O6 → 3 CO2 + 3 CH4.
This transformation requires successive actions of four populations of micro-organisms that degrades complex molecules in simpler compounds:
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hydrolysis of biological polymers into monomers (glucides, fatty acids, amino acids) by an hydrolytic microflora that
Methods for estimating activities
Methods to estimate CH4 production, consumption, and emission by soils should be used with caution, while keeping in mind that they measure complex microbial activities, integrating a larger number of environmental parameters. To be significant, measurements must take into account the spatial and temporal variations as well as the low sensitivity of the methods, especially for CH4 measurement at atmospheric concentration 〚211〛.
Estimation of activities in various environments
As already indicated, flux and activity measurements present a very large variability that may partly refrain interpretation or comparisons. However, using large sets of data allows to draw general conclusions. Estimates presented in this section are from a data base we established from 57 references presenting individual or aggregated data 〚56〛, 〚58〛, 〚73〛, 〚79〛, 〚80〛, 〚84〛, 〚86〛, 〚90〛, 〚93〛, 〚100〛, 〚103〛, 〚105〛, 〚110〛, 〚115〛, 〚116〛, 〚120〛, 〚123〛, 〚126〛, 〚133〛, 〚137〛, 〚150〛, 〚153〛, 〚156〛, 〚161〛
Environmental factors that affect methane emission
Factors that affect CH4 emission by soils are those that affect:
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gas diffusion in relation with the oxydo-reduction level and CH4 transfer, in particular the water content, the nature of the clays and the type of vegetation;
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microbial activities in general: temperature, pH, Eh, substrate availability, physicochemical properties of soils, etc.;
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methanogenesis and in particular the competition with denitrification and sulphate-reduction;
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methane-mono-oxygenase activity: content in H2, CH4, ammonium,
Effects of cultural practices in wetlands
Cultural practices in wetlands (planted mostly with rice but also with other aquatic plants such as jute or waterchestnut) affect CH4 emission through their effects on methanogenesis, methanotrophy and CH4 transfer.
Effects of cultural practices in uplands and forests
Cultural practices in upland soils mostly affect their potential to oxidise atmospheric CH4. Nitrogen fertilisation that lead directly or indirectly to an increase in the NH4 content of the soil has an inhibitory effect on CH4 oxidation, through competition at the level of the methane-mono-oxygenase towards nitrification 〚30〛, 〚146〛 and the toxicity of NO2 produced. Cultural practices that destroy micro-aerophilic niches suitable for CH4 oxidisers also reduces atmospheric CH4 oxidation 〚86〛,
Cultivated methanogenic soils: ricefields
Cultivated wetlands are mostly ricefields. Strategies to reduce CH4 emission by ricefields may be oriented toward (i) reducing CH4 production, (ii) increasing CH4 oxidation, and (iii) reducing CH4 transport through the plant. Potential techniques include water, fertiliser management, cropping pattern, varietal selection, and, possibly, the use of selective inhibitors.
Related web sites
Additional information can be found at the following WEB sites:
The Intergovernmental Panel on Climate Change (IPCC): http://www.ipcc-nggip.iges.or.jp/
Publications IPCC: http://www.ipcc.ch/pub/pub.htm
The World Resource Institute: http://www.wri.org/
National Council for Science and the Environment: http://www.cnie.org/
National Library for the Environment: http://www.cnie.org/nle/
United States Environmental Protection Agency: http://www.epa.gov/globalwarming/
MIT Joint Program on the Science and
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