Biomass burning

Biomass burning accounts for 20-40 Tg CH4/ year. CH4 emissions arising from biomass burning are a result of incomplete combustion and huge amounts can be produced during large-scale burning of woodlands, savannah and agricultural waste. In savannah regions of the world, burning is often carried out every few years to promote regeneration of the vegetation. The importance of CH4

Fig. 9.3. Sinks for methane (CH4).

(methanotrophs), which use the CH4 as a source of carbon in the process of biological CH4 oxidation. Methanotrophs are fairly ubiquitous in soil, but two distinct types have been reported.

The first are commonly described as high-capacity-low-affinity methanotrophs and are adapted for growth at high CH4 concentrations (several thousand parts per million in air). Such high CH4 concentrations arise from waterlogged soils and sediments. The second are known as low-capacity-high-affinity methanotrophs and are able to make use of the trace amounts of CH4 in the atmosphere (~1800 ppb). Although several of the 'low-affinity' methanotrophs have been identified and cultured in laboratories, the 'high-affinity' methanotrophs remain poorly understood.

The key as to whether a soil acts as a sink or source of CH4 tends to be water. Forest soils are often strong sinks for CH4, because transpiration by the trees keeps the soil water content from becoming too high. In such soils the aerobic conditions required by methanotrophs tend to predominate, giving rise to a net CH4 sink. When soils do become waterlogged, such as often happens in winter, the availability of oxygen falls and the balance shifts from methano-trophs to anaerobic CH4-producing bacteria (methanogens), with the soil becoming a CH4 source.

It is not uncommon for both high-affinity and low-affinity methanotrophs to be active in the same area, but at different depths. The high-affinity methanotrophs make use of the atmospheric CH4 diffusing into the soil from the surface, and so tend to be most active in the top few centimetres of the soil. Active populations of the low-affinity methano-trophs, on the other hand, can often be found in deeper soil layers, making use of the high concentrations of CH4 arising from soil meth-anogenesis. Indeed, the bulk of low-affinity methanotroph activity can be confined to a very narrow band in the soil. This band, occurring where the balance between CH4 diffusing up through the soil and oxygen diffusing down through it, is just right. Changes in the water table and rainfall throughout the year mean that such bands of activity can move up and down through the soil fol lowing the conditions most suitable for CH4 oxidation.

In addition to water content of the soil, factors such as pH, soil temperature and concentration of inorganic nitrogen (e.g. nitrate and ammonium) can be crucial in determining whether a particular soil will act as a sink for CH4 or not (Hutsch et al., 1994). For instance, though the soil under alder trees may be well aerated and seemingly ideal for methano-trophs, the high concentrations of nitrate that arise from nitrogen fixation in the alder's roots mean that methanotroph activity is slowed or completely stopped (Reay et al., 2001).

Changes in human land use can have a huge impact on the capacity of soils to act as a CH4 sink. Conversion of wooded and fallow land for agricultural use tends to result in increased nitrogen concentrations in the soil which then inhibit CH4 oxidation (Steudler et al., 1989). Similarly, the increased deposition of nitrogen from the atmosphere due to human activities can also reduce, or completely inhibit, CH4 oxidation in soil.

Rapid, and often very significant, changes in CH4 flux arise from alterations in soil drainage and soil structure caused by land-use change. Deforestation can result in higher water tables and soil water contents, due to the loss of transpiration by the trees. Soil compaction, such as that caused by agricultural machinery, can also result in anaerobic conditions developing in the soil and consequently turn the soil from a CH4 sink to a CH4 source.

Our potential for control of the soil CH4 sink lies primarily in our ability to alter land use practices. The better targeting of fertilizer application, avoidance of soil compaction, and more thoughtful land conversion could all help to avoid the destruction of soil CH4 sinks. Likewise, reducing the amount of atmospheric nitrogen pollution we produce could also help to maintain levels of CH4 oxidation in existing soil CH4 sinks around the world.

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