Woodland soils can act as effective sinks for both atmospheric methane, and for methane produced in deeper soil layers. The Intergovernmental Panel on Climate Change (IPCC) estimate that soils represent a methane sink of around 30 million tonnes per year. The methane is predominantly used by bacteria in the soil (methanotrophs) which use the methane as a source of carbon in a process called methane oxidation. Two distinct types of methanotroph can often be found in soil:

The first type are called 'high capacity - low affinity' methanotrophs and are adapted for growth at high methane concentrations (several 1000 parts per million in air), such as those arising from waterlogged soil layers. The second type, often called 'low capacity - high affinity' methanotrophs, are able to make use of the trace amounts of methane in the atmosphere (around 1.8 parts per million in air). Though many of the 'high capacity' type methanotrophs have been identified and cultured in laboratories, the other 'high affinity' methanotrophs remain poorly understood.

As with wetland soils, the key as to whether a soil acts as a sink or source of methane tends to be water. Forest soils tend to be good sinks for methane because the trees help keep the water table well below the surface and allow the methanotrophs to grow. Where the soil become waterlogged, such as sometimes happens in winter, the balance shifts from methanotrophs to anaerobic methane producing bacteria (methanogens) and the soil becomes a methane source. As well as water content of the soil, soil temperature and the concentration of nitrogen can also be crucial factors in determining whether a particular soil will act as a sink for methane.

Human Impact

Changes in human land-use can have a huge impact on the capacity of soils to act as a methane sink. Conversion of woodland to agricultural land tends to result in increased nitrogen concentrations in the soil, which then inhibit methane oxidation. Similarly, the increased deposition of nitrogen from the atmosphere, due to the activities of man, can also reduce or completely inhibit methane oxidation in soil. Changes in soil drainage can also be crucial in determining the size of the soil methane sink.

Potential for control

Our potential for control of the soil methane sink lies primarily in our ability to change land-use practices. The better targeting of fertilizer application and land conversion could help to avoid the destruction of large soil methane sinks unnecessarily. Likewise, reductions in the large amounts of atmospheric nitrogen pollution we produce could also help to maintain levels of methane oxidation in existing soil methane sinks.