Soil and greenhouse gases

Of the four greenhouse gases mentioned above, the soil is both a source and sink of three of them, CO₂, N₂O, CH₃. Growing plants take in CO₂ from the atmosphere and use it in the process of photosynthesis. It is likely that increasing temperature will lead to greater plant growth, therefore more CO₂ being taken from the atmosphere. When the plant parts die they add carbon to the soil where it can be stored. Thus increasing plant growth could lead to more CO₂ leaving the atmosphere and becoming resident in the soil. However even within the soil there is a carbon cycle. Incoming vegetation remains are broken down by the soil microorganisms, nutrients released for further plant growth, but some of the carbon is released back to the atmosphere where it contributes to the CO₂ supply in the atmosphere. Thus the soil can act both as store for carbon and also has the potential to release CO₂ into the atmosphere. We thus need to learn how to manage the soil organic matter resource well.

Methane (CH₄) is a much more potent greenhouse gas than CO₂. A molecule of methane traps heat more than 30 times more effectively than CO₂. Over half of the CH₄ released globally originates in the soil. The main sources of CH₄ to the atmosphere are flooded soils, particularly rice paddy fields and the world's wetlands. The digestive systems of termites that live in the soils of the tropics are also a source of methane to the atmosphere The main opportunities to reduce CH₄ emission would be to drain much of the wetland but this hardly seems feasible on the scale necessary, and to develop rice plants that can be grown without the need for inundated paddy fields.

Over half of the global emission of N₂O is from soils under agriculture or under natural and semi-natural vegetation. Nitrogen, supplied in fertilisers and from organic matter is converted to N₂O, in which form it can be emitted to the atmosphere. Much of the recent increase in the atmospheric concentration of N₂O has been attributed to the global trend of increased use of nitrogen fertiliser. Fertiliser applications can be managed to lower N₂O emissions although on a global scale amounts applied are likely to rise in the quest for higher productivity. Much can be done, however, to reduce loss of N₂O to the atmosphere by more efficient fertiliser use, with better timing of applications, and the use of slow-release fertilisers.