Soils are an important part of the complex and carefully balanced chemical cycles that keep the planet functioning in a manner that is comfortable to humans and human development. Soils are stores of key nutrients such as carbon, nitrogen and phosphorus which they pass on to plants and microorganisms to use. Most soils receive nutrients from elsewhere; from dead plant and animal matter, from the atmosphere, from rivers and groundwater, from the bedrock below and from animals moving about.

Some nutrients are used by microorganisms to produce or consume greenhouse gases. Organisms that live in the soil need a range of nutrients to survive and, like humans, they ‘breathe out’ waste products. Microorganisms that ‘breathe’ like us are called heterotrophic microorganisms. They survive in soils with plenty of oxygen which they use to decompose carbon forms in the soil and they release carbon dioxide. These organisms like drier soils, with a nice moisture film to give them the water they need (see my previous post) but enough oxygen to respire. When a soil is very rich in carbon, e.g. peat, allowing these organisms to function by draining the land means that more of that carbon is released into the atmosphere in the form of carbon dioxide and that an important carbon store is lost. Rather than taking in oxygen from the air, some other heterotrophic microorganisms use oxygen that is attached to nitrogen in a common nutrient form called nitrate. These are called denitrifying microorganisms and they can turn a nutrient that is useful for plants into a greenhouse gas which is far less useful in terms of land productivity.

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On the other hand, some microorganisms like to live without oxygen, and they ‘breathe’ in a different way, using chemical respiration (chemotrophs). Some examples of these are bacteria that produce methane (a greenhouse gas 25 times more powerful than carbon dioxide) and some that produce nitrous oxide (300 times more powerful a greenhouse gas than carbon dioxide). These bacteria are called methanogens (methane generators) and nitrifiers (bacteria that ‘breathe’ by turning ammonium ions into nitrous oxide) respectively.

There are far more bacteria types, that have all kinds of intesting metabolisms (iron oxidisers, sulphur oxidisers etc.) but I will continue to focus on the greenhouse gases for now. So, once a bacteria has made some gas – what happens to it? And how does this relate to soil porosity? Well, if a gas is produced in a wet pore – it may well stay there for some time, building up in a bubble until development of a pressure or concentration gradient releases it. Sometimes, if a gas stays in the soil long enough, other bacteria with different metabolisms from those that made the gas might want to use it. For example, some denitrifying bacteria can use nitrous oxide in their metabolism and they put out nitrogen gas instead. Nitrogen gas makes up nearly 80% of the atmosphere and is preferable in terms of global warming potential than nitrous oxide. Other bacteria consume methane, these are called methane oxidisers, so they need oxygen to function. So imagine you have a soil with a groundwater level of 10cm below the surface. In here, methane is being produced. On top, methane is being oxidised. So how much methane actually makes it out of the soil and into the atmosphere to contribute to greenhouse gas accumulation? Well, this depends on the extent of the oxidising contition and in turn on the soil porosity.

In my next article I will talk about how human land use change can affect the porosity of the soil and the nutrients that are available to bacteria. Soil’s contribution to greenhouse gas production is a natural process that can not (and should not) be artificially halted, however, when humans make substantial changes to the way the soil system naturally operates then the soil’s contribution is no longer in balance with other processes. This is where problems can arise.

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