ABSTRACT
When organic matter is added to soils, it is used as a source of energy and nutrients by microorganisms. The carbon is thereby unlocked from chain-like molecules from which plants are composed. Microorganisms such as fungi and bacteria get energy by breaking down these often long molecules into smaller units such as sugars, which are in turn broken down to provide a source of energy and carbon. While some of the carbon is used by microorganisms as a building block in multiplication and reproduction, another part of the carbon is oxidised by reaction with oxygen in the soil to create the greenhouse gas (GHG) carbon dioxide (CO2). As microorganisms reproduce and die rapidly, CO2 is also produced as a result of microbial decomposition soon after organic matter is added to soil. This process by which carbon locked in organic molecules is converted into the gas CO2 is called ‘mineralisation’. The speed of mineralisation varies greatly depending upon soil temperature – a higher temperature (say, between 15 and 30°C) is more conducive to microbial growth than the lower temperatures in temperate climates (between 20 and 15°C). This is the main reason why soil organic carbon (SOC) levels are generally higher in cooler climates than in the (sub)tropics, though other factors such as water logging (creating very low-oxygen conditions) are very important.
