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Soil processes are an integral part of life on Earth. With constant fluxes of carbon between plants, the atmosphere and the ground, soil also plays a vital role in the global climate.
Made up of minerals, residues from plants and animals, water, air and living organisms, the soil beneath our feet is a complex and efficient ecosystem operating at a tiny scale.Carbon cycling
Because soil organic matter contains around 60% carbon, it is the defining factor in soil’s influence on the global carbon cycle.
With around 1 500 billion tonnes of carbon found in the organic matter in soil worldwide, soils are the second largest active store of carbon after the oceans (40 000 billion tonnes). There is more carbon stored in soil than in the atmosphere (760 billion tonnes) and in vegetation (560 billion tonnes) combined.
However, as with other carbon cycles, there are constant transfers of carbon between the soil and the atmosphere and vice versa, through plants. In fact, the emissions of CO2 (carbon dioxide) from soils to the atmosphere are around ten times those from fossil fuels, but under natural conditions this is balanced by a similar flux in the other direction.
Plants absorb CO2 from the atmosphere through the process of photosynthesis and use it to build their roots, stems or leaves. Carbon is mainly transferred into the soil through the release of organic compounds into the soil by plant roots or through the decay of plant material or soil organisms when they die.
Microbial breakdown of the organic matter finally releases the nutrients which plants use to grow. During this process of decomposition, some carbon is released as carbon dioxide through soil respiration, whilst other carbon is converted into stable organic compounds that are locked into the ground. How fast this happens depends on factors including temperature and rainfall, the soil-water balance, and the composition of the organic material.
Different organic matter sources decompose at different rates. If the addition of new material is less than the rate of decomposition, soil organic matter declines and, conversely, if the rate of addition is greater than the rate of decomposition, soil organic matter will increase. However, in general the process leading to losses of carbon from the soil happens more quickly and easily than the process of rebuilding carbon stocks.
The processes that take place underground in the soil are vital in providing many of the raw materials we depend upon – from the food we eat, to the clothes we wear, to the materials we build with. Soil processes are essential for clean water and ecosystem health, without which life as we know it would be impossible.
The fertility of soil and its ability to perform its key functions depend to a large extent on the levels of organic matter within it. Humus, besides giving the soil its brownish or dark colour, is one of the most complex elements of soils. Without this mixture of different organic components, including very stable and old humified substances, as well as more changeable plant and animal residues in the process of decay, soils would not support the wide variety of organisms that live within it.
These creatures – ranging from bacteria to worms and insects – recycle organic matter and make available the nutrients that plants depend on. Organic matter is the key to healthy soil, maintaining soil structure, providing nutrients from leaching, fixing pollutants and improving water infiltration and retention.