Carbon content of agricultural soils
The organic matter in soil (humus) originates from the decomposition of vegetable and animal waste by soil organisms. The functioning of soil (agricultural and forestry production, regulation of hydrological, nutrient, climate cycles), its structural stability (resistance to erosion) and its biodiversity depend on the quantity and quality of the organic matter it contains. Carbon content, the main component of the organic matter, is a general indicator of soil quality and fertility.
Organic carbon content varies considerably by soil type: it is influenced by natural factors, for example the physical properties of the soil (e.g. the clay content) and climate conditions, and by the use made of the soil. Agricultural and forestry practices have a direct (modification of the organic matter cycle) or indirect (erosion, compaction) on the levels of organic carbon in soil.
The results of the five sampling campaigns (from 1990-1994 to 2000-2014) show that carbon levels remained stable overall, despite increases and decreases in some plots.
There is no national or international guide value for the carbon content of soil, hence this indicator is not assessed.
The decline in organic carbon levels is considered to be one of the eight main threats to soil identified in the European Thematic Strategy for Soil Protection. Compared to other countries, the levels of organic carbon in arable soils surveyed in Switzerland are relatively high.
Every five years since 1985, a composite sample has been taken from the top 20 centimetres of the soil in each of the 103 permanent plots operated by the Swiss Soil Monitoring Network (NABO).
The analyses are carried out on the topsoil, as this is the part of the soil in which organic matter (of which carbon is the main component) accumulates. The organic carbon is analysed using the standard Swiss method (oxidation with potassium dichromate solution and sulphuric acid) or the “carbon and nitrogen elemental analysis” method.
The indicator was calculated for the 29 arable sites (crop rotation areas). Although the carbon content of soil develops slowly, agricultural practices can trigger perceptible variations in the shorter term. The results have been presented in the form of average content per measurement cycle since the second soil monitoring cycle (1990-1994).