Carbon content of arable soils

Comment

Organic carbon content varies considerably by soil type. It is influenced by natural factors, for example the physical properties of the soil (e.g. clay content) and climate conditions, and by how humans use the soil. Agricultural practices influence organic carbon levels directly (modification of the organic matter cycle) or indirectly (erosion, compaction). National and international research is being conducted on guide values for typical carbon content in soils, but these values have not yet been determined.

The results of the seven sampling campaigns (every five years from 1985 to 2019) show that carbon levels in arable soils have increased slightly on average. The following points are important for understanding this trend:

• Carbon levels vary greatly among the NABO arable sites that were sampled, and the graph only shows the average in the top 20cm. In addition to sites with rising or stable carbon levels, there are also sites that show a decline.
• The graph shows the average value across all sites that were sampled. Depending on the soil and cultivation methods, the carbon content can range from approximately 1.2 to around 4 per cent by weight.
• Continuous time series for carbon content are only available for around 30 arable sites. A representative picture of Switzerland as a whole cannot be made based on this small data set.

As there are currently no reference values for the carbon content of Swiss soils, it is not possible to definitively characterise the content and development of this indicator. Overall, however, the trend in organic carbon content in the top 20cm of soil appears to be slightly positive, which is why the trend is also cautiously assessed as positive.

More detailed information can be found in the publication 'Results of the National Soil Observation 1985–2019' (available in german and french). 

Method

Since 1985, composite samples have been taken every five years from the top 20cm of soil at all 114 permanent plots of the Swiss Soil Monitoring Network (NABO). The analyses are carried out on the topsoil because that is where organic matter (whose main component is carbon) accumulates.

The organic carbon content (as a percentage of dry matter (DM)) is determined from the difference between the total carbon content (TC) and inorganic carbon (IC). To do this, TC is determined using a carbon/nitrogen (C/N) analyser and IC using the Scheibler method, and the difference is then calculated. During the first four surveys (1985–2004), organic carbon was analysed by wet oxidation with potassium dichromate and subsequent retitration using the FAL method. As this method determines slightly lower concentrations than measurement using a C/N analyser, samples from the first four surveys were remeasured and the correction factors were applied to all sites in the first to fourth surveys.

The indicator is calculated for arable sites (arable soils) with continuous time series. Although the carbon content in soils changes only slowly, agricultural practices can lead to noticeable fluctuations in a shorter period of time, especially in arable farming. The results are given as the average content per survey cycle.

Basis for assessment of the trend