Underestimate of Greenhouse Gas Emissions from Switzerland’s Farmed Transition Soils

Greenhouse gas emissions from drained carbon-rich soils are being underestimated in GHG accounting. To resolve this, their distribution and emissions need to be better assessed. Accordingly, improvements for the GHG reporting system are suggested.

Organic soils are carbon-rich soils that formed under water-logged conditions, in fens and raised bogs. They are known to have high greenhouse gas (GHG) emissions when drained, e.g. for agriculture. The emissions from carbon-rich organic soils are accounted for in Switzerland’s GHG inventory, which is compiled and submitted annually under the framework of the United Nations Framework Convention on Climate Change. There are however other soils, termed here ‘transition soils’, that are not carbon-rich enough to be considered organic soil for the GHG inventory, but also contain a lot of carbon which accumulated under water-logged conditions. They include carbon-poor organic soils, carbon-rich mineral soils, as well as any carbon-rich soils overlaid with a mineral layer, a practice carried out by some farmers e.g. to mitigate the negative effects of heavily subsided soils.

Transition soils lose carbon when drained

It is likely that transition soils also lose carbon when drained. Indeed, there is emerging evidence from other countries suggesting that their per hectare GHG emissions can be substantial, sometimes as high as those from carbon-rich organic soils. As in most other countries, these emissions are not accounted for in Switzerland’s GHG accounting, meaning we underestimate soil-borne emissions. Emissions from transition soils are often overlooked in part because of knowledge gaps, and in part because the reporting system is over-simplified, classifying soils only as either ‘organic’ or ‘mineral’ (i.e., non-organic soils).

GHG inventory may recognise additional soil categories

For GHG reporting, we suggest that as a mid-term solution, the GHG inventory recognises additional soil categories, so that the spectrum of soils –with respect to the carbon content– is better represented. These soil categories would need to have their own sets of emission factors, which could initially be derived from the scientific literature if sufficient data are available. In the long-term, we suggest that GHG accounting of soil-borne carbon-related emissions abandons the use of categories and associated emission factors, and instead makes use of models predicting GHG emissions as a function of carbon content and water level. Both these approaches require closing large knowledge gaps.

Soil mapping and field measurements needed

Firstly, more research is needed to assess where transition soils occur. With the new soil mapping methodology developed by the Swiss Soil Competence Centre (ccsols.ch), such soil properties will be quantified and recorded. The Swiss Federal Council has approved 2023 the concept for a national soil mapping survey, which is scheduled to begin in 2029. Secondly, field measurements are needed to assess how high the emissions of the different transition soils are, and how these are related to the main drivers of emissions, including carbon stocks and the ground water level. Lastly, measurements and / or estimates of peat depth, carbon density and ground water level across the landscape are necessary.