To reach net zero greenhouse gas emissions by 2050, Switzerland will depend on domestic negative emissions of about 7 million tons of CO2-equivalents per year (Mt CO2-eq yr-1). Soil carbon sequestration is one of the cheapest and technically least demanding technologies. It is defined as a net uptake of atmospheric carbon dioxide (CO2) that leads to an increase in soil organic carbon storage on the same unit, where CO2 was taken up by photosynthesis. Compared to other technologies it has the great advantage that it rarely competes with food production and is often associated with environmental benefits. Furthermore, higher soil organic carbon stocks increase soil fertility and improve the resilience of the soil system to climate change. The main disadvantage however, is that soil carbon sequestration does not lead to a permanent storage of carbon and most measures are only effective for a few decades. This report addresses questions 1 and 2 of the postulate Nr. 19.3639 ‘Kohlenstoffsequestrierung in Böden’ by national council Jacques Bourgeois. In part 1, which addresses question 1, we assess the potential to store additional carbon in Swiss soils and show what would be necessary to improve our understanding of the actual soil carbon sequestration potentials. In part 2, which addresses question 2, we discuss advantages and disadvantages of specific measures to enhance soil organic carbon stocks. Because there are great differences between different land use types or soil categories, we discuss the measures separately for organic and mineral soils and distinguish unmanaged, agricultural, forest and settlement soils. In Switzerland soil carbon sequestration potentials are largest on agricultural mineral soils. As a result of historic land use conversions (mainly deforestation and drainage) and an intensification of agricultural use, these soils have lost significant amounts of carbon and current soil organic carbon stocks are rather low, especially on cropland. Part of the lost carbon could be regained by measures that increase soil organic carbon stocks. Permanent grassland and forest soils have higher soil organic carbon stocks and the potential for sequestration is therefore small. However, these high stocks might be at risk under climate change and efforts should focus on maintaining soil organic carbon stocks. Due the small area, settlement soils offer a limited potential for carbon sequestration. Organic soils store significant amounts of carbon but drainage-induced loss rates are high. Efforts should focus on reducing these emissions before their potential to store additional carbon can be considered. Generally, the potential for additional carbon storage is site specific and depends on current soil organic carbon stocks and management. National-scale estimates of soil carbon sequestration potentials are still highly uncertain. To improve estimates, we rely on soil organic carbon maps and spatially explicit management information. On agricultural mineral soils the measure with the highest potential is conservation agriculture (0.52–1.05 Mt CO2-eq yr-1) as it could be applied on a large area. Agroforestry and cropland to grassland conversions lead to a reduction of cropping areas and their application is only recommended for selected areas. The estimated potentials are 0–0.12 Mt CO2-eq yr-1 for agroforestry and 0.05 Mt CO2-eq yr-1 for cropland to grassland conversions. In the case of agroforestry, a significant carbon sink is expected in wood, which is not included here. Which measures would be most effective on grassland soils is not clear yet. Generally, it is important to note that additions of organic fertilizer, which can be an integral part of several measures, only count as a true sequestration measure if the biomass was produced on-farm (also excluding feed imports). Furthermore, it is important to add that the sequestration potentials presented only refer to topsoils due to a lack of data. For a full carbon accounting, effects on subsoils (below 30 cm depth) would need to be included. Biochar as another option for agricultural soils is not considered in this report, but in an accompanying study. Measures to enhance soil organic carbon stocks on forest soils include the selection of tree species, liming or wood ash application. However, they are all expected to have small effects on total soil organic carbon stocks. Afforestation on former cropland is the only measure that could lead to significantly higher soil organic carbon stocks, but would conflict with food production. However, afforestation generally leads to additional carbon storage in woody biomass. In settlements, creating new areas for carbon accumulation such as green roofs offers a potential for soil carbon sequestration of 0.07 Mt CO2-eq yr-1. This measure can have positive effects on urban climate and local biodiversity. The inclusion of biochar underneath newly built roads, could sequester 0.37 Mt CO2-eq yr-1. Biochar could also be used in tree substrates and would have positive side-effects on water uptake and retention. Drained organic soils emit 0.51–0.69 Mt CO2-eq yr-1. Measures should focus mainly on reducing these losses as soil carbon sequestration is difficult to achieve on degraded peatlands. The most promising measure to reduce emissions is rewetting, but the consequence is a severe impairment of the production function. Most likely soil covering and soil mixing cannot reduce CO2 losses. Overall, most measures to sequester carbon in mineral soils and reduce carbon losses from organic soils are relatively well known and several measures are ready to be implemented. However, careful selection of sites and measures is highly recommended as the potential to sequester carbon is strongly-site specific and any potential side-effects such as yield reductions need to be factored in. In summary, soil carbon sequestration in Switzerland could offset an upper maximum of 24% of the domestic negative emissions based on the presented measures.
