Carbon storage in urban soils of Lausanne and Zürich, Switzerland

Some urban soils are likely to store large amounts of organic carbon, due to their perennial vegetation cover; yet we have very little empirical data on the organic carbon stocks of urban soils. In this study, we aimed to quantify the carbon stocks of urban topsoils developed from different geological substrates and under different land uses.

We selected the Lausanne-Morges agglomeration and the city of Zürich with a constrasting urban and geomorphological context. In order to quantify the carbon content, soils developed on 6 different parent materials and 7 primary vegetation types were sampled. In total, 107 soils were sampled from October 2022 to April 2023. In addition, data from the project Better Gardens (Tresch et al., 2018) and the master thesis of Eloïse Singer were also used to have a more representative view of the urban green surfaces (248 sites).

The samples were collected with a 2.5 cm auger by mixing 5 sub-samples in an area of 2 x 2 m, from a 0 to 20 cm depth. The analyses conducted were bulk density, soil organic carbon (SOC) content, texture by laser granulometry and carbon to nitrogen ratio (C:N). Soil parent material was assigned from existing geomorphological maps. Land use (vegetation type) was initially determined from urban maps then checked on-site.

We investigated differences in soil properties between cities, parent material and land uses using analyses of variance. The 4.3.1 version on R Studio was used to create exploratory graphs (boxplots and QQ-Plots) and for all statistical analyses.

Overall, Lausanne had siltier soils than Zürich. The C:N ratio was also narrower in Lausanne, with an average of 8 for Lausanne and 13 for Zürich. The mean bulk density results showed that the sampled soils were not as compacted as expected in an urban area (µ = 1.09 g/cm³). The highest bulk density was found on a lawn site in Zürich (µ = 1.20 g/cm³). Geomorphology class as determined from the maps was not found to have a significant effect on any of our soil properties. This could be explained by human soil disturbance and the overarching effect of management practices in urban areas.

The SOC topsoil stock of Lausanne’s public green spaces varied between 9.1 and 149.0 t/ha with an average of 61.8 t/ha, while those of Zurich varies between 29.5 and 141.2 t/ha with an average of 75.6 t/ha. The private gardens were found to have a significantly higher SOC % than the public green spaces, which was consistent with findings of previous studies. 

This study provides new empirical data on the carbon stocks in urban soils developed from different geomorphological substrates and under numerous land uses. Overall, results suggest that the ecosystem services supported by the soils are still functional. The carbon sequestration capacity of the urban soils should not be underestimated as part of the solution for global climate change mitigation and adaptation. It should be taken into account in decision making processes towards a sustainable urbanisation.