Storing carbon in urban soils: review and prospects

Today, urbanization is at the heart of environmental concerns. While it has existed for millennia, it has accelerated steadily since the industrial revolution. On a global scale, it is predicted that between 2000 and 2030, the equivalent of the surface area of the city of Paris will be urbanized every day, i.e. a total of around 1.5 million km ² in 30 years. Historically, urban development strategies have taken very little account of soil as a resource, leading to soil degradation. In addition, the many human activities that require urban soils to perform various functions (e.g., supporting buildings and infrastructure, supporting vegetation and biodiversity, storing and filtering water and pollutants) make them spatially very heterogeneous, with rapid temporal evolution. Despite the significant potential it represents in terms of leveraging sustainable urban development, the capacity of soils to provide ecosystem services (e.g., regulating biodiversity or water quality) is still given very little consideration in urban development projects.

To enable urban soils to provide ecosystem services, one of the levers for action is to maintain, or even increase, the quantity of organic matter in soils (SOM). Indeed, SOM plays a predominant and positive role in the physical (aggregation, aeration, water retention, root development), chemical (buffering capacity, pH regulation) and biological (microbial, animal and plant activity and diversity) fertility of soils. What's more, as Pierre Barré and Lauric Cécillon explain in the landmark article "Carbon storage potentialities in soils" (ref. [GE 1 061] ), SOM is mainly composed of carbon (C) (on average 58%): the phenomena of organic C storage/unstorage in soils (COS) therefore have a direct effect on CO ₂ concentration in the atmosphere, and consequently, on climate.

Studying the stocks of MOS, and de facto COS, in urban soils is becoming essential in view of increasing urbanization. Urban soils, like agricultural and natural soils, constitute an essential resource, but one that is limited and non-renewable on human timescales. However, the scientific community's interest in urban soils is only very recent (since the 1990s in France). One of the limits to the study of these soils is their high degree of spatial and temporal heterogeneity, making it difficult to generalize. As a result, current knowledge of COS stocks in urban soils is based mainly on one-off case studies. So what do we know about these soils and their capacity to store SOC?