Less is more, soil and substrate quality as an opportunity for urban greening and biodiversity conservation

Soil is a key component of the water and nutrient cycles and a major contributor to global carbon sequestration. It can remediate pollution and provides habitat for almost all terrestrial plants, as well as a large proportion of terrestrial fauna. Yet, despite soils global importance, it is estimated that one third of worldwide soil is degraded (FAO, Status of the World's Soil Resources (SWSR) - main report. Food and Agriculture Organization of the United Nations and Intergovernmental Technical Panel on Soils, Rome, 2015). In urban environments, soil is often overlooked despite its potential to alleviate problems, such as flash flooding, and its vital role in supporting vegetation, which in turn contributes to the urban landscape by, for example, reducing the urban heat island. Because of the changing nature of cities, soils undergo many disturbance actions such as manipulation, compaction and pollution. These processes degrade their important properties, leading to loss of fertility and function. Infertile soils are, however, a potentially valuable resource for the creation of species-rich, native plant communities, as the most biodiverse herbaceous vegetation is often found on infertile soils. When habitats in cities are being created, such as for extensive green roofs, a bigger research effort is required in developing soils that meet the economic and engineering needs of industry, whilst also functioning as a successful habitat.