Using recovered road sweepings in manufactured soils: a Betula pendula growth trial

Background and aim Sustainable soil management practices are required to reverse degradation and maintain or create ‘healthy soils’. Reclaimed and treated street cleansing residues (SCRs), which are produced in large quantities and often added to landfill, can potentially provide a suitable ‘circular economy’ alternative to virgin soils. Methods The response of silver birch ( Betula pendula; N = 60 ) – a fast-growing and common street tree – was tested in a range of manufactured soils derived from SCRs ( N = 4) and compared against a range of commercially-available comparator soils ( N = 4) to assess the suitability of the manufactured soils and identify any potentially deleterious elements within the waste-derived soils to either be removed within the process or mitigated in subsequent soil refinements. Key soil parameters (including pH, organic matter and carbon and nitrogen contents) together with tree-health parameters (using non-destructive techniques, e.g. tree growth, chlorophyll estimation/extraction and leaf nitrogen contents) were monitored over a 12-months period. Results Different physico-chemical properties were recorded for the tested soils, with higher pH, elevated C:N ratios and organic matter contents in the waste-derived (-blend) soils, which performed comparably to commercial soils in supporting tree growth, but also showed significant differences in tree growth. Smartphone-based techniques, e.g. contact-image chlorophyll estimation approaches showed suitability to monitor ‘overall tree health’, whereas extracted chlorophyll and leaf nitrogen contents showed healthy tree growth in B. pendula . Conclusion This research demonstrates for the first time that recovered road sweepings can grow healthy trees and can therefore be used successfully as a soil in urban planting schemes. If scalable and pending regulatory approval and more extensive testing (i.e. additional species, longer growing periods), use of road sweeping derived soil could be a significant benefit to the circular economy of urban regions, providing a valuable use for otherwise discarded waste, supporting urban greening agendas, and reducing the demand for natural soils.