Black carbon influence on urban soil ecosystem services: from its contribution to the soil carbon cycle to its role in mitigating the risks of heavy metals exposure into urban horticulture produce

Urban soils underpin the provision of all ecosystem services delivered by urban greenspaces which are essential in strengthening urban resilience and mitigating many of the environmental and health challenges faced by urban populations. Understanding how to enhance ecosystem service provision by urban soils is crucial to support future greenspace management strategies, such as urban horticulture expansion or the increased multifunctionality of urban greenspaces. Through different experiments and field studies the role of urban soils in carbon sequestration, pollutant bioavailability and mitigation, urban food and nutritional security has been explored, highlighting the crucial contribution of soil black carbon across all these. Black carbon in the form of soot was demonstrated to play an active role in urban soil carbon dynamics by both suppressing the mineralisation of soil labile organic carbon and contributing to soil CO2 effluxes. Field experimental results revealed that soil application of an engineered form of black carbon (biochar) at the rate of 20 t ha-1 to a clayey loam urban soil under three different vegetation covers did not influence urban soil ecosystem service provision. The first UK-wide assessment of heavy metals and metalloids concentrations (total and bioavailable) across UK urban horticultural soils demonstrated that growing food across these soils poses a low risk to the urban grower’s health and that soil black carbon contributes to mitigating the risk of heavy metals and metalloids uptake into urban horticulture produce. A large-scale field study showed that the long-term exposure to heavy metals and metalloids through consumption of urban horticulture produce is unlikely to pose detrimental human health risks. It also demonstrated that the consumption of urban horticulture produce contributes to the daily intake of all essential minerals, but their concentrations is often lower than those found in equivalent commercial horticultural crops. Future research possibilities are discussed along with the key findings of this research.