Letter to the editors: Phyto-P-mining-secondary urban green recycles phosphorus from soils constructed of urban wastes
Résumé
Cities are hotspots of consumption of matter, energy, and water and hotspots of production of wastes, which are also secondary resources. Nutrients such as phosphorus are hardly extracted and recycled from these wastes, except from sewage sludge. This paper discusses a concept for the recycling of P from a great variety of urban wastes (phyto-P-mining). Phyto-P-mining is based on the plant extraction of P from waste materials, which were used to produce planting substrates. They are intended for the greening of urban structures, which were de-vegetated during urbanization or which were not intended to be vegetated before (secondary urban green). After the newly established plants have extracted P, their biomass can be used to produce bioenergy (biogas, wood) or compost. Phosphorus could then be recycled from digestion residues and ashes or directly from compost. Phyto-P-mining is based on otherwise wasted nutrients and on the greening of a high number of not yet vegetated plots, including public or private plazas, sidewalks, roofs, and fallows. Greening is a major goal for urban planning, as functioning soil-vegetation-complexes provide ecosystem services such as climate regulation, dust absorption, wind brake, or aesthetic improvement. Especially in the dense inner city quarters, where vegetation is rare, new green improves public health and well-being. However, due to the lack of available horizontal but the high abundance of vertical structures like walls and facades in city centers, vertical green will be very important for phyto-P-mining. It can efficiently extract P from wastes due to its high ratio of biomass to ground area. Like the vertical areas, the vertical greens are often private properties. Although private greening is primarily conducted for social and cultural reasons, direct market benefits such as bioenergy or fertilizers may reduce costs for the greening. This will foster private urban greening to the benefit of the community and also the recycling of nutrients from urban resources. Phyto-P-mining based on secondary urban green will reestablish soil functions and natural cycling mechanisms in artificial urban systems. The approach has a great potential (i) to improve the urban living environments and to deliver benefits such as (ii) the recycling of phosphorus and other nutrients from urban wastes for the application in urban or rural agri- or horticulture and (iii) the ethically and ecologically sound production of bioenergy.