Urban agriculture development often faces the problem of soil pollution. Soil engineering consisting in the addition over polluted soils of a top layer made of recycled wastes is a promising solution. This study was co-constructed with urban farmers and aimed at testing in situ the feasibility of growing vegetables safe for consumption in substrates consisting of organic and inorganic waste, directly overlaying soil polluted by trace elements (TE). Two plants were tested: radishes and tomatoes. Three substrates were tested: 1) sheep manure mixed with composted ramial chipped wood (SHW); 2) biowaste compost mixed with mushroom compost and ramial chipped wood (BMW); and 3) deep excavated subsoils mixed with green waste compost (EXC). Only radishes grown in EXC presented levels of TE below the threshold values. For all the other cases, cadmium levels were above the threshold values. This result concerning plant contamination by TE is consistent with a contamination of SHW and BMW substrates by the polluted soil underneath. EXC contained lower TE content, suggesting that mineral materials limited the transfer from the polluted soil towards the substrate overlay. We concluded that adding a combination of mineral and organic waste on top of polluted soils may better mitigate vegetables contamination than adding only organic waste. However, this result was not observed for all tested vegetables. More research is needed to evaluate the best substrate candidate and its adequate thickness, to study its physico-chemical evolution over a longer period of time and to test a larger panel of vegetables.

Urban agriculture has been of growing interest for a decade because it can address many economic and societal issues in the development of modern cities. However, urban agriculture is often limited by the availability of fertile and non-contaminated soils in the cities. Recycling excavated mineral wastes from building activities to construct fertile soils may be a more sustainable alternative than the importation of topsoils from rural zones. The present study aims to evaluate the possibility to grow green vegetables on soils made with excavated deep horizon of soils and green waste compost. During three consecutive seasons, we tested in situ the effects of different amounts of compost (10, 20, and 30%) and the addition of an earthworm species (Lumbricus terrestris) on the production of lettuce (Lactuca sativa L.), arugula (Eruca sativa Mill.), and spinach (Spinacia oleracea L.) in mono- and co-culture. Our results demonstrate that it is possible to reuse mineral and organic urban wastes to engineer soils adapted to agriculture. Here, we observed that higher doses of compost significantly increased plant biomass, especially when earthworms were introduced. For example, in the autumn, going from 10 to 30% of compost in the soil mixture allows to multiply by 2 the arugula biomass, and even by 4 in the presence of earthworms. These results were partly due to the positive effects of these two factors on soil physical properties (micro- and macro-porosity). This preliminary study also showed that some plants (arugula) are more adapted than others (lettuce) to the soil properties and that it only takes few months to get the highest yields. These promising results for the development of urban agricultures encourage to test many other combination of plant and earthworm species but also to conduct experiments over long-term periods.