In situ management of urban gardens soils moderately contaminated by lead

Conscient des risques de pollution métallique pouvant toucher les sols des jardins familiaux urbains, le Service des Espaces Verts et de l'Environnement de la ville de Nantes (SEVE) a lancé depuis 2012 un programme de vérification de la qualité des sols. La pollution au plomb de plusieurs parcelles a été mise en évidence sur plusieurs sites, avec l'obligation d'établir un plan de gestion, malheureusement coûteux et peu durable.Face à cette situation, le SEVE souhaite tester des méthodes biologiques, telle que la phytoextraction, moins agressives pour l'environnement...Soil quality is of concern regarding the development of kitchen gardens in urban allotments gardens. Although lead (Pb) is poorly available in the soil, some vegetables can accumulate it above maximum permitted concentrations. This study focuses on the development of methods for the in situ sustainable management of kitchen garden soils moderately Pb-contaminated (200 mg/kg DM), including the phytoextraction option. The first option, in situ assessed on kitchen gardens, combines the phytoextraction using Brassica juncea (L.) Czern. and the cultivation of metal-excluder vegetables, allowing the crop production during the soil clean-up. Tomato, cabbage, potato tubers, leek and green bean were cultivated either in a crop rotation or co-cropped with B. juncea (vs. only vegetables). The harvested vegetables are indeed Pb-excluders, except green beans. Due to a low soil Pb availability (geogenic Pb), Pb phytoextraction was inefficient (max. 2 mg Pb/kg). The second option, tested at the bench scale, aims to increase the phytoextraction by adding chelatants, either organic chemicals (i.e. EDTA) or via a bacterial inoculum (bioaugmentation) notably selected based on its siderophore production, into the soil. Extraction potential of B. juncea and Lycopersicon esculentum was evaluated in hydroponics with a Pb exposure similar to the DTPA extracted fraction. Shoot Pb concentration for L. esculentum cv. « Marmande » (max. 2000 mg/kg) was 20 to 100 times higher than for B. juncea (max. 45 mg/kg) reaching the threshold value for Pb hyperaccumulator. EDTA incorporation into the soil confirmed the high extraction potential of tomato when Pb availability is increased (114 to 615 mg/kg; anthropogenic or geogenic Pb). However, use of the bioaugmentation was not conclusive