In situ management of a moderately lead contaminated allotment gardens with a secure cropping system

The preservation of the agronomic quality of soils in allotment gardens is of great concern. In case of soil contaminated by metals or metalloids, phytoextraction may be relevant for in-situ remediation. Considered as an environmental-friendly solution, this method generally requires nonetheless many years to achieve the cleaning-up of soils. For this reason phytoremediation fails to develop significantly. To avoid this constraint, this study aimed at developing a secured cropping system in a lead (Pb) contaminated allotment garden, including both the cultivation of vegetables unable to accumulate Pb and Pb phytoextraction.To achieve this goal, an in situ and participative 3-year experiment has been launched since July 2015 on an allotment garden (Nantes, France). With a geogenic origin, Pb in topsoil averages 170 mg/kg. Vegetables (tomato, winter cabbage, potatoes and leeks) on the one hand and Indian mustard (Brassica juncea) used for phytoextraction on the other hand were cultivated in rotation or as crop co-cultures and were compared to a control (without phytoextraction).The first two seasons of culture showed that Pb concentrations in edible parts of tomatoes and winter cabbages were under the EEC regulatory threshold set at 0.1 and 0.3 mg/kg of fresh matter respectively. Analysis of Pb in potatoes and leeks are in progress. For the 'co-culture', Pb in Indian mustard shoots reached 0.9 mg/kg dry matter (DM) with a mean translocation factor (TF) of 1.5 for the season 1 (Tomato culture). It was about 0.2 mg/kg DM with a mean TF of 0.03 for the season 2 (Cabbage). Thus, phytoextraction efficiency was very low. For 'rotation', Pb concentration in B. juncea shoots, ca. 0.5 mg/kg DM with a mean TF of 0.2, was significantly lower than what was recorded with co-culture. Then the culture of tomatoes and cabbages could be maintained on this site in association with phytoextraction, expecting nevertheless higher performances of Pb-extraction by B. juncea. In such a prospect, this association of non-accumulating vegetables and metal accumulating crops appears as a promising solution to secure cropping on contaminated soils - with some recommendations however. It is a way to avoid the systematic closure of gardens, especially when soil contamination is slightly over the local reference value.