Maintaining the cultivation of vegetables with low Pb accumulation while remediating the soil of an allotment garden (Nantes, France) by phytoextraction.

Lead (Pb) is commonly found in urban soils and can transfer to vegetables. This entails a health risk for consumers of garden crops. The increasing demand of gardening on urban soil linked to the population increase and concentration in urban areas induces an increase in the risk, as people could be forced to cultivate contaminated soils. The aim of this study was to evaluate the performance of a cropping system that allows simultaneously (i) growing eatable vegetables that accumulate few Pb and (ii) cleaning up the soil with other plants by phytoextraction. The tests were carried out in an allotment garden (Nantes, France) where soils are moderately enriched by Pb from geogenic origin (178 mg.kg-1 of dry soil on average). Four vegetables known to accumulate slightly Pb (Solanum lycopersicum, Brassica oleracea cv. "Capitata," Solanum tuberosum, and Phaseolus vulgaris) were grown. The in situ ability of Brassica juncea L. to progressively absorb the phytoavailable Pb of the soil was assessed during four seasons. Analyses of the edible parts of the four vegetables confirmed that they can all be safely cultivated. The accumulation of Pb in B. juncea shoots was too low (ca. 1 mg.kg-1 of dry matter at best) for phytoextraction purposes. Our results confirm that it is possible to grow very low Pb-accumulating vegetables on soils moderately contaminated with Pb, although it was not possible to reduce phytoavailable Pb rapidly enough with B. juncea. This study identifies possible avenues of research to improve this cropping system by using appropriate vegetables that will allow food production to continue on moderately contaminated soil while cleaning it up.

Selection of soil health indicators for modelling soil functions to promote smart urban planning.

The contribution of soil health to global health receives a growing interest, especially in urban environment. Therefore, there is a true need to develop methods to evaluate ecological functions provided by urban soils in order to promote smart urban planning. This work aims first at identifying relevant soil indicators based either on in situ description, in situ measurement or lab analysis. Then, 9 soil functions and sub-functions were selected to meet the main expectations regarding soil health in urban contexts. A crucial step of the present research was then to select adequate indicators for each soil function and then to create adapted reference frameworks; they were in the form of 4 classes with scores ranging from 0 to 3. All the reference frameworks were developed to evaluate soil indicators in order to score soil functions, either by using existing scientific or technical standards or references or based on the expertise of the co-authors. Our model was later tested on an original database of 109 different urban soils located in 7 cities of Western Europe and under various land uses. The scores calculated for 8 soil functions of 109 soils followed a Gaussian distribution. The scoring successfully expressed the strong contrasts between the various soils; the lowest scores were calculated for sealed soils and soils located in urban brownfields, whereas the highest were found for soils located in city parks or urban agriculture. Despite requiring a soil expertise, the proposed approach is easy to implement and could help reveal the true potential of urban soils in order to promote smart urban planning and enhance their contribution to global health.

Drivers to improve metal(loid) phytoextraction with a focus on microbial degradation of dissolved organic matter in soils.

Bioaugmentation of soils can increase the mobilization of metal(loid)s from the soil-bearing phases. However, once desorbed, these metal(loid)s are mostly complexed to the dissolved organic matter (DOM) in the soil solution, which can restrict their availability to plants (roots mainly taking up the free forms) and then the phytoextraction performances. Firstly the main drivers influencing phytoextraction are reminded, then the review focuses on the DOM role. After having reminding the origin, the chemical structure and the lability of DOM, the pool of stable DOM (the most abundant in the soil) most involved in the complexation of metal(loid)s is addressed in particular by focusing on carboxylic and/or phenolic groups and factors controlling metal(loid) complexation with DOM. Finally, this review addresses the ability of microorganisms to degrade metal(loid)-DOM complexes as an additional lever for increasing the pool of free metal(loid) ions, and then phytoextraction performances, and details the origin of microorganisms and how they are selected. The development of innovative processes including the use of these DOM-degrading microorganisms is proposed in perspectives.

This review focuses on the available drivers to increase the pool of free (i.e. phytoavailable) metal(loid)s in the soil solution, with a specific focus on the ability of microorganisms to degrade dissolved organic matter for enriching this pool, and then to substantially improve phytoextraction performance.

Chelate-assisted phytoextraction of lead using Fagopyrum esculentum: laboratory vs. field experiments.

The development of more sustainable remediation techniques has been receiving greater attention, as an alternative to soil excavation plan in urban gardens. An in situ phytoextraction experiment with buckwheat (Fagopyrum esculentum) was performed with a 5 mmol kg-1 citric acid (CA) application. Joint experiments under laboratory conditions were conducted using various cultivars of F. esculentum in two soils with a Pb contamination of either geogenic or anthropogenic origin and various chelate concentrations. Results show that a minimum dose of 50 mmol kg-1 of CA is required to lower soil pH and raise the concentration of mobile Pb-CaCl2 for both soils. Consequently, Pb shoot uptake is increased from 6.3 to 8.9 times depending on soil type. Phytoextraction efficiency is found to be 1.3 to 2.0 times higher in the anthropogenic contaminated soil than in the soil with geogenic Pb. A scale effect has also been identified since Pb root accumulation under laboratory conditions was 2.4 times higher than in the field experiment. Despite an increase in the Pb extraction rate with CA, buckwheat appears to lack the efficiency needed to remove Pb in moderately contaminated soils. The calculated remediation period would last 166 years to remove the mobile Pb fraction.

Points of attention in designing tools for regional brownfield prioritization.

The regeneration of brownfields has been increasingly recognized as a key instrument in sustainable land management, since free developable land (or so called "greenfields") has become a scare and more expensive resource, especially in densely populated areas. However, the complexity of these sites requires considerable efforts to successfully complete their revitalization projects, thus requiring the development and application of appropriate tools to support decision makers in the selection of promising sites where efficiently allocate the limited financial resources. The design of effective prioritization tools is a complex process, which requires the analysis and consideration of critical points of attention (PoAs) which has been identified considering the state of the art in literature, and lessons learned from previous developments of regional brownfield (BF) prioritization processes, frameworks and tools. Accordingly, we identified 5 PoAs, namely 1) Assessing end user needs and orientation discussions, 2) Availability and quality of the data needed for the BF prioritization tool, 3) Communication and stakeholder engagement 4) Drivers of regeneration success, and 5) Financing and application costs. To deepen and collate the most recent knowledge on the topics from scientists and practitioners, we organized a focus group discussion within a special session at the AquaConSoil (ACS) conference 2017, where participants were asked to add their experience and thoughts to the discussion in order to identify the most significant and urgent points of attention in BF prioritization tool design. The result of this assessment is a comprehensive table (Table 2), which can support problem owners, investors, service providers, regulators, public and private land managers, decision makers etc. in the identification of the main aspects (sub-topics) to be considered and their relative influences and in the comprehension of the general patterns and challenges to be faced when dealing with the development of BF prioritization tools.