Cadmium uptake by vegetables in market gardens : investigating between generic and site-specific effects in a field experiment

Food safety has become a major concern in Belgium since excessive levels of Cd and Pb were measured in vegetables grown in private and market gardens, even in areas with low soil contaminations. The importance of soil Cd concentration and soil pH in controlling plant uptake is widely recognized. However, achieving a neutral or slightly alkaline pH does not appear to be not sufficient to produce vegetables that comply with European legislation in areas affected by low atmospheric contaminations. Moreover, existing models often struggle to provide satisfactory predictions of measured concentrations in vegetables, especially in field trials and at low to moderate levels of contaminations. As a cost-effective remediation approach, the addition of amendments has been reported in the literature to mitigate Cd uptake by vegetables. However, some studies have reported contradictory conclusions regarding the effect of the same factor on cadmium uptake by vegetables. This highlights the need for more quantitative research to clarify the interactions between soil characteristics, amendment types, and their combined effects on reducing cadmium bioavailability. A field experiment was conducted in seven market gardens in the Province of Liège in order to take into account effect of site variability on soil-plant interactions. Past industrial activities in central Belgium were responsible for large atmospheric depositions in the environment, especially along the Meuse – Vesdre valleys. The study sites represented a fairly wide range of soil properties and moderate levels of contamination (from <1 to 5 mg.kg-1 of Cd). The effects of two organic amendments, biochar and compost, and lime on soil pH, CaCl2-extractable metals and uptake by Swiss chard and lettuce were studied. Total organic carbon, texture, cation exchange capacity and available nutrients were also quantified for each soil. After one growing season, no significant effects of organic and lime amendments were observed due to severe drought conditions and the short-term cultivation period. Longer periods of time are needed for amendments to fully interact with the soil as shown by pot experiments. No significant trend in soil content was observed and plant uptake was mainly influenced by initial soil pH. However, soil pH and cadmium content could not fully explain the measured plant concentrations due to site-specific soil-plant functioning. The influence of soil properties on the efficiency of amendments to control soil-plant transfer requires further investigation to identify the key factors controlling Cd bioavailability, especially in the field. Our latest results in that topic will be presented.