Investigation of trace elements content in organic wastes used for market gardening

While agricultural recycling is recognized as an alternative to stockpiling or incineration, the benefits of the use of organic wastes as fertilizers and soil amendments should be assessed together with potential environmental and toxicological impacts due to the presence of trace elements (TE). While these considerations are common in northern countries, issues and problems involved in waste management are increasing in developing countries. Within the framework of the ANR project ISARD, designed to set up methods to ensure suitable agricultural intensification based on the recycling of organic wastes, this study investigated the contents of major chemical elements and TE in various composts, from sewage sludge, household refuses, animals manure and garden rubbish, applied on market-garden crops on the outskirts of cities in various countries (Saint Denis, La Réunion, France; Majunga, Magacascar; and Dakar, Senegal). Organic waste contents are various and depend on the geographic origin and type of wastes (e.g., Pb = 0.82-2100 mg kg-1 dry matter). Half of the organic wastes that were examined exhibit very high TE concentrations, and are above the limits set by European legislation and found in the literature data for organic wastes designed for market gardening. Size fractionation of organic wastes exhibited a fairly large enrichment in TE in the smaller solid fraction (0.2-20 ?m) in comparison with raw wastes. This result suggests that TE were potentially associated with organic matter in the 0.2-20 ?m fraction, which is the most reactive to degradation of micro-organisms. The use of such organic wastes for market gardening could consequently be potentially harmful with respect to TE phytoavailability and phytotoxicity. However, total concentrations of TE in organic wastes and of TE dynamic in amended soils will be crucial to predict TE phytoavailability. (Résumé d'auteur