Home and community composts in Nantes city (France): quality and safety regarding trace metals and metalloids.

Home and community composting are key strategies for local organic waste management. The quality and safety of industrial composts are controlled, but those of home and community composts are not, and this could make them unsafe for use in kitchen gardens. Home (n = 20) and community (n = 41) composts, from urban and suburban areas including mildly Pb-contaminated allotment gardens, were analyzed for quality and safety regarding trace metals and metalloids (TMM) using mid-infrared Fourier transform spectrometry (FT-MIR) and portable X-ray fluorescence spectrometry, respectively. Home composts had a significantly higher Pb content (98 mg.kg-1  ± 10 mg.kg-1 ) than community composts (21 mg.kg-1  ± 2 mg.kg-1 ). Numerous home composts (85%) and a few community composts (17%) exceeded the organic farming thresholds for Pb (45 mg.kg-1 ) and Zn (100 mg.kg-1 ). The high mineral matter content and the relative abundance of chemical functions attributable to silicates (up to 35%) highly paralleled with TMM contents, mostly concentrated in the fine fraction. Co-inertia analysis highlighted strong and significant links between TMM contents and the whole chemical signature delivered by FT-MIR spectrometry. Pb-contaminated soil could be carried into home compost by green waste or by voluntary addition. Covariance analyses indicated that mineral matter and chemical functions only partly explained the variability in Pb content, suggesting a more complex combination of drivers. Community composting appears as a suitable local solution resulting in high-quality compost that complies with European organic farming regulations, while home composting from allotment gardens should be seriously evaluated to comply with such safety requirements.