The Effects of Rabbit-Manure-Derived Biochar Co-Application with Compost on the Availability and Heavy Metal Uptake by Green Leafy Vegetables

The use of organic amendments to enhance plant growth is increasing due to horticulture activities and vegetable cultivation in urban areas. Consequently, as organic amendments impact heavy metal solubility and plant uptake of unknown contaminants, the risk of human exposure to potentially toxic elements from contaminated soils and compost is increasing. Biochar co-application with compost may reduce the risk-related increased metal uptake by edible plants. To verify this thesis, a greenhouse experiment was established to examine the effects of rabbit-manure-derived biochar (RBC) on Cu, Cr, Cd and Pb uptake by five green leafy vegetables (lettuce—<i>Lactuca sativa</i> L., spinach—<i>Spinacia oleracea</i> L., corn salad—<i>Valerianella locusta</i> L., kale—<i>Brassica oleracea</i> L., mustard greens—<i>Brassica juncea</i> L.) cultivated in compost substrate and soil amended with a 30% (<i>v</i>/<i>w</i>) mix of compost and biochar. The results indicated that the addition of biochar decreased Cu, Cr, Cd and Pb availability in the tested substrates, reducing the uptake of Cd in spinach by 61% and Pb in mustard greens by 73%. The application of RBC also had some adverse effects, such as enhanced accumulation of Cr by kale, lettuce and mustard greens cultivated in compost. Compost co-application with biochar to soil decreased the availability of metals, reducing the content of Pb and Cd in tissues of the tested vegetables, while uptake of Cu and Cr was enhanced in spinach and lettuce by 20%. In conclusion, the application of compost and biochar can be beneficial in improving the quality of urban soil used for horticulture purposes. However, more attention by gardeners should be paid to soil and compost testing in terms of heavy metal contamination and possible adverse effects of organic amendments application for green leafy vegetable cultivation.