Application of biochar from sewage sludge to plant cultivation: Influence of pyrolysis temperature and biochar-to-soil ratio on yield and heavy metal accumulation

Applying biochar products from sewage sludge (SS) pyrolysis as soil amendment for plant cultivation was investigated in this study with special attention paid to heavy metal accumulation in the plants when pyrolysis temperature and biochar-to-soil mass ratio (C:S) were changed. Biochar obtained at four different temperatures were adopted as soil amendment for Allium sativum L. garlic plant cultivation. Experimental results revealed that biochars were rich in nutrient contents and they improved garlic yields. Although contents of heavy metals including As, Zn, Pb, Ni, Cd, Cr and Cu, etc. were elevated in the biochars compared to local soil, they fell within the acceptable limits for land application and SS is a suitable biochar resource, especially biochar produced at 450 °C had rich micropores, relatively stable functional groups in structure and rugged surface to contact well with soil, conducive to its usage as a biochar. The garlic grew faster when planted in the biochar-amended soil and had higher final dry matter yields than those planted in the reference soil, especially biochar produced at 450 °C corresponding to the highest final yields. The C:S ratio related to the highest garlic yields changed when the pyrolysis temperature was changed and this ratio was 1:4 for the biochar produced at 450 °C. General heavy metal accumulation in the garlic occurred only for the most enriched Zn and Cu, and mainly in the roots & bulbs; in addition this bioaccumulation was increasing as leaching from biochar increased but not increasing with C:S ratio. The garlic planted in soil amended with biochar of 450 °C contained the lowest level of heavy metals compared to other biochars. Those results indicated that heavy metal accumulation in plants can be inhibited through proper pyrolysis temperature choice and prevention of heavy metal leaching from the SS biochar.