Impacts of biochar concentration on the growth performance of a leafy vegetable in a tropical city and its global warming potential

Urban farming has been gaining popularity but the poor urban soil quality has raised concern regarding soil productivity. Biochar generated from waste wood has been widely applied as a soil conditioner, but few studies investigated its effects when used in high concentrations on urban soil. In this study, the effects of biochar on the growth of the leafy vegetable, xiao bai cai (Brassica rapa Cultivar Group Pak Choi Green-petioled Form) were investigated with its concentrations in urban soil varying from 0 to 100% (v/v). The results demonstrated that the 30% biochar and soil mixture (v/v) was best in terms of fresh weight, dry weight and leaf number gain over those of the control (0% biochar). Analysis of the growing medium showed that by increasing the biochar concentrations, available nitrate and potassium as well as cation exchange capacity increased accordingly, while available phosphate decreased. A life cycle assessment showed that the global warming potential could be reduced when applying waste wood-generated biochar as an urban soil amendment for vegetable production.