Urban Biochar Alters the Chemical and Physical Properties of Plant Growing Media.

Municipal waste management is an ever-increasing problem for many cities. Biochar produced from urban biosolid and green-waste and used as a soil conditioner (urban biochar) can be a sustainable way of managing these waste streams although production and transport costs are currently prohibitive for broad scale agriculture. A more likely market for urban biochar is the horticultural industry where biochar may also have the potential to replace environmentally unfriendly substrates such as peat in growing media while simultaneously sequestering carbon. This study assessed the effect of periodic wetting and drying (over 110 days) on the chemical and physical properties of growing media amended with varying levels of urban biochar. Urban biochar was produced through slow pyrolysis (600°C) of a mixture of green waste and biosolid. The treatments were: a mixture of coir peat (20%) and composted pine bark (control); 20, 40, 60% biochar and composted pine bark and; 100% biochar. Water retention curves were established over four sampling times. In a parallel experiment with the same treatments, nutrient mobility was assessed. Chemical and physical stability of biochar in media was measured using Mid-Infrared Spectroscopy and particle size analysis. Nutrients present in pore water of biochar treatments were found to be higher than control, mainly through release of the inherent nutrient content of biochar and increased sorption of nutrient into biochar surface. Biochar significantly decreased bulk density of media treatments. Higher rate of biochar used in treatment of this experiment was not good for water retention property of media. Therefore, lower rate than 20% biochar is recommended.