Urban biochar improves nitrogen and phosphorus availability in growing media

The purpose of this study was to substitute sphagnum peat from plant growing media with urban biochar (UB) and evaluate its impact on nitrogen (N) and phosphorus (P) availability. The UB was produced from the pyrolysis (650°C) of a 2 : 1 ratio of biosolids to green waste. We compared three mixes – 20% sphagnum peat mixed with composted pine bark (B0), 20% UB mixed with composted pine bark (B20) and 60% UB mixed with composted pine bark (B60) – for their ability to promote plant growth and minimise leaching losses in a greenhouse experiment using silverbeet (Beta vulgaris ssp. cicla). Plants were grown in 4.0-L custom-made chambers with the capacity to collect leachate and measure nitrous oxide gas flux. Both biochar mixes increased media pH, air filled porosity, bulk density and nutrient content relative to B0. The B0 had the highest cation exchange capacity and electrical conductivity. The UB-based mixes, B20 and B60, had no significant effect on silverbeet biomass after 11 weeks of growth but had higher N use efficiency and P availability than B0. These results indicate that UB can completely replace sphagnum peat from growing media and can be used at the rate of 60% on volume basis while improving N and P availability. Using a higher rate of biochar in growing media has the additional advantage of sequestering more carbon and reducing urban waste streams and landfill costs.