Agronomic effectiveness of urban biochar aged through co-composting with food waste

Terra preta soils have been shown to develop after considerable modification of soil through char addition and over time natural ageing has led to increase in fertility of those soils. A co-composting experiment was conducted to accelerate the artificial ageing of urban biochar (UB) with the aim of achieving similar terra preta effect. UB was produced through the pyrolysis of 2:1 ratio of biosolids and green waste and then composted with food waste (10% v/v) until compost maturity at around 75 days. A portion of the UB was placed in litterbags within the composting biomass in order to examine the effects of co-composting more closely. Addition of 10% UB to food waste accelerated the composting process. As measured from the litter bags, co-composting UB with foodwaste increased CEC, pH, EC and nitrogen loading of composted UB relative to the un-composted UB. However, the composting process reduced BET surface area and porosity of UB most probably due to clogging of pores by the organics released during composting. The agronomic value of UB, UB co-composted with foodwaste and foodwaste compost was evaluated in a greenhouse pot experiment with sorghum plants on a sandy acidic topsoil. Results of the pot experiment showed higher plant growth, lower emissions of N₂O and higher nitrogen use efficiency in soil amended with UB than the soil amended with compost and co-composted UB.