Closing the circle for urban food waste anaerobic digestion: The use of digestate and biochar on plant growth in potting soil

Food waste can be used via anaerobic digestion (AD) to produce biogas. The liquid by-product (digestate) contains nitrogen (mainly as ammonium) which is susceptible to N loss as a fertiliser and also faces logistical constraints for agricultural use. Biochar can adsorb and retain nutrients and alter microbial N-cycling processes. We investigated the use of digestate as a nutrient source and its interactions with biochar when applied to potting mix targeted for localised urban use. Tomato (Solanum lycopersicum L.) plants were grown for 45 days in potting mix amended with five rates of food-waste-derived digestate (0, 2, 4, 6, 8 and 10% v/v), with and without biochar (10% v/v). At harvest, plant growth parameters and potting soil characteristics were measured. The rhizosphere bacterial community was profiled by amplicon sequencing followed by an in-silico analysis of putative functional genes. Plant biomass and shoot N concentration increased with digestate addition in a typical nutrient dose-response curve. While growth lagged when digestate was combined with biochar, both treatments reached a similar maximum shoot yield. Biochar application decreased soil mineral N content and two putative ammonium nitrification cycling genes, AmoA and AmoB, which prevent the conversion of NO3- from NH4+. Rhizosphere bacteria beta-diversity separated into two distinct clusters, with and without biochar, at all levels of digestate. Addition of biochar with digestate decreased the availability of soil mineral N by decreasing the mineralisation of nitrogen which likely reduced the risk of N loss. The rhizosheath bacterial community mediated this effect. Further quantification of these processes may aid in optimising of the benefits for the environment and plant growth from the transformation of blended organic urban wastes into organic fertilisers for potting soils.