Interaction of Urea Ammonium Nitrate with Food Waste Organic Amendments on Gaseous Emissions, Nitrogen Dynamics and Soil Properties

Approximately one-third of all food produced globally goes to waste, promoting a growing demand to recycle and recover wasted nutrients using sustainable waste management technologies such as composting and anaerobic digestion. Organic end-products from composting and anaerobic digestion have low nutrient contents making agricultural utilisation challenging (economically and practically) relative to synthetic fertilisers. However, organic end-products can enhance physical and biological properties of soil that can have syngeneic effects when combined with synthetic N. This study investigated synergistic relationships between food waste-derived organic soil applied at a rate of 50 kg ha–1 total N [compost or anaerobic digestion effluent (liquid digestate (LD) or solid digestate (SD))] and two rates (50 and 100 kg ha–1 total N) of synthetic N [urea ammonium nitrate (UAN)] to produce sustainable waste-derived fertiliser products. Greenhouse gases (CO2, N2O), soil chemistry (NH4+-N, NO3–N, pH) and microbial biomass C (MBC) during 56 days of soil incubation were quantified. The application of organic soil amendments produced lower N2O emissions than UAN. LD + UAN 50 decreased cumulative N2O emissions by 23% compared to the UAN 100 despite having the same total N (100 kg ha–1) and the same available N rate. Therefore, replacing UAN for LD in farming practices has the potential to not only supply an equivalent amount of available N in the soil, but also lower N2O emissions. SD produced the highest CO2 emissions, followed by LD and compost. SD +UAN 50 increased MBC levels due to the high levels of carbon and increased labile carbon and available nitrogen due to the application of UAN. The major drawback of using SD compared to LD is that the evaporation of LD caused high ammonia volatilisation rates, reducing available N in SD. Therefore, future studies should explore strategies to reduce ammonia volatilisation of LD