Value of food waste-derived fertilisers on soil chemistry, microbial function and crop productivity

Food waste management through composting and anaerobic digestion are sustainable waste management processes that can promote sustainable agricultural practices by recycling nutrients, reducing reliance on synthetic fertilisers, and conditioning soils. However, compost and digestate (anaerobic digestate effluent) are low nutrient value fertilisers, limiting their use within agriculture due to the large volumes required for land application. Alternatively, food waste-derived fertilisers could be used alongside chemical fertilisers to improve crop growth further and reduce demand on synthetic fertilisers. Therefore, this study evaluated the impact of food waste-derived fertilisers (compost, liquid digestate (LD), and solid digestate (SD)) on plant growth (Ryegrass) when applied at a rate of 50 kg N·ha−1 in combination with synthetic fertiliser (Urea Ammonium Nitrate (UAN)) at a rate of 0 or 50 kg N·ha−1. Ryegrass growth, soil chemistry, and rhizosphere microbial communities were investigated. SD in combination with UAN yielded the largest shoot biomass, while sole UAN at any application rate was not significantly different from the control. Beneficial plant growth phyla were in higher abundance in all treatments amended with food waste-derived fertilisers, and putative genes encoded for N fixing were found in higher abundances in all soil amendments compared to control. This highlights the value of food waste-derived fertilisers within agriculture. However, while drying would logically reduce costs associated with digestate transport, ammonia volatilisation led to significant N losses, reducing the fertiliser value of SD. Future studies should explore methods to mitigate ammonia valorisation to enhance the final fertiliser value of SD.