Optimising the application of digestate within intensive agriculture has the potential to reduce the synthesis and use of inorganic fertilisers. Decoupling future agricultural production from a reliance on inorganic fertiliser would bring a wide range of economic, environmental and geopolitical benefits, especially since the global demand for food is predicted to rise sharply by 2050. However, there remain many uncertainties surrounding the environmental impact of digestate usage in agriculture, especially after application of different physical fractions of digestate to land (whole [WD], liquid [LD] and solid [SD]). These include effects of digestate on soil microbial communities, greenhouse gas (GHG) emissions and the leaching of nutrients to the subsurface. In this context, the thesis aimed to evaluate environmental impacts of different digestate fractions using a range of laboratory and field experimental approaches, focussing on: a) the soil microbial community composition and carbon use efficiency (CUE); b) GHG emissions and how digestate application rates can affect GHG emissions; and c) the export of multiple phosphorus (P) and nitrogen (N) fractions via leaching. All digestate fractions increased GHG emissions from grassland soils compared to control treatments, especially when application rates of LD and WD fractions were increased. However, the application of SD positively increased the soil fungal and bacterial biomass, resulting in a positive CUE when compared to WD. Surface application of WD and SD to grassland soil significantly increased the leaching of a range of P fractions, compared to both control and LD treatments, although all digestate fractions reduced the leaching of N compared to inorganic fertiliser treatments. The results reported in this thesis highlight the need to plan the application of different fractions of digestate to land carefully, following best agricultural practices to minimise adverse environmental impacts, improve a broad range of soil health
parameters and, ultimately, derive maximum agronomic benefit from the return of digestate to land.