Optimising the use of organic amendments, such as livestock slurry, on commercial farms represents one route through which the reliance of agricultural production on inorganic fertiliser use might be reduced. For economic, environmental and geopolitical reasons, decoupling future agricultural production from inorganic fertiliser use is desirable, particularly if increases in future demand for food at global scale are to be met sustainably. However, there remains substantial uncertainty surrounding the impacts of organic amendments on many of the key physico-chemical and microbial properties of agricultural soils. This uncertainty reduces the likelihood that land owners and land managers will adjust farming practices in order to deliver more widespread use of organic amendments to support production. In this context, the research reported in this thesis sought to understand how the management of livestock slurry within intensive grassland systems can be optimised to support production. The thesis had a particular focus on understanding how the soil microbial community mediates the input of livestock slurry, in terms of the influence of this community on the cycling and crop-availability of macronutrients within soil. The thesis first examined the impact of a biological slurry additive, SlurryBugs, on the nutrient content of livestock slurry during storage, finding positive effects of the additive particularly with respect to the total phosphorus (P), where an increase by 27% was observed compared to the control slurry treatment, and the total solids contents of slurry during storage. It was hypothesised that the SB additive may have altered the emission of phosphine (PH3) from slurry during storage. Subsequently, the impacts of slurry application, both with and without the biological additive, on soil organic matter (SOM), as well as on the nitrogen (N) and P content of grassland soils were examined, in comparison to inorganic fertiliser and control treatments. Positive effects following slurry application were observed, spanning SOM, Olsen P, mineral N and soil pH conditions.
Finally, the impacts of applying slurry alongside a range of carbon (C) substrates of different quality (glucose, glucose-6-phosphate (G6P), and cellulose) to a grassland soil were examined, in terms of the partitioning of C within soil as mediated by the microbial community and in terms of changes in the structure and biomass of the soil microbial community. The results revealed an increase in the soil microbial biomass, as well as a decrease in the cumulative respiration, following the application of both slurry types, alongside a carbohydrate, compared to the treatment with the carbohydrate alone, likely due to a microbial metabolic mechanism known as preferential substrate utilisation. In addition, a bacterial predominance within the soil microbial community was observed in all treatments, with increasing dominance of fungi toward the end of the 49-day incubations. This thesis also revealed that the quality of C substrates represented a major factor affecting both the extent of mineralisation and of incorporation of externally-derived C into microbial biomass. The application of 14C-glucose or 14C-G6P to soil resulted in a significantly greater incorporation of 14C into microbial biomass by 68 or 57%, respectively, compared to 41% following the 14C-cellulose application. Further, the addition of US slurry alongside 14C-glucose generated a significantly greater extent of mineralisation by 30%, compared to the treatments with AS slurry or with only 14C-glucose added with 19 and 21%, respectively. Taken together, the data reported within this thesis have potentially important implications for the way in which livestock slurry is managed as a nutrient resource on commercial farms, as well as for broader environmental concerns including the acidification of agricultural soils and the impact of agricultural soils on the global C cycle.