Slurry stores are an important source of both methane (CH4) and ammonia (NH3) emissions. Strategies to reduce these emissions include modifying the slurry environment through use of natural and biological additives. In this thesis, two such strategies were explored. The first three experimental chapters explored the potential to mitigate emissions by using inorganic acid (50% concentrated sulphuric acid H2SO4), fermentable carbohydrates at 10% w/v to reduce slurry pH, and biological additives (effective microorganisms - EM) at 5% v/v, to suppress methanogenesis. The addition of a carbohydrate source resulted in ‘self-acidification’ of cattle slurry from average pH 6.8 to pH’s as low as pH 3.5 in laboratory and pot-scale experiments, under both warm and cool conditions. The reduced slurry pH inhibited both CH4 and NH3 emissions significantly (by between 72% and 84% for CH4, and 57% and 92% for NH3). The use of agriculture food-chain by-products as sources of available carbohydrates, such as brewery spent grains, successfully promoted self-acidification by producing large amounts of lactic acid and reducing the slurry pH to 4.0. As a result, the greenhouse gas (GHG) carbon dioxide equivalent (CO2 eq.; CH4+CO2+N2O) emission was inhibited by 86%. However, NH3 emissions were not significantly inhibited. Meanwhile, methane oxidation by methanotrophs within slurry crusts was also explored, but bio-augmentation with EM showed no clear effect on this potential CH4 sink. The application of self-acidified slurry to ryegrass in a pot experiment demonstrated significant NH3 and CH4 inhibition compared to the untreated slurry during the first 48 hours after application. However, plant health was severely affected by the acidic slurry treatment, resulting in plant death in most cases. The mechanism for the self-acidification of slurry was elucidated by a conducting a metagenomic analysis of the slurries immediately after carbohydrate addition, and at the end of the 30 day storage period. This linked the dominance of Lactobacillales in the self-acidified slurry to the high lactic acid production and reduced pH in the treatments that received the carbohydrate source. The metagenomic analysis also identified groups of hydrogenotrophic methanogens of the Order Methanobacteriales, Methanomicrobiales and Methanosarcinales, as the dominant methanogens in both treated and untreated slurries. The low CH4 emission from the self-acidified slurry was associated with low abundance of methanogens. Whilst this study demonstrated the potential for self-acidification of slurry to reduce CH4 production and emission, as well as reduction in NH3 emissions from slurry stores and following land spreading, further research is necessary to test the strategy at a larger scale, and develop methods that minimize the negative effect of the acidified slurry on soil and plant health, e.g. via slurry injection or incorporation.