Mitigating ammonia and greenhouse gas emissions from stored cattle slurry using agricultural waste, commercially available products and a chemical acidifier

The production of bovine slurry and its subsequent storage are significant sources of ammonia (NH₃) and greenhouse gases (GHGs). Chemical acidification of manures has been shown to significantly reduce these emissions. Waste products, derived from food processing and on-farm practices, may be used as “natural” acidifiers. However, the efficacy of these products in reducing pH and any subsequent emissions are unknown. Commercial “slurry improvers” or “additives” may also be a viable mitigation option; however, their effectiveness is questionable. This study investigated the efficacy and cost of a range of waste and commercial amendments and a chemical acidifier, ferric chloride (FeCl₃), to identify the most effective amendment for NH₃ and GHG emissions reduction. Ammonia abatement potential was observed for 5% sugar beet molasses (67% reduction), 7% apple pulp (49% reduction), and 7% grass silage (38% reduction). Methane (CH₄) emissions were reduced only by spent brewers’ grain, sugarbeet molasses, and grass silage effluent at the higher inclusions (i.e. amounts added), with reductions ranging from 15% to 70%. Carbon dioxide (CO₂) emissions were significantly increased with the addition of waste amendments. Commercially available additives had little impact on emissions, with the exception of one treatment, which reduced CH₄ by approximately 10%. Ferric chloride reduced NH₃ emissions by 20%–68%, CH₄ by 6%–65%, and CO₂ by 6%–38%, depending on the inclusion. All waste amendments had low marginal abatement costs ranging from -€0.46 to €0.88 kg⁻¹ NH₃ abated compared to FeCl₃ and commercial amendments (€1.80 to €231 kg⁻¹ NH₃). This incubation experiment demonstrated that a range of on-farm and industry waste streams could be valorised to reduce NH₃ emissions. However, many of these may result in higher CH₄ and CO₂ emissions due to input of labile carbon sources. Therefore, based on the results of the current study, it is recommended that sugarbeet molasses and ferric chloride, at 5% and 1.1% inclusions respectively, be examined in field experiments.