Effect of slurry dry matter content, application technique and timing on emissions of ammonia and greenhouse gas from cattle slurry applied to grassland soils in Ireland

Agriculture contributes to 98% and 33% of ammonia and greenhouse gas (GHG) emissions in Ireland, respectively. Those emissions are mainly associated with livestock production. The Gothenburg Protocol and the EU National Emissions Ceilings Directive are policy drivers which set new and more demanding targets from 2020 to reduce these gaseous emissions. A field experiment was set up in Wexford (Ireland) between April 2009 and August 2010, on a grassland site established with a uniform ryegrass (Lolium perenne) sward for more than ten years. The objective was to investigate the impact of slurry dry matter (DM) content, application technique and timing of application on the overall GHG balance from cattle slurry applied to grassland soils. The treatments on plots were a control, calcium ammonium nitrate (CAN) and cattle slurry, either grass-based or maize-based and with varying DM contents, applied by mimicking trailing shoe and splash plate application. The dry matter contents were varied by mixing different ratios of faeces and urine. The results showed that, while ammonia (NH3) volatilisation losses were significantly increased on slurry spread plots, cumulative direct nitrous oxide emissions, and corresponding emission factors, were significantly higher when applying CAN. In terms of GHG field balance, the potential decrease in indirect nitrous oxide (N2O) emissions, calculated from a reduction of ammonia volatilisation losses using trailing shoe as opposed to splash plate, could be easily offset by an increase in direct N2O emissions and ecosystem respiration. Switching from summer to spring application was much more efficient for mitigation of both NH3 and GHG emissions, due to favourable soil and climatic factors which enhanced crop growth. Any potential trade-off between NH3 and N2O emissions was cancelled, leading to an overall positive effect on reactive nitrogen losses and offering agronomic benefits to farmers.