Precision application techniques reduce ammonia emissions following food-based digestate applications to grassland

Abstract

The anaerobic digestion of source-segregated food waste for the production of biogas has increased significantly in recent years. The digestate produced is a valuable source of plant nutrients such as nitrogen and phosphorus. However, minimising ammonia (NH₃) emissions following land application of digestate is important to maximise crop available N supply and reduce environmental impacts. The objectives of this study were (1) to compare NH₃ emissions from food-based digestate to those from cattle slurry applied to grassland under comparable conditions and (2) to evaluate the effect of precision application techniques (shallow injection and trailing shoe) on NH₃ emissions compared with the conventional surface broadcast method. The results showed that NH₃ emissions from broadcast-applied food-based digestate (mean 31% total N applied) were higher than from cattle slurry (mean 21% total N applied), reflecting its higher total N and NH₄–N contents, and high pH (mean pH 8.4). Both precision application methods reduced NH₃ emissions from food-based digestate by 40–50% in comparison with the surface broadcast treatments, with shallow injection more effective than trailing shoe (P < 0.05). Precision application techniques could be an effective method for reducing NH₃ emissions when applying food-based digestate to grassland, providing soil conditions are suitable. Practitioners should be encouraged to use precision application techniques when spreading high N digestates. Given that increasing quantities of food-based digestate are predicted to be produced in the future, failure to mitigate emissions effectively could adversely impact our ability to meet national NH₃ reduction targets.