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

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.