Explaining variation in ammonia emission after field application of digested and untreated slurry using physical and chemical characteristics

Abstract Anaerobic digestion alters slurry properties with implications for ammonia (NH 3 ) emission after field application. While pH and dry matter (DM) are known drivers, they do not fully explain observed variability. This study quantified the extent to which chemical and physical characteristics explain NH 3 emission from digestates and untreated pig and cattle slurry. Five field trials were conducted over two years using nine digestates and six untreated slurries. Emission was measured over 150 h following surface band application using dynamic flux chambers and cavity ring-down spectroscopy. Slurries were characterized for pH, DM, rheological parameters (consistency coefficient $$K$$ K ; flow behavior index $$m$$ m ), particle size distribution, and exposed surface area. Mixed-effects models were used to evaluate correlation between subsets of these variables and cumulative emission. Cumulative NH₃ emission was 41% of applied TAN on average for digestate, 27% for cattle, and 13% for pig slurry. Emission varied among trials and was consistently positively correlated with pH. Similar positive associations were found for DM, viscosity ( $$K$$ K or $$m$$ m ), and the fraction of particles > 2 mm; however, these variables were collinear and provided comparable model support. Emission was negatively correlated with exposed surface area, likely reflecting reduced spreading and horizontal flow at higher DM, and showed limited predictive value. Overall, variation in NH 3 emission was largely attributable to pH and DM or correlated physical properties. Mechanistically, this reflects pH-dependent NH 3 speciation and reduced infiltration due to increased viscosity. Additional physical characterization beyond pH and DM offers limited predictive gain.