Acidification with acetic or sulfuric acid: Effects on NH3 and N2O emission from cattle and pig slurry applied to grassland.

Ammonia (NH₃) emissions from slurry application contribute to air pollution, ecosystem acidification and nitrogen loss, while nitrous oxide (N₂O) emissions contribute to climate change. Sulfuric acid is commonly used to reduce NH₃ emissions, but safety concerns and sulfur accumulation in soil have prompted interest in alternatives such as acetic acid. We compared the effects of both acids on NH₃ and N₂O emissions following cattle and pig slurry application to grassland in Denmark. Sulfuric acid was added at 3.0 and 2.9 kg t-1 to cattle and pig slurry, respectively; corresponding acetic acid doses were 3.86 and 3.23 kg t-1. Acidification reduced slurry pH to approximately 6.0 for cattle and 5.8 for pig slurry.Separate field experiments used block designs with three replicates for NH₃ and four for N₂O. Slurry was applied manually. NH₃ emissions were measured using dynamic flux chambers and cavity ring-down spectroscopy, and N₂O emissions using static chambers and mid-infrared spectroscopy. The first 6 h of cattle slurry NH₃ emissions were extrapolated following an equipment malfunction. Cumulative emissions were analysed by ANOVA accounting for blocks. Both acids reduced cumulative NH₃ emissions relative to untreated slurry, with no statistically detectable difference between acids. The reduction was statistically significant only for cattle slurry: 36% based on observed data and 41% when extrapolated data were included. Cumulative direct N₂O emissions showed no statistically detectable treatment effect for either slurry type, although mean emissions from cattle slurry were numerically higher after acidification with either acid.Under these field conditions, acetic acid achieved NH₃ mitigation comparable to sulfuric acid and may be an alternative where sulfur inputs are constrained.Implication: This study presents a field-scale comparisons of acetic and sulfuric acid for slurry acidification under grassland conditions. The results show that acetic acid can achieve ammonia emission reductions comparable to sulfuric acid without increasing nitrous oxide emissions, providing new evidence to support alternative manure management strategies aimed at reducing agricultural air pollution while maintaining efficient nitrogen use.