Ammonia Emission from Field-Applied Pig Slurry and Digestate: Effects of Solid–Liquid Separation and Biochar Addition

Abstract

Solid–liquid AC:separation is proposed to mitigate ammonia emission after field application of slurry, but evidence for pig slurry and digestate remains limited, particularly when emission from both fractions is included. This study quantified ammonia emission from unseparated pig slurry and digestate and their corresponding liquid and solid fractions across six field trials using dynamic flux chambers. Additional trials assessed biochar amendment to separated fractions. Application of liquid fractions reduced emission by 58% (digestate) and 71% (pig slurry) relative to unseparated materials, while emission from solid fractions was highly variable and, in some cases, exceeded those from unseparated slurry (% TAN). Aggregated ammonia emission, calculated by combining fraction-specific losses with separation efficiency, was consistently lower than from unseparated slurry under most scenarios (reductions: 29–88%). Because separation efficiencies could not be measured directly in this study, these aggregated, system-level estimates rely on literature-derived TAN partitioning scenarios. In the tested materials and at the tested amendment rates, biochar addition tended to increase ammonia emission, likely due to increased pH and dry matter concentration. Results demonstrate that separation reduces ammonia emission when evaluated at the system level but requires appropriate management of solid fractions.