Greenhouse Gas and Ammonia Emission Factors from Raw and Digested Cattle Slurry: A Field-Based Synthesis for Cattle Systems

Anaerobic digestion (AD) of cattle slurry is widely promoted as a manure-management strategy, yet its effects on greenhouse gas (GHG) and ammonia (NH3) emissions remain uncertain. This review provides the first field-based synthesis of paired methane (CH4), nitrous oxide (N2O), and NH3 emission factors (EFs) from raw and digested cattle slurry during storage and land application. A total of 98 paired observations from 16 studies were analyzed using paired bootstrap geometric mean ratios. AD reduced dry matter, volatile solids, and carbon content while increasing total ammoniacal nitrogen (TAN) and pH. Consequently, digestate CH4 EFs during storage were 51% lower than those of raw slurry (p = 0.001). In contrast, NH3-N EFs increased by 63% during storage (p = 0.071) and by 40% following land application (p = 0.003). However, TAN-normalized NH3 EFs did not differ between manure types, suggesting similar volatilization potential. AD had no significant effect on N2O EFs. A system-level analysis showed that AD reduced total GHG emissions across the storage-to-land-application continuum by 49%, although this benefit was partially offset by increased NH3 losses. These findings identify AD as an effective CH4 mitigation strategy for cattle slurry when combined with targeted NH3 abatement measures to maximize environmental benefits.