Variable rate precision application of feedlot cattle manure mitigates soil greenhouse gas emissions

Solid cattle manure amendments provide a low-cost alternative nutrient source to inorganic fertilizers, while providing a carbon input to the soil. The augmented soil organic carbon levels, however, may be largely offset by manure-related greenhouse gas (GHG) emissions. Soil nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2) emissions were measured at the landscape-scale in a Canadian prairie agricultural field supporting silage barley (Hordeum vulgare L.) production. Manure was applied to meet barley P requirements, while total N rate was supplemented using anhydrous ammonia. A non-manured control (NMC) also was included, to calculate N2O emission factors. The NMC zone consisted of an annual application of anhydrous ammonia at 80 kg N ha−1. In addition to solid cattle manure at a constant (CRM; 45 Mg ha−1) or variable (VRM; 0–72 Mg ha−1) rate, the manured treatment zones also received 80 kg N ha−1 of anhydrous ammonia. The VRM treatment included set-backs from the watershed basin centers in ephemeral wetlands that did not receive solid cattle manure. Gas samples were collected using chamber-based methodology, with chambers installed at 130 locations across six watershed basins (n = 2 per zone) during 2019–2021. Cumulative N2O emissions were 76 % (CRM) and 62 % (VRM) higher following manure addition. The normalized N2O emissions for CRM were 24 % greater than VRM and NMC, with CRM having 31 % larger manure-induced N2O emissions than VRM. Though all soils were net CH4 sinks, manure application reduced CH4 consumption by 33 % (CRM) and 25 % (VRM) compared with the NMC. Manure addition did not impact cumulative CO2 emissions. Although VRM application mitigated manure-related GHG emissions, enhanced GHG intensity following manure addition highlights the importance of ensuring balanced soil fertility, to support optimal crop growth and maximize yield-scaled GHG performance metrics in manured landscapes.