Plant secondary metabolites in beef cattle diets affect reactive nitrogen and greenhouse gas emissions during manure soil application: A laboratory incubation

Feed additives in beef cattle diets can reduce enteric greenhouse gas (GHG) emissions. However, subsequent effects on soil carbon (C) and nitrogen (N) after land application of manure from additive-fed animals remain largely unknown. In this study, manure (mainly feces) from beef cattle fed either an un-supplemented diet (UN) or a diet containing one of two essential oil-based feed additives, Agolin® (AG; 1 g/steer/day) or Mootral® (MT; 23.5 g/steer/day), were collected and applied to soils with different textures (clay or sandy loam). The soil-feces mixtures were incubated in a completely randomized block design at two moisture levels, 50 % and 90 % water holding capacity (WHC). The AG treatment, versus MT and UN, yielded lower cumulative mineralized N (N_min; p < 0.01) in clay at 90 % WHC, but not at 50 % WHC. Similarly, AG, versus MT and UN, had lower carbon dioxide (CO₂) emissions in clay at 90 % WHC (p < 0.01), but higher emissions in sandy loam at 90 % WHC (p < 0.01). There were no differences in CO₂ among treatments at 50 % WHC. Nitrous oxide (N₂O) emissions were only affected by soil type (p = 0.01) regardless of soil moisture or feed additives. Methane (CH₄) emissions were affected by soil moisture (p = 0.03) and the interaction soil × moisture × manure (p < 0.01), however the feed additives had no effect. These findings indicate that the feed additives tested may affect C dynamics and N_min in a soil and/or moisture-dependent manner.