Molecular speciation and aromaticity of biochar-manure: Insights from elemental, stable isotope and solid-state DPMAS...

The use of biochar (BC) in feedlot cattle diets has recently been explored as an approach to simultaneously improving animal production and reducing enteric methane (CH₄) emissions. This study examines the impact of BC on manure properties and whether BC affects manure composition and carbon (C) and nitrogen (N) outputs from feedlot steers offered a barley-based diet with BC at 0.0, 0.5, 1.0 and 2.0% (BC₀, BC_0.5, BC₁ and BC₂) of diet dry matter. Manure was sampled three times over a 235 day feeding trial conducted in southern Alberta, Canada. Results showed that BC₂ increased total C and the C/N ratio by 5.7 and 6.6% relative to BC₀, respectively (P < 0.05), while total N exhibited a quadratic response from BC₀ to BC₂ (P = 0.005). Manure ¹⁵δN signatures, ranging from +3.83 to +7.34‰, were not affected (P > 0.05) by BC treatment. DPMAS ¹³C NMR revealed similar structural features among BC₀ and BC₂; indigestible BC had a minor impact on the bulk-C speciation of manure organic matter (OM). Compositional changes were limited to the aromatic-C region of the ¹³C NMR spectra. Fused-ring domains, mainly pyrogenic-C, were increased by 1.56-fold at BC₂ relative to BC₀. Overall, results demonstrated that BC stabilizes recalcitrant-C in manure OM, potentially sequestering soil-C when applied to croplands. This approach provides an added value to its use in ruminant diets, mainly from a nutrient cycling perspective. However, whole-farm studies are further required to validate the incorporation of BC into beef production systems.