Production of N₂ and N₂ O from nitrate ingested by sheep

Supplementing ruminants with nitrate (NO3-) reduces their enteric methane (CH₄ ) emissions; however, the greenhouse gas (GHG) mitigation achieved can be partially offset by small emissions of nitrous oxide (N₂ O), a more potent GHG. Sheep were dosed intraruminally with ¹⁵ NO3- to investigate whether dietary NO3- is a precursor of N₂ O and/or di-nitrogen gas (N₂ ), and to quantify the amounts of NO3- recovered as N₂ O and N₂ in gas emissions from sheep adapted or not adapted to dietary NO3-. Ruminally cannulated sheep were adapted to a hay diet supplemented with NO3- (n = 3; 10 g NO3-/kg DM) or urea (n = 3; 5.3 g urea/kg DM). On the day of the experiment all sheep were dosed intraruminally with ¹⁵ NO3- and quickly moved into gas-tight chambers to enable recovery of ¹⁵ N in N₂ O and N₂ to be measured. Measurements of gases accumulating in the chambers were made over 10 successive 50 min periods; this enabled the amount of N₂ O produced, and the recovery of ¹⁵ NO3--N in N₂ O and N₂ to be determined over a total of 10 hr. Only 0.04% of labelled NO3--N was recovered as N₂ O, and this was not dependent (p > .05) on whether or not the animals had been adapted to dietary NO3-. Approximatively 3% of ¹⁵ NO3--N was recovered as ¹⁵ N₂ , which was also not dependent (p > .05) on whether sheep had been adapted to NO3-. Because the kinetics of rumen ammonia (NH₃ ) were uncertain, the recovery of ¹⁵ N from NO3- in rumen NH₃ could not accurately be quantified, but our results suggest that approximately 76% of dietary NO3- was converted to NH₃ in the rumen. We conclude that the small amount of NO3- recovered in N₂ was evidence of denitrification, previously thought not to occur in the rumen.