Production of N 2 and N 2 O from nitrate ingested by sheep

Summary Supplementing ruminants with nitrate ( NO 3 − ) reduces their enteric methane ( CH 4 ) emissions; however, the greenhouse gas ( GHG ) mitigation achieved can be partially offset by small emissions of nitrous oxide (N 2 O), a more potent GHG . Sheep were dosed intraruminally with 15 NO 3 − to investigate whether dietary NO 3 − is a precursor of N 2 O and/or di‐nitrogen gas (N 2 ), and to quantify the amounts of NO 3 − recovered as N 2 O and N 2 in gas emissions from sheep adapted or not adapted to dietary NO 3 − . Ruminally cannulated sheep were adapted to a hay diet supplemented with NO 3 − ( n  = 3; 10 g NO 3 − /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 NO 3 − and quickly moved into gas‐tight chambers to enable recovery of 15 N in N 2 O and N 2 to be measured. Measurements of gases accumulating in the chambers were made over 10 successive 50 min periods; this enabled the amount of N 2 O produced, and the recovery of 15 NO 3 − ‐N in N 2 O and N 2 to be determined over a total of 10 hr. Only 0.04% of labelled NO 3 − ‐N was recovered as N 2 O, and this was not dependent ( p  > .05) on whether or not the animals had been adapted to dietary NO 3 − . Approximatively 3% of 15 NO 3 − ‐N was recovered as 15 N 2 , which was also not dependent ( p  > .05) on whether sheep had been adapted to NO 3 − . Because the kinetics of rumen ammonia ( NH 3 ) were uncertain, the recovery of 15 N from NO 3 − in rumen NH 3 could not accurately be quantified, but our results suggest that approximately 76% of dietary NO 3 − was converted to NH 3 in the rumen. We conclude that the small amount of NO 3 − recovered in N 2 was evidence of denitrification, previously thought not to occur in the rumen.