Production of N₂ and N₂O 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 (N2O), a more potent GHG. Sheep were dosed intraruminally with 15NO3 to investigate whether dietary NO3 is a precursor of N2O and/or di-nitrogen gas (N2), and to quantify the amounts of NO3 recovered as N2O 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 15NO3 and quickly moved into gas-tight chambers to enable recovery of 15N in N2O 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 N2O produced, and the recovery of 15NO3-N in N2O and N2 to be determined over a total of 10 hr. Only 0.04% of labelled NO3-N was recovered as N2O, and this was not dependent (p > .05) on whether or not the animals had been adapted to dietary NO3. Approximatively 3% of 15NO3-N was recovered as 15N2, 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 15N 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.