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 (NO3-)-N-15 to investigate whether dietary NO3- is a precursor of N2O and/or di-nitrogen gas (N-2), and to quantify the amounts of NO3- recovered as N2O and N-2 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 (NO3-)-N-15 and quickly moved into gas-tight chambers to enable recovery of N-15 in N2O 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 N2O produced, and the recovery of (NO3-)-N-15-N in N2O and N-2 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 (NO3-)-N-15-N was recovered as N-15(2), 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 N-15 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 N-2 was evidence of denitrification, previously thought not to occur in the rumen.