Production of N2 and N2O from nitrate ingested by sheep

Abstract

Supplementing ruminants with nitrate (NO− 3 ) 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 15NO- 3 to investigate whether dietary NO− 3 is a precursor of N2O and/or di-nitrogen gas (N2), and to quantify the amounts of NO− 3 recovered as N2O and N2 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 15NO− 3 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 15NO− 3-N in N2O and N2 to be determined over a total of 10 hr. Only 0.04% of labelled NO− 3-N was recovered as N2O, and this was not dependent (p > .05) on whether or not the animals had been adapted to dietary NO− 3. Approximatively 3% of 15NO− 3-N was recovered as 15N2, which was also not dependent (p > .05) on whether sheep had been adapted to NO− 3. Because the kinetics of rumen ammonia (NH3) were uncertain, the recovery of 15N from NO− 3 in rumen NH3 could not accurately be quantified, but our results suggest that approximately 76% of dietary NO− 3 was converted to NH3 in the rumen. We conclude that the small amount of NO− 3 recovered in N2 was evidence of denitrification, previously thought not to occur in the rumen.