Production of N₂ and N₂ O from nitrate ingested by sheep

Area of Science:

• Agricultural Science
• Environmental Science
• Animal Science

Background:

• Nitrate (NO3-) supplementation in ruminants reduces enteric methane (CH4) emissions, a key greenhouse gas (GHG).
• However, this mitigation can be offset by nitrous oxide (N2O) emissions, a more potent GHG.
• Understanding the fate of nitrate in ruminant digestion is crucial for optimizing GHG mitigation strategies.

Purpose of the Study:

• To investigate if dietary nitrate (NO3-) is a precursor for nitrous oxide (N2O) and/or di-nitrogen gas (N2) emissions in sheep.
• To quantify the recovery of NO3- as N2O and N2 in gas emissions.
• To determine if adaptation to dietary nitrate influences these N2O and N2 emissions.

Main Methods:

• Sheep were dosed intraruminally with 15N-labeled nitrate (15NO3-).
• Animals were housed in gas-tight chambers for 10 hours to measure N2O and N2 accumulation.
• Nitrate adaptation (NO3- or urea supplemented diet) was compared to non-adapted states.

Main Results:

• Only 0.04% of labeled nitrate-nitrogen was recovered as N2O, irrespective of adaptation status.
• Approximately 3% of nitrate-nitrogen was recovered as N2, also independent of adaptation.
• An estimated 76% of dietary nitrate was converted to ammonia (NH3) in the rumen.

Conclusions:

• Dietary nitrate is not a significant precursor for N2O emissions in sheep.
• The recovery of N2 suggests denitrification occurs in the rumen, contrary to previous assumptions.
• Nitrate supplementation offers a viable GHG mitigation strategy with minimal N2O byproduct.