Nitrous Oxide

Nitrous oxide is another greenhouse gas (GHG) emitted from Canadian farms, accounting for about half the warming effect of agricultural emissions. This gas, familiar to us as laughing gas, is produced in nature by microbes as they process nitrogen in soils. All soils emit some nitrous oxide (N2O), but farm soils often emit more than others because of the nitrogen that is added to soil in the form of fertilizers, manures and other inputs.

Without these additional inputs to replace the nitrogen removed from farms in harvested grain, milk, meat, and other products, crop yields would soon decline. But as the amounts of added nitrogen increase so do potential losses into the environment, including losses of nitrogen to the air as N2O.

Typically, scientists assume that about one per cent of the nitrogen added to farm fields is emitted as N2O, though this can vary widely with soil water content which is influenced by the hilliness of the land and soil clay content (eg: wetter soils tend to have higher emissions).

Aside from the N2O released directly from soils, farms can also produce indirect emissions. This is where N2O is produced outside the farm boundaries from nitrogen leached from fields or emitted into the air as ammonia gas.

This nitrogen, once lost from the farm, can find its way into adjacent environments where it can be converted and emitted as N2O. Although not produced on farms, this N2O is from nitrogen used on the farm; hence, it must be counted as farm-derived N2O. An illustration of how nitrogen is cycled in a farm environment is presented in the diagram below.

Conceptual View of the Nitrogen Cycle on Canadian Farms
Description of this image follows.

Description – Conceptual View of the Nitrogen Cycle on Canadian Farms

Figure shows pathways of nitrogen compounds in a farm system. Movement includes transport within, import to, and export from the system. Nitrogen is imported to the farm in the form of ammonium and nitrate fertilizers. Nitrogen is exported from the system in the harvested crops and livestock products, ammonia and nitrous oxide losses to the air, and nitrates leached from the soil. Within the system, nitrogen from manure, fertilizer and soil organic matter is taken up by plants. Dinitrogen from the atmosphere may be fixed by legumes and be added to the soil as organic matter after plant residues decompose.

Nitrous oxide be produced at many points in the cycle. The term dinitrogen refers to the naturally-occurring nitrogen gas (N2) that is a major component of our atmosphere.

Nitrous Oxide Reduction

Since N2O is produced mostly from excess available nitrogen in soils, one way to suppress emissions of this gas is to apply fertilizer judiciously: adding just enough, at the right place and time, to meet crop demands, but avoiding excess amounts left over. This can reduce fertilizer costs to producers and reduce the amount of nitrogen lost through excess fertilizer application.

Fertilizer can be used more efficiently by:

  • adjusting fertilizer rates to coincide with plant needs
  • placing fertilizer near plant roots (but not too deep in the soil)
  • applying fertilizer several times each year, rather than only once
  • using slow-release forms

Similarly, using manure efficiently can also help limit N2O emissions-not only because less is released from the manure, but also because less fertilizer now needs to be used. Perhaps the most fundamental way of reducing N2O from manures is to alter feeding rations so that less nitrogen is excreted in urine and feces in the first place.

Other practices that can sometimes reduce N2O emissions from farms include:

  • greater use of legumes as a nitrogen source
  • use of cover crops (sown between successive crops) to remove excess available nitrogen
  • avoiding use of summer fallow (leaving the land unplanted, with no crop nitrogen uptake, for a season)
  • adjusting tillage intensity (sometimes, but not always, no-till practices can reduce emissions)

Most methods of reducing N2O emissions depend on improving the efficiency of nitrogen use on farms. Progress toward this aim has many other benefits:

  • it reduces the cost of production because less fertilizer is used
  • it saves on fossil fuel use (and hence CO2 emissions) because producing nitrogen fertilizer is energy intensive
  • it lessens the amounts of nitrates, ammonia and other nitrogen pollutants entering the environment

Despite much progress, the nitrogen cycle on farms still results in the leakage of N2O and reducing these leaks remains a research priority.

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