Nitrous Oxide – no laughing matter

A great deal of focus is given to CO2 emissions and their role in the greenhouse effect, however the public is less aware of other powerful greenhouse gases.  Atmospheric nitrous oxide (N2O) concentrations have been rising since the Industrial Revolution due to anthropogenic activities. It is estimated that N2O accounts for 6% of anthropogenic greenhouse gas emissions. Due to its atmospheric lifetime of approximately 120 years combined with its heat-trapping effects,  N2O has approximately 310 times more warming power than CO2 on a per molecule basis.
Sources of nitrous oxide can be both natural and anthropogenic.  Natural sources, produced from various biological processes in soil and aquatic habitats, account for approximately 60% of total global annual N2O emissions (IPCC, 2001c). The primary anthropogenic source of N2O is application of nitrogen fertilizer in agriculture, accounting for approximately 35% of total global annual N2O emissions. In the 20th century, rapid expansion of agriculture combined with intensified land use contributed to an 80% net increase in atmospheric N2O. An accelerated rate of N2O accumulation in the atmosphere is predicted for the coming decades due to increased nitrogen fertilizer application required to feed an ever-growing population (Kroeze et al. 1999).
The main soil processes involved in N2O production are nitrification (an aerobic process where ammonia is converted to N2 and N2O) and denitrification (an anaerobic where nitrate is converted to N2 and N2O).  Nitrification is common with the use of organic N fertilizers, and denitrification is common in wetlands and riparian zones. Factors affecting N2O production are soil wetness, oxygen availability and additions of nitrogen to the soil.  Ideally, plants uptake the nitrogen and grow, but as a general rule, 1% of applied fertilizer gets emitted as nitrous oxide.  The more fertilizer that gets applied, the higher the N2O emissions, so fertilizer-intense crops such as corn yield higher N2O emissions than crops with less external N application.
Kroeze and Mosier (2000) estimate that more efficient nitrogen use could decrease agricultural soil N2O emissions by as much as 35% globally, with greater savings in the input-intensive systems of North America, Europe, and Russia. Management practices to increase nitrogen-use efficiency include:

•    use of inorganic nitrogen (nitrate) fertilizers (less N2O leaked to the atmosphere)
•    timing fertilizer application during the cool moist months
•    injection of fertilizer in no till systems reducing N2O emissions due to surface runoff
•    keeping farm equipment off wet soil to minimize soil compaction,
•    planting more nitrogen-efficient crops.

These management practices are required to balance nitrogen flows for water and air quality as well. Nitrogen introduced to aquatic systems by agricultural runoff and atmospheric deposition changes the activity of nitrifiers and denitrifiers. The water filtration capacity of wetlands and riparian buffers removes nitrogen from the water and emits it to the air.  This function of wetland soils is vital for water quality as it filters farm runoff, but it also increases N2O emissions as a result.  The best solution is to prevent excess nitrogen from reaching riparian and aquatic systems by improving agricultural nitrogen management.