Nitrous Oxide is the number three greenhouse gas in the atmosphere in terms of effect (quantity x potency)
Annual Nitrous Oxide levels (parts per billion)
Global Nitrous Oxide Levels by Month
Chart obtained from the NOAA Earth System Research Laboratory http://www.esrl.noaa.gov/gmd/hats/combined/N2O.html
Chart updated here each year, usually in late January
Since the levels of Nitrous Oxide have been monitored starting in the late 1970's, the data shows a relentless mostly linear increase.
While the trend is up, the rate of increase is far less than CO2 and there is no huge stockpile of stored Nitrous Oxide to be unleashed if the trend continues, the way there is with Methane.
According to the EPA (http://epa.gov/climatechange/ghgemissions/gases/n2o.html) the sources of Nitrous Oxide emissions in the United States are:
75% - Agricultural Soil Management
6% - Industry or Chemical Production
5% - Stationary Combustion
5% - Other
5% - Manure Management
4% - Transportation
January 22, 2018: The chart produced by NOAA in January 2018 has been posted - The gradual increase continues at about the same linear rate since the late 1970's.