Soil N2o Emissions From Irrigated Corn Amended With Manure and Inorganic N.

Nitrous oxide (N₂O) is a potent greenhouse gas that also impacts atmospheric chemistry and stratospheric ozone levels. Nitrogen (N) amendments to soil are required to maintain high crop yields but also provide the substrate for microbial processes (nitrification and denitrification) that result in N₂O production. We measured N₂O emissions during two growing seasons (2-3 times per week) for continuously cropped irrigated corn in northern Colorado amended with dairy manure, urea, Super U (a stabilized N source), and a check treatment that received no added N.  Manure was applied in March of both years while the mineral N fertilizers were applied at crop emergence in May, and watered into the soil the day after application with ~20 mm irrigation water. All plots were roto-tilled after manure application in March, and mineral N (urea, Super U) was broadcast applied at a rate of 179 kg N/ha in May 2012 and 2013. In the case of manure, we assumed that  ~40% of the added N would be converted to mineral N form annually.  A total of 428 and 315 kg N/ha of manure N was applied in 2012 and 2013 respectively.  N₂O emissions were highest from the urea, followed by dairy manure, Super U, and the check in 2012. After subtracting emissions from the check treatment, N₂O emission factors per unit of applied N were approximately 0.8% for urea, 0.6% for manure, and 0.45% for the Super U in 2012. Grain yield did not vary with N sources but was significantly lower for the check treatment. The study shows that judicious choice of N can reduce N₂O emissions in Central Great Plains cropping systems.