Multiple Site On-Farm Comparison of Nitrous Oxide Flux Response to Nitrogen Rate.

High nitrogen (N) fertilizer rates have been increasingly scrutinized given their potential to lead to increases in atmospheric nitrous oxide (N₂O).  Previous studies have indicated large increases in N₂O flux at N rates greater than that required to maximize corn grain yield.  Thus, environmentally significant reductions in N₂O emissions may be possible with relatively little impact on grain yield or economic return.  To test this hypothesis, experiments were conducted in Michigan at three farms and one experiment station, all planted to corn, in 2007 and in 2008.  Six rates of nitrogen fertilizer (0-225 kg N ha^-1) were broadcast and incorporated prior to planting.  Field measurements included N₂O flux via static chambers, soil temperature, soil moisture, soil inorganic nitrogen (0-10 cm), and corn grain yield.  Measurements were taken immediately prior to fertilization, then on a 7-10 day frequency throughout the growing season.  Additional measurements were collected immediately following fertilization and rain events at the frequency of every 2-3 days.  Within a week of fertilization, an N₂O flux response to fertilizer could be detected.  The majority of the total N₂O emissions occurred within 50 days of fertilization.  On an individual site basis, linear and nonlinear increases in N₂O flux as a function of N rate were observed.  However, a nonlinear type of flux response better described all sites and years.  Background N₂O flux was similar across sites and years (at 0 kg N ha^-1, 4.6 ± 0.7 g N₂O-N ha^-1 day^-1) while much variability among sites was observed at high N rates (at 225 kg N ha^-1 mean rates ranged from 6.2 to 98.9 g N₂O-N ha^-1 day^-1).  Application of N rates at or slightly below that required for maximum grain yield significantly reduced total N₂O emissions. 

 

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