Split Application of Urea Does Not Decrease and May Increase Nitrous Oxide Emissions in Rainfed Corn.

Modification of nitrogen (N) fertilizer application timing within the growing season has the potential to reduce soil nitrous oxide (N₂O) emissions but limited data are available to assess its effects. We compared cumulative growing season N₂O emissions (cN₂O) following urea applied to corn (Zea mays L.) in a single application (SA) at planting or in three split applications (SpA) over the growing season.  For both SA and SpA, granular urea was broadcast and incorporated at six fertilizer N rates in the corn phase of a corn-soybean [Glycine max (L.) Merr.] rotation and in a continuous corn system over two growing seasons. Daily N₂O flux was measured using chambers on 35 dates in 2012 and 40 dates in 2013 and soil nitrate-N concentration was measured weekly.  Split application did not affect grain yield and did not reduce cN₂O.  Across N rates and rotations, cN₂O was 55% greater with SpA compared with SA in 2012.  Increased cN₂O with SpA in 2012 likely resulted from a prolonged dry period prior to the second split application followed by large rainfall events following the third split application.  Across years and rotations, SpA increased cN₂O by 57% compared with SA when the maximum N rate was applied.  Exponential relationships between cN₂O and fertilizer N rate explained 62 to 74% of the variance in area-based cN₂O and 54% of the variance in yield-based cN₂O.  Applying urea to coincide with periods of high crop N demand does not necessarily reduce and may increase N₂O emissions.