Split application of urea does not decrease and may increase nitrous oxide emissions in rainfed corn

Modification of 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 nitrous oxide 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 before 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.