Nitrogen Source and Timing of Application: What We Have Learned.

Unused agricultural applications of nitrogen (N) by corn (Zea mays L.) can contribute to nitrous oxide (N₂O) emissions, water quality concerns, and overall reduction in farmer’s profitability.     The objective of the study was to investigate the potential of different N sources to reduce N₂O emissions and to evaluate various N sources and timings of application on N use efficiency. Several studies were conducted near Urbana, Illinois on highly productive mollisols between 2007 and 2012 using various combinations of N sources: anhydrous ammonia (AA) with and without NServe; urea; urea-ammonium nitrate (UAN) with and without Instinct, and ESN (polymer coated urea), and application times: fall, spring, and sidedress. Nitrous oxide fluxes using vented non-steady state closed chambers were measured on selected treatments one to three times per week early in the growing season and less frequently at the end of the season between 2009 and 2011. Grain yield was collected from all treatments. Grain yield response to time and source of N was largely affected by yearly conditions, illustrating the complexity and challenge of N management under field conditions. Averaged over six distinct growing seasons, corn yield was 7% greater for spring AA than fall AA with NServe. Sidedress applications did not improve yield over preplant applications, and in dry years, sidedressing or splitting the application between preplant and sidedress reduced yield and N use efficiency. Regardless of N source, N₂O-N emissions were near background levels except immediately after substantial (>20 mm) single or 2-3 day cumulative rains during June and July. Emission differences for treatments occurred in 2010; the only year with high N₂O-N emissions. In 2010, cumulative emissions were lowest for the check (2.21 kg N₂O-N ha^-1) followed by ESN (9.77 kg N₂O-N ha^-1), and ESN was significantly lower than urea (14.07 kg N₂O-N ha^-1) and AA (16.89 kg N₂O-N ha^-1). Across years, mean cumulative emission for ESN was 4.95 kg N₂O-N ha^-1and 39% lower than AA, while urea emitted intermediate amounts. The data indicate that ESN can maintain high-corn yield levels while mitigating N₂O-N emissions relative to AA.