Nitrification's Role in Nitrogen Loss of Urea Applied Preflood in Delayed-Flood Rice Production.

Nitrogen source and application time recommendations for rice are currently limited to applying an NH₄-forming N source to a dry soil surface at the 5-leaf growth stage for producers utilizing the delayed-flood production system.  Fertilization trials were conducted in Dewitt and Calhoun silt loams to compare rice yield response to urea and Super U (urea+DCD,Agrotain International) applied from planting to preflood at two N rates. Laboratory incubations were conducted to determine inorganic N concentrations across time in the Dewitt and Calhoun soils amended with urea or Super U.  For the Dewitt soil, the main effects or their interaction did not affect rice grain yields suggesting nitrification of N fertilizer was slow.  For the Calhoun soil, only the main effects of N rate and application time affected rice grain yields. Yields increased as N rate increased and were 78% greater when N fertilizer was applied 2 d before flooding (DBF, 6436 kg ha^-1) than 7 DBF (3609 kg ha^-1).  Nitrogen fertilizer applied 7 DBF was incorporated with water the same day as application suggesting significant N losses via nitrification followed by denitrification at the time of flooding.  Laboratory incubations showed that recovery of fertilizer N was similar between soils.  Nitrification of added urea-N, regardless of source, was very rapid in the Calhoun soil and relatively slow for the Dewitt soil.  The majority (>90%) of the recovered fertilizer-N was present as NO₃-N by 10 d after incorporation (DAI) for urea and 15 DAI for Super U in the Calhoun soil. For the Dewitt soil, 90% of the urea-N and 46% of the Super-U N was present as NO₃-N by 30 DAI. The nitrification inhibitor DCD slowed nitrification of urea-N in the Dewitt soil, but had minimal effect on nitrification in the Calhoun soil. Results show nitrification rate and inhibition by DCD differs among soils and appears to play an important role in fertilizer N use efficiency for rice.