Floodwater Nitrogen Concentration of Drill-Seeded, Delayed Flood Rice Fertilized Preflood and Midseason.

Nitrogen (N) fertilizers have the potential to enter surface waterbodies via irrigation return flow from rice [Oryza sativa (L.)] fields.  Fertilizer-N rate and application timing (preflood v. midseason) effects on N floodwater concentrations were investigated in a dry-seeded, delayed-flood cultural system on a Dewitt silt loam. Nitrogen was applied preflood onto dry soil (0, 67, 134, and 202 kg N/ha) arranged in a randomized block replicated four times.  The midseason N study was a split-plot arrangement with two preflood-N rates (67 and 134 lb kg N/ha) and midseason treatments consisting of single application rates (67 [with and without rice] and 134 kg N/ha), a control (0 kg N/ha), and a split application (34 + 34 kg N/ha). Water samples were collected daily for the first week and then once per week for the remainder of the season.  Floodwater N concentrations (18.9 mg N/L maximum) from preflood-N rates of 67 and 134 kg N/ha decreased to control levels (0 kg N/ha) within 6 days of application, but 202 kg N/ha at preflood caused floodwater N concentrations to remain above control levels for up to 11 days. Midseason N fertilizer applications had higher floodwater N concentrations compared to preflood-N fertilizer applications.  When 34 kg N/ha was applied at midseason, floodwater concentrations reached a maximum 1 day after fertilization (~14 mg N/L) and decreased to control levels within 4 days after fertilization. The 67 kg N/ha rate applied at midseason reached peak concentration (32 mg N/L) 1 day after fertilization and reached control levels within 5 days. Based on this study, floodwater retention to minimize N loss is needed for a minimum of 6 days for typical preflood and midseason applications, and 11 days for application rates above 134 kg N/ha.