Nitrogen Loss Potential by Nitrification/Denitrification As Affected by N-Source in Rice.

Nitrification and subsequent denitrification loss of applied N in rice production have been studied minimally. Hypothetically, this loss mechanism could greatly reduce agronomic N use efficiency. Laboratory incubations were conducted to quantify the nitrification potential of a Sharkey clay soil. Within 2 d of incubation, 37% of the recovered N was in the NO3- form and increased to 79 and 93% for 9 and 15 d, respectively. Based on these findings, a field study was conducted in 2011 to evaluate the effectiveness of two nitrification inhibitors (NI), nitrapyrin and dicyandiamide (DCD). Furthermore, a sulfur-polymer coated urea product was also evaluated. Nitrapyrin was applied at 0.5, 1.0, and 1.5 kg ha-1, whereas DCD was applied at 5, 10, and 15% (N basis). Both were delivered with urea liquor applied at 84 and 168 kg N ha-1 14 days before permanent flood establishment (dbf). The controls included N applied at the same rates without nitrapyrin and DCD 14 dbf, as well as 1 dbf. DCD applied at a minimum of 12.5 kg ha-1 resulted in grain yields being 12% greater than the urea alone applied 14 dbf. Increasing the DCD rate to 19 kg ha-1 resulted in a 16% yield advantage. However, all DCD applications resulted in yields less than the standard 1 dbf application. Nitrapyrin applied with 84 kg N ha-1 resulted in 12 and 10% greater yields for 1.0 and 1.5 kg nitrapyrin ha-1, respectively, compared to urea without nitrapyrin. Yield reductions were observed relative to the 1 dbf application. The sulfur-polymer coated urea proved to be the best at stabilizing urea from nitrification loss; however, at both 84 and 168 kg N ha-1, a 5% yield loss still occurred relative to the 1 dbf application. Research is on-going to address nitrification/denitrification loss in rice production.