Nitrous oxide and nitric oxide emissions and nitrogen use efficiency as affected by nitrogen placement in lowland rice fields

Urea deep placement (UDP) has demonstrated its benefits of saving N fertilizer and increasing nitrogen use efficiency (NUE) and grain yields. However, studies on its environmental impacts, particularly on nitrous oxide (N2O) and nitric oxide (NO), are limited. We conducted multi-location field experiments in Bangladesh to determine the effects of UDP versus broadcast prilled urea (PU) on N2O and NO emissions, NUE, and rice yields. N2O and NO emissions were measured from three N fertilizer treatments-no N, UDP, and PU-using automated gas sampling and analysis systems continuously for two rice-growing seasons-Aus (May-August) and Aman (August-December). Fertilizer-induced peaks in N2O emissions were observed after broadcast application of PU but were rarely observed after UDP. Total seasonal N2O and NO emissions, yield-scaled emissions, and fertilizer-induced emissions were affected by fertilizer treatments and sites. Though nitrogen fertilizer increased emissions significantly over the control, emissions resulting from UDP and PU were similar. Effects of N placement on grain yields and NUE were site- and season-specific. Of the N placement methods, UDP increased grain yields by 13% (p < 0.05) during the Aman season and gave similar yields in spite of lower N application during the Aus season. UDP increased N recovery from 25 and 16% of broadcast PU to 61 and 73% during the Aus and the Aman seasons, respectively in one site, but was similar in another site. On the other hand, alternate wetting and drying irrigation reduced grain yield and N recovery at the BRRI site during the Aman season.