Title: Late-season nitrogen applications improve grain yield and fertilizer-use efficiency of dry direct-seeded rice in the tropics
Abstract: Efficient nitrogen (N) management plays an important role in rice cultivation. Previous studies in the tropics mainly focused on split-N applications before panicle initiation in transplanted rice. In this study, we examined the effects of top-dressing at panicle initiation and flowering of dry direct-seeded rice on yield and fertilizer-use efficiency in field experiments at the International Rice Research Institute, the Philippines, in the dry and wet seasons. Two high-yielding indica cultivars (Rc222 and Rc282) were drill-seeded under four split-N application regimes and a control with no supplemental N. The timing of the split-N applications significantly affected grain yield, even though the total N input was unchanged. The late-season N application at heading increased yield by 6% (0.49 t ha−1) in the dry season and 7% (0.49 t ha−1) in the wet season. The N recovery efficiency also increased with an increasing proportion of N applied at later growth stages. The yield advantage of the late-season N application over the early-season N application (seeding to panicle initiation) may be attributable to delayed senescence (maintenance of more and greener leaves toward maturity), higher canopy photoassimilation, greater grain-filling, and a higher harvest index. Our results underline the importance of late-season N application in dry direct-seeded rice under favorable lowland conditions in the tropics. The optimal split-N regimes during the vegetative stage may depend on the season or temperature regime, so this subject requires additional investigation.