UPPER THRESHOLDS OF NITROGEN UPTAKE RATES AND ASSOCIATED NITROGEN-FERTILIZER EFFICIENCIES IN IRRIGATED RICE

Measurements of the N uptake rate of rice (Oryza sativa L.) hape usual ly been conducted over a period of 7 to 14 d. Such a sampling interval may not accurately determine the maximum rate, since information conc erning N uptake dynamics during the first meek after N topdressing is limited, In this study, N uptake dynamics and fertilizer N recovery ef ficiency following N topdressing were determined with more frequent sa mplings. Field experiments were conducted at International Rice Resear ch Institute (IRRI) and at the Philippine Rice Research Institute (PRR I) during the 1995 dry season. Treatments consisted of 0, 50, and 100 kg N ha(-1) applied at midtillering (MT) or panicle initiation (PT), T issue N concentration and N uptake mere measured 0, 2, 4, 6, 9, and 14 d after N application. Fertilizer N recovery efficiency was estimated using the difference method, At PI, maximum N uptake rates of 9 to 12 kg ha(-1) d(-1) were measured over the 4-d period following applicati on of 100 kg N ha(-1), which is up to double the previously reported m aximum value of 6 kg ha(-1) d(-1). The much larger estimates of maximu m N uptake rates in our study probably resulted from more frequent sam pling than in previous studies. Nitrogen uptake rates at PI were simil ar, regardless of N rate at MT. Recovery efficiency of N applied at PI reached 74 to 78%, and was significantly higher than that at MT. Reco very efficiency was relatively insensitive to the amount of N applied at MT and PI, which suggests that high N fertilizer efficiency can be achieved with large rates of applied hi when growth conditions are fav orable.