UPTAKE AND UTILIZATION OF 5-SPLIT NITROGEN TOPDRESSING IN AN IMPROVED AND A TRADITIONAL RICE CULTIVAR IN THE BHUTAN HIGHLANDS

Abstract

SUMMARY: The uptake of urea fertilizer (N_DFF), applied with 150 kg nitrogen (N) ha⁻¹, topdressed in five splits of 30 kg N ha⁻¹ (30 N) each at 7, 26, 45, 70 and 83 days after transplanting (DAT) of rice (Oryza sativa L.), was investigated in an improved (Khangma Maap, KM) and a traditional (Janam, JN) cultivar in Bhutan highlands, using enriched ¹⁵N stable isotope. The treatments were arranged in a split–split plot design, with N fertilizer levels as main plots, cultivars as subplots and topdressing treatments as sub-subplots, with all the sub-subplots receiving the same dose except different timing of one split of enriched ¹⁵N to determine partial N fertilize use efficiency at each split dose. Although cultivar differences were not recorded in soil N accumulation and in total dry matter N, KM produced 21% higher grain yields compared to JN due to higher grain harvest index and partial factor productivity of N. Irrespective of the cultivars, topdressing timing had significant effects on N_DFF, with highest mean N recovery (REN) of 29% of applied 30 N at 45 DAT during active tillering stage, resulting in mean N_DFF total (grain + straw) uptake of 8.71 kg N ha⁻¹ compared to least effective topdressing timing at 7 DAT with mean REN of 12% and N_DFF total of 3.51 kg N ha⁻¹. In similarity to topdressing at 45 DAT, topdressing at 70 DAT (panicle initiation stage) was equally effective with mean REN of 27% across the cultivars. Hence, fertilizer N topdressing recommendations that combine use of improved cultivars with N applications timed to coincide with maximum crop demand at 45 and 70 DAT, could enhance N fertilizer use efficiency for increased rice yields as well as reduce N losses downstream, which can cause adverse off-site environmental effects.