Fertilizer Deep Placement as One Way of Increasing Nitrogen Use Efficiency and Grain Yield in West African Irrigated Rice Systems

Nitrogen (N) is the main nutrient that is limiting rice yields (Oryza sativa L.) in West Africa, and it loss can be very high particularly in the irrigated rice systems with poor water control. Previous studies have reported very low (30%) fertilizer N use efficiency by broadcasting in irrigated cropping systems. And N deficiency can negatively affect phosphorus and potassium plant uptake and reduce rice yields. The effect of fertilizer urea (46% nitrogen – N) broadcast or deep placed (FDP) using urea super granules (USG) on rice yield performance and nutrient uptakes was investigated in West Africa. Fields and pot experiments were carried out in Burkina Faso in the wet and dry season of 2012 and 2013, respectively. The main objective of the experiments was to evaluate how fertilizer N application methods – using prilled urea broadcast (PU) or briquettes (urea supergranules – USGs) affect N use efficiency in irrigated rice systems. PU was broadcast applied, and USGs were point placed deeply into the soil at 5–7 cm to two different Nerica rice varieties (FKR 19 and NERICA 62N), using same fertilizer N rates (52 kg N ha−1). The pot experiments investigated the effect of soil pH on N use efficiency, and phosphorus (P) and potassium (K) uptakes for both PU and USG application methods. Field experiments clearly indicate higher rice fertilizer nutrient N, P and K uptakes with USG than PU, resulting in significantly higher rice yields in USG-plots. Average rice grain yield was 5146 kg ha−1 with USG and 4583 kg ha−1 with PU in the wet season, and 7000 kg ha−1 and 6644 kg ha−1, respectively, in the dry season. NERICA 62N was much more responsive to USG as compared with FKR 19 variety as it produced higher tillers and panicles numbers under USG, leading to significantly higher yields as compared with FKR 19. In the wet season, USG significantly increased N agronomic efficiency (NAE) by 39.43% and physiological efficiency by 24.23% over PU. In the dry season, however, differences in N use efficiency (NUE) between USG and PU were not significant. Pot experiments indicate that soil total N was higher in acid than in alkaline soils. And rice N, P and K nutrient uptakes were significantly higher with USG than PU. These confirm the superiority of USG over PU in terms of increasing N, P and K nutrient uptakes and N use efficiency as observed in the field experiments. These studies suggest that in West African irrigated rice systems, fertilizer deep placement could be more effective in improving nutrient N, P and K uptakes, N use efficiency and irrigated rice yields. And smallholder rice farmers in West Africa may derive more benefits from using fertilizer deep placement technology than the conventional urea broadcast application method.