Lowland rice response to nitrogen fertilization

Nitrogen deficiency is one of the most important nutritional disorders in lowland rice producing areas around the world. Nitrogen fertilizer recommendations for lowland rice (Oryza sativa L.) varieties grown on Inceptisols are limited. The objective of this study was to evaluate the response of lowland rice (cv. Metica 1) to added N and to determine N use efficiency and nutrient accumulation during the crop growth cycle. A field experiment was conducted during 3 consecutive years in the central part of Brazil on a Haplaquept Inceptisol. Nitrogen levels used were 0, 30, 60, 90, 120,150, 180, and 210 kg N ha(-1). Nitrogen fertilization significantly increased dry matter and grain yield. Ninty percent of the maximum grain yield (6400 kg ha(-1)) was obtained with the application of 120 kg N ha(-1) in the first year of experimentation. In the second and third years, 90% of the maximum yields (6345 and 5203 kg ha(-1)) were obtained at 90 and 78 kg N ha(-1), respectively. Yield components were also significantly affected by N treatments. Among yield components, panicle length and panicle number m(-2) had highest correlations with grain yield (r = 0.70** and 0.78**); maximum grain yield across the 3 years was achieved at about 583 panicles m(-2). Dry matter production and grain yield at the highest N level (210 kg N ha(-1)) across the 3 years were 9423 and 6389 kg ha (1), respectively. At this grain + straw yield, the rice crop accumulated 139 kg N, 26 kg P, 218 kg K, 36 kg Ca, 24 kg Mg, 850 g Zn, 5971 g Mn, 125 g Cu, 4629 g Fe, and 104 g B. Nitrogen use efficiency defined in several ways, decreased with increasing N rates. Agronomic efficiency across 3 yeras averaged over N rates, was 23 kg of grain produced per kg of N applied. Physiological efficiency was 146 kg biological yield (grain + straw) produced per kg of N accumulated across the 3 year and N rates. Average agrophysiological efficiency was 63 kg grain produced per kg of N accumulated in the grains plus + straw. Apparent N recovery efficiency was 39% across the 3 years and N rates. Average nitrogen utilization efficiency was 58 kg of grain produced per kg N utilized by the rice crop. Soil pH and calcium concentration in the soil decreased significantly at higher N rates, whereas, soil Al3+ level was significantly increased after the harvest of the third rice crop.