Effects of Time and Rate of Nitrogen Fertilizer Application on NUE and Yield of Rapeseed )Brassica napus L.( as Second Crop in Paddy Fields

To determine the proper nitrogen fertilizer rates and its application time for achieving to high nitrogen use efficiency and grain yield of rapeseed cultivar Hyola 401, a research work was carried out at paddy fields of Rice Research Institute of Iran in Rasht during 2005-2007 cropping seasons. A factorial experiment was conducted in randomized complete blocks design with three replications. The first factor was nitrogen fertilizer rate at five levels of 0, 60, 120, 180 and 240 kg N ha-1 from urea source. Second factor was fertilizer application time at five levels of all fertilizer at planting; 1/3 at planting + 2/3 in stem elongation; 1/3 at planting + 1/3 in stem elongation + 1/3 before flowering; 1/3 in 3-4 leaf stages + 1/3 in stem elongation +1/3 before flowering and 1/4 at planting + 1/4 in 3-4 leaf stages +1/4 in stem elongation + 1/4 before flowering.  In this experiment, some important traits such as oil content, oil and grain yields, nitrogen use efficiency (NUE), nitrogen agronomic efficiency (NAE), nitrogen physiological efficiency (NPE) and nitrogen apparent recovery fraction (NRF) were calculated. The results of the combined analysis of variance showed Significant difference was observed between nitrogen rates in all traits except in seed oil content. Application of 60 kg N ha-1 produced the maximum NAE, NUE and NPE with values of 12.45, 25.54 and 35.92 kg kg-1, respectively and application of 240 and 180 kg N ha-1 with average production of 2505 and 2596 kg ha-1, respectively showed the highest grain yield. Nitrogen application time of 1/3 at planting + 1/3 in stem elongation + 1/3 before flowering had the highest grain yield and NRF with average of 2155.3 kg ha-1 and 38.91 %. According to the results of the present experiment, it seems application of 180 kg N ha-1 produced the maximum NRF and grain and oil yields. Also, third application time decreases nitrogen loss resulting from leaching, denitrification and sublimation due to availability and proper use of nitrogen during plant growth and increases rapeseed yield by increase in nitrogen absorption.