Abstract: 【Objective】 This study aimed at the frequent drought at the stem elongation stage of winter oilseed rape (Brassica napus L.) in northwest China, and local farmers usually applied heavy irrigation and nitrogen (N) fertilizer for high benefits, thus led to serious environmental problems. The objective of present study was to determine the reasonable application amount of irrigation and N fertilizer at stem elongation stage of winter oilseed rape cultivated in northwest China. 【Method】 Ten treatments, including three nitrogen levels (N₀: 0 kg·hm^-2, N₁: 80 kg·hm^-2, N₂: 160 kg·hm^-2) and three irrigation levels (I₀: 0 mm, I₁: 60 mm, I₂: 120 mm) at the beginning of stem elongation stage, and a control (CK) with no irrigation and no nitrogen (no basal nitrogen, no topdressing nitrogen) during the whole growing stage of winter oilseed rape were conducted in this two-year experiments. This study determined the effects of different irrigation and nitrogen levels on aboveground dry matter (ADM), seed yield, oil production, nitrogen uptake, soil nitrate content and nitrogen utilization efficiency of winter oilseed rape. 【Result】 Irrigation or nitrogen application at stem elongation stage significantly improved aboveground dry matter, seed yield, oil production and nitrogen accumulation amount of winter oilseed rape. The depth of soil layer with the peak of soil nitrate was moved down obviously with the increase of irrigation amount, and the peak value was increased with the increase of nitrogen application amount, which showed a clear trend of nitrate leaching with the increase of irrigation and nitrogen amount. Total soil nitrate-N accumulation amount in I₁N₁ was not significantly different with I₀N₀; however, was significantly decreased by 41.9 kg·hm^-2 in comparison to I₂N₂. Soil nitrate-N was mainly distributed in 0-40, 40-80, and 80-160 cm soil layer. Seed yield varied from 1 534 to 3 024 kg·hm^-2 and from 2 318 to 3 746 kg·hm^-2 in 2012-2013 and 2013-2014, respectively. The highest seed yield and oil production were always occurred in I₂N₁, and the lowest were always occurred under CK for both seasons. Compared with I₂N₁, seed yield in I₁N₁ was significantly decreased in 2012-2013, a drought season, while no significant differences were found between I₂N₁ and I₁N₁ in 2013-2014, a wet season. No significant differences were found between I₂N₁ and I₁N₁ in both seasons. Average seed yield and oil production in I₁N₁ were 3 264 and 1 358 kg·hm^-2 for both seasons, respectively, which were just 3.6% and 4.7% lower than I₂N₁, respectively. Nitrogen absorption amount in I₁N₁ was 7.3% significantly lower than in 2012-2013, while there were no marked differences between I₁N₁ and I₂N₁ in 2013-2014. Mean nitrogen agronomic efficiency in I₁N₁ was reduced by 7.2% in comparison with I₂N₁. 【Conclusion】 From the perspective of improving seed yield and nitrogen utilization efficiency of winter oilseed rape, and simultaneously alleviating the downward trend of soil nitrate, I₁N₁ (60 mm irrigation and 80 kg N·hm^-2) treatment can be recommended as a suitable irrigation and nitrogen schedule for winter oilseed rape at stem elongation stage.

Key words: irrigation, nitrogen application, stem elongation stage, oil production, nitrogen absorption, soil nitrate nitrogen