Yield and nitrogen leaching in rapeseed field under different nitrogen rates and water saving irrigation

Irrigation water is limiting for crop production in arid and semi-arid areas and excess nitrogen (N) application is a source of groundwater contamination. Therefore, alternate furrow irrigation can be used as water saving irrigation (WSI) and a controlling measure of groundwater N contamination. The objectives of this investigation were to evaluate the effect of ordinary furrow irrigation (OFI), variable alternate furrow irrigation (VAFI) and fixed alternate furrow irrigation (FAFI) and different N application rates (0, 100, 200, and 300kgha⁻¹) on rapeseed yield and yield quality, drainage water, N leaching, uptake and N use efficiency. Results indicated that in terms of seed yield the VAFI is superior to FAFI and it is equivalent to the full irrigation (OFI). Therefore, VAFI is a water saving irrigation in the study region (25% reduction in water use) even under drought conditions with 175mm of rainfall occurred mostly in winter. Furthermore, based on the seed yield, nitrogen use efficiency (NUE) and water use efficiency (WUE), it was concluded that VAFI with 200kgNha⁻¹ is appropriate irrigation and N fertilizer management for rapeseed in the study region. However, based on the seed oil, protein yields, oil yield based WUE and apparent N recovery, VAFI with 300kgNha⁻¹ is best treatment. Leaching during the growing season could be reduced by using VAFI especially under conditions with low rainfall in winter. Only in FAFI, N uptake decreased and soil residual N was increased as compared with OFI and VAFI. Thus, in order to avoid N losses, the amount of N fertilizer should be reduced in proportion to the amount of soil water available for plant water uptake under water saving irrigation (VAFI).