One-Time Root-Zone Nitrogen Fertilisation Allows High Yields and Labour Savings During Oilseed Rape Production in the Yangtze River Delta Region of China

The one-time root-zone application of urea (RZU) technique has been proved to have great potential in increasing cereal crops (e.g. rice and summer maize) yields and nitrogen-use efficiency (NUE). However, the beneficial effects of the RZU technique on oilseed rape ( Brassica napus L.) production have not been documented. Thus, this study was conducted to explore the potential of RZU technique in improving seed yield and NUE in oilseed rape production. The effects of the RZU technique for two oilseed rape planting methods (transplanted oilseed rape (TOR) and directly sown oilseed rape (DOR)) on seed yield, NUE, economic benefit, and N recovery of 15 N labelled urea were compared with the conventional split application of urea (CSU) in 2019 and 2020. The results showed that the RZU technique for TOR and DOR significantly ( p < 0.05) increased seed yields by 7.55–17.4% and 11.5–19.7%, NUE by 32.6–33.8% and 28.0–30.5%, and economic benefit by 14.4–32.0% and 12.6–26.8%, respectively, compared with the CSU technique at the same N application rate. In addition, 15 N-urea recovery efficiency increased from 36.7% in CSU to 62.0% in RZU for TOR and from 44.0% in CSU to 78.0% in RZU for DOR. These results confirm that urea application using the RZU technique can be effective as a one-time application for the oilseed rape growth period. The RZU technique has broad application prospects when this technique is further optimised and combined with suitable fertilisation equipment.