Objectives Rapeseed production efficiency heavily relies on reasonable nitrogen (N) supply. The objective was to unravel the soil N turnover and rapeseed N uptake in two common rotation systems, which would provide guidance for the N fertilizer management.

Methods A split plot experiment was conducted at the Experiment Farm in Huazhong Agricultural University, Wuhan, China. The main treatments were composed of two rotation systems, that was rice–rapeseed rotation (RR) and cotton–rapeseed rotation (CR). The sub-treatments were the application season of nitrogen fertilizers, included: 1) no nitrogen application in both seasons(N_0-0); 2) N 150 kg/hm² only in rice or cotton(N_150-0); 3) N 150 kg/hm² only in rapeseed season(N_0-150). The net soil nitrogen mineralization amount in different growing stages of rapeseed were measured using in situ mineralization culture method, and the N uptake in the corresponding stages of rapeseed were measured at the same time.

Results The capacity of soil N supply and the total N uptake of rapeseed were considerably enhanced by N fertilizer application. Compared to zero N fertilizer input, the net soil N mineralization accumulations were increased by 101.2 kg/hm² in RR and 110.0 kg/hm² in CR, respectively, and rapeseed N uptake were increased by 76.8 kg/hm² in RR and 96.2 kg/hm² in CR, respectively, when N 150 kg/hm² was applied only in rapeseed season. From the distribution proportion, the net soil N mineralization during rapeseed growth period (seeding−stem-elongation period) in CR was more than that in RR, which accounted for 64.5% and 52.3%, respectively; while this tendency was inverted in the late growth stage of rapeseed (flowering−maturity period). The trend of rapeseed N uptake was similar to the net soil N mineralization. The rapeseed N uptake in CR was 37.1 kg/hm² more than that in RR during rapeseed growth period (seeding−stem-elongation period), while 18.2 kg/hm² less than that in RR during the late growth stage of rapeseed (flowering−maturity period), suggesting CR is conducive for rapeseed N uptake in early stages while RR is beneficial to the rapeseed N uptake in late season.

Conclusions In the cotton−rapeseed rotation system, the rapid release of nutrients from the residual cotton leaves is beneficial to the N supply in the early growth stage of rapeseed; while in the rice−rapeseed rotation, the slow release of nutrients from rice root residues is beneficial to the N supply in the late growing stage of rapeseed. Thus the N fertilizer should be more applied on the early growing stages of rapeseed in the rice−rapeseed rotation, and in the late stages of rapeseed in the cotton-rapeseed rotation.