Differences in soil nitrogen transformation and the related seed yield of winter oilseed rape (Brassica napus L.) under paddy-upland and continuous upland rotations

Abstract Differences in crop rotations significantly affect abiotic and biotic soil properties, thereby influencing soil nitrogen (N) transformation and crop growth. A three-year field experiment involving different N fertilizer inputs under rice-oilseed rape (RO) and cotton-oilseed rape (CO) rotations was implemented. 15 N-labeled approaches were used to quantify soil gross nitrogen fluxes during the different growth periods of the oilseed rape season and to estimate the nitrogen fertilizer residual effects of preceding crops on subsequent oilseed yield and shoot N uptake. The results showed that under the same N input, seed yield in the RO rotation decreased by an average of 16.9% compared with that under the CO rotation. Soil gross N transformation in oilseed rape seasons revealed different characteristics between these two rotations. During the seedling period, soil in the RO rotation revealed higher gross N immobilization rates and lower gross N mineralization rates; howev er, in the flowering period, soil N transformation showed the opposite trend. More N after cotton harvest remained in the soil layer, and 7.8% of N fertilizer applied during the cotton season could be taken up by oilseed rape in the CO rotation, which was higher than the proportion in the RO rotation. Consequently, greater soil gross N immobilization rates and lower soil N residue were more considerable for inducing lower indigenous soil N supplies and seed yield in the paddy-upland rotation. Optimizing N management depending on soil N transformation in different rotations is critical for achieving high yield and fertilizer use efficiency.