Potential for improving nitrogen use efficiency in rice-wheat rotation with soil fertility increases in the Taihu Lake region

Purpose Soil fertility and nitrogen (N) fertilizer application level are both important factors affecting the grain yield (GY) and N use efficiency (NUE) of crops, but knowledge of their integrated effects remains poor. This study aimed to reveal the comprehensive relationship between the GY and NUE of wheat and rice with the integrated soil fertility (IFI) and N application level (NA). Then, we can reveal the potential for optimizing fertilization by improving the limiting factors to IFI to reduce the NA and improve NUE in the study area. Materials and methods Eight test fields with different IFI in the Taihu Lake region of China were selected to carry out 2-year in situ tests to obtain the comprehensive quantitative relationship between NA and IFI with GY and NUE. Based on the comprehensive quantitative relationship models, the NA reduction ratio with IFI improvement for pursuing the maximum yield, maintaining the current maximum yield, and maintaining the NUE of the current maximum yield for wheat-rice rotation were calculated. The three kinds of NA reduction ratios were spatially coupled with IFI improvements to obtain the spatial distribution of NA reduction, and GY and NUE increase based on IFI increase scenario. Results and discussion The results show that increases in NA and the IFI were both beneficial to GY but inhibited NUE. The improved soil fertility had slightly larger impact as NA on the yield and NUE of the rice-wheat rotation. The soil fertility indicators, SOM, total nitrogen, and available potassium had the most significant correlations with the IFI, which were the main obstacle factors for soil fertility in study area. When the NA reduction ratios of wheat and rice were selected at 0.50 and between 1.22 and 2.34, respectively, a maximum reduction in 9.3% of NA in rice-wheat rotation can be achieved, and a maximum increase of 1.20% of GY or 1.24% of NUE can be achieved simultaneously by improving the obstacle factors of IFI in study area. Conclusions It is difficult to improve the GY and NUE of crop production simultaneously by improving the soil IFI, particularly when pursing the maximum yield. Only by enhancing the soil IFI, reducing the NA significantly and sacrificing an appropriate portion of the maximum yield can high levels of NUE be achieved in the rice-wheat rotation system. Under this hypothesis, the NA reduction with increasing efficiency of the wheat-rice rotation system, was primarily contributed by rice.