Response of Crop Yield to Nitrogen Application and Optimal Nitrogen Application Rate in a Rice–Wheat Rotation System

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To provide a scientific basis for rational nitrogen application under the rice–wheat rotation (R-W rotation) system, this study examined crop yield, NUE, and changes in soil nitrogen pools in response to different nitrogen application rates for rice (0–420 kg ha−1) and wheat (0–360 kg ha−1) from 2020 to 2022 in Jiangsu Province, China. Rice and wheat yields, along with their yield components, exhibited similar responses to nitrogen fertilization. In both cropping seasons, not only did current-season nitrogen application (CN) significantly affect yields but previous-season nitrogen application (PN) and the interaction between CN and PN also had notable effects. For both crops, the impact of CN on yield was greater than that of the CN × PN interaction, which in turn exceeded the effect of PN alone. These effects diminished with an increasing number of rice–wheat rotation cycles. The yields of rice and wheat initially increased and then declined with rising rates of both CN and PN. The maximum combined yield (14,459.9 kg ha−1) was achieved with nitrogen application rates of 265.7 kg ha−1 in the rice season and 257.1 kg ha−1 in the wheat season. Yield responses to CN were primarily driven by panicle number, spikelets per panicle, and crop-specific traits, grain filling efficiency for rice, and grain weight for wheat. PN effects were mediated through its interaction with CN, with panicle number serving as the main pathway. However, when nitrogen application rates fell below 300 kg ha−1 for rice and 240 kg ha−1 for wheat, a reduction in soil nitrogen content was observed. Therefore, to achieve high yields while maintaining soil nitrogen pool stability, recommended nitrogen application rates are 300 kg ha−1 for the rice season and 257.1 kg ha−1 for the wheat season.