Predicting the Nitrogen Quota Application Rate in a Double Rice Cropping System Based on Rice–Soil Nitrogen Balance and 15 N Labelling Analysis

Excessive nitrogen (N) fertilization, low use efficiency, and heavy pollution are the dominant issues that exist in intensively cultivated double rice cropping systems in China. Two-year field and 15 N microregion experiments were conducted to evaluate the N fate in a soil-rice system under a series of different N rate treatments from 2020 to 2021. The economic N application rate that simultaneously improved rice yield and N use efficiency in the rotation system was also investigated. Results demonstrated that soil residues and mineralized N accounted for more than 58.0% and 53.2% of the total N input in the early and late rice seasons, respectively. Similarly, most of the total N input was absorbed by rice, ranging from 43.7% to 55.6% in early rice and from 36.8% to 54.7% in late rice. Rice N use efficiency significantly decreased with increasing N application, while rice grain yield and its N uptake increased when the N application rate was below 150 kg ha −1 in early rice and 200 kg ha −1 in late rice. Exceeding this point limited rice N uptake and yield formation. The apparent N recovery rate, N residual rate, and N loss rate were 23.5–34.4%, 17.0–47.1%, and 26.0–47.8% for the early rice, and 32.8–37.3%, 74.2–87.0%, and 71.5–92.1% for the late rice. The linear plateau analysis further indicated that the recommended N application rate (118.5–152.8 kg ha −1 for early rice and 169.9–186.2 kg ha −1 for late rice) can not only maintain a relatively higher rice yield and N utilization but also significantly reduce soil N residue. Our results provide theoretical guidance for improving N management in double-cropping rice fields in southern China.