Post-Anthesis Nitrogen Dynamic Models and Characteristics of Rice Combined with Sowing Date and Nitrogen Application Rate

In order to explore the effect of the relationship between the combination of sowing date and nitrogen application rate on the nitrogen status of rice plants and nitrogen uptake and transfer after anthesis, three sowing dates were set—23 May (S1), 2 June (S2), and 12 June (S3)—and four nitrogen fertilizer treatments—no nitrogen (N0), 180 (N1), 270 (N2), and 360 (N3) kg N/hm2—were applied in a field experiment. The dynamic characteristics of nitrogen in rice post-anthesis under different treatments were analyzed by model fitting. The results showed that the three-leaf SPAD values of rice under different treatments varied, exhibiting a slow–fast–slow inverted S-shaped curve on the days after anthesis. However, the maximum SPAD value (ks), the time to enter the rapid period of decline (t1s), and the time to reach the maximum rate (Ts) were different between the different treatments. The maximum SPAD (ks) values of each sowing date increased with the increase in nitrogen fertilizer application; the t1s of each treatment was 15–29 days after spike development, and the S3 treatment entered the rapid decline period the earliest. It was beneficial to the transfer of leaf nitrogen to grain, and the nitrogen content, dry matter, and nitrogen uptake of stem sheaths under different treatments varied with days after anthesis; the S3 treatment exhibited the highest values. Leaf nitrogen content and dry matter decreased linearly in different treatments, and leaf nitrogen uptake showed an exponential downward trend. The parameters alnc, aldm, alnu, blnc, bldm, and blnu all increased gradually with the delay in sowing date and the increase in nitrogen, and the maximum values were obtained in the S3N3 treatment. The dry matter accumulation and nitrogen uptake of all treated grains showed a slow–fast–slow S-shaped upward trend over time; the maximum dry matter accumulation (kgdm) of grains was the greatest at 9652.7 kg/hm2, and the duration of the rapid grouting period (t2gdm − t1gdm) was the longest, lasting 32 days. The maximum nitrogen absorption of grains (kgnu) was highest in the S3N3 treatment, whereas the rapid nitrogen absorption duration of grains (t2gnu − t1gnu) was the longest in S1N0. These results provide a basis for the development of optimized nitrogen fertilizer application, real-time nitrogen fertilizer management, and post-anthesis nitrogen uptake and distribution models for rice with different sowing dates.