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.

We aimed to elucidate the color changes of rice leaves after anthesis and create an algorithm for monitoring the nitrogen contents of rice leaves and of the whole plant. Hence, we aimed to provide a theoretical basis for the precise management of rice nitrogen fertilizer and the research and development of digital image nutrition monitoring equipment and reference. We selected the leaf colors of the main stems of four major rice varieties promoted in production, including Huaidao 5 (late-maturing medium japonica rice), Yangjing 4227 (early maturing late japonica rice), Changyou 5 (late japonica hybrid rice), and Yongyou 8 (late japonica hybrid rice). Under different nitrogen levels, the leaf R, G, and B values of the four rice varieties at different stages after anthesis, the dynamic changes in RGB normalized values, the correlations between RGB normalized values and leaf SPAD values, the leaf nitrogen content and whole plant nitrogen content, and the nitrogen prediction model were studied. The research results demonstrate the following: (1) regardless of nitrogen levels, the leaf of R, G, B, NRI, NGI and NBI of different rice varieties after anthesis followed the order, G > R > B. R, G, NRI, NGI, and days after heading could be fitted according to a logarithmic equation, y = aebx (0.726 ≤ R2 ≤ 0.992); B, NBI, and days after heading could be fitted using a linear equation, y = a + bx (0.863 ≤ R2 ≤ 0.992). Both fitting effects were significant (except NGI). (2) A quadratic function (Y = −1296.192x2 + 539.419x − 10.914; Y = −1173.104x2 + 527.073x − 12.993) was adopted to construct a monitoring model for the NBI and SPAD values of japonica rice and hybrid japonica rice leaves after anthesis and the R2 values were 0.902 and 0.838, respectively. Exponential functions (Y = 5.698e7.261x; Y = 3.371e9.326x) were employed to construct monitoring models of leaf nitrogen content, and the R2 values were 0.833 and 0.706, respectively. Exponential functions (Y = 5.145e4.9143x; Y = 3.966e5.364x) were also used to construct a monitoring model for the nitrogen content of the whole plant, and the R2 values were 0.737 and 0.511, respectively. The results obtained from prediction tests by using Determination Coefficient (R2), Relative Percent Deviation (RPD), and Root Mean Square Error (RMSE) showed that it was feasible, accurate, and efficient to use a scanner for measuring the nitrogen content of rice.