Objectives: In order to find out the differences in source–sink characteristics of ratooning rice and main season rice and find approaches to increase the grain yield of ratooning rice, a sowing-by-stage method was adopted to synchronize main season rice with ratooning rice and induce their key growth stages under similar environmental conditions. The source–sink characteristics of four varieties under similar ecological conditions of main and ratooning rice were examined in 2019 and 2020. Results: The main results were: (1) The leaf area index (LAI) of the ratooning rice ranged from 0.54 to 1.44, while that of the LAI of main season rice was 4.67–7.71. The LAI of ratooning rice was much smaller than that of the main season rice; the former was only approximately 1/7–1/5 of the latter. (2) The photosynthesis (Pn) and net assimilation rate (NAR) of the ratooning rice were significantly higher than those of the main season rice before the milking–maturing stages. Still, at the late maturing stage, no definite tendency existed. (3) Ratooning rice transported ¹⁴C-assimilate from the flag leaf to the panicle at an estimated 81.43%, while main season rice transported 63.95%. The main stem’s top first and second internodes have been observed to be a major location for the ¹⁴C-assimilate in main season rice. (4) The grain yield of main season rice was 6029–7929 kg ha⁻¹ while the grain yield of ratooning rice ranged from 2363–3297 kg ha⁻¹. The sink capacity of the main season rice was approximately 2.4–3.6 times that of ratooning rice. The catalase activity of the rachis branches of the ratooning rice was higher than that of the main season rice. (5) The grain/leaf area (sink/source) ratio in the ratoon season rice was 1.69–2.46 times higher than that of the main season rice. Conclusions: The grain yields of ratooning rice were determined by the interaction of source and sink capacity while those of main season rice were mainly increased by enhancing sink capacity. Choosing varieties with heavier 1000 grain weight, exerting the advantages of higher photosynthetic rate and net assimilation rate of ratooning rice, promoting leaf area, and improving the transportation capacity of carbohydrate are the main approaches to increase the grain yield of ratooning rice.

Double-cropping rice cultivation reduces soil fertility, and the extensive use of chemical fertilizers has harmful effects on both the environment and grain yield. The application of organic materials could be used as a practical strategy to maintain soil fertility and improve grain yield in a double-season rice cropping system. For this purpose, field experiments with six growing seasons over three years, from 2016 to 2018, were conducted to assess the effects of five organic materials (biochar, Chinese milk vetch, rice straw, rapeseed cake fertilizer, and manure) on the grain yield and soil fertility, aiming to save about 25% of the chemical nitrogen (N) fertilizer required for all rice growing stages. The result showed that, compared with CK (the most common dose of fertilizer in this study region; 100% chemical fertilizer without organic fertilizer), the grain yield and soil fertility of double-cropped rice were increased after applying organic fertilizers for three consecutive years. Specifically, the CRC treatment (Chinese milk vetch (10.77 t ha⁻¹ in fresh)/rice straw (26.51 t ha⁻¹ in fresh) + 75% chemical fertilizer) showed significantly higher rates of effective panicles (4.65–10.92%) and annual grain yield (8.00–8.82%). The total N, total phosphorus (P), total potassium (K), alkaline N, and available P content in the CRC soil were significantly increased by 11.85%, 12.22%, 15.08%, 23.32%, and 41.04%, respectively, relative to CK. The decomposition of the applied Chinese milk vetch and rice straw combined with 75% chemical fertilizer resulted in more soil humus (9.50 g kg⁻¹), humic acid (3.19 g kg⁻¹), fulvic acid (3.26 g kg⁻¹), and active organic carbon (5.78 g kg⁻¹) and a significantly higher carbon pool management index (13.5%), as well as significantly higher soil urease activity (18.10%) and acid phosphatase activity (17.64%). Therefore, in this study, Chinese milk vetch (10.77 t ha⁻¹ in fresh) in the early rice season/rice straw (26.51 t ha⁻¹ fresh) in the late rice season + 75% chemical fertilizer treatment was the optimal dose for the double-season rice cropping system. It resulted in higher rice yields and has the potential to be used for more sustainable soil fertility.