Through field experiments, four treatments were set up: no nitrogen fertilizer (CK), conventional fertilization (N100), 25% organic nitrogen replacing inorganic nitrogen (N75), and 50% organic nitrogen replacing inorganic nitrogen (N50) to analyze the effects of different proportions of organic nitrogen substitution on the yield and composition of double-cropping rice, nitrogen fertilizer absorption and utilization efficiency, and soil nitrogen supply capacity. The experimental results showed that under the condition of no nitrogen fertilizer, rice could produce a grain yield of 9 971 kg hm-2 (early rice 5 457 kg·hm-2 and late rice 4 514 kg·hm-2). With a nitrogen fertilizer contribution rate of 25.6%, nitrogen fertilization significantly increased rice grain yield. Compared with conventional fertilization treatment, the organic substitution treatments significantly increased rice grain yield, especially with 25% organic nitrogen substitution showing better results. Whether early rice or late rice, the N75 treatment had the highest effective panicle number, followed by N50 and N100, with CK being the lowest. However, there were no significant differences in rice harvest index, thousand-grain weight, and grains per panicle among the treatments. The combination of organic and inorganic fertilizers also increased nitrogen absorption by rice. Compared with N100, the nitrogen uptake of grains and straw in early rice increased in N75 and N50 treatments, and the nitrogen uptake of grains and straw in late rice under N75 treatment significantly increased by 25.5% and 43.7%, respectively. Nitrogen fertilization caused nitrogen accumulation in the soil, especially in the N100 treatment, with much higher nitrogen surplus than in the organic fertilizer treatments (N75 and N50). Whether early rice or late rice, the apparent nitrogen use efficiency of the N75 treatment was the highest, followed by N50 and N100 being the lowest. There were no significant differences in internal nitrogen use efficiency and nitrogen fertilizer productivity among the nitrogen treatments. Nitrogen fertilization increased the contents of total nitrogen, microbial biomass nitrogen, water-soluble nitrogen, ammonium nitrogen, and nitrate nitrogen in the soil. Compared with N100, the soil contents of total nitrogen, microbial biomass nitrogen, water-soluble nitrogen, ammonium nitrogen, and nitrate nitrogen under the N75 treatment significantly increased by 14.0%, 42.8%, 68.8%, 25.1% and 107.1%, respectively. However, there were no significant differences in soil nutrient contents between the N75 and N50 treatments. Under the condition of conventional nitrogen fertilization, compared with single chemical fertilizer application, the combined application of 25% organic fertilizer + 75% inorganic fertilizer can significantly improve soil nitrogen supply capacity without reducing rice grain yield, nitrogen uptake, and nitrogen use efficiency. Therefore, from the perspective of long-term production in the southern rice-rice cropping system, the combined application of organic and inorganic fertilizers has greater advantages and potential compared with pure chemical fertilizer treatment.
