Objective The study investigated the effects of long-term combined application of organic and inorganic nitrogen (N) on rice yield, N uptake and utilization, pH, and soil carbon (C) and N content in the rice-wheat rotation system in southern China, which aimed to provide theoretical basis and scientific basis for improving rice yield and nutrient utilization efficiency as well as scientific application of organic and inorganic fertilizers.

Methods Relying on the rice-winter wheat rotation system, based on the conventional nitrogen application amount (225 kg N hm⁻²), six treatments were set in the rice season, including no fertilizer (CK), 100% fertilizer nitrogen (CF), 75% fertilizer nitrogen + 25% organic fertilizer nitrogen (25%M), 50% fertilizer nitrogen + 50% organic fertilizer nitrogen (50%M), 25% fertilizer nitrogen + 75% organic fertilizer nitrogen (75%M) and 100% organic fertilizer nitrogen (100%M). The rice yield, nitrogen uptake, nitrogen use efficiency, nitrogen balance, and soil carbon and nitrogen content were analyzed in the rice season of 7th rotation cycle.

Result Compared with the CK treatment, the yield and N uptake of rice were increased under the different fertilization treatments. With the increase in the proportion of organic fertilizer, the rice yield and N uptake showed a trend of first increasing and then decreasing. 25%M treatment had the highest rice yield and N uptake. Under fertilization treatments, the N recovery efficiencies were 29.4 ~ 50.0%; the N partial factor productivities were 51.5 ~ 57.1 kg kg⁻¹. Among all treatments, 25%M treatment had the highest N efficiency. Compared with CK treatment, different fertilization applications significantly reduced the internal N utilization rate, but there was no significant difference among the different fertilization treatments. Compared with the control, CF treatment decreased soil pH, while soil pH was increased in all amounts combined application of organic and inorganic fertilizer treatment (cf. CF treatment). The contents of total C, total N, and inorganic N in soil were increased with the proportion of organic fertilizer. According to grey correlation analysis, soil factors from high to low correlation with rice yield were ammonium N, total C, total N, nitrate N and pH.

Conclusion Under the condition of conventional N application amount, compared with inorganic fertilizer application, the measurement of 25% organic fertilizer + 75% inorganic fertilizer could increase soil C and N content and alleviate soil acidification on the promise that the grain yield, N uptake and N use efficiency of rice did not decrease. Therefore, from the perspective production of a rice-wheat rotation system in southern China, organic and inorganic combined application has greater advantages and potential in yield and fertilizer reduction than pure inorganic fertilizer treatment.