Objectives We explored the spatial-temporal variation characteristics and driving factors of relative yield gap of rice across China in 15 years (2004−2019) and the response of relative yield gap to nitrogen application rate under different soil productivity levels, so as to provide theoretical basis for rational application of nitrogen fertilizer in rice.

Methods The data were collected from 408 monitoring sites set up by the Center of Arable Land Quality Monitoring and Protection, Ministry of Agriculture and Rural Affairs from 2004 to 2019 in major rice regions of China, including 24 in northeast China, 110 in the Yangtze River Delta, 138 in the middle reaches of the Yangtze River, 56 in southwest China and 80 in south China. Based on the long-term monitoring database of Farmland Fertility, the Ministry of Agriculture and Rural Affairs of China, the difference of rice yield between fertilized and non-fertilized plot was defined as relative yield (RY), the highest relative yield (HRY), the average relative yield (ARY) and the relative yield gap (G_RY) were obtained by using the statistical method of high-yielding households, and the effects of fertilization and soil factors on the relative yield gap were determined using the random forest model, and soil productivity level was divided according to the yield of non-fertilized area. The relationship between the relative yield gap of rice and nitrogen application rate under different soil productivity levels was quantified.

Results Within the 15 years of observation, the HRY of rice in China was 4.98−6.86 t/hm², ARY was 3.06−3.47 t/hm², and G_RY was 1.92−3.41 t/hm². In terms of different regions, HRY and G_RY trends were in order of southwest China (rice−other crop)>Yangtze River Delta (rice−other crop)>south China (early rice)>northeast China (single rice)>middle of the Yangtze River (rice−other crop)>south China (late rice)>middle of the Yangtze River (early rice)>middle of the Yangtze River (late rice). In low and medium soil productivity levels, the G_RY decreased and then stabilized with increasing nitrogen fertilizer application rate, but it did not change significantly in high productivity soils. Except the low and medium productivity soils in the southwest China and the low productivity soils in the northeast China, the inflection points of nitrogen application rate appeared in the low and medium productivity soils in other regions. For low and medium productivity soils, the inflection points of nitrogen application rate for early and late rice in the middle of the Yangtze River were 187.5, 165.0 kg/hm² and 183.5, 152.5 kg/hm², respectively; early and late rice in south China were 195.0, 153.0 kg/hm² and 169.0, 157.0 kg/hm², respectively; rice-other crop rotation system in the middle of Yangtze River and Yangtze River Delta were 199.5 kg/hm², 184.5 kg/hm² and 202.0 kg/hm², 171.0 kg/hm², respectively; and 146.5 kg/hm² for the medium productivity soil in northeast China. Nitrogen application rate (NF), soil organic matter (SOM) and total nitrogen (TN) were relatively important factors affecting G_RY in low and medium productivity soils. In high productivity soils, SOM and TN had significant effects on G_RY, and potassium application (KF) significantly affected G_RY of early rice in the middle of the Yangtze River and south China.

Conclusions The relative yield gap of rice in southwest China (rice-other crop) was the highest, and that of late rice in middle reaches of Yangtze River was the lowest. Nitrogen application rate, soil organic matter and total nitrogen are important factors affecting the relative yield of rice. Nitrogen fertilizer should be reduced appropriately in high productivity soil as nitrogen application rate has no significant effect on relative yield gap. For low and medium productivity soils, increasing nitrogen application could significantly reduce the relative yield gap and increase yield potential within the inflection point (146.50−202.00 kg/hm²). Potassium fertilizer application requires more attention at high soil productivity in middle of Yangtze River and south China.