## 4 Discussion  
### Response of Double Cropping Yield to the N Rate and Plant Density in Different Ecological Regions  
N fertilizer is an important limiting factor affecting crop yield [41]. Increasing the N application rate can enhance the rice yield [42]. The results showed that, in the two ecological regions, the yields in the early and late season under the N\({}_{2}\) treatment enhanced compared with those under the N\({}_{1}\) treatment. The reason for this was that the higher N application increased the biomass, the panicle number and the photosynthetic rate at the heading stage, which led to enhanced rice productivity. A consistent result was discovered by a previous study [43]. This study found that the yield increases in double cropping rice that were achieved by changing the N application rate from N\({}_{1}\) to N\({}_{2}\) at Shanggao were lower than those at Xingguo, and the soil organic matter and available nutrient contents of Shanggao were higher than those of Xingguo. The ecological region with high soil fertility had a high soil available nutrient content, which led to a decrease in the dependence of the rice yield on inorganic N fertilizer [44; 45]. Therefore, the high N fertilizer input decreased the yield remuneration of double cropping rice in the ecological region with high soil fertility. Planting density affects the rice population structure, yield components and dry matter [25; 46]. [PERSON] et al. [47] showed that reasonably dense planting reduced the number of individual tillers but increased the panicle number of the population, which was more conducive to achieving full rice yield potential. The current results showed that, in the two ecological regions, compared with the D\({}_{1}\) treatment, the biomass and nutrient uptake of double cropping rice under the D\({}_{2}\) treatment increased because of the good complementarity between the panicle number, the grains per panicle and the seed setting rate. The result is similar to that observed through the previous study [31]. When the planting density was too high and the population was too large at the early growth stage, fierce intraspecific competition and unbalanced yield components occurred [28; 48]. The results  
Figure 9: Correlation between the photosynthetic rate at the heading stage and the N, phosphate and potassium uptake at the mature stage of the double cropping rice yield (**A**–**D**). * and ** represent significant difference at the 5% and 1% probability levels. Pn, photosynthetic rate; Y, yield; NU, N uptake; PU, phosphate uptake; KU, potassium uptake. showed that, compared with the D\({}_{2}\) treatment, the D\({}_{3}\) treatment increased the panicle number but reduced the grains per ear, and the yield in the double cropping season did not change significantly. Moreover, there was no difference between the two ecological regions. The reason for this may be that the self-regulation ability of a population under dense planting conditions may be closely related to the genotype of the variety. The N fertilizer application and plant density had important influences on the crop production, and there was a coupling effect between them in an ecological scenario [49; 50]. [PERSON] et al. [31] showed that, in double cropping machine-transplanted rice, reduced N application treatment led to declines in the growth rate and dry matter, but the yield negative effect of the N reduction was compensated for through the increased plant density. The results showed that dense planting could effectively make up for the yield reduction due to N reduction in double cropping rice at Shanggao. At Xingguo, the yield reduction of the late rice caused by decreased N application could not be compensated for by dense planting. The reason for this may be that the different soil fertilities affected the yield compensation ability of the rice, and the insufficient supply of soil nutrients led to a decline in rice population productivity [51].