Nitrogen rate and plant density interaction enhances radiation interception, yield and nitrogen use efficiency of mechanically transplanted rice

With the development of hybrid rice cultivars and mechanized planting technologies, it is hypothesized that the grain yield and nitrogen (N) use efficiency of mechanically transplanted rice could be further improved by harnessing the compensatory effects from the N rate and plant density on individual plants and populations. We evaluated the interactive effects of the N rate and plant density on the grain yield and N use efficiency of rice in two field experiments during 2014 and 2015. The results indicated the significant interaction between the N rate and the plant density on the grain yield. The application of 165 kg N ha(-1) along with a plant density of 24-27 x 10(4) hills ha(-1) significantly increased the grain yield. A correlation analysis showed that the spikelet per hectare had a strong correlation with the grain yield. With an increase in the plant density, the N recover efficiency (NRE) was increased by 12.7-40.0% in 2014. However, the higher plant density, as in 33 x 10(4) hills ha(-1) (D33), decreased the NRE by 5.8% in 2015. The N rate and plant density also significantly influenced the leaf area index (LAI) and the interception of photosynthetically active radiation (IPAR), the two important factors for grain yield formation. The LAI values of the 82.5 kg N ha(-1) (N82.5), 165 kg N ha(-1) (N165) and 82.5 kg N ha(-1) (N247.5) treatments was increased by 26.3%, 101.4% and 116.0%, respectively, compared with that of the 0 kg N ha(-1) (NO) treatment. Similarly, the LAI of the 21 x 10(4) hills ha(-1) (D21), 27 x 10(4) hills ha(-1) (D27) and D33 was increased by 17.8%, 20.5% and decreased by 12.8%, respectively, compared with that of 15 x 10(4) hills ha(-1) (D15). The highest LAI was recorded when N247.5 was combined with D27. The N application increased the photosynthetically active radiation (IPAR) by 4.1%, 12.4% and 14.4%, respectively, compared with that of the N0 treatment. However, the IPAR increased by 0.8%, 2.6% and 3.3%, respectively, in comparison with that of D15. A moderate increase in the IPAR significantly increased the grain yield, while too much IPAR decreased the photosynthetically active radiation use efficiency (PARUE), leading to the lower synthesis of photosynthetic products. Hence, various combinations of the N rate and plant density are practicable for reducing the N inputs in addition to improving the grain yield and NRE.