Yield and photosynthetic characteristics of mechanical-transplanted rice under different slow-release nitrogen fertilizer rates and plant population

Objectives The recommended nitrogen (N) application rate for high rice yield in Wenjiang area of Sichuan Province of China is 180 kg/hm2,in which 70% of N is from controlled release fertilizer and 30% from commercial urea. The plant population, slow-release N fertilizer addition rate and the total N input were optimized for mechanical transplanted rice production in this paper.Methods A split plot field experiment was conducted using mechanical transplanted hybrid rice as material in Wenjiang area, Sichuan Province. Keeping the row space of 30 cm, three plant spaces of 16 cm, 18 cm and 20 cm were set up. Under the urea N application rate of 54 kg/hm2, four slow-release fertilizer N rates of 36, 66, 96 and 126 kg/hm2 were added and the corresponding total N input were 90, 120, 150 and 180 kg/hm2. All the nitrogen fertilizer was side top-dressed in once at the same time of transplanting. The photosynthetic indices and growing index of rice were measured at the main growing stages.Results The total N input and the plant spacing showed significant or extreme significant regulating effect on the biomass accumulation, photosynthetic characteristics and grain yield of mechanical transplanted rice in the main growing stages, and their interaction effect was significant as well. The regulating effect of controlled release fertilizer was more significant than that of plant population. In the planting space of 18 cm and the total N input of 150 and 180 kg/hm2, the LAI, the culm and sheath weight per shoot and the leaf weight were increased significantly, and the decrease rate of leaf area was slowed down, and the high photosynthetic potential and high growth rate were kept longer, and the dry matter accumulation during jointing-full heading stage was enhanced as a result. The above two treatments significantly increased the spikelet number, helped maintaining a stable seed setting rate and 1000-grain weight, brought significant improvement of grain yield. When the plant space was 16 cm and the total N input was 150 kg/hm2, the number of tiller was increased fast and decreased slowly after jointing, formed adequate effective panicles, although spikelets were decreased significantly. There were higher seed setting rate and 1000-grain weight, which might lead to higher yield. When the plant space was 20 cm and the total N input was 180 kg/hm2, adequate effective panicles were ensured, the formation of strong stalk and big panicles was promoted, grains per panicle and dry matter accumulation were increased. Furthermore, high grain filling was benefitial to the 1000-grain weight. The correlation analysis revealed that on the basis of the combination in mechanical-transplanted spacing with the slow-release N fertilizer, the photosynthetic potential at the jointing-full heading stage, high valid LAI at the full heading stage would benefitial to the increment of yield. Conclusions The setting of the row space in 30 cm, plant spacing in 18 cm, the N for 150 kg/hm2 added by the slow-release N fertilizer (96 kg/hm2) and conventional urea (54 kg/hm2) could maximize the hybrid rice yield in the local research area, and improve photosynthetic production and yield.