Effects of Nitrogen Forms and Application Rates on Nitrogen Uptake, Photosynthetic Characteristics and Yield of Double-Cropping Rice in South China

Nitrogen is an important element that affects the growth and yield of rice obviously. To explore the effects of nitrogen (N) on the growth of double-cropping rice, four consecutive trials were conducted in South China Agricultural University. Four N forms and five N application rates were set up. Rice N uptake, leaf anatomic structure, leaf fluorescence parameters and yield were studied. The results showed that rice leaf SPAD value and N content with urea treatment were the highest among 4 N forms at the heading and flowering stage. The order of rice yield and N use efficiency treated with different N forms were urea > ammonium sulfate > ammonium nitrate > sodium nitrate. The N application rate test showed that photochemical quantum efficiency and chemical quenching coefficient of rice leaves showed an inverted-V-type changing tendency with the highest value at 180 kg/hm2 N treatment. Treatment with less than 180 kg/hm2 N decreased the area and perimeter of small vascular bundles of the lateral leaves. Yield of early rice and later rice with 180 kg/hm2 urea treatment was 17.42 to 33.28% and 6.17 to 21.28% higher than those of other N levels, respectively. The above results suggested that 180 kg/hm2 of urea N are suitable and recommended for double-cropping rice planting in South China.