Agronomic responses to nitrogen application and nitrogen utilization in rice fields

Field experiments were conducted to explore the optimum N application rate based on agronomic responses to different N application rates, and the leaf SPAD values at the critical growth stages were measured for guiding the N management practices in the Qianjiang region. The results indicate that there are no significant differences in grain yield when N application rates range from 90 to 180 kg/hm2, while grain yield is decreased when N rate is over 180 kg/hm2. The equation between grain yield of rice and N application rate is established: y=-0.0728x2+22.335x+6811.5，R2=0.9442. Then the economic optimum nitrogen rate (EONR), 138 kg/ha, is evaluated in the study area by the equation between economic benefit of rice and N application rate (y=-0.134x2+37.097x+12533-M，R2 = 0.9331) based on the input of N fertilizer and output of grain yield, and the N recovery efficiency, agronomic efficiency and N partial factor productivity are 40.9%，11.5 kg/kg and 63.2 kg/kg, respectively. The produced N application rate (140 kg/ha) guided by SPAD values at the critical growth stages is matched perfectly with EONR under the maximum economic benefit, higher nitrogen use efficiency and low nitrogen apparent loss.