Application of relative SPAD values for nitrogen fertilizer management of different cultivars of summer maize

The field experiments were conducted to investigate the differences of SPAD values among different nitrogen (N) fertilizer treatments, maize cultivars and critical growth stages, the effects of N fertilizer management methods based on relative SPAD threshold values on N rates, grain yield, N use efficiency and change of soil N content. The results show that SPAD values of maize leaves at the critical growth stages were increased significantly with the increase of N rates (the N fertilizer rate is less than N 210 kg /ha), and the correlation between relative SPAD values at the flare opening and grain yield can be simulated by the linear plus plateau model, and the plateau relative SPAD values of Zhengdan958 and Jinongyihao are 0.976 and 0.981, respectively. The differences of absolute SPAD values among maize cultivars and growth stages are significant, while the differences of relative SPAD values are not significant. The rates of topdressing N and total fertilizer N are increased with the increase of relative SPAD threshold values. Compared with farmer fertilizer practices (FFP), the N fertilizer management methods based on relative SPAD threshold values could decrease N application rates, field N apparent loss and soil inorganic N residual after maize harvest, and increase N use efficiency while keeping higher grain yield. In this experiment condition, the range of feasible relative SPAD threshold values for ensuring higher grain yield and higher N use efficiency of two maize cultivars is 0.95～0.98. The grain yields of Zhengdan958 and Jinongyihao are not decreased under the N management with this threshold value range, while total N rate is decreased by 42%, and the N recovery efficiency and N agronomic efficiency are increased by 18.6, 20 percentage points and 6.0, 6.5 kg/kg, respectively.