Comparison of RQ flex and SPAD methods of diagnosing maize (Zea mays) nitrogen nutrient

Precise and prompt approach of crop nutrient diagnosis is the prerequisite in determining crop N content and the recommendation of reasonable amount of N fertilizer. In this study, a field experiment was carried out under drip irrigation condition, the chlorophyll equipment (SPAD-502 Plus) and nitrate reflectrometer (RQ flex10) was employed in terms of diagnosis of maize N nutrition during key growth stages. The aim of the study was to build maize fertilization mode by using the most suitable diagnosis approach. Five N fertilizer application rates were arrayed, including N₀ 0 kg(N).hm^-2, N₂₂₅ 225 kg(N).hm^-2, N₃₃₀ 330 kg(N).hm^-2, N₄₃₅ 435 kg(N).hm^-2 and N₅₄₀ 540 kg(N).hm^-2. The difference of diagnostic effect between SPAD-502 Plus and RQ flex10 methods on maize plant N nutrient were compared based on correlation analysis. The analysis compared the relationships between corn leaf SPAD and leaf sheath NO₃ concentration for various N fertilizer application rates, plant total N content and yield during different maize growth stages. Also the sensitivities of the RQ flex 10 and SPAD methods of diagnosis of maize plant N nutrient were compared. The results were as follows: 1) Maize leaf SPAD and leaf sheath NO₃^- concentration significantly increased with increasing N fertilizer application rate and the most sensitive period was from jointing stage to either tassel-emergence stage or grain-filling stage. The response of sheath NO₃^- concentration to N application rate was more sensitive than that of SPAD. This resulted into a higher degree of fit between NO₃^- concentration determined by RQ flex 10 method and N application rate or maize yield. RQ flex 10 method was more sensitive in diagnosis of maize N nutrition. 2) The total N content in maize had a significant linear correlation with leaf SPAD, while there was linear and then a constant correlation between the total N and sheath NO₃^- concentration. That was, total N content was linearly correlated with NO₃^- concentration as corn sheath NO₃^- concentration less than 186 mg·L_-1, however, total N content varied slight when sheath NO₃^- concentration exceeding 186 mg·L_-1 . 3) The recommended economic fertilizer application rate in maize under drip irrigation was 402.5 kg·hm^-2, which produced a yield of 17 049 kg·hm^-2. The critical NO₃^- concentration was 729.3 mg·L_-1 at jointing stage, 536 mg·L_-1 at tassel emergence stage and 81.2 mg·L_-1 at grouting stage, respectively. In conclusion, our study showed that both the chlorophyll meter and RQ flex 10 were suitable for maize N nutrient diagnosis. Moreover, the RQ flex 10 approach was a more sensitive method than the chlorophyll meter in diagnosing maize N nutrient status. The study suggested that the RQ flex method was highly suitable for the recommendation of N fertilizer as well as better N resource management in maize fields.