Optimizing Fertiliser Application Rates in Wheat Using Nitrogen Nutrition Index

Crop response to nitrogen (N) is usually evaluated by N use efficiency and diminishing return curves between yield and applied N fertilizers. However, both methods are highly variable due to environmental conditions and are dependent to the amount, timing and type of N fertilizers. Extending the results of such studies will led to overestimation of crop N requirements as a result of differences in precipitation, temperature and radiation across locations and years. Therefore optimizing fertilizer use for maximum productivity should be based on methods with higher certainty. Nitrogen nutrition index i.e. the ratio of actual N concentration in plant tissues to the critical N concentration is more reliable measure for fertilizer recommendation because of its stability over environmental fluctuations. However, for development of N nutrition index the critical dilution cure should be specifically established for each crop. In this papers N nutrition index is estimated for different wheat cultivars and tested for evaluation of grain yield in response to N application rates. To determine N nutrition index for wheat cultivars, a field experiment was conducted with factorial arrangement based on complete randomized block design with three replications. Experimental factors included three wheat cultivars (Chamran, Gaskogen and Sionez) and four N application rates (0, 55, 110 and 170 kg N ha-1). Shoot dry matter and N concentration was measured in five sampling during vegetative growth period. Using these data critical N concentrations and critical N dilution curves were calculated and compared with the previously established reference dilution curve for wheat crop. Nitrogen nutrition index (NNI) was then calculated as the ratio between measured shoot N (%) and the critical N concentration. Using NII grain yield and protein content of wheat cultivars was compared under different levels of N fertilizer. Critical concentration of nitrogen was slightly underestimated by the curve obtained in this study compared to reference curve. However, estimated coefficients of the dilution curve were closed to those of reference curve of wheat crop. Overall, nitrogen deficiency, sufficiency and excess in shoot dry matter were properly described by the estimated critical dilution and critical uptake curves. All wheat cultivars were nitrogen limited in no fertilized control and with application of 55 kg N ha-1 however, in 170 kg N ha-1 shoot nitrogen content was more than amount required for optimal growth and in 110 kg N ha-1 nitrogen uptake was closed to critical values. In unfertilized control and 55 kg N ha-1 NNI was lower than 1 during the whole vegetative growth period. However, in 110 kg N ha-1 NNI was almost 1 and in 170 kg N ha-1 varied between 1-1.3. A significant relation was obtained between NNI at flowering and relative yield of wheat cultivars and grain yield of three cultivars reached to its maximum at NNI between 0.9-1. Grain protein was also linearly correlated with NII at flowering. Based on the results nitrogen nutrition index calculated from critical dilution curve could be used as a powerful tool for precise estimation of crop growth rate, dry matter accumulation, grain yield and protein and to optimize the amount of N fertilizer required for any predefined yield level