Establishment and validation of critical nitrogen dilution curve based on the total plant biomass and nitrogen concentration

Objectives The precision of critical nitrogen dilution curve for potato is not always satisfactory when used in the practical nitrogen nutrition diagnosis, due to the differences in nutrient characteristics of potato cultivars. We tried to construct and validate a critical nitrogen dilution curve that was available for most potato cultivars under drip irrigations in Inner Mongolia.

Methods Three separate field experiments were conducted with different N rates and three potato cultivars under drip irrigation from 2014 to 2016. The above-ground shoots and under-ground tubers were sampled to determine the biomass and nitrogen concentration in five key periods of potato including seedling, tuber formation, tuber expansion, starch accumulation and harvest. The biomass and N concentration of whole potato plant were calculated according to a formula. The critical N dilution curve and the corresponding N nutrition index were established based on the biomass and N concentration of the whole plant.

Results The biomass and N concentration of both the above-ground shoot and the whole plant showed a negative power function relationship with advance of growth stages. The average coefficient of determination (R²) of critical N dilution curve based on the biomass and N concentration of the above-ground shoot is 0.52, whereas the average R² of the critical N concentration dilution curve based on the biomass and N concentration of the whole plant was 0.94, which was 80% higher than the former. Using the curve to predict the N requirement, the R² of critical N dilution curve in Kexin 1, Shepody and Holland 14 all reached 0.95, indicating the availability of the models in the diagnose of potato N nutrition.

Conclusions Compared with the critical N dilution curve established based on the biomass and N concentration of the above-ground part of a tuber crop like potato, the model based on the biomass and N concentration of the whole plant is more precise for the N nutrition diagnose. Under drip irrigation in Inner Mongolia, the N dilution model is Nc = 4.57W^–0.41. According to this curve, the reasonable N application rate for Kexin1 is N 170–180 kg/hm², for Shepody is 190–200 kg/hm², and for Holland 14 is 215–225 kg/hm², which are highly agreeable with the field experiment results.