Abstract:

Several field experiments with different levels of N application (0, 120, 240, 360, and 480 kg ha^-1) were performed to investigate the responses of cotton growth and lint yield to a range of applied nitrogen treatments in Nanjing and Anyang, standing for the ecological conditions in the middle lower reaches of Yangtze River Valley and Yellow River Valley in China, respectively, and provide information better assessing the requirement of nitrogen and improving the fertilizer-nitrogen use efficiency for cotton. We established a model for cotton fruit-branch leaf and investigated its relationship with yield based on Justes, a critical nitrogen concentration dilution curve, which can be defined as the minimum nitrogen concentration required for maximum growth rate at any time. The results indicated that the biomass and nitrogen accumulation were greatly affected by nitrogen level for cotton fruit-branch leaf, and exhibited a sigmoid behavior when expressed as a function of time. The beginning time of fast accumulation period for nitrogen was 2–4 days earlier than that for biomass. The cotton lint yield was correlative with nitrogen concentration in fruit-branch leaf, which would cause the lint yield reduction when it was lower or higher than critical nitrogen value in fruit-branch leaf. Therefore the phenomenon of nitrogen luxurious consumption existed for fruit-branch leaf of cotton after flowering. The nitrogen concentration in fruit-branch leaf increased with the increasing of applied N rates, and decreased in the growing process. The relationship between the nitrogen concentration and dry matter of fruit-branch leaf can be described by a power equation. The patterns of nitrogen concentration dilution model were consistent at both experimental sites, but with different values for parameters. Therefore, the results presented in this paper backup the viewpoint that a critical nitrogen concentration dilution curve for cotton fruit-branch leaf is independent of ecological region. The maximum and minimum nitrogen concentration dilution curves can also be described with power equation, and are independent of ecological region. Relationships between total nitrogen accumulation and accumulated dry matter in fruit-branch leaf fit very well to the model Nupt=10 aW 1－b for each nitrogen fertilization level at two experimental sites. The cotton fruit-branch leaf in Anyang had a higher capacity of nitrogen accumulation than that in Nanjing for the same dry matter. The lower position of the critical nitrogen concentration dilution curve for the same fruit-branch leaf biomass in Nanjing indicated that the nitrogen biomass productivity was higher than that in Anyang. Because of having the biological sound for the critical N concentration dilution curve, it can be a reliable tool for diagnosing the nitrogen nutrition status of fruit-branch leaf. Therefore, the diagnosises made by the models of allometry and nitrogen nutrition index (NNI), which based on the critical nitrogen concentration dilution model, had a sound biological basis. According to the allometric coefficient, NNI and dynamic nitrogen accumulation rate under critical nitrogen concentration, the optimal rate applied nitrogen fertilization in Anyang which was 240–360 kg ha^-1 and more does to 360 kg ha^-1 should be higher than that in Nanjing which was 240 kg ha^-1.

Key words: Cotton, Fruiting branch leaves, Biomass, Nitrogen accumulation, Critcal nitrogen concentration, Model