Effect of nitrogen applied levels on the dynamics of biomass,nitrogen accumulation and nitrogen fertilization recovery rate of cotton after initial flowering

Quantitative information regarding biomass accumulation,nitrogen uptake,and nitrogen use efficiency for cotton plant under varing environmental and growth conditions is required for the development of plant growth models and also for the assessment of the contribution of cotton to nitrogen balances in cotton cropping systems.In order to quantitatively study the dynamic characteristics of accumulations of dry matter,nitrogen uptake and nitrogen fertilization recovery rate of cotton plant after initial flowering,two field experiments with different levels of nitrogen application(0,120,240,360,and 480 kg·hm~(-2)) were carried out 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.The results show that the total dry matter and nitrogen accumulation of cotton were affected significantly by the application rate of nitrogen fertilizer throughout the growth period after initial flowering.With the increase of nitrogen application rate,the dry matter firstly increased,and then reached its highest value at 360 kg·hm~(-2) in Anyang and 240 kg·hm~(-2) in Nanjing,and the nitrogen uptake amount reached its highest value at 360 kg·hm~(-2) in Anyang and Nanjing,respectively.However,the total dry matter(including roots) and nitrogen uptake amount continuously increased with the progress of plant growth after initial flowering,and the general pattern of crop growth followed a sigmoid function of time.The eigenvalues of the dynamic accumulation model with the nitrogen application level of 360 kg·hm~(-2) and 240 kg·hm~(-2) for Anyang and Nanjing,respectively,were more harmonious than that with other treatments.The maximum nutrient uptake rate appeared earlier than did the dry matter in all treatments.Cotton lint yield was significantly affected by the nitrogen rate of supply at the two experimental sites.Among the nitrogen treatments,lint yield was lower in the control plot(0 kg·hm~(-2)) than in the fertilized plots.At the two experimental sites,the yield data shows that nitrogen application significantly increased yield.Lint yield increased with the increase of nitrogen application rate up to 360 kg·hm~(-2) in Anyang,and up to 240 kg·hm~(-2) in Nanjing, and the highest yield was 1789.7 kg·hm~(-2) and 1467.1 kg·hm~(-2),respectively.The relationship between lint yield and nitrogen application rate can be described with a quadratic equation.The dynamic developments of nitrogen fertilization recovery rate behaved in a very similar way as the cotton yield did at the two experimental sites.The nitrogen recovery rate and momentary nitrogen recovery rate reached their highest value at the nitrogen application level of 360 kg·hm~(-2) and 240 kg·hm~(2) for Anyang and Nanjing,respectively.Excessive nitrogen delayed cotton maturity and increased the proportion of vegetative organs,thereby reduced there quality and lint yield.