Effects of reduced nitrogen rate on cotton yield and nitrogen use efficiency as mediated by application mode or plant density

Nitrogen (N) fertilization plays an important role in yield formation of field-grown cotton (Gossypium hirsutum L.), but little is known of its interaction with mode of application or plant density under irrigated production. Our objective was to determine the effects of N application rate on cotton yield, leaf senescence and N use efficiency as mediated by mode of application and plant density. To achieve this goal, two field experiments which were conducted from 2015 to 2016 using a split-plot design in randomized complete blocks. In the first experiment, the main plots were assigned to N application modes (conventional application and drip fertigation) and the subplots to N rates (375, 319, 264 and 0 kg N/ha). In the second experiment, the main plots were assigned to plant density (12 plants/m2-low density and 19.5 plants/m2-high density) and the subplots to N rates (330, 264 and 0 kg N/ha). The N rate of 264 kg/ha under drip fertigation or high plant density did not reduce cotton yield. Agronomic nitrogen use efficiency (aNUE) and nitrogen recovery efficiency (NRE) were the highest at 264 kg N/ha under drip fertigation and high plant density. Although a reduced N rate increased boll load, fertigation or high plant density relatively reduced boll load and delayed late-season leaf senescence as indicated by the increased photosynthetic rate and chlorophyll content as well as the reduced malondialdehyde concentration compared to conventional application or low plant density. The yield stability across N rates (264-375 kg N/ha) was probably due to the delayed leaf senescence and improved N use efficiency. The results suggest that the N rate could be reduced to 264 kg/ha, or 20-30% from the traditionally recommended rate, without sacrificing yield under high plant density or drip fertigation. These results are beneficial to the formulation of a scientific and rational use of N fertilizer for sustainable cotton production and environmental health.