Effects of drought stress on cotton growth and intraspecific relationship under different nitrogen application rates

Soil, water, and nitrogen fertilizer are the main factors which affect crop growth and intraspecific relationships in the desert oasis region, but little is known about the effects of drought stress on interspecific or intraspecific relationships under different nitrogen application rates. This study aimed to investigate the response of yield to drought stress under different nitrogen application rates and analyze changes in the intraspecific relationship of cotton under different treatments. A field experiment was conducted in Gansu Province of China, the growth parameters and yield of cotton were measured, and the changes in intraspecific relationships were analyzed under three nitrogen applications (300 kg·hm^-2, 225 kg·hm^-2 and 150 kg·hm^-2) and water treatments (normal water, moderate drought stress, and serious drought stress). The results showed that:1) under the same nitrogen application rate, the plant height of cotton significantly decreased under serious drought stress in 2016 and 2017, and the stem diameter significantly reduced as the drought stress intensified, but the leaf area increased slightly with increasing drought stress and was highest in moderate drought stress conditions. 2) The stem biomass significantly decreased in conjunction with decreased water conditions under the same nitrogen application rate, and the leaf biomass and seed cotton yield were highest under moderate drought stress, and decreased under serious drought stress. 3) Under the local nitrogen application rate of 300 kg·hm^-2, the relative neighbor effect (RNE) changed from positive to negative with increasing drought stress. The RNE first increased and then decreased with increasing drought stress under the 225 kg·hm^-2 treatment, and the RNE significantly reduced with increasing drought condition and the values were all negative in the 150 kg·hm^-2 treatment. In summary, the combination of local nitrogen application rate of 300 kg·hm^-2 and moderate drought stress resulted in high cotton yield, and intraspecific facilitation. The cotton yield decreased under the other treatments and the interactions all demonstrated intraspecific competition.