Effects of Nitrogen Conservation Measures on the Nitrogen Uptake by Cotton Plants and Nitrogen Residual in Soil Profile in Extremely Arid Areas of Xinjiang, China

This study researched the effects of using various nitrogen (N) conservation measures on the residual characteristics of nitrate and ammonium N in soil and the associated N uptake by cotton plants. A field experiment with six treatments was conducted, as follows, no N application (DT1), conventional N application (DT2), 60% conventional N application combined with DCD (DT3), 60% conventional N application combined with NBPT (DT4), 60% conventional N application combined with cotton straw returning (DT5), and 60% conventional N application combined with DCD, NBPT, and cotton straw returning (DT6). The results showed that the cotton straws in the DT5 treatment were beneficial for the vegetative growth of cotton seedlings. However, it was observed that the later performance of the plants in this sample was poor in terms of height, biomass, and yield of cotton. The plant height in the DT6 sample increased by 15 cm compared with those in DT1, and the soil and plant analyzer development (SPAD) values of the fourth leaf from the top of the DT6 plants were higher than those in the DT1 and DT4 samples. The DT6 plants (60% Urea + DCD + NBPT + cotton straw) increased N use efficiency by up to 47%, and no significant decrease in biomass and cotton yield was observed compared to the DT2 sample. The residual content of nitrate N in the tillage layer increased gradually over time between two rounds of drip irrigation treatment applications. Compared with the DT2 treatment, the other treatments resulted in lower residual nitrate N contents. In summary, the application of N fertilizers at a reduced rate combined with N conservation measures may increase N use efficiency and decrease the risk of non-point source N fertilizer pollution, while maintaining the cotton yield.