Optimal irrigation method in pre-sowing irrigation with saline water and determination of mineralization degree index for cotton seedling

Abstract: Due to freshwater resources shortage and annual rainless spring, cotton cultivation in Hebei Low Plain is faced with the problems of using the shallow underground saline water. In order to evaluate the impact of long-term saline water irrigation on cotton seedling, and suggest the suitable irrigation method and irrigation water salinity to ensure optimal cotton full stand establishment, the 6 years field experiment under border irrigation and furrow irrigation with mineralized water before sowing was conducted. The response relations among different salt stress levels and cotton full stand establishment and seedling growing were analyzed in this study. The results showed that the cotton full stand establishment decreased when the salinity of irrigation water reached a threshold value, while the threshold had a large year-to-year fluctuations due to the sprout and emergence stage of cotton which was easy to be affected by multiple factors. Under the same salinity of irrigation water, the full stand establishment of border irrigation treatment was higher than the corresponding furrow irrigation treatment by annual mean 6.9%-13.1%. Under these two irrigation methods, the full stand establishment were significantly related to the soil salinity (P<0.01), and the slopes of the dependent equations were similar. However, the ground temperature and some other soil characters were both better in border irrigation treatments, which brought a 5.5-7.7 percentage point higher full stand establishment occurred in the border irrigation treatments under the same soil salinity. Although the salinity of topsoil was easily affected by factors such as rainfall, the soil salinity of treatments presented an accumulation trend with the increase of irrigation years when the salinity of irrigation water beyond 4 g/L, therefore, there is a cumulatively negative impact of long-term irrigation upon cotton emergence. With the increasing salinity of irrigation water, cotton seedling process was delayed, and plant height, leaf area and weight of dry matter etc. were all inhibited, especially for the leaf area. Cotton seedlings grew better in border irrigation treatment when the salinity of pre-sowing irrigation water was below 8 g/L rather than in furrow irrigation treatment. To keep the full stand establishment in consistent with the fresh water irrigation treatment, suitable salinity of irrigation water for border irrigation and furrow irrigation should be controlled below 6 g/L and 4 g/L, respectively. Under such condition, cotton seedling process might be delayed, and the growth of cotton seedling was overall inhibited. In conclusion, to obtain higher cotton full stand establishment and better seedling growth, border irrigation and irrigation water with salinity of lower than 6 g/L were recommended for pre-sowing irrigation; or to save water, furrow irrigation and irrigation water with salinity of lower than 4 g/L were recommended. The study provides an important reference for directing local cotton production under pre-sowing irrigation with saline water.