Spatiotemporal dynamics of water, salt, and heat in the root zone of cotton fields under mulch drip irrigation in southern Xinjiang

[Objective] To investigate the soil water flow system pattern and its influence mechanism on the dynamic characteristics of soil water, salt and heat in the root zone of cotton fields under mulched drip irrigation in southern Xinjiang. [Methods] Field irrigation experiments under freshwater and brackish water conditions were conducted at the Water Conservancy Research Institute of Bayingolin Authority of Tarim River Basin, in Korla City. High frequency and high spatial resolution monitoring of soil water, salt and heat dynamics in the root zone was carried out using portable instruments and in-situ real-time monitoring devices, to analyze the soil water flow system pattern. [Results] During one irrigation period and the entire growth period, the variation trends of soil moisture under freshwater and brackish water treatments were generally consistent. The variation amplitude of soil moisture in the shallow layer (0-30 cm) was significantly higher than that in the deep layer (30-65 cm). Under the combined effects of drip emitter water supply, soil evaporation, and root water uptake, a soil water flow system structure was formed, with the drip emitter acting as the “source” and inter-mulch evaporation and root uptake as the “sink”. Soil salinity was redistributed with water movement. Influenced by plastic mulching, soil texture, and structure, salt mainly accumulated at depths of 15-30 cm beneath the mulch gaps and 5-15 cm beneath the wide rows. Soil temperature in the shallow layer exhibited significant diurnal variation due to the warming effect of plastic mulch, especially in wide-row areas, forming hydrothermal potential gradients that promoted lateral water movement and localized salt accumulation. In contrast, the deep soil temperature showed smaller fluctuations and remained relatively stable. [Conclusion] The study reveals the control mechanism of the soil water flow system on water-salt-heat dynamics under mulched drip irrigation, and provides a scientific basis for precise regulation of water and salt in the root zone.