Pullout Behaviour and Influencing Mechanisms of Desert Plant Roots in Clayey Sand During Thawing

In cold and arid regions, the mechanical properties and influencing mechanisms of the root-soil interface during the thawing stage remain poorly understood. This study focuses on Alhagi sparsifolia root-clayey sand composites to investigate the effects of temperature (-10 °C to 25 °C), initial soil water content (4-12%), and naturally varying root diameter (4.50-5.05 mm) on root pullout behaviour, and integrates endoscopic macro-observation, environmental scanning electron microscopy (ESEM), soil water migration tests, and nuclear magnetic resonance (NMR) techniques to reveal the dominant influencing mechanisms. Key findings reveal the following: (1) An increase in soil water content from 4% to 12%, and a temperature rise from -10 °C to 25 °C led to a maximum reduction in the average peak pullout force (F_T) of roots exceeding 95%. (2) There is a non-monotonic relationship between root diameter and pull-out force, which can be attributed to two distinct failure modes: a newly observed failure mode known as root bark peeling, occurring under high soil moisture conditions (≥8%), and a commonly observed failure mode referred to as partial soil detachment, occurring under low soil moisture conditions (≤6%). (3) The coupling effects of temperature and water content reveal that the increase in temperature predominantly contributes to strength loss (>63%) during the ice-water phase transition (-10 °C to 0 °C), while soil water content primarily influences root pullout behaviour in the liquid water stage (5 °C to 25 °C). (4) As the temperature rises, in soils with low water content (4-6%), the reinforcing effect of roots appears to stabilize at -1 °C, whereas in soils with high water content (8-12%), stabilization occurs only beyond 5 °C. These findings enhance the understanding of root-soil interactions in thawing environments and provide a theoretical basis for soil bioengineering techniques aimed at slope stabilization in cold and arid regions.