Influence of Artemisia ordosica on soil freeze-thaw processes in Mu Us Sandy Land

The seasonal freeze-thaw process in the unsaturated zone can play an important role in the water and heat balance of the ground surface in arid and semi-arid regions. Accurately assessing the dynamics of seasonal freeze-thaw cycles is essential for the effective management of water resources, improved agricultural productivity, and ecological environmental conservation. While significant efforts have been made to understand the seasonal freeze-thaw process, the influence of the vegetation on this process remains unclear. In this study, a field experiment was carried out in the Mu Us Sandy Land at "Jizi Bay" in the Yellow Basin, China, focusing on Artemisia ordosica, a native shrub species. Two weighing lysimeters were installed at the experimental site: one was bare soil, while the other was covered by Artemisia ordosica. A systematic measurement was then made of the soil water content, soil temperature, heat flux, evapotranspiration, as well as weather conditions during the period from October 2020 to September 2023. The results show that the soil water content on the ground surface shared the minimal variation between Artemisia ordosica and bare soil. During the freeze-thaw period, the average fluctuation of the daily soil temperature on the ground surface under Artemisia ordosica was smaller than that of bare soil. Additionally, under the Artemisia ordosica condition, the occurrence of the minimum temperature was, on average, delayed by 1.11 hours, and the maximum temperature was delayed by 1.72 hours, compared to bare soil. The presence of Artemisia ordosica has been demonstrated to exert a dual effect on the freeze-thaw process. The dual effect is characterised by both promoting and inhibitory characteristics, which have the capacity to inhibit the freezing and thawing process of shallow soil and promote the freezing and thawing process of deep soil. Once the deep soil thawed at the relatively stable meteorological conditions, while the shallow soil failed to experience the repeat freeze-thaw cycles, the end time of the freeze-thaw process was determined by the deep soil. In this case, the water consumption by the roots of Artemisia ordosica promoted thawing, resulting in an earlier ending of the freeze-thaw cycle. Conversely, once the shallow soil experienced repeat freeze-thaw cycles under unstable meteorological conditions after the deep soil was thawed, the end time of the freeze-thaw process was influenced by the shallow soil. At this stage, the insulating effects of the Artemisia ordosica canopy, dead leaf layer, and the shallow root system inhibit the freezing and thawing processes of the soil, resulting in a delayed completion of the freeze-thaw cycles or minimal difference compared to bare soil conditions. There was also the influence of Artemisia ordosica on the soil water content and temperature, as well as the evapotranspiration rates during the freeze-thaw cycle. Furthermore, the Artemisia ordosica was relatively small in the early stage of the experimental period. There was a relatively small difference in the cumulative evapotranspiration rate under the Artemisia ordosica and the bare soil condition. However, the magnitude of the difference in evapotranspiration rates between Artemisia ordosica and bare soil was influenced by the initial soil water content at the onset of the freeze-thaw period, as the Artemisia ordosica grew. The findings can also provide a strong reference to promote ecological protection and high-quality development in the Yellow River Basin.