Water Use Strategies of Pure Versus Mixed Plantations Under Degradation in Edge Shelterbelt of the Kubuqi Desert, China

China's “Three‐North” Shelterbelt Program is strategically vital for combating desertification. Yet, persistent drought and water stress have degraded both pure and mixed forests, shifting their water use strategies; However, the underlying interaction mechanisms remain unclear. This study employed hydrogen‐oxygen isotope technology to elucidate the coupled effects of afforestation patterns (Pure vs. Mixed) and degradation degrees (Light vs. Severe) on poplar water use strategies in the north “edge” shelterbelt of the Kubuqi Desert in China. The results showed: (1) Soil water in plantations was shaped by afforestation patterns and degradation. In pure plantations, severe degradation increased water loss in shallow (0–120 cm) and deep (200–400 cm) layers; in mixed plantations, it raised soil water across (0–300 cm). Furthermore, mixed plantations always had higher shallow (0–120 cm) but lower deep (300–400 cm) water than pure plantations, while middle‐layer (120–300 cm) water varied markedly with degradations. (2) Under light degradation, poplars water source was 80–350 cm in pure and 100–250 cm in mixed plantations. Severe degradation shifted intersection down to 100–400 cm for pure and up to 40–120 cm for mixed plantations, implying pure plantations absorbed deeper, mixed plantations shallower with intensification. (3) The main water uptake depth of pure plantations shifted from 200 to 300 cm (78.4%) under light degradation to 300–400 cm (47.7%) under severe degradation. For mixed plantations, the pattern shifted from the balanced utilization of the 120–400 cm layer (83.2%) to a dual‐source utilization of 0–120 cm (41.8%) and 300–400 cm (49.0%). In particular, afforestation patterns and degradation degrees have the greatest impact on the absorption and utilization of 200–300 cm soil water, with the contribution rate ranging from 4.10% to 78.4% (89.42% of coefficient of variation). This study found that as degradation intensified, pure plantations formed a “deep‐layer conservative” strategy, while mixed plantations developed a “shallow‐deep flexible” strategy. Mixed plantations can better cope with the risk of forest degradation, maintaining higher water conservation capacity and diversified soil water use strategies, which provided a theoretical basis for the precise restoration and water management of degraded plantations in arid regions.