Biological Salt‐Pump Effect: Long‐Term Stand Development of Haloxylon ammodendron and Its Role in Reshaping Soil Hydrothermal and Salinity Regimes

Afforestation in the desert–oasis ecotone induces soil desiccation and secondary salinization. However, the seasonal dynamics of soil water–heat–salt interactions across plantations of different stand ages in arid regions remain unclear. Here, we investigated the seasonal patterns and coupled relationships of soil water–heat–salt dynamics in seasonally frozen desert soil along a long‐term plantation chronosequence. From 2018 to 2024, we monitored these parameters at depth intervals of 0–40, 40–80, 80–120, 120–160, and 160–200 cm in 20‐, 30‐, and 50‐year Haloxylon ammodendron plantations in the Hexi Corridor, Northwest China. With increasing stand age, soil hydrothermal and salinity profiles shifted systematically, with higher temperatures, increased shallow‐soil drying and deep‐water accumulation, and progressive salinization within 0–200 cm. The biological salt‐pump effect was the core mechanism regulating soil salinization in the H. ammodendron plantations. Salt accumulation was most pronounced in the 160–200 cm soil layer of 30‐year plantations, particularly during the summer. The salinity dynamics were well‐fitted to a Boltzmann function. The predominant mechanisms of soil water–salt transport shifted from evaporation to co‐regulation by biological and physical processes: in the 20‐year stands, surface salinization was primarily promoted by evaporation, as shown by a linear mode; in the 30‐year stands, an intensified biological salt‐pump effect resulted in salts being transported into the 160–200 cm deep soil layer; and in the 50‐year stands, the combined action of deep‐root salt pumping and evaporation‐induced upward migration enhanced salt redistribution. These findings reflect the joint influence of biological and physical processes across the soil profile, providing insights into the seasonal water–salt dynamics of H. ammodendron plantations of different ages. This can help to identify critical periods and soil layers vulnerable to salinization, supporting the sustainable management of H. ammodendron plantations in desert–oasis transition zones.