Phosphorus Loss Characteristics in Typical Watershed Ecosystems of the Qilian Mountains, China

The Qilian Mountains, located in the arid regions of northwest China, are typical high-altitude mountains and represent a significant ecological security barrier for the country. In recent years, these regions, influenced by climate change and human activities, have faced a series of ecological and environmental challenges, including land degradation, vegetation reduction, increased runoff, nutrient loss, and soil quality deterioration. However, the causes of soil quality decline from the perspective of phosphorus migration and loss have not been thoroughly investigated. Colloidal phosphorus is considered to play a crucial role in the migration process of phosphorus in forest soils, and this fraction of phosphorus requires in-depth research and quantification. This project takes the typical forest soil-hydrological system on the northern slope of the Qilian Mountains as the research object, with a primary focus on the following aspects: (1) The distribution characteristics of colloidal phosphorus in forest soils. (2) The forms and distribution characteristics of phosphorus, as well as the spatio-temporal characteristics of ecological stoichiometric ratios in the water of the forest ecosystem. (3) Risk assessment of phosphorus loss in Qilian Mountain ecosystem. The research results indicate that the average total phosphorus content in shrub soils is the highest, while that in Picea crassifolia soils is the lowest. The available phosphorus content in Picea crassifolia forests is significantly higher than that in shrub and grassland areas. Although the content of colloidal phosphorus is the lowest among all vegetation types, it accounts for only 10%-27% of total water-dispersed phosphorus, but this fraction is relatively active and may dominate the migration and transformation of phosphorus in the soil. With increasing soil depth, the content of colloidal phosphorus significantly decreases, suggesting that colloidal phosphorus in the soil may undergo leaching loss. Higher altitudes are associated with higher total dispersible phosphorus content in the soil, while colloidal phosphorus content is lower, indicating that most colloidal phosphorus is concentrated at lower altitudes. This also implies that colloidal phosphorus may dominate the top-down migration of phosphorus within the watershed. The average total phosphorus concentration in various water bodies within the watershed ranges from 0.01 to 0.721 mg L^-1, primarily in the form of particulate phosphorus. The average total phosphorus concentration in most water bodies exceeds the total phosphorus standard for eutrophic surface water (0.02 mg L^-1), posing a potential threat to the downstream water bodies of the watershed. Phosphorus loss restricts the growth and recovery of vegetation in the study area.