Effects of ecological restoration projects implementation on soil structural stability and nutrients on the Qinghai-Tibet plateau: a case study of the mid-reaches of the Yarlung Tsangbo River

The Qinghai-Tibet Plateau, often revered as the “Asian Water Tower” and regarded as China's foremost ecological safeguard, is now confronting mounting environmental pressures marked by accelerating soil erosion, declining grassland health, and advancing desertification. In response, ecological restoration projects (ERPs) have been implemented in recent years, including afforestation, pasture conservation and development, soil and water conservation, and desertification control. ERPs are crucial for curbing land degradation and improving soil quality. However, the coupled responses of soil quality and nutrients cycling under ERPs remain poorly understood, limiting precise enhancement of ERP effectiveness. This study focuses on a typical ERPs area in the Mid-reaches of the Yarlung Tsangbo River, South Qinghai-Tibet Plateau. A total of 11 representative sampling sites were chosen, encompassing 5 distinct vegetation-based engineering interventions: Sophora moorcroftiana (SM), Hedysarum scoparium (HS), Artemisia wellbyi (AW), Populus spp. (PL), and Hippophae rhamnoides (HR). This study aimed to elucidate ERPs effects on soil structural stability and nutrients, and reveal their interactions with implementation duration and soil depth. The results showed that (1) PL demonstrated the most effective improvement in soil structural stability (Soil aggregate stability ranking: PL > HR > AW>HS > SM; Soil bulk density: PL > SM > AW>HR > HS) and the greatest enhancement of soil nutrients (Soil stoichiometric ratios: PL > SM > AW>HR ≈ HS; Soil organic carbon components: PL > AW>HR > SM > HS). (2) The strength of the correlations among soil aggregate stability, carbon‑nitrogen‑phosphorus pools, available nutrients, and stoichiometric ratios exhibited a decreasing trend with soil depth. Organic carbon regulates pH and bulk density, which in turn influence soil stoichiometric ratios and available nutrient content, ultimately determining soil aggregate stability. (3) The duration of ERPs implementation significantly promoted soil structural improvement and increased nutrient content (0.487 < R 2 < 0.783, P < 0.05). With increasing soil depth, soil structural stability tended to decline, and nutrients content showed a decreasing trend (0.469 < R 2 < 0.791, P < 0.05). (4) PL demonstrated the highest improvement in the soil quality index (SQI = 0.83), while HS showed the lowest (SQI = 0.31). This study provides scientific support for the effectiveness assessment and optimization of ERPs in the Qinghai-Tibet Plateau, as well as for sustainable regional management. • Impacts of ERPs on soil stability and nutrients in high-altitude river valleys are revealed. • Populus spp. is best for soil structure stability and nutrients, but Hedysarum scoparium is worst; the same is true for SQI. • Longer ERPs duration boosts soil structure stability and nutrients. • Soil structural stability and nutrient levels decrease with increasing soil depth. • Soil organic carbon drives soil aggregate stability by modulating pH, density, and nutrient availability.