Effects of Different Ecological Restoration Treatments on Plant Functional Group Characteristics and Nitrogen and Phosphorus Stoichiometry in Alpine Semi-Humid Sandy Land

Ecological restoration of alpine sandy lands plays an important role in promoting vegetation recovery and enhancing ecosystem functions. However, the response characteristics of plant functional groups and nitrogen (N) and phosphorus (P) dynamics under different restoration treatments remain unclear. Taking three typical ecological restoration treatments in the alpine semi-humid sandy land on the eastern edge of the Qinghai–Tibet Plateau—fencing + sand barrier establishment + mixed grass seeding (MG), fencing + sand barrier establishment + shrub planting alone (SA), and fencing + sand barrier establishment + shrub–grass combination (SG)—as the study objects, and unrestored natural sandy land as the control (CK), this study investigated the characteristics of plant biomass and diversity, as well as the content and stoichiometric characteristics of N and P in the plant–litter system under different restoration treatments, and analyzed the response relationships between plant diversity and changes in N and P in the plant–litter system. The results showed that all three restoration treatments increased plant community biomass and diversity indices. Among them, the SG treatment had the highest aboveground biomass and diversity, whereas the MG treatment had the largest belowground biomass. All three restoration treatments increased the N and P contents of herbaceous plants and litter, with the greatest increases observed under the SG treatment. The results for plant functional groups showed that legumes had the highest N content, while forbs had the highest P content. The N:P ratios of aboveground plant parts ranged from 5.85 to 8.82 and were lower in the SA and SG treatments, suggesting that plants under the restoration treatments may be N-limited. In conclusion, the SG treatment is the most effective restoration approach for promoting vegetation recovery and nutrient content in alpine sandy lands. These results provide a scientific basis for selecting ecological restoration treatments for alpine sandy lands and for improving restoration effectiveness and efficiency.