Abstract: This research investigated the effects of fertilization on degraded alpine steppe in the Tibetan Plateau, and aimed to comprehensively assess the quality of this alpine grassland based on the index of grassland quality (IGQ) and soil properties. A field experiment was conducted in Bange County with three different N and four different P fertilization levels [N: 0 (N₀), 7.5 (N₁), 15.0 (N₂) g N·m^-2; P: 0 (P₀), 7.5 (P₁), 15.0 (P₂), 30.0 (P₃) g P₂O₅·m^-2] in a factorial combination, with 12 N-P combinations, and N₀P₀ as control. The aboveground biomass of different vegetation communities and soil physical and chemical properties were assessed within the controlled experiment. It was found that combined application of nitrogen and phosphorus improved the IGQ, but only the treatment N₁P₁ (N: 7.5 g N·m^-2, P: 7.5 g P₂O₅·m^-2) had a significant effect on IGQ, which under tis treatment was increased by 67.16% compared with control. Data for soil organic matter, total nitrogen and phosphorus, available nitrogen and phosphorus, and pH under the 12 different treatments for three soil depths (0-10 cm, 10-20 cm, 20-30 cm) were subjected to principal component analysis. The results showed that soil pH is negatively correlated with total nitrogen and available nitrogen, and fertilization had a greater impact on soil quality in the 0-10 cm soil layer than in the 10-20 cm and 20-30 cm soil layers. The high P level (30.0 g P₂O₅·m^-2) of fertilizer application increased soil total phosphorus and available phosphorus content but had an adverse effect on plant absorption rate. In summary, N : P fertilization (except treatments N₀P₁, N₁P₀) improved grassland quality, and N₁P₁ had the highest relative score. Hence, moderate N : P fertilization plays a positive role in improving grassland quality and soil fertility in the alpine steppe on the Tibetan Plateau. Further study is necessary to understand the restoration effect of manure.

Key words: alpine steppe, IGQ, N : P fertilization, soil properties, PCA