Evaluation of Soil Fertility in Alpine Shrub Communities of the Qilian Mountains, Northwest China

Understanding soil fertility is significant for the restoration and scientific management of shrub vegetation in the Qilian Mountains. This study focused on the soils associated with five common alpine shrub species—Salix gilashanica, Potentilla fruticosa, Caragana jubata, Caragana tangutica, and Berberis diaphana. We examined soil fertility characteristics, analyzed the relationships among soil fertility indicators, and evaluated the comprehensive fertility status of soil within shrub communities using principal component analysis. The results indicated that (1) the mean values of soil organic matter, total nitrogen, and total phosphorus were 77.94, 3.85, and 0.74 g kg−1, respectively; (2) the soil organic matter and total nitrogen content were significantly higher than the national averages, while the total phosphorus content was slightly lower; and (3) the soil pH and total potassium showed weak variability, the total phosphorus content exhibited moderate variability, and other nutrient indicators (including soil organic matter, total nitrogen, alkali-hydrolyzable nitrogen, available phosphorus, available potassium, soil bulk density, and soil water content) exhibited strong variability. Soil organic matter exhibited a significant positive correlation with total nitrogen, alkali-hydrolyzable nitrogen, and soil water content but negatively correlated with soil pH and bulk density. Total nitrogen was positively correlated with alkali-hydrolyzable nitrogen, available phosphorus, and soil water content but negatively correlated with soil pH and soil bulk density. Total phosphorus demonstrated a positive correlation with total potassium and soil bulk density, whereas total potassium was negatively correlated with available phosphorus. The order of soil fertility of the five communities was Salix gilashanica > Potentilla fruticosa > Berberis diaphana > Caragana jubata > Caragana tangutica. The soil fertility index, based on PCA, indicated that Salix gilashanica exhibited the highest fertility status. The findings of this study provide a theoretical reference for the restoration and reconstruction of shrub vegetation, the enhancement of soil fertility, and the improvement of fragile ecosystems in the Qilian Mountains. It also provides essential insights for soil restoration and sustainable ecosystem management in alpine environments.