Soil Quality Evaluation and Analysis of Driving Factors of Pinus tabuliformis in Loess Hilly Areas

The selection of suitable tree species and the reasonable allocation of planting areas are important measures for improving soil quality. To evaluate the soil quality (SQ) and its driving factors of Pinus tabuliformis forests in loess hilly areas where forestry ecological projects, such as returning farmland to forest (grass), have been implemented, this study selected P. tabuliformis forests with different restoration years (1a, 6a, 11a, 18a, and 22a) in Wuqi County and used grassland before afforestation (PRG) and abandoned grassland (AG) with 22 years as controls. In this study, soil physicochemical indices, soil fauna indices, and herbaceous plant indices obtained via principal component analysis were used to establish a soil quality evaluation model via the fuzzy comprehensive evaluation method to comprehensively evaluate SQ. Structural equation modeling (SEM) was used to identify the key factors affecting the SQ of P. tabuliformis forests. The goal was to create a model that could effectively evaluate the SQ while considering all relevant factors. The findings of the study showed that: (1) by performing a principal component analysis on the 27 indicator factors, the first six principal components had eigenvalues > 1, and the cumulative contribution rate was 90.028%, effectively encompassing the information of the original variables. (2) The highest soil quality index (SQI) was 0.592 (p < 0.05) in the restored 6a P. tabuliformis forest, whereas the lowest SQI was 0.323 in the restored 1a P. tabuliformis forest. As the number of years of restoration increased, the SQ of the P. tabuliformis plantation forest progressively approached that of the long-term abandoned grassland, with only a 1.8% difference after 22 years of restoration. The SQI of the P. tabuliformis woodland in restored 6a was 83% higher than that of 1a, and following 6a of restoration, the SQI showed a decreasing trend with increasing restoration years. Nevertheless, the SQI increased by >52% compared with the early stage of restoration (1a) and by 31% compared with the grassland before afforestation (PRG). (3) SEM revealed that the SQ of P. tabuliformis forest land was mainly driven by soil physical and herbaceous plant indicators, and soil fauna indicators and restoration years had a negative effect on the evolution of SQ in P. tabuliformis forests. The driving factors of P. tabuliformis forests of different restoration years were different, and with the increase in restoration years, t