Evaluation of Soil Quality in Five Ages of Chinese Fir Plantations in Subtropical China Based on a Structural Equation Model

Abstract

Soil quality evaluation provides necessary and fundamental data and information for understanding the current situation of the soils and for promoting the sustainable use of land resources. In this study, soil quality was assessed by developing a structural equation model (SEM) in five aged Chinese fir plantations, located in the same soil type, with similar site conditions, in Subtropical China. A total of 150 soil samples were taken from the five aged stands of Chinese fir forests: 8-year-old young forests (stand I), 14-year-old middle-aged forests (stand II), 20-year-old near-mature forests (stand III), 26-year-old mature forests (stand VI), and 33-year-old over-mature forests (stand V). Fifteen soil indicators, including soil bulk density (BD), capillary porosity (CP), total capillary porosity (TCP), water holding capacity (WHC), soil organic matter (SOM), total nitrogen (TN), available nitrogen (AN), available phosphorus (AP), available potassium (AK), soil pH, soil acid phosphatase (ACP), invertase (INV), urease (URE), and catalase (CAT), were measured. The SEM was used to determine the weight of each soil indicator, and the soil quality index (SQI) was estimated for the Chinese fir plantations. Results showed that soil physical indicators, such as BD, CP, TCP, WHC, and chemical indicators, including SOM, TN, and AN, significantly degraded in stand II groups compared with the stand I groups, but were significantly recovered in the stand III groups. However, the enzyme activity of soil biological indicators had different patterns with changes in soil physical and chemical properties. The calculated SQI in the studied Chinese fir forests ranged from 0.4084 to 0.7298, which was significantly higher in the stand V and lower in the stand II (middle-aged stand) than in the other four aged stands (p < 0.05). The SEM weight analysis showed that the BD, SOM, and ACP were the most important indicators affecting the physical, chemical, and biological properties of the soils in Chinese fir forests in the study area. This study provided an innovative scientific approach for estimating the weight of SQI in forests and a theoretical basis and practical application for sustainable management of Chinese fir forest ecosystems.