Soil quality assessment of oak forests invaded by moso bamboo (Phyllostachys edulis) in the northern subtropics

Soil quality is closely related to the evolution of forest ecosystems, especially in the context of global warming; therefore, it is essential to study the changes in soil quality caused by the invasion of moso bamboo in its habitat. In this study, we selected plantation forests and natural secondary forests with typical vegetation of the northern subtropics, Quercus acutissima plantation (QAP) and oak secondary forest (OSF), collected plant community and soil physicochemical data, and established a soil quality index (SQI) to assess how moso bamboo invasion affects soil quality. In addition, the mechanisms driving the moso bamboo invasion effects on soil quality were investigated using a partial least squares structural equation model (PLS-SEM). The results demonstrated that: (1) Moso bamboo invasion significantly affected community structure and soil physicochemical properties, there were inconsistent changes in soil physicochemical indicators in oak forests of different origins, and in general the effects of moso bamboo expansion on soil physicochemical properties were concentrated in the topsoil (0–20 cm). (2) Soil bulk density (BD), total soil porosity (TSP), total nitrogen (TN), and available potassium (AK) comprised the minimum data set (MDS) used to calculate the SQI, and the SQI of oak forests of different origins ranged from 0.48 to 1, and the invasion of moso bamboo led to a significant decrease in the SQI of the surface layer of QAP. A redundancy analysis (RDA) showed that soil quality assessment factors were closely related to community structure and the PLS-SEM indicated that moso bamboo invasion indirectly affected surface soil quality through its effects on community and soil factors. The results showed that moso bamboo invasion had different effects on oak forests of different origins. They also showed that soil quality was strongly affected by aboveground biomass (AGB), the tree Shannon-Wiener index (TH'), and available potassium (AK).