Vertical differentiation of soil properties and microbial communities in long-term Eucalyptus grandis × urophylla plantations: surface diversity recovery does not offset deep-layer functional degradation.

AIMS: Long-term Eucalyptus cultivation causes soil degradation, yet the vertical stratification of soil physicochemical properties and its impact on the vertical succession of microbial communities remain poorly understood. This study aimed to clarify the effects of long-term Eucalyptus planting on the vertical succession of soil properties and microbial communities. METHODS: This study investigated first-rotation (5-year, E1) and third-rotation (21-year, E3) E. grandis × urophylla plantations, with mixed Cunninghamia lanceolata and Pinus massoniana forest serving as the control (CK). Soil samples from 0 to 20 cm, 20-40 cm and 40-60 cm layers were collected to explore vertical variations in soil physicochemical properties, enzyme activities and microbial communities. RESULTS: Total potassium (TK) content was significantly higher in the third-rotation plantation (E3) than in the control (CK), and increased with soil depth across all plantation types. Available phosphorus and organic carbon first rose briefly then declined overall, and both decreased with soil depth. Urease, acid phosphatase and sucrase activities declined obviously with stand aging and increasing soil depth. Bacterial α-diversity declined initially, then recovered and reached its peak in the E3 topsoil. Microbial community structure changed greatly; dominant microbes showed distinct age- and depth-dependent distribution patterns. Plantation age positively affected soil pH and total potassium but negatively reduced aggregate stability. CONCLUSION: Successive Eucalyptus planting altered soil physicochemical and enzymatic profiles, and reshaped the vertical microbial community structure.