Effects of close-to-nature tranformation of typical plantation forests on soil physicochemical properties in Hainan Tropical Rainforest National Park

With the focus on four typical plantation forests (Acacia mangium, Chinese fir, Caribbean pine, and Eucalypt) and natural secondary forests within Hainan Tropical Rainforest National Park, this study investigates the restoration mechanisms of close-to-nature transformation measures on soil in tropical rainforest regions.A comparative analysis was conducted between control plots of plantation forests and their close-to-nature transformation plots (ring girdling and selective logging) to examine changes in soil physicochemical properties at 0—20 cm and 20—40 cm depths. The results indicate that: (1) Compared to natural secondary forests, surface soil total potassium (TK) content in the control plots of Acacia mangium and Caribbean pine was significantly higher. Specifically, the Acacia mangium control plot also showed significant increases in total phosphorus (TP), available potassium (AK), and TK content across 0—40 cm soil depth (with TK increasing by 9.8 to 11.9 times). In Chinese fir control plots, TK content was significantly elevated in all layers, while deep-layer AK content markedly decreased. Conversely, the Eucalyptus control plots exhibited clear signs of soil nutrient degradation and reduced pH levels. (2) Compared to the control plots, the close-to-nature transformation of Acacia mangium significantly increased deep-layer TK content, improved soil mechanical composition (evident by decreased sand and increased silt content), and significantly raised the proportion of large aggregates. For Chinese fir, close-to-nature transformation notably enhanced deep soil TK content and the mass proportion of large aggregates. However, selective logging in Eucalyptus plantations significantly accelerated soil acidification and decreased aggregate stability (with mean weight diameter reducing by 55.8%). (3) Redundancy analysis revealed that clay content, alkaline nitrogen (AN), and available phosphorus (AP) are major driving factors for aggregate stability. There is a highly significant positive correlation between the proportion of large aggregates and aggregate stability. In summary, the close-to-nature transformation of Acacia mangium, as well as the control plots of Caribbean pine and Chinese fir, contribute positively to soil quality improvement, aligning it more closely with the state of natural secondary forests. In contrast, the transformation of Eucalyptus poses risks of soil acidification and potassium loss. These findings provide scientific evidence for the transformation and sustainable management of plantation forests in Hainan Tropical Rainforest National Park.