Impact of Close-to-Nature Transformation on Soil Fertility Restoration in Chinese Fir Plantation Forest: A Case Study of Cili County, Hunan Province

This study aimed to address soil land degradation caused by long-term monoculture in Chinese fir plantation forests in southern China. A 10-year close-to-nature transformation experiment was conducted in Cili County, Hunan Province. We systematically evaluated the vertical differentiation effects and restoration mechanisms of different "retention density–afforestation method" combinations on soil quality. The results showed that high retention density (170–180 trees/acre) significantly increased surface layer (0–10 cm) soil organic carbon content (+13.8%) and available phosphorus (AP) content (+126.7%) by enhancing litter input and rhizosphere exudates, making it the optimal strategy for the rapid recovery of surface-layer soil fertility. In contrast, low retention density (150–160 trees/acre) improved the physical structure of the subsurface layer (10–20 cm), enhancing microbial phosphorus cycling (microbial biomass phosphorus, +34.4%) and increasing non-capillary porosity (+19.1%), thereby effectively supporting the ecological functions of the mid-layer soil. Mixed afforestation with Phoebe zhennan and Betula luminifera showed limited short-term improvement but activated deep-layer (20–40 cm) AP potential (+273.7%), suggesting its capacity to mobilize deep nutrient pools over medium to long terms. We developed an integrated assessment framework for soil quality based on principal component analysis and a random forest model. This framework used a comprehensive index that incorporated physical, chemical, and microbial indicators, thus identifying phosphorus supply and microbial activity as the key drivers of soil fertility restoration. This study proposed a hierarchical synergistic pathway—"retention density drives shallow restoration whereas afforestation strategies unlock deep potentials"—thereby providing both a theoretical basis and scalable transformation models for the sustainable management of subtropical plantation forest ecosystems.