Alleviating monoculture-induced soil degradation in Chinese fir plantations in southern China: Optimizing understory mixtures balances stoichiometry and microbial diversity

Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.), a key subtropical plantation species in China, suffers severe soil degradation under monoculture systems, evidenced by productivity decline and nutrient depletion. Mixed forests enhance nutrient availability and microbial function. However, systematic optimization of species configurations of mixed forests remains unexplored. In this study, based on a 30-year-old Chinese fir plantation (retained density: 225 trees/ha), four treatments were established with a tree ratio of 3:7 (Chinese fir to planted species): mixed planting a Phoebe bournei (M1) understory; planting a Phoebe bournei and Taxus wallichiana var. chinensis understory (M2); planting a Phoebe bournei, Taxus wallichiana var. chinensis; planting a Schima superba understory (M3); and a pure plantation of Chinese fir (M0). This study analyzed biotic factors (enzyme activity, microbial metabolic limitations, and community structure), abiotic factors (chemical properties and stoichiometric ratios), and soil quality to select the optimal tree species mixture configuration. Mixed planting significantly enhanced soil nutrient levels, balances stoichiometric constraints, optimizes enzyme activities and microbial diversity, and improved soil quality. Mixed planting showed 29.9–72.6 % higher TN, TP, AN, AK, and SOC in the 0–20 cm layer versus monoculture. Hydrolytic enzymes (sucrase, urease and β-1,4-glucosidases) activity peaked in mixed forests, while monoculture (catalase activity) exhibited elevated oxidative enzymes. M2 had the highest soil quality index (SQI). M2 reduced phosphorus limitation by 12.1 % and alleviated subsoil carbon constraints and increased nitrogen hydrolyses activity. Microbial diversity and keystone taxa were enriched in mixed forests, driven by tree configuration effects on total phosphorus (path=0.68), N/P ratio (0.71), SOC (0.33), pH (0.34), and microbial diversity (0.60). Among the treatments, M2 optimally restores degraded monoculture by balancing SOC accumulation, microbial diversity, and phosphorus availability. In M2, Dependentiae serves as a key soil health biomarker during forest conversion. This study provides a theoretical basis for the sustainable and healthy management of plantations.