Mixed With Broadleaf Tree Species Promote Soil Humus Accumulation and Aggregate Stability in Chinese Fir Plantations

Optimizing soil aggregate structure is an effective approach to enhance the productivity of Chinese fir ( Cunninghamia lanceolata ) plantations. While humic substances are recognized as a crucial binding agent, its role at the scale of governing the composition and stability of soil aggregates remain uncertain. Therefore, this study aims to investigate the effects of mixed plantation of Chinese fir with broadleaved tree species, including Chinese fir × Michelia macclurei (stand type I), Chinese fir × Mytilaria laosensis (stand type II), and pure stand of Chinese fir (stand type III), on the soil aggregate composition and stability and the humus component contents within aggregates of differing sizes (> 2, 1–2, 0.25–1, and < 0.25 mm). Our findings revealed that the mixed stand types, particularly stand type I, exhibited greater levels of soil aggregate‐associated humus components (including humic acid [HA], fulvic acid [FA], humin [HM], humin linked to iron oxides [HMI], humin linked to clay [HMC], and humin residue [HMR]) and stronger soil aggregate stability by promoting diverse litter input and root interactions, which strengthen the accumulation and cementation of soil organic matter. Soil aggregate composition of the mixed stand types was predominantly composed of > 2 mm aggregates, while the pure stand type was primarily composed of < 0.25 mm aggregates. In all stand types, soil aggregate‐associated humus components increased significantly as aggregate size decreased, mainly concentrated in < 0.25 mm aggregates. The higher accumulation of soil humus (especially HA) contents in the mixed stand types is a key factor in promoting the formation of > 2 mm aggregates and ultimately affecting soil aggregate stability. Moreover, the partial least squares path model (PLS‐PM) demonstrated that stand type directly governed soil aggregate composition and stability by modifying the humic and humin fractions, especially in > 2 mm aggregates (Goodness‐of‐Fit = 0.868). Therefore, to help mitigate soil aggregate stability decline and nutrient depletion, recommended management approaches (i.e., establishing mixed forests and selecting suitable mixed tree species) are required to counteract soil environment and quality deterioration, thereby ensuring sustainable soil resource management in Chinese fir plantations ecosystems.