Soil aggregates are the basic units of soil structure, and their stability is a key indicator of soil quality and capacity to support ecosystem functions. The impacts of various environmental factors on soil aggregates have been widely studied. However, there remains elusive knowledge on the synergistic effects of changing forest stand structure on soil aggregate stability (SAS), particularly in subtropical China where soil erosion remains a critical issue.

We investigated variations in the components of soil humus (HS), including humic acids (HAs), fulvic acids (FAs), and humins (HMs), under pure Chinese fir (Cunninghamia lanceolata) plantation (PP) and multi-layered mixed plantation (MP) comprising C. lanceolata, Castanopsis hystrix, and Michelia hedyosperma. The state of soil aggregate stability, was determined by three separate methods, i.e., dry-sieving, wet-sieving, and the Le Bissonnais. High-throughput sequencing was used to determine the diversity and composition of microbial communities under PP and MP. We then built partial least squares path models (PLS-PM) for assessing the responses of SAS to the variations in soil microorganisms and HS components.

The MP stands had significantly greater SAS (P ​ < ​0.05), higher content of HAs and more rapid organic matter humification within aggregates, than the PP stands. High-throughput sequencing confirmed that the Pielou and α-diversity index values (Chao1 and Shannon) for fungi were all significantly higher under MP than under PP, while no marked difference was found in bacterial α-diversity between the two plantation types. Moreover, there were markedly greater abundance of three bacterial phyla (Verrucomicrobia, Chloroflexi, and Gemmatimonadetes) and three fungal phyla (Ascomycota, Kickxellomycota, and Glomeromycota), and significantly less abundance of two bacterial phyla (Planctomycetes and Firmicutes) and four fungal phyla (Basidiomycota, Mortierellomycota, Mucoromycota, and Rozellomycota) under MP than under PP. The Chloroflexi and Ascomycota phyla appeared to be the primary drivers of soil aggregate distribution. Our findings revealed that the promotion of SAS under MP was mainly driven by increased soil organic matter (SOM) content, which altered bacterial communities and enhanced fungal diversity, thereby increasing HAs content and the rate of organic matter humification.

Considering the combined effects of enhanced soil quality, productivity, and relevant economic costs, introducing broadleaved tree species into Chinese fir plantations can be an effective strategy for stabilizing soil structure against erosion in subtropical China. Our study elucidated the controls on variations of SAS in Chinese fir-dominated plantations and demonstrated the benefit of converting pure Chinese fir plantation to multi-layered mixed plantations in increasing soil structural stability and improving site quality.