Dynamic changes in soil characteristics, enzyme activity, and microbial communities during montane riparian forest succession

Montane riparian forests offer crucial ecosystem services and showcase a diverse array of successional vegetation dynamics, yet the belowground ecosystem processes underlying these services remain poorly understood. While aboveground succession patterns have been documented through stand structure analysis and chronosequence approaches, critical knowledge gaps persist in understanding how belowground systems drive vegetation reshaping. In particular, the dynamics of belowground ecosystems, including soil microbial communities, soil physicochemical properties, and enzyme activities, and how their assembly responds to vegetation succession and site conditions across successional gradients remain poorly quantified. In this study, five representative vegetation communities were selected, representing secondary successional stages (Gra: grassland; Shr: shrubs; Pio[sbnd]W: pioneer woods; Lat[sbnd]W: late successional woods; Top-W: top successional woods) across floodplains and terraces within riparian zones on the Loess Plateau of China. Shifts in microbial structure during secondary succession and across site conditions were examined using 16S and ITS rDNA Illumina sequencing, along with the assessment of eight soil parameters and three soil enzyme activities. Our findings showed that during the process of succession, Top-W stage exhibited elevated levels of TN, TP, NH₄⁺, NO₃⁻, and AP compared to earlier stages, accompanied by reduced pH. Soil enzyme activities varied significantly among different successional stages and site conditions. Bacteria communities demonstrated greater alpha diversity (Shannon and Chao 1) variability across successional stages than fungi. Specifically, the relative abundance of Firmicutes and Actinobacteriota (cipiotrophs) decreased from Shr to Lat-W stages, while Chloroflexi (oligotrophs) increased. Functional categories associated with C cycling were more prevalent at Gra and Shr stages, whereas nitrogen fixation and N cycling were more prevalent at Lat-W and Top-W stages. Notably, Shr stages were dominated by arbuscular mycorrhizal fungi, whereas the Lat-W and Top-W stages showed increased Basidiomycota abundance and ectomycorrhizal associations. Furthermore, floodplains maintained higher soil moisture but lower enzyme activities compared to terraces, reflecting distinct microbial communities between these habitats. Soil pH, MO, and TN contents were key factors driving bacterial dynamics, while SOC and Pro enzyme activity were pivotal in shaping fungal dynamics. The study provides a foundation for understanding the “microbial communities-soil properties-vegetation succession” interaction mechanism and reveals the linkages between soil microbial community and riparian ecosystem functions.