Soil microbial communities play a crucial role in terrestrial ecosystem functioning, driving processes like organic matter turnover, pollutant breakdown, nutrient regulation, nitrogen fixation, and mycorrhiza formation. Their interactions with soil properties are complex and bidirectional; microbial communities influence soil properties (e.g., nutrient content, hydrolase activity), and soil properties in turn shape microbial communities (e.g., pH, moisture, nutrient availability). Prior research has demonstrated these two-way interactions, highlighting the need for further characterization of soil microbial communities, soil properties, and their relationships. Boreal forests, characterized by specific tree genera (Pinus, Picea, Larix, Abies, Betula, Populus), cover a significant portion of the Earth's land surface and are vital to the global carbon budget. The Greater Khingan Mountains in China possess one of the world's largest and best-preserved primeval boreal forests, representing the southernmost extent of this biome. While research exists on various aspects of boreal forests (regeneration, nutrient content, invasive species), relatively little is known about the relationships between soil microbial communities and soil properties within these ecosystems. This study aimed to characterize soil fungal and bacterial communities in southern boreal forests of the Greater Khingan Mountains and investigate their relationship with soil properties. Considering that a stable soil microbial community is typically formed 30–50 years after afforestation, soil samples from stands older than 50 years were preferentially selected. The study area, the Mohe Observation and Research Station, provided broadleaf (birch and aspen, average age 50 years) and coniferous (larch and pine, average age 96 years) stands suitable for comparison.

Existing literature highlights the significant role of soil microbial communities in nutrient cycling and other ecosystem processes. Studies have shown the influence of soil pH and moisture on microbial communities, and the positive effects of phosphorus addition on microbial respiration and biomass. The two-way interaction between soil microbial communities and soil properties (e.g., nutrient content, moisture, pH) is well-established. Research in boreal forests has explored the impact of vegetation composition on microbial activities and the relationship between bacterial abundance and soil properties. However, a comprehensive understanding of the relationship between soil microbial communities and soil properties in southern boreal forests remains limited, making this study crucial for advancing our knowledge.

The study was conducted in the Mohe Observation and Research Station in the Greater Khingan Mountains, characterized by a mean annual temperature of -5.5 °C and mean annual precipitation of 425 mm. Four natural forests dominated by single tree species (birch, aspen, larch, pine) were selected. Topsoil samples (17 cm diameter, 5 cm height) were collected using the five-diagonal point sampling method after litter removal. Samples were sieved (2 mm mesh), mixed, and subjected to various analyses. Microbial DNA was extracted using the FastDNA SPIN Kit for Soil. Soil bacterial 16S rRNA gene (V3-V4 region) and fungal internal transcribed spacer (ITS1-ITS2 region) were amplified using specific primers. Sequencing was performed using the Illumina TruSeq DNA Sample Prep Kit. Soil chemical properties (total N, NH₄-N, NO₃-N, total and dissolved organic C, total and available K and P, pH, and enzyme activities – urease, protease, sucrase, cellulase) were analyzed using various techniques (AutoAnalyzer 3, Vario TOC cube, flame spectrophotometer, spectrophotometry, colorimetric assays). Data analysis involved quality filtering of raw sequences, OTU classification using RDP classifier, rarefaction analysis, Venn analysis, relative abundance bar charts, PCoA and PERMANOVA analyses (for β-diversity), Student’s t-test, PCA and PERMANOVA (for soil properties), RDA/CCA (for relationships between microbial communities and soil properties), and VPA (to quantify the contributions of soil properties to microbial community differences). Functional classification of microbial communities was performed using FUNGuild and PICRUSt.

Soil fungal ITS sequencing yielded approximately 805,820 clean reads, and bacterial 16S rRNA gene sequencing yielded approximately 641,184 clean reads. Fungal reads were clustered into 386 OTUs, and bacterial reads into 1460 OTUs. Significant differences in microbial community composition and diversity were observed between broadleaf and coniferous forests. Dominant fungal and bacterial OTUs differed across forest types. PCoA ordination and PERMANOVA analysis confirmed significant differences in beta diversity between forest types. Significant differences in functional fungal and bacterial composition were also observed between forest types. Soil properties (total organic C, total N, total and available P, available K, protease activity, pH) varied significantly between broadleaf and coniferous forests. RDA/CCA analysis revealed strong associations between soil microbial communities and several soil properties. Soil protease activity strongly correlated with fungal communities in both forest types, but not with bacterial communities. Soil NH₄-N and total P content strongly correlated with fungal and bacterial communities in broadleaf forests but not coniferous forests. Soil K content showed strong correlations with both fungal and bacterial communities across all forest types. Variance partitioning analysis indicated that K nutrition, NH₄-N content, total P content, and protease activity explained a significant portion of the variance in soil fungal and bacterial communities.

This study demonstrates the significant influence of tree species on soil microbial communities and soil properties in southern boreal forests of the Greater Khingan Mountains. The findings are consistent with previous research showing tree species effects on soil C and N stocks and soil microbial communities. The consistent strong association between soil protease activity and fungal communities highlights the role of fungi in organic matter decomposition. The differing relationships between soil properties and microbial communities in broadleaf versus coniferous forests suggest distinct ecological processes at play. The strong association between soil K content and both fungal and bacterial communities across all forest types emphasizes the importance of K in soil and forest management. Contrasting findings with subtropical forest studies might be attributed to differences in sampling depth, forest type, and latitude. The influence of plant litter composition on soil microbial communities and properties is likely a critical factor driving the observed differences between forest types. This study emphasizes the complex interplay between tree species, litter composition, soil properties, and soil microbial communities in shaping boreal forest ecosystems.

This study revealed significant differences in soil microbial communities and properties between broadleaf and coniferous southern boreal forests in the Greater Khingan Mountains. Key relationships between microbial communities and soil properties included a strong correlation between soil protease activity and fungal communities, a strong correlation between soil NH₄-N and total P with microbial communities in broadleaf forests but not coniferous forests, and strong correlations between soil K content and microbial communities across all forest types. The study underscores the effect of tree species on soil microbial community characteristics and soil properties. However, the generalizability of these findings to other boreal forests requires further investigation due to limited comparable data. Future research could focus on the effects of litter composition on the soil-microbiota-litter microecosystem.

The study focused on a specific geographic location and a limited number of tree species. The relatively small sample size might limit the generalizability of findings to other boreal forests. The study did not explicitly investigate the causal relationships between soil properties and microbial community composition; further experiments would be needed to establish causality. The reliance on high-throughput sequencing may have introduced biases related to primer selection and bioinformatics analyses. The study's focus on topsoil only may not fully represent the complexity of soil microbial communities and their interactions with soil properties across different soil depths.