Abstract:

Aims：The rhizosphere, as a critical interface for plant-soil interactions, shapes forest soil nutrient cycling through the differentiation of microbial community structures between rhizosphere and non-rhizosphere zones and their regulatory roles in carbon (C) and nitrogen (N) dynamics. This study aimed to characterize the structural and functional divergence of microbial communities in rhizosphere and non-rhizosphere soils of Quercus mongolica pure forests, and to elucidate their responses to soil C and N gradients. 

Methods：Soil samples were collected from rhizosphere (R) and non-rhizosphere (NR) soils of pure Quercus mongolica forests in eastern Liaoning, China. Bacterial 16S rRNA and fungal ITS regions were sequenced, and amplicon sequence variants (ASVs) clustered with 97% similarity. α diversity (Chao1, Shannon, Simpson, and Pielou indexes) and β diversity (NMDS analysis based on Bray-Curtis distance and PERMANOVA test) were analyzed to assess microbial community species richness and heterogeneity. Microbial function was predicted based on the MetaCyc database by PICRUSt2 analysis. Soil total organic carbon (TOC), total nitrogen (TN), organic carbon (SOC), and C:N ratios were measured using a standard protocol. Mantel tests and Pearson correlations were applied to assess the microbial-carbon and nitrogen content linkages. 

Results：Rhizosphere soil microbial endemic ASV was more than that of non-rhizosphere soil. At the phylum level, rhizosphere bacteria were dominated by Proteobacteria and Acidobacteria. In the fungal community, the abundance of Ascomycota in the rhizosphere was significantly higher than that in the non-rhizosphere, while the abundance of Basidiomycota showed an opposite trend. The eutrophic bacteria in rhizosphere soils are mainly used by the simple carbon source secreted by the roots, while the non-rhizosphere oligotrophic bacteria are adapted to the low trophic environment. The fungal community exhibited function-driven niche differentiation, and the rhizosphere ascomycetes formed a symbiotic network based on the carbon source secreted by the host, while the rhizosphere basidiomycetes degraded complex organic matter. Alpha diversity analysis showed that the Shannon index and Chao1 index of rhizosphere fungi were significantly higher than those of non-rhizosphere, while there was no significant difference in bacterial diversity. Beta diversity analysis showed that there were significant differences in the composition of rhizosphere and non-rhizosphere microbial communities. Functional prediction showed that the metabolic pathways of organic matter-degrading bacteria (such as Candidatus udaeobacter) and nitrogen cycle functional bacteria (such as Bradyrhizobium) in rhizosphere soil may be dominated by amino acid biosynthesis. Soil carbon and nitrogen gradients had an impact on microbial diversity and abundance through resource competition and metabolic adaptation: rhizosphere bacterial diversity was positively correlated with organic carbon, and fungal community abundance was significantly correlated with total carbon and C:N. 

Conclusion：Quercus mongolica rhizosphere effects drive microbial community assembly through resource competition and metabolic adaptation, fostering copiotrophic bacteria and symbiotic fungi that enhance nutrient acquisition. The tight coupling between microbial diversity and soil C-N gradients underscores the ecological significance of rhizosphere microbiomes in regulating forest soil fertility. These findings provide a mechanistic basis for leveraging rhizosphere engineering in the sustainable management and ecological restoration of degraded Q. mongolica forests in northeastern China.

Key words: Quercus mongolica, Rhizosphere soil, Non-rhizosphere soil, Microbial community structure, Soil carbon and nitrogen coupling