Spatiotemporal differentiation of soil bacterial communities and their environmental associations across the on- and off-year cycle of Moso bamboo

Objective This study aimed to reveal the spatiotemporal differentiation of soil bacterial communities across different root-associated soil compartments (rhizoplane, rhizosphere, and bulk soils) during the on- and off-year cycle of Moso bamboo and seasonal changes, and to explore their relationships with soil environmental changes.

Method A total of 139 soil samples, including rhizoplane soil, rhizosphere soil and bulk soil, were collected from a Moso bamboo forest in the Shunan Bamboo Sea, Sichuan Province, China, across on- and off-years and during the growing and defoliation seasons. Based on 16S rRNA amplicon sequencing, soil bacterial community structure was analyzed at the ASV level. Combined with Tax4Fun2 functional prediction and environmental factor association analysis, the diversity, composition, differentially abundant taxa and co-occurrence network characteristics of soil bacterial communities were compared among different year types, seasons and soil compartments.

Result (1) The α-diversity of soil bacterial communities across the three soil compartments was generally higher in off-years than in on-years. PCoA and PERMANOVA analyses showed that year type was the dominant driver of community structure differentiation, with soil compartment and season also showing significant effects; (2) Bacterial community composition differed markedly among soil compartments. The number of shared ASVs in rhizoplane and rhizosphere soils was substantially higher than that in bulk soil, indicating higher compositional conservatism in root-associated soil compartments. Significant differential taxa between on- and off-years were detected in all three soil compartments. These taxa mainly involved Chloroflexi and its related lineages in the on-year, whereas Actinobacteria and its related lineages were more frequently enriched in the off-year. However, the composition and taxonomic coverage of differential taxa showed clear soil compartment-specific patterns. In addition, microbial co-occurrence networks were generally larger and more tightly connected during the on-year; (3) Total nitrogen, ammonium nitrogen and organic carbon were generally higher in the off-year, and community differentiation was closely associated with environmental variation. Tax4Fun2-based prediction indicated that all three soil compartments showed differences in potential functional pathways between on- and off-years, involving nitrogen metabolism, carbon metabolism, amino acid metabolism, membrane transport and signal transduction, but the predicted functional shifts differed among soil compartments.

Conclusion The effects of the on- and off-year cycle on soil bacterial communities in Moso bamboo showed clear soil compartment dependence. Year type was the dominant factor driving community differentiation (PERMANOVA, R² = 0.158), and the rhizoplane was an important root-associated interface responding to year-type shifts. In rhizosphere soil, total nitrogen showed the highest independent explanatory power for community variation, indicating that nitrogen-related factors are important environmental variables linking resource fluctuations under the on- and off-year cycle with bacterial community responses.