Maintaining a balanced soil carbon cycle is crucial for mitigating climate change. Soil heterotrophic respiration, primarily driven by microorganisms, plays a vital role in this cycle. Microbial respiration is closely linked to microbial community properties, which are in turn shaped by both biotic (e.g., plant diversity) and abiotic (e.g., soil chemical properties) factors. Plant diversity can influence soil carbon input through litter and root exudates, potentially enhancing microbial biomass and activity. However, the precise mechanisms by which plant diversity affects microbial communities and soil carbon dynamics remain unclear. Abiotic factors such as soil carbon, nitrogen, phosphorus levels, pH, and moisture also significantly impact microbial community assembly and functioning. Different facets of the soil microbial community—abundance (biomass), taxonomic diversity, and functional diversity—can be assessed through various methods (PLFA, 16S rRNA gene sequencing, functional gene quantification, MicroResp, etc.). These facets may be correlated but not necessarily always coupled, due to factors like functional redundancy and differing sensitivities to environmental change. Understanding the relationships between these facets and their influence on microbial functions, such as respiration, is critical for improving soil carbon cycle models. This study, conducted in a subtropical forest experiment in China, aimed to understand the mechanistic effects of tree diversity and soil chemical properties on microbial functions by integrating different facets of the microbial community into a common framework. The hypotheses were: H1: Tree diversity would drive microbial community facets and increase soil microbial functioning. H2: Soil microbial biomass, taxonomic and functional profiles would be correlated and drive microbial functions. H3: Microbial physiological potential would link microbial biomass, taxonomic and functional profiles to microbial respiration. H4: Environmental conditions (tree diversity and soil chemical properties) would co-determine soil respiration.

Existing literature suggests a strong link between plant diversity and soil microbial communities. Studies have shown that increased plant diversity can lead to enhanced soil carbon storage by increasing microbial biomass and activity. However, the relative importance of microbial biomass, taxonomic diversity, and functional diversity in driving soil carbon dynamics has been debated. Some studies have emphasized the role of taxonomic diversity in predicting microbial functions, while others highlight the importance of functional diversity. The relationship between these microbial facets and their contribution to key functions like microbial respiration is not fully understood. Furthermore, the influence of abiotic factors, particularly soil chemical properties, on these relationships remains an important area of investigation. Prior research has established correlations between soil organic carbon content and microbial activity, but the specific mechanisms and the interplay between biotic and abiotic factors are not always clear. The current study aims to address these gaps by considering different facets of the microbial community within a unified framework.

The study was conducted at the BEF-China experiment in southeastern China. In 2018, 150 soil samples were collected from 52 plots across a gradient of tree species richness (1, 2, 4, 8, 16, and 24 species). Samples were collected between pairs of trees, avoiding spatial autocorrelation. Soil chemical properties (moisture, pH, total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), C:N ratio, C:P ratio) were measured. Soil microbial biomass was assessed using PLFA analysis, distinguishing bacterial and fungal biomass. Active microbial biomass was measured using substrate-induced respiration (SIR). Microbial taxonomic profiles (bacterial and fungal communities) were determined by 16S and ITS amplicon sequencing, respectively. Microbial functional profiles were quantified using qPCR of genes involved in carbon catabolism. Microbial physiological potential was assessed using the MicroResp method with 14 substrates. Soil microbial respiration was measured without substrate addition. Statistical analyses included linear models, Pearson correlations, structural equation modeling (SEM), and variance partitioning. Linear models were used to assess the effect of tree species richness on microbial community facets and functions. Correlations were used to examine relationships between microbial facets. SEM was employed to determine the combined effects of microbial facets on microbial physiological potential and respiration and the influences of tree diversity and soil quality. Variance partitioning was used to separate the effects of microbial biomass, taxonomic and functional profiles.

The study revealed several key findings: 1. **Positive effect of tree diversity:** Tree species richness positively influenced total microbial biomass (+25% from monocultures to 24-species plots), bacterial diversity (+12%), and physiological potential (+12%). Microbial respiration also showed a positive trend, though not statistically significant at p<0.05. 2. **Key drivers of microbial respiration:** Microbial biomass and physiological potential were identified as the main drivers of microbial respiration, while microbial diversity showed a weaker influence. 3. **Strong correlations between microbial facets:** Positive correlations were observed between total and active microbial biomass, as well as between functional profile variables. The bacteria-to-fungi ratio (B:F) negatively correlated with microbial biomass and fungal diversity. Fungal diversity showed a positive correlation with active microbial biomass. 4. **Microbial community facets drive soil microbial functions:** Microbial community facets explained up to 50% of the variance in microbial respiration, with microbial biomass being the most important driver. Taxonomic and functional profiles had much smaller effects on microbial respiration. 5. **Interaction of microbial facets in mediating microbial respiration:** SEM analysis showed combined positive effects of microbial biomass, fungal diversity, and physiological potential on microbial respiration, with microbial biomass having the strongest effect (total effect size: 0.672). Microbial physiological potential was strongly influenced by microbial biomass and functional gene evenness. 6. **Influence of soil chemical properties:** Soil chemical properties, particularly total organic carbon (TOC), were the strongest drivers of the relationships between microbial community facets and functions, affecting all facets of the microbial community and respiration. TOC had strong positive effects on microbial biomass and physiological potential. Soil pH also affected several microbial facets, while soil humidity and the C:P ratio had more specific, less pervasive effects. 7. **Tree diversity's indirect effect:** Tree species richness indirectly influenced microbial respiration by increasing microbial biomass and physiological potential.

This study provides strong evidence that tree diversity significantly impacts soil microbial communities and their functions, ultimately affecting ecosystem-level processes like soil carbon cycling. The findings highlight the importance of microbial biomass as a primary driver of microbial respiration, challenging the sole focus on microbial diversity in some previous research. The study also emphasizes the considerable influence of soil chemical properties, particularly TOC content, on microbial communities and their functioning. The indirect effect of tree diversity on microbial respiration, mediated through increases in microbial biomass and physiological potential, is a notable finding. This suggests that strategies aimed at enhancing tree diversity could be effective in improving soil health and carbon sequestration, but the quality of soil organic carbon and soil conditions also play a critical role. The observed interactions between different facets of the soil microbial community and their combined effects on respiration provide valuable insights for improving soil carbon cycle models. The findings underscore the need to consider the complex interplay between biotic and abiotic factors and the different aspects of the microbial community when studying soil carbon dynamics.

This research demonstrates that tree diversity and soil carbon content are significant drivers of microbial respiration, primarily through their effects on microbial biomass. The study's integrated framework successfully links different microbial community facets to ecosystem functions, highlighting the critical role of microbial biomass in predicting microbial respiration. The findings suggest a positive feedback loop where tree diversity enhances soil carbon storage via increased microbial biomass and activity. Future research should focus on refining soil carbon cycle models to incorporate these findings, further investigate the role of different soil organic carbon pools, and incorporate temporal dynamics into models to improve predictive capabilities.

The study is limited by its focus on a single site and time point, which might limit the generalizability of findings across other biomes and seasons. While the researchers used a diverse array of techniques, certain microbial processes and interactions might not have been fully captured. The influence of other environmental variables beyond those measured could also impact the relationships explored. Further research is needed to validate these findings across different environmental contexts and under various climatic conditions.