|dc.description.abstract||Soil microbial communities play a dominant role in global biogeochemical cycles, with profound effects on agriculture, ecosystem stability, human health, and global climate. As a result, assessing their biogeographic patterns can help to further reveal mechanisms shaping their diversity and function in the environment. Furthermore, due to extensive spatial heterogeneity and environmental gradients, there is potential for overlooking key biogeographical patterns, critical metabolic processes, and novel bacterial taxa existing within deeper soil horizons that can be highly dependent on changes in land-usage. Additionally, an active area of research in soil microbial biogeography is assessing the extent to which current environmental or past historical factors constrain microbial community assemblages.
The objectives of this study were to examine and characterize depth-dependent bacterial community characteristics across multiple land-use types to explore subsurface biogeographical patterns. I collected soil samples across seven distinct land-use types to depths of 45 cm, including old-growth and mature forests, decommissioned, and active agricultural fields from the rare Charitable Research Reserve (Cambridge, Ontario). Bacterial communities were characterized by sequencing of bacterial 16S rRNA gene amplicons coupled with multivariate statistical analyses from 376 soil samples. In addition, to explore functional and metabolic characteristics of collected soils, the PICRUSt algorithm was used to predict metagenomes of uncharacterized taxa.
Soil bacterial communities across all sites were strongly influenced by depth. Upper soils (0–15 cm) and open field sites maintained higher bacterial alpha-diversity than deeper soils and forested sites. The magnitude of soil depth effects appeared to differ across environment types highlighting that land-use type also plays a significant role in shaping communities; bacterial communities across the field sites (i.e., grasslands and agricultural sites) were shown to be more strongly affected than forested sites. Soil pH, which exhibited a large gradient across samples, appeared to be largely responsible for differential shifts in communities with depth across land-use types especially considering that C, NH4+, NO3‾, moisture, and texture showed generally consistent trends with depth across all sites. This observation was further corroborated by NPMANOVA and CCA, which highlighted that pH was among the top explanatory variable explaining >15% of the variation in the dataset. This finding further emphasizes that pH is a strong predictor of bacterial community composition, not only across surface soils, but also within the soil subsurface. Overall, the impact of pH on soil bacterial community composition exceeded that of depth.
The effect of land-use type on subsurface bacterial communities was found to be largely attributed to differences in dominant plant communities. Field sites were characterized by tall grasses whereas forest sites were characterized by woody tree species. Considering that plant inputs (i.e., root exudates, litter) are translocated through soils over time and affect the physicochemical environment, these findings further enforce that plants play important roles in structuring soil bacterial communities across environment types. In addition, contrary to evidence from the aboveground plant communities and site histories, there was no direct evidence of bacterial community succession throughout soils across the field sites sampled in this investigation. Instead, edaphic factors including soil texture, particularly sand, silt, clay, and moisture, appeared to govern changes in overall community composition across the field sites, highlighting the importance of the immediate physicochemical environment in shaping soil bacterial communities.
Soils across all sites and depths were dominated by members of the Proteobacteria (33.2%), Actinobacteria (27.8%), Acidobacteria (14.9%), Chloroflexi (6.6%), Gemmatimonadetes (4.7%), Bacteroidetes (3.0%), Nitrospirae (2.1%), Firmicutes (2.3%), Verrucomicrobia (1.7%), and Latescibacteria (formerly WS3; 1.2%). In addition to observing trends in specific phyla with depth (e.g., Proteobacteria and Bacteroidetes), data also highlighted consistent depth-specific changes in OTU relative abundances. Although the majority of significant correlations were negative (indicating a decrease in abundance with increasing depth), Spearman’s correlation analysis found evidence for consistent positively correlated OTUs with depth. Notably, all positively depth-correlated OTUs were affiliated with uncultivated bacteria, further highlighting that subsurface environments are poorly studied. Correlation analyses were also conducted for pH. Nitrospirae and Chloroflexi members were among the top strongly and positively correlated taxa with pH, consistent with previous studies. Acidomicrobiia and Solibacteres classes, members of the Acidobacteria phylum, were found to be strongly and negatively correlated with pH, which is also consistent with previous research. These results further demonstrate the importance of pH in shaping soil bacterial communities considering that many taxa are adapted to narrow and specific growth and pH ranges.
The PICRUSt results reflected observations noted in the taxonomy-based analysis. “Transporter” associated genes appeared to show differential abundances across land-use type. Forest sites, in particular site CA, a mature forest environment, had the lowest abundance of “transporter” associated genes. This result may further highlight pH effects on soil bacterial communities, considering that site CA had samples with the lowest pH and, consequently, the lowest species diversity.
Overall, this research has set up baseline observations of bacterial community dynamics at the rare Charitable Research Reserve expanding on the few studies that have included soil depth as an environmental gradient and paving the way for future investigations. In addition, this study exemplifies important global environmental gradients including depth, land-usage, and soil biogeochemistry operating at smaller geographical scales across consistent underlying geology. Furthermore, this work has added insight concerning the interplay of the immediate physicochemical environment and past historical legacies in shaping soil microbial communities. Future research with the dataset generated will further explore bacterial taxa that vary in relation to pH and depth, in addition to phylogenetically novel taxa existing at low relative abundance, providing additional insight into the unexplored biodiversity of soil microbial communities.||en