Divergent Responses of Forest Soil Microbial Communities under Elevated CO₂ in Different Depths of Upper Soil Layers

Numerous studies have shown that the continuous increase of atmosphere CO₂ concentrations may have profound effects on the forest ecosystem and its functions. However, little is known about the response of belowground soil microbial communities under elevated atmospheric CO₂ (eCO₂) at different soil depth profiles in forest ecosystems. Here, we examined soil microbial communities at two soil depths (0 to 5 cm and 5 to 15 cm) after a 10-year eCO₂ exposure using a high-throughput functional gene microarray (GeoChip). The results showed that eCO₂ significantly shifted the compositions, including phylogenetic and functional gene structures, of soil microbial communities at both soil depths. Key functional genes, including those involved in carbon degradation and fixation, methane metabolism, denitrification, ammonification, and nitrogen fixation, were stimulated under eCO₂ at both soil depths, although the stimulation effect of eCO₂ on these functional markers was greater at the soil depth of 0 to 5 cm than of 5 to 15 cm. Moreover, a canonical correspondence analysis suggested that NO₃-N, total nitrogen (TN), total carbon (TC), and leaf litter were significantly correlated with the composition of the whole microbial community. This study revealed a positive feedback of eCO₂ in forest soil microbial communities, which may provide new insight for a further understanding of forest ecosystem responses to global CO₂ increases.IMPORTANCE The concentration of atmospheric carbon dioxide (CO₂) has continuously been increasing since the industrial revolution. Understanding the response of soil microbial communities to elevated atmospheric CO₂ (eCO₂) is important for predicting the contribution of the forest ecosystem to global atmospheric change. This study analyzed the effect of eCO₂ on microbial communities at two soil depths (0 to 5 cm and 5 to 15 cm) in a forest ecosystem. Our findings suggest that the compositional and functional structures of microbial communities shifted under eCO₂ at both soil depths. More functional genes involved in carbon, nitrogen, and phosphorus cycling were stimulated under eCO₂ at the soil depth of 0 to 5 cm than at the depth of 5 to 15 cm.