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 highthroughput 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.