Atmospheric CO2 concentration is continuously increasing, and previous studies have shown that elevated CO2 (eCO2) significantly impacts C3 plants and their soil microbial communities. However, little is known about effects of eCO2 on the compositional and functional structure, and metabolic potential of soil microbial communities under C4 plants. Here we showed that a C4 maize agroecosystem exposed to eCO2 for eight years shifted the functional and phylogenetic structure of soil microbial communities at both soil depths (0-5 cm and 5-15 cm) using EcoPlate and functional gene array (GeoChip 3.0) analyses. The abundances of key genes involved in carbon (C), nitrogen (N) and phosphorus (P) cycling were significantly stimulated under eCO2 at both soil depths, although some differences in carbon utilization patterns were observed between the two soil depths. Consistently, CO2 was found to be the dominant factor explaining 11.9% of the structural variation of functional genes, while depth and the interaction of depth and CO2 explained 5.2% and 3.8%, respectively. This study implies that eCO2 has profound effects on the functional structure and metabolic potential/activity of soil microbial communities associated with C4 plants, possibly leading to changes in ecosystem functioning and feedbacks to global change in C4 agroecosystems.
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