Elevated CO2 alters the structure of the bacterial community assimilating plant-derived carbon in the rhizosphere of soya bean

Elevated CO2 (eCO(2)) increases rhizodeposits, which in turn alters the soil microbial community. However, it is not really known how the microbial community metabolizes plant-derived carbon (C) in the rhizosphere under eCO(2), especially in agricultural soils. This study used a (CO2)-C-13 labelling technique combined with DNA-stable isotope probing (SIP) to fractionate the C-13-DNA and C-12-DNA from the rhizosphere of soya bean plants (Glycine max (L.) Merr. cv. Suinong 14) grown for 54 days under ambient CO2 (aCO(2)) (390 ppm) or eCO(2) (550 ppm). The DNA fractions were then subjected to Illumina Miseq sequencing. The results showed that eCO(2) decreased the richness and diversity of the C-13-assimilating bacterial community compared to aCO(2) (p < 0.05). Elevated CO2 decreased the abundances of genera, including Pseudarthrobacter, Gaiellales_uncultured, Microlunatus, Gemmatimonas, Gemmatimonadaceae_uncultured, Ramlibacter, Massilia, Luteimonas, Acidobacteriaceae_uncultured, Bryobacter and Candidatus_Solibacter. These genera were probably fast-growing bacteria and sensitive to labile C. In contrast, eCO(2) stimulated the growth of genera Novosphingobium, Acidimicrobiales_uncultured, Bacillus, Flavisolibacter and Schlesneria, which were able to assimilate complex C compounds. Moreover, the increased population of Novosphingobium under eCO(2) might have accelerated electron flow from the oxidation of organic C. Correspondingly, eCO(2) did not affect the concentration of the dissolved organic C but increased the plant-derived C-13 in the rhizosphere. These results indicated that an eCO(2)-induced increase in non-labile C in rhizodeposits contributed to the increase in population size of a number of the plant-C-metabolizing genera that might become the mechanism for the turnover of fresh C in the rhizosphere, modifying the soil C cycle under eCO(2) environments.