Long-term CO2 enrichment alters the diversity and function of the microbial community in soils with high organic carbon

The response of soil microorganisms to elevated atmospheric CO2 (eCO(2)) has the potential to alter the regulation of soil biogeochemical processes including carbon and nutrient cycling. A mechanistic understanding of this microbial response in agricultural systems is essential due to the potential impact on soil quality. This study used an eight-year free-air-CO2 enrichment (SoilFACE) experiment to assess the microbial response to eCO(2) in three major agricultural soils (Chromosol, Vertosol, and Calcarosol) planted annually with grain crops. Elevated CO2 increased the number of operational taxonomic unit (OTU) by 14.3%, 13.3% and 4.1% and the Shannon diversity by 3.7%, 4.4%, and 2.6% in the top 5-cm soil layer of the Chromosol, Calcarosol and Vertosol, respectively. The relative abundance of the oligotroph Acidobacteriaceae Subgroup 1 in the top 5-cm soil of the Chromosol and Vertosol was significantly increased by eCO(2). Elevated CO2 did not affect community diversity in the 5-10 cm soil layer. The functional attribute analysis of the bacterial communities showed that eCO(2) increased pectin and benzene degradation, the pentose phosphate pathway and the production of phytase-6 in the top 5-cm soil of the Chromosol. These results suggest that eCO(2) increases the presence of oligotrophs in the bacterial community and overall mineralization of soil organic carbon (SOC) in surface soils with high SOC. Changes in microbial function due to eCO(2) likely impact the stability of SOC and, consequently, the quality of farming soils for sustainable crop production.