Soil purple phototrophic bacterial diversity under double cropping (rice-wheat) with free-air CO2 enrichment (FACE)

Soil purple phototrophic bacterial (PPB) communities and their responses to elevated atmospheric carbon dioxide (CO2) concentration and nitrogen (N) fertilizer were investigated under a rotation of paddy rice (Oryza sativa L.) and winter wheat (Triticum aestivum L. cv. Yangmai 14) cultivation in a FACE (free-air CO2 enrichment) system. Community structures and abundances of PPB were determined by denaturing gradient gel electrophoresis (DGGE) and real-time quantitative PCR respectively, targeting the pufM gene, which encodes a protein in the light reaction centre of PPB. Soil PPB communities were more diverse and larger under rice than under wheat cultivation, which may be attributed to the flooding of the paddy field and soil moisture changes. Elevated atmospheric CO2 concentration significantly increased the abundance and biodiversity of PPB in soils under rice cultivation, while N fertilizer application rate had less effect on the abundance and diversity. Phylogenetic analysis showed that two common dominant DGGE bands belonged to Bradyrhizobium-and Rhodopseudomonas palustris-like PPB in both rice and wheat soils. The results demonstrated a significant shift in soil PPB communities during the rice-wheat rotation, and a strong positive response of PPB communities to an elevated atmospheric CO2 concentration. Our results also indicated that a diverse and abundant soil PPB community could occur in upland crop fields as well as in aquatic environments and paddy-rice fields. These findings extend our understanding of the ecological significance of PPB in terrestrial soil environments and their responses to future climate change.