Influence of alternate wetting and drying water-saving irrigation practice on the dynamics of Gallionella-related iron-oxidizing bacterial community in paddy field soil

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The redox cycle of iron (Fe) is a key process in the biogeochemistry of paddy field soil. It has been recently remarked that Fe(II)-oxidizing bacteria belonging to the family Gallionellaceae (Gallionella-related FeOB) are one of the key players in the Fe(II) oxidation in the soil. The present study aimed to investigate the influence of alternate wetting and drying (AWD) irrigation for water-saving rice cultivation on the dynamics of the community of Gallionella-related FeOB in the paddy field soil over three seasons (wet-dry-wet seasons) of rice cultivation. The AWD irrigation brought about the reduction of Fe(II) contents in the soil, compared with the continuously flooding (CF) irrigation, especially in the last half of the rice cultivation period in the dry season. In contrast, a trend of higher copy number of 16S rRNA genes of Gallionella-related FeOB was observed in the last half of the rice cultivation period of the AWD paddy soil. As a result, a significant negative correlation was observed between the Fe(II) contents and the copy numbers of 16S rRNA genes. PCR-DGGE analysis of 16S rRNA genes of Gallionella-related FeOB showed that the band patterns were different between the CF and AWD paddy soils. Meanwhile, the results of clone library analysis indicated that differences in the taxonomic composition of Gallionella-related FeOB were not observed between the CF and AWD paddy soils, suggesting that Gallionella-related FeOB responded to the AWD water-saving irrigation on the individual strain levels. The potential of Fe(II)-oxidizing activity and its apparent contribution of the activity of Gallionella-related FeOB to the Fe(II) oxidation in the soil of the investigated field were estimated to be 2.4 × 10−3 to 1.7 × 10−1 mg Fe(II) g−1 dry soil day−1 and 4.6%, respectively, based on the reported Fe(II) oxidation rates of cells. The present study showed that the fluctuation of the redox status of Fe caused by the repeated cycle of wetting and drying of the field brought about the changes in the community structure of Gallionella-related FeOB in the soil. Furthermore, it was suggested that Gallionella-related FeOB were certainly involved in the Fe(II)-oxidizing process in the AWD paddy field soil.