Temporal dynamics of total and free-living nitrogen-fixing bacterial community abundance and structure in soil with and without history of arsenic contamination during a rice growing season

Despite the fact that the nitrogen (N) fixers act as the key regulator of ecosystem process, a detailed study of their abundance, diversity, and dynamics in arsenic (As)-contaminated rice fields is missing so far. DNA extracted from soil followed by 16S rRNA and nifH gene-based real-time qPCR, clone library analysis, and DNA sequencing were used to examine the status of the total and diazotrophic communities in two agricultural fields with and without arsenic contamination history during one rice cultivation season. In general, higher nifH and 16S rRNA gene copy numbers were observed in rice growing soils with lesser As than that with higher As. Elevated levels of 16S rRNA and nifH genes in soil is directly associated with total and nitrogen fixers abundance in the agricultural land without As contamination history through the cultivation period, but the copy number of 16S rRNA gene was decreased, and the nifH gene remained unchanged in the As-contaminated land. Additionally, Canonical Correspondence Analysis (CCA) indicated the possible suppression of nifH gene abundance by soil pH, phosphate, and As content. Increased abundance of total and Acidobacterial lineages in low As-containing soil and the detection of several uncultured groups among nifH gene sequence in higher frequency indicated the presence of novel nifH bearing bacterial groups. Conversely, the abundance of copiotrophic Proteobacterial lineages gradually increased in soil with higher As. Herein, our study demonstrated that the dynamics of free-living nitrogen-fixing bacterial communities were perturbed due to As contamination in agricultural land.