Water regime is important to determine cadmium toxicity on rice growth and rhizospheric nitrifier communities in contaminated paddy soils.

Purpose: It is unknown how the Cd-mediated changes in the rhizospheric nitrifier community affect plant growth under different water management practices. This study examined the effect of Cd on microbial N transformation and its association with rice growth in two water regimes, continuous flooding (CF) and alternate wetting and drying (AWD), in a paddy soil. Methods: The 16S rRNA, amoA genes in the rhizosphere and endosphere, and OsNTRs were determined using qPCR. Nitrifier community analysis was performed by sequencing the amoA genes. Concentrations of N and Cd in shoots and roots, and IAA in the roots were determined by flow injection analyzer, ICP-MS and enzyme-linked immunosorbent (ELISA), respectively. Results: The direct and indirect negative effects of Cd bioavailability in the rhizosphere on plant growth and ultra-structure of roots and leaves were greater under AWD than CF after 56-day growth. Nitrifier community was changed by water and Cd treatments. Crenarchaeota, Nitrosopumilus, Nitrosopira, and Nitrosovibrio tenuis related species were the dominant nitrifier players. Increasing Cd level decreased plant N uptake, root-IAA concentration, and the abundance of 16S rRNA gene in the rhizosphere and endosphere to a greater extent under AWD than under CF. This in turn decreased the colonization of endophytes and the surface area of the roots, leading to decreased N uptake from the soil. Conclusion: The results imply that appropriate water management and rhizospheric engineering in Cd-contaminated paddy fields may be important approaches to decreasing plant Cd uptake and rhizospheric nitrification. [ABSTRACT FROM AUTHOR]