Niche Differentiation of Arsenic-Transforming Microbial Groups in the Rice Rhizosphere Compartments as Impacted by Water Management and Soil-Arsenic Concentrations

Abstract

Arsenic (As) bioavailability in the rice rhizosphere is influenced by many microbial interactions, particularly by metal-transforming functional groups at the root-soil interface. This study was conducted to examine As-transforming microbes and As-speciation in the rice rhizosphere compartments, in response to two different water management practices (continuous and intermittently flooded), established on fields with high to low soil-As concentration. Microbial functional gene composition in the rhizosphere and root-plaque compartments were characterized using the GeoChip 4.0 microarray. Arsenic speciation and concentrations were analyzed in the rhizosphere soil, root-plaque, pore water, and grain samples. Results confirmed several As-biotransformation processes in the rice rhizosphere compartments, and distinct assemblage of As-reducing and methylating bacteria was observed between the root-plaque and rhizosphere. Results confirmed higher potential for microbial As-reduction and As-methylation in continuously flooded, long term As-contaminated fields, which accumulated highest concentrations of AsIII and methyl-As concentrations in pore water and rice grains. Water management treatment significantly altered As-speciation in the rhizosphere, and intermittent flooding reduced methyl-As and AsIII concentrations in the pore water, root-plaque and rice grain. Ordination and taxonomic analysis of detected gene-probes indicated that root-plaque and rhizosphere assembled significantly different microbial functional groups demonstrating niche separation. Taxonomic non-redundancy was evident, suggesting that As-reduction, -oxidation and -methylation processes were performed by different microbial functional groups. It was also evident that As transformation was coupled to different biogeochemical cycling processes (nutrient assimilation, carbon metabolism etc.) in the compartments and between treatments, revealing functional non-redundancy of rice-rhizosphere microbiome in response to local biogeochemical conditions and As contamination. This study provided novel insights on As-biotransformation processes and their implications on As-chemistry at the root-soil interface and their responses to water management, which could be applied for mitigating As-bioavailability and accumulation in rice grains.

Recent trends in nitrogen cycle and eco-efficient nitrogen management strategies in aerobic rice system

“…Comparing the relative abundance of soil microbes, it has been observed that bacteria are the dominant group over archaea and fungi existing in the rhizosphere soil that surrounds plant roots, whereas among the bacterial phyla, Proteobacteria is the predominant phylum found in most paddy rhizosphere soils ( Somenahally et al, 2021 ; Zhang et al, 2021 ). China-based research studies have demonstrated that the most abundant bacterial genera affiliated with the core microbial communities of rice rhizosphere soils under standard crop management are Anaeromyxobacter , Arenimonas , Arthrobacter , Bacillus , Bellilinea , and 15 other bacterial genera, all of which are well-known to share in the overall growth and health of rice plants ( Zhou et al, 2020 ; Sahu et al, 2022 ).…”

“…Comparing the relative abundance of soil microbes, it has been observed that bacteria are the dominant group over archaea and fungi existing in the rhizosphere soil that surrounds plant roots, whereas among the bacterial phyla, Proteobacteria is the predominant phylum found in most paddy rhizosphere soils (Somenahally et al, 2021;Zhang et al, 2021).…”