Soil redox status and dissolved organic matter control the biogeochemical transformations of arsenic in paddy soils

Biogeochemical processes in paddy soils are strongly influenced by redox oscillations, but the linkage between redox status, abundance and transcription of microbial functional genes and their hosts, and biogeochemical transformations of arsenic (As) is not fully understood. We conducted incubation experiments with a constructed soil slope, which was partially submerged to generate a redox potential (Eh) gradient. As Eh decreased down the slope, the abundance of the genes representing iron (Fe) reducing bacteria (Geo), and encoding dissimilatory sulfite reductase (dsr), arsenate [As(V)] reductase (arrA and arsC) or arsenite [As(III)] methylation (arsM) increased, consequently enhancing the reductive mobilization of Fe and As, sulfate reduction, and microbial As methylation. Flooding increased the transcription of arrA and arsM by 14.9-fold and 4.5-fold, respectively. Draining followed by reflooding markedly decreased dissolved organic carbon (DOC) and hindered Eh decline, suppressing microbial functional genes and reductive processes of Fe and As, and of As methylation. Reflooding with a rice straw extract partially restored the abundance of microbial functional genes, reductive processes and As methylation, whereas removal of DOM by the addition of activated carbon (AC) produced opposite effects. Structural equation modeling revealed that porewater DOM directly affected Eh, subsequently impacting the abundance of functional genes and biogeochemical transformations of Fe and As. Responsive microbial hosts for transcribed arrA, arsC and arsM were identified. This study shows that porewater DOM and soil Eh critically control reductive mobilization of Fe and As biogeochemical transformations in paddy soil through impacting functional genes abundance, transcription and composition of microbial hosts.