Details of the protocols followed for sampling of the paddy soil, its processing and microcosm set up

Paddy soil samples were collected from ten arsenic-impacted agricultural fields of Bethuadahari (23.5989 N, 88.3973 E), an As hotspot in West Bengal, India. After removing the topsoil, samples were collected from depths of 20–30 cm, with multiple subsamples (n = 5; 0.5 kg each) per field, which were pooled and mixed. Composite soil mixture (CSM1 and CSM2) were prepared using equal amounts of soil from each field and were used for further analysis. For the microcosm setup, 10 g of soil mixture was added in 40 mL of Tris-buffered minimal salt medium supplemented with graded As concentrations (As³⁺: As⁵⁺ = 1:4 ratio) in 100-mL serum vials, and incubated under sub-oxic conditions for 60 days. Total DNA extracted form CSM as well as post-incubation microcosms was used for metagenomic analysis.