Geochemical Drivers Govern Redox-Mediated Arsenic Transformation in Multivariate Sourced Organic-Amended Paddy Soils

Sustainable remediation of toxic metal-(loid)-contaminated paddy soils using biowastes is of great importance from both agricultural and environmental perspectives. The redox-mediated interactions between organic amendments, with multivariate sources (i.e., biogas slurry (BGS), rice husk-biochar (RH-BC), cow dung (CD)) and geochemical drivers may influence arsenic (As) mobilization under dynamically changing redox situations, such as in paddy soils. Here, we explored the impact of BGS, RH-BC, and CD on the mobilization pathway of As in a contaminated paddy soil under a wide range of soil redox potentials (<i>E</i> _h: -252 mV to +512 mV), using an automated biogeochemical microcosm system. The partial least-squares-path model (PLS-PM) was used to identify geochemical drivers, which govern As mobilization under reduced and oxidized conditions. Results revealed that As mobilization in the unamended (control) soil was higher under reduced conditions (<i>E</i> _h ≤ +100 mV; dissolved As = 4.2-9.2 mg L^-1) than that in oxidized conditions (<i>E</i> _h ≥ +100 mV; dissolved As = 3.8-6.7 mg L^-1). With CD addition, the concentration of dissolved soil As decreased significantly by 19-62% at <i>E</i> _h < 0 mV, followed by RH-BC (12-54%) and BGS (34-49%) compared to control. Temporal increase in pH under moderately reduced conditions (<i>E</i> _h > +100 mV) led to a maximum decrease in dissolved As concentration with CD (36-77%), and it ranged from 18 to 75% and 29 to 49% for RH-BC and BGS, respectively, over control. These findings highlight that the addition of BGS, RH-BC, and CD, particularly CD, to As-contaminated paddy soil can decrease As mobilization under slightly reduced to oxidized conditions, which occur in the natural paddy soil-rice system. This research advanced our understanding to employ multivariate tools to identify the most potent organic amendment to immobilize As under paddy soil conditions.