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 (E _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 (E _h ≤ +100 mV; dissolved As = 4.2-9.2 mg L^-1) than that in oxidized conditions (E _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 E _h < 0 mV, followed by RH-BC (12-54%) and BGS (34-49%) compared to control. Temporal increase in pH under moderately reduced conditions (E _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.