Dynamics of sulfate reduction regulate arsenic mobilization and speciation in paddy soils in response to root exudates

Background and Aims Root exudates have been proposed to influence arsenic (As) mobility and speciation in paddy soils. However, how sulfur-rich conditions interacting with root exudates affect iron (Fe)-sulfur-driven As mobilization and speciation remains unclear. Methods As speciation and Fe mineralogy were characterized in an 28-day anoxic microcosm experiment using paddy soils, in which typical root exudates including citric acid, oxalic acid and glucose were introduced under under varying sulfate concentrations of 0 mM , 5 mM and 25 mM . Results All root exudates accelerated the reduction of Fe (oxy)hydroxides, and a corresponding increase in As concentrations of 11.6–46.3% in pore water of paddy soils. Oxalic acid had the strongest promoting effect, followed by citric acid and glucose. Although sulfate reduction further enhanced Fe (oxy)hydroxides dissolution, the concurrent formation of Fe sulfides sequestrated a portion of released As, resulting in 4.6%-22.5% lower pore water As compared to exudate alone. Elevated sulfide fluxes promoted greater As immobilization via Fe sulfides formation. Compared to the sulfate-free treatments, sulfate reduction facilitated the formation of dimethylarsenic and dimethylmonothioarsenate by 52.6-127.5% and 14.3–99.9%, respectively. Conclusions Root exudates can enhance As bioavailability in the rhizosphere by promoting its mobilization. In contrast, sulfate reductions may partially counteract As release through incorporating into sulfide minerals. The extent of As immobilization via sulfate reduction appears to depend on sulfate fluxes. This study highlights the critical role of sulfate reduction in regulating As mobilization and speciation in rhizosphere soils.