Simultaneous application of rice straw and sulfate in paddy soil under flood-drain conditions and its consequence on arsenic mobility

This study investigated the impact of the simultaneous application of rice straw (RS) and sulfate, mimicking actual agricultural practices, on arsenic (As) mobility in paddy soils under repetitive redox oscillation conditions. While previous studies primarily focused on the individual effects of organic amendments or sulfate on As mobility, this study explored their combined effects to provide a more comprehensive understanding of As dynamics in paddy soils. Microcosm experiments were conducted to observe changes in As mobility during two flood-drain cycles, analyze As and iron (Fe) fractionation using Wenzel sequential extraction, and examine changes in the relative abundance of microorganisms involved in Fe reduction. The simultaneous application of RS and sulfate increased the relative abundance of microorganisms involved in Fe reduction and promoted reducing conditions, significantly increasing Fe oxide reductive dissolution and As concentrations in porewater during the first flood-drain cycle. Arsenic concentrations in porewater ranged from 0.4 to 5.3 mg/L without applications and from 4.3 to 20.7 mg/L with simultaneous application of RS and sulfate. Meanwhile, redox oscillations decreased As mobility by approximately 68 %–87 %, attributed to decreased dissolved organic carbon concentrations and increased Fe oxide crystallinity. These findings highlight the dynamic nature of As mobility in response to environmental factors associated with actual agricultural practices and provide valuable insights into understanding As contamination in paddy soils.