Arsenic mobilization and accumulation in rice plants (Oryza sativa L.) produced in alluvial Tejo river Basin: effects of water and soil composition, and agricultural practices

Rice ( Oryza sativa L.) cultivation under flooded conditions enhances arsenic (As) mobilization and accumulation, raising concerns about food safety and human health risks. This study aimed to investigated As dynamics and mobilization in water, soil properties, fertilization practices, and in plant organs (root, culm/leaves and grain) in a paddy field located in the Tejo River Basin (Portugal), during two cropping seasons (2018 and 2019). Soil As concentrations remained relatively stable throughout both seasons, ranging between 19–23 mg kg⁻¹, with spatial variability showing persistent enrichment in southern field areas. The Hydrochemical analysis revealed contrasting redox environments between floodwater (oxidizing) and soil solution (reducing), with the stability diagrams confirming the arsenate (As(V)) dominance in floodwater and arsenite (As(III)) prevalence in soil solution, with the latter being more mobile and bioavailable for plant uptake. Irrigation water was classified as C3S1, evolving to C3S2 at field outlets and plant analysis demonstrated that As accumulation was predominantly confined to root tissues, retaining > 96% of total As across growth stages, while culm/leaves accumulated much less and grains showed limited translocation (< 0.2%). Arsenic concentration in grains ranged between 200 to 420 µg.kg⁻¹, exceeding the current European threshold for inorganic As. The morphological assessment indicated normal plant development without visible stress symptoms, indicating that current As levels do not severely compromise rice growth. Additionally, the fertilizer analysis revealed minimal As inputs (4.439 mg.kg ^− 1 ), suggesting the presence of As is not attributed to irrigation water, but rather to the progressive accumulation in soil through plant roots year after year, combined with small annual inputs from basal fertilizers. These findings provide crucial insights into As dynamics in European rice systems, demonstrating that while root accumulation is substantial, grain contamination remains important to assess food safety under current cultivation practices. Moreover, the results support the need for designing management strategies that minimize grain As while sustaining productivity under Mediterranean conditions.