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 investigate 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 ranging between 19 and 23 mg kg−1. The Hydrochemical analysis revealed contrasting redox environments with the stability diagrams confirming the arsenate (As(V)) dominance in floodwater and arsenite (As(III)) prevalence in soil solution. 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 (> 96%), while culm/leaves accumulated much less and grains showed limited translocation (< 0.2%). Arsenic concentration in grains ranged between 200 and 420 µg kg−1, exceeding the current European threshold for inorganic As. The morphological assessment indicated 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 attributed to progressive accumulation in the soil through plant roots year after year, combined with 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.