Contrasting Effects of Phosphate on Arsenic Mobility and Human Exposure in Agricultural Soils

Arsenic in agricultural soils may pose human health risks due to elevated concentrations and increased soil ingestion associated with dust generation. However, the actual exposure depends on the fraction of As that becomes bioaccessible under gastrointestinal conditions. This study evaluates As mobility, bioaccessibility, and associated health risks in 57 agricultural topsoils from seven regions affected by geogenic enrichment and historical mining. Arsenic partitioning was characterized using operationally defined extractions and mineralogical analyses, and bioaccessibility was determined by an in vitro gastric assay. The total As concentrations range from 25 to 2007 mg kg^-1, exceeding national agricultural soil thresholds (40 mg kg^-1) in ∼80% of samples. Nevertheless, relative As bioaccessibility ranges from 2% to 38%, with most soils exhibiting values below 10%. Arsenic is predominantly associated with Fe (oxyhydr)oxides, while discrete As-bearing minerals are rare. Surface-bound As, defined by phosphate extraction, strongly controls both mobility and bioaccessibility. Long-term phosphate enrichment due to repeated fertilization exerts contrasting effects on As behavior. Readily soluble P enhances both As mobility and bioaccessibility, whereas higher Olsen-extractable P is associated with lower As bioaccessibility, suggesting progressive occupation of reactive sorption sites. A risk assessment based on a conservative rural ingestion scenario indicates that most soils did not exceed non-carcinogenic thresholds when bioaccessible As was considered. These findings demonstrate that bioaccessible As and associated exposure risk are controlled primarily by reactive surface-bound As fractions rather than by total soil As concentrations, whereas phosphate exerts contrasting secondary effects depending on its geochemical availability and partitioning.