Effects of fly ash and steel slag on cadmium and arsenic accumulation in rice grains and soil health: A field study over four crop seasons in Guangdong, China

• Fly ash and steel slag amendments mitigated Cd and As accumulation in rice grains. • Mitigation effects declined over time and lasted for one to three crop seasons. • Soil pH and labile Si were crucial in determining and sustaining amendment effects. • Steel slag significantly increased soil inorganic nutrients and some toxic elements. • Steel slag altered enzyme activities and bacterial community composition in soil. Fly ash and steel slag can potentially mitigate the cadmium (Cd) and arsenic (As) accumulation in rice grains but their long-term effectiveness and impact on soil health are unclear. By running a four-crop-season field trial, we found that the concentrations of Cd, As and inorganic As in rice grains were significantly reduced by steel slag and (consecutively applied) fly ash. For both amendments, decreased soil extractable Cd by increased pH was crucial in reducing grain Cd, but soil re-acidification diminished their effects. Increased soil extractable silicon played a key role in alleviating grain As accumulation. Steel slag had a more persistent effect on reducing grain Cd than fly ash but the sustainability of their effects on reducing grain As depended on rice cultivars. Steel slag improved soil fertility by increasing soil calcium, magnesium, manganese and zinc but chromium and nickel were also increased; it also enhanced the activities of soil urease and alkaline phosphatase, shifted soil bacterial community composition, and increased bacterial diversity. Fly ash had little effect on soil health. Our results indicated that steel slag had positive and sustainable effects on mitigating grain Cd and As accumulation but its potential negative impact on soil health requires in-depth monitoring.