The Impacts of Applying Metal(loid) Enriched Wood Ash to Soils on the Growth and Elemental Accumulation of Rice

The mobility and uptake of phosphorus, manganese, arsenic, copper, zinc and several other heavy metal(loid)s to rice shoots and grains were measured under controlled irrigation [flooded and non-flooded] conditions in a pot experiment. Doses of 0.1–1% (w:w) of a metal(loid)-rich wood ash containing ≤ 13,000 mg kg−1 phosphorus, manganese, arsenic, copper and zinc were applied to a study soil to determine the impacts of the ash on rice grain quality and the fate of metal(loid)s. Pore water and rice shoots and grains were analysed for beneficial and toxic elements derived from the ash, and Food-chain exposure modelling was applied to the experimental data thereafter to predict risk of onward impacts to human health. Concentrations of phosphorus and manganese in pore water increased with ash addition, though this did not enhance grain phosphorus or manganese. Zinc mobility was largely reduced over the course of the experiment, but appeared unrelated to ash dose. Arsenic presented the greatest mobility in pore water of all measured metal(loid)s (~ 2500 µg l−1), with shoot concentrations displaying clear dose and irrigation response. Although rice concentrations of arsenic in grain were the lowest of all measured metal(loid)s, risk-modelling of the data showed that even a 0.1% addition of the study ash to soil could expose a high rice consuming cohort to a vastly increased probability of deleterious health impacts. Whilst it is unlikely that contaminated ash would be knowingly applied to land, this study highlights that even singular applications of metal(loid) rich ash to agricultural soils could have negative effects on crops and forward impacts to human health. Other application scenarios, whereby lesser contaminated ashes are repeatedly applied to soils, warrant further work with regard to potential risks of accumulated metal(loid)s entering the food chain.