The uptake of arsenic species by commonly grown Australian rice varieties cultivated utilising two widely used agronomic practices (straw incorporation and nitrogen fertilisation) and the role dimethyl arsenic plays in inducing straighthead disease

Environmental context Rice is a staple diet for many people worldwide. Rice accumulates arsenic naturally from soils and is thought to cause straighthead disease. This disease causes sterility in rice resulting in a lack of grain formation and hence the head of the rice remains upright at maturity. Rationale Rice is inherently efficient at accumulating arsenic (As) due to it being grown under anaerobic conditions. Arsenic in rice plants can pose a significant risk to human health. In this study we aim to determine if two agronomic practices, i.e. straw and nitrogen addition to rice paddies, influence the bioavailability of different As species within the soil and their incorporation in the various components of the rice plant and whether these practices lead to straighthead disease. Methodology A field trial was conducted to grow commercially grown rice varieties with the application of wheat straw and nitrogen individually and in combination. Results The incorporation of straw into the soil resulted in the formation of dimethyl arsenic (DMA) and an increase of As content in the grain as DMA. DMA accumulated linearly with total As concentration in the grain to become the major As species, whereas inorganic arsenic (Asi) plateaued at 0.2 µg g-1. Discussion Rice plants cannot control the accumulation and distribution of DMA within the plant, and DMA uptake increases the severity of straighthead disease. The plants with straighthead disease recorded higher DMA concentrations in the husks and leaves when compared to healthy plants. This study showed that rice has some capacity to control Asi concentrations in the grain, whereas DMA accumulation is unregulated. Straw incorporation into soil promotes DMA formation and uptake into rice; this agronomic practice should be avoided.