Soil Warming Increases Arsenic Availability in the Rice Rhizosphere

Abstract

Photo source: Flickr/WorldFish Arsenic is a widely distributed toxic element that naturally occurs in minerals. One of the most common pathways for exposure is when arsenic leaches into drinking water supplies, often through natural weathering/breakdown. People are also exposed to arsenic through food and dust. Long-term exposure is linked to health problems, including skin lesions, cardiovascular disease, and cancer. Research from Bangladesh shows long-term exposure through drinking water increases the rate of morbidity by up to 60% (http://bit.ly/2fFhq6w). Understanding exposure routes and minimizing human consumption and contact with arsenic has widespread human health benefits. One crop known to take up arsenic when the element is available in soils or irrigation water is rice (Oryza sativa L.). Arsenic accumulates throughout the plant tissues including the grain that is consumed. This is concerning for people whose diets include a high proportion of rice or rice products such as regions and cultures where rice is served with most meals, those on a gluten-free diet using rice substitutes (e.g., rice milk or rice flour), and young children who are given rice crackers and rice cereals. Rice plants may be exposed to arsenic through soil or irrigation water. Because of this, the hydrological and biogeochemical processes that take place in the rhizosphere, or the soil zone around the roots of plants, influence arsenic uptake, according to Rebecca Neumann, Assistant Professor in the Department of Civil and Environmental Engineering at the University of Washington. Specifically, rice plants release oxygen from their roots when flooded, which reacts with iron, forming “plaques” along root surfaces. The iron oxide plaques scavenge arsenic, and the plants take up arsenic released from the plaques or dissolved in the soil solution. One factor that can influence rhizosphere processes, which ultimately affect arsenic accumulation in the rice grain, is soil temperature. Because global climate models predict an increase in average temperatures, Neumann and colleagues set up an experiment to determine if elevated soil temperatures would have an effect on plaque formation and arsenic uptake. Their results were recently published in Agricultural & Environmental Letters (see http://bit.ly/2vCtRUO). “We purposefully grew our plants in a system where we could image and visualize the rhizosphere,” Neumann says. The researchers grew plants in rhizoboxes (30 cm 3 20 cm 3 2.5 cm), which enabled them to sample soil porewater and perform synchrotron imaging on the plant root zone at the end of the experiment. Rice plants were kept in growing chambers, where the atmospheric and ambient soil conditions were designed to mimic conditions in Bangladesh. The elevated soil temperature treatment was 5°C higher, which is the upper limit of expected warming in the region. Globally, Bangladesh does not have the highest concentration of arsenic in rice grains; however, “arsenic contamination of rice poses a large human health problem [in Bangladesh] because rice is the staple food. So that was part of the motivation for focusing on and trying to mimic conditions that are relevant in that context,” Neumann explains. X-ray fluorescence data collected for (a,c) one plant from the 26.1°C soil-temperature treatment and (b,d) one plant from the 30.5°C soil temperature treatment. (a–b) Solid-phase iron concentrations in counts per second (cps); concentrations in both panels scale with color bar in panel (a). (c–d) Solid-phase arsenic concentrations in counts per second; concentrations in both panels scale with color bar in panel (c). The synchrotron imaging provided additional information about arsenic in the rhizosphere. The authors report that with warmer soil temperature, there was more iron plaque on roots and more arsenic sequestration. Neumann explains, “There is more arsenic mobilized and made available in the warmer temperatures. This excess arsenic, some of it [is] sequestered in the plaques, and then some of it [is taken up through] the roots.” At the end of the experiment, plants were destructively sampled to test for arsenic levels in plant tissues. The authors report the plants grown in warmer soil temperatures had greater concentrations of arsenic in the porewater, root tissue, straw tissue, and husk. However, arsenic concentrations in the grain did not change with soil temperature. Read the full study in Agricultural & Environmental Letters http://bit.ly/2vCtRUO. While this is a small data set, using a single rice cultivar and focusing only on elevated soil temperature in laboratory conditions, Neumann says the results show, “it's not just plant physiology that can change [with increased temperatures], but actually these processes in the rhizosphere.” Having established this response, Neumann hopes to investigate rhizosphere interactions in other rice cultivars and to scale up to field experiments. Understanding what changes may occur under future climate conditions enables researchers to help mitigate arsenic exposure through rice before it happens.