ELEVATED SOIL TEMPERATURE INCREASES ARSENIC UPTAKE BY RICE AND ARSENIC CONCENTRATION IN RICE TISSUES: A POT EXPERIMENT

Arsenic uptake by rice threatens rice yields and contaminates rice grain, jeopardizing food security and exposing people worldwide to unsafe levels of arsenic. A significant body of work has amassed elucidating the dynamic biogeochemical processes involved in arsenic release from soil to solution, re-sequestration of arsenic into root-iron plaques, uptake of arsenic by rice roots, and accumulation of arsenic in edible tissues. However, a dearth of knowledge exists on how these processes will be affected by a changing climate; thus, it is unclear how hazards associated with arsenic contaminated rice will change in the future. We present results from a pot experiment testing the affect of elevated soil temperature on arsenic biogeochemistry and arsenic uptake by rice. We reasoned that elevated soil temperature would: 1) increase arsenic concentrations in anoxic porewater by increasing the rate of microbially-mediated reductive dissolution of arsenic bearing iron minerals in anoxic soil, and 2) decrease oxygenation of the rhizosphere and thus diminish oxidative formation of arsenic scavenging iron plaques around rice roots. We knew oxygen solubility would decrease with increasing temperature and we expected that increased rates of root respiration would result in less oxygen delivery to the rhizosphere. We tested these hypotheses by growing rice plants under identical atmospheric conditions, but in soils with different temperatures. We found that a ~4 ^oC increase in soil temperature increased both arsenic uptake and arsenic concentration in rice tissue. The increase was related to a greater amount of soil-released arsenic in the elevated soil-temperature treatment. Contrary to our expectations, increased soil temperature did not decrease oxygenation of the rhizosphere; in fact, more arsenic was sequestered on/in root plaques in the elevated soil-temperature treatment. However, this greater sequestration of arsenic did not offset the greater availability of arsenic in the elevated soil-temperature treatment. Our results show that elevated soil temperature increases arsenic availability to and uptake by rice plants grown in pots. As the climate system warms, arsenic concentration in rice tissue could increase, potentially exacerbating the threats that arsenic contaminated rice poses.