Double Threat: Toxic Arsenic and Climate Change Plummet Rice Production
Muehe, E. M., Wang, T., Kerl, C.F., Planer-Friedrich, B. and S. Fendorf. 2019. Rice production threatened by coupled stresses of climate and soil arsenic. Nature Communications, 10: 4985. https://doi.org/10.1038/s41467-019-12946-4
Over half of the world’s population depends on rice for their daily diet. That dependency puts rice high on the list of crops to study when it comes to climate change. Several projections have already been done to understand how increased temperature and carbon dioxide (CO2) will affect how much rice we can grow. However, rice has another issue that could impact our ability to feed everyone: arsenic. We don’t currently understand how future climate conditions will affect how much arsenic is in soil or taken up by plants.
Arsenic is a naturally occurring element present all over the world. It can be found in the groundwater we drink and some of the foods we eat. Rice paddy soil irrigated with water high in arsenic accumulates more arsenic with each irrigation cycle, meaning the amount of arsenic plants are exposed to gets higher over time.
Arsenic reduces plant growth and can impact how much rice crop we can produce over time, but it’s also toxic to humans. Long-term exposure to arsenic has been linked to cancer, cardiovascular disease, and diabetes. We will have a hard time feeding our global population if we have less rice, but if that rice is also not safe for us to eat that’s an even bigger problem.
In order to understand how global rice production might be affected by both climate change and arsenic at once, researchers turned to greenhouses to test future scenarios and measure rice production. Greenhouses allow researchers to control all the variables that affect how plants grow – sunlight, temperature, soil nutrients, water, etc. In this experiment, researchers tested four different scenarios by manipulating the temperature, carbon dioxide in the air, and the level of arsenic in the soil the plants grew in. Plants were grown in one of four scenarios:
- Today’s climate: 33°C and 415ppmv CO2
- Potential Future Climate: 38°C and 850ppmv CO2
- Potential Future Climate with elevated temperature only: 38°C and 415ppmv CO2
- Potential Future Climate with elevated CO2 only: 33°C and 850ppmv CO2.
All four of these treatment groups were grown in soil with low and high levels of arsenic. The total amount of rice crops grown under all these conditions was measured, as was the amount of arsenic was taken up by plants.
Final Grade: Arsenic Flunks Rice Production
When comparing rice production in current climate conditions to future ones, researchers found that the potential future climate (scenario 2 above) decreased the amount of rice produced by 16%. Growing the plants in higher arsenic alone decreased the crop yield by a whopping 40%. When the stressors were combined, i.e. higher temperature, higher CO2, and higher arsenic, the crop yield decreased by 42%, suggesting that arsenic may have a higher effect on the crop yield than the conditions of climate change.
Researchers also found that the quality of the rice decreased with higher levels of arsenic in the soil. The amount of arsenic that was taken up by the plant increased, and the amount of inorganic arsenic – the more toxic form – doubled in the plant compared to plants grown in low arsenic soil.
Rethinking Our Models
Previous studies of climate change impacts on rice have not considered arsenic. This study suggests arsenic may in fact be a more important driver of rice production than increased temperature and CO2. Higher amounts of arsenic may lead to less rice produced overall, and higher levels of arsenic in the rice that does grow – posing a toxicity risk to humans that consume it. A lot of the world depends on rice. To understand how we can feed everyone with this key crop, we will have to understand how arsenic fits into the model.