Risk of selenium deficiency due to climate change

Selenium is an indispensable trace element that we take in through food, for example cereals. The selenium content of food depends strongly on the selenium concentration in the soil. Studies showed that selenium concentration is lower when pH and oxygen availability are high and the proportion of clay and organic carbon in the soil is low. In Europe, selenium-poor soils are found mainly in Germany, Denmark, Scotland, Finland and some Balkan countries. This is known on the basis of regionally limited investigations.

Precipitation is central

The global distribution of selenium, on the other hand, was previously largely unknown. However, thanks to the analysis of a large amount of data collected for other purposes (data-mining), Eawag and five other institutes, including ETH Zurich, have now succeeded in reconstructing the global distribution. The researchers compiled information from 16 data sets collected from 1994-2016 and evaluated a total of 33,241 soil samples. They analyzed selenium concentrations in the top 30 centimeters of the soil layer and 26 other environmental variables. It was shown that above all the interactions between climate and soil play a role in the distribution of selenium.

The greatest influence on the selenium concentration in the soil is precipitation and the ratio between precipitation and evaporation (drought index). Precipitation washes out the soil and causes a loss of selenium. At the same time, precipitation can have a positive effect on selenium content. This is because wet soils have a lower oxygen content and a lower ph value, so that the negatively charged selenium remains better bound to soil particles. Areas with low to moderate rainfall and high clay content are most likely to have high selenium content. Dry, alkaline soils with little clay are more likely to contain little selenium.

Based on these findings, the researchers modeled the average selenium concentration of soils for the periods 1980-1999 and 2080-2099. Parts of Australia, China, India and Africa will see an increase in selenium content due to climate change, they said. Overall, however, selenium content in soils will decrease, the researchers conclude: Compared to 1980-1999, by the end of this century (2080-2099), 66 % of agricultural land will have an average selenium loss of about 9 %. Affected areas are mainly croplands in Europe and India, China, southern South America, South Africa and the southwestern United States (see map).

These losses can also be significant for our health. Because already today, up to one billion people are affected by insufficient selenium intake. The authors see their study as an early warning for humanitarian organizations and the agricultural industry. To counteract selenium deficiency, selenium-containing fertilizer could be used. Finland has been doing this since 1984, and selenium could also be used as an additive in animal feed.

Health significance of selenium

Selenium (Se) is essential for human health and is absorbed through food. Due to its antioxidant effect, it intercepts free radicals and thus plays an important role for the immune system. It also serves the body as a building block for numerous proteins. Today, up to one billion people are affected by a selenium deficiency. This can result, for example, in a disease of the heart muscle. But too much selenium can also be harmful and lead to vomiting, liver damage or a disturbance of taste.

Text: Eawag