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USA | Research
What are the long-term effects of the Fukushima disaster on local agronomy? Soil fertility remains low in spite of removal of radioactive soils. 3/8/2021
 An experiment testing which green manure was suitable to recover land. /M. Y. Matsushima
On March 11, 2011, Japan experienced the unprecedented Tohoku earthquake. It was the largest in Japanese history and created a massive tsunami that hit the northeast coast of Japan. The result of the earthquake and tsunami was the collapse of the Fukushima Daiichi nuclear power plant. This collapse had immediate and long-term impacts on the area. On February 13, 2021, just a decade after the disaster, Japan suffered a 7.3 magnitude earthquake, a replica of the same earthquake that caused the Fukushima Daiichi disaster. At the time of this article's publication, Japanese officials have not identified any wrongdoing at its nuclear power plants, but residents along the coast are cautious. The Soil Science Society of America and the American Society of Agronomy are conducting an information campaign to raise awareness about the 10th anniversary of this disaster. Immediately after the 2011 accident, the Japanese central government designated the Yamakiya area of Fukushima Prefecture as a "Planned Evacuation Zone". About 1,200 people had to move from their beloved hometown, to avoid being exposed to the radioactive cesium that fell in their area. Yamakiya is a quiet little mountain town. Many of them used to be in agriculture before the Fukushima disaster. Soils in the area were also contaminated with radioactive cesium. This radioactive cesium has a half-life of thirty years. This means that it will take 30 years for half of the cesium-137 to lose its radioactivity and become the safest form of barium. Unfortunately, if food is grown on that radioactive cesium-contaminated soil, the food would also be contaminated with radioactive cesium. As residents continued to be evacuated, the Japanese Ministry of the Environment carried out a decontamination process. Surface soils contaminated with radioactive cesium were removed mechanically and new uncontaminated soil was added. The Chiba University research group led by Miwa Yashima Matsushima began studying the soils of the Yamakiya region, Fukushima, in 2015. This is one of the areas where the land was severely contaminated by radioactive cesium, as well as other pollutants. The evacuation order was lifted in 2017. But by January 2021, only 343 people returned to their homes in Yamakiya. The lands are left without management. Fallow areas tend to be covered with grass and bushes. They found that many areas of uncultivated rice paddies were invaded by common reeds. Why have so few returned and why are they not farming? The government decontamination process gave Yamakiya two different consequences: recovery from radioactive contamination and greatly diminished soil fertility. As for radioactive contamination, the news is good. In most of the areas they monitored in Yamakiya after decontamination, soils in residential and agricultural areas had safe amounts of radioactive cesium. This means that the removal of the contaminated soils solved the problem of continuous radiation exposure in the Yamakiya area and was safe for human residents. That is why they were able to return in 2017. On the other hand, the soil surface was not healthy for growing food from the point of view of soil fertility. For example, a farmer's vegetable field in Yamakiya showed very low carbon content after the decontamination process. The total amount of carbon in the soil decreased by 75%. Soil carbon, known as soil organic matter, plays several critical roles in growing healthy crops. In addition, the ability of the soil to retain and release cations decreased by 48%. This is also important because fertilizers are applied as salts (usually nitrates) that are retained and released in an electrical exchange between soil particles and plants. Soil organic matter plays many important roles in an ecosystem. It provides the soil with a chemical buffer, provides elements for plants, supports the physical environment, and provides food for microbes. In soils where decontamination took place, this content of soil organic matter is extremely low. To restart cultivation in such areas, we must increase the content of organic matter in the soils. In many suburban areas of Japan, farmers use cattle manure that contains a lot of organic carbon and other nutrients. These areas have nearby sources of manure that make it an inexpensive means of improving soil health. However, in areas affected by radioactive contamination from the Fukushima disaster, there are few places where the livestock industry has restarted and returned to its pre-2011 level, and the availability of livestock manure tends to be low. The team thought they could use green manure instead. In 2017, experiments began to test the soil reclamation effects of green manure in Yamakiya. They grew and incorporated rye, hairy pea, and sorghum four times over 2 years. The radioactive cesium concentrations were mainly below the detection level for these decontaminated plants and soils. Green manure incorporations were effective in improving the physical conditions of the soil. The incorporation of rye increased the pore space by approximately 20%. In addition, the available nitrogen was increased through the incorporation of hairy peas. On the other hand, the organic matter content of the soil and the capacity to retain and release cations did not change significantly. They concluded that green manure helps restore soil fertility to some extent. But for a substantial recovery, they needed to apply more organic matter to the soil in the decontamination zones. Based on this idea, they compared the effects of chemical fertilizer and cow manure on plant growth in typical Japanese highland soil (Andosol) versus soil in Yamakiya after decontamination. They did a small-scale experiment in pots. They found that even if they provided chemical fertilizers to the nutrient-poor Yamakiya soil, they would not be able to grow healthy plants. The application of cow manure increased the root and aerial biomass of the plants. To do this, they needed more studies to see how organic matter can recover these decontaminated soils over a longer period. During the three-year research project, the researchers were impressed by how friendly the people at Yamakiya were. In such a quiet and peaceful area, people used to hang out with nature. They went to the surrounding forests to collect leaves to put in their gardens. Forest mushrooms were his favorite food. These lives were never the same again. The researchers were unknown when they first visited Yamakiya, some young students and a middle-aged woman from the Tokyo area. However, they always welcomed them and let them investigate in their land. Furthermore, they took care of them, told them how to cook the food of their country and taught them something very important. They never give up and stand up for something better and keep their lives at Yamakiya: "Our full respect for these people motivates us to progress and take a step towards a better world based on soil science." Source: ASA / SSSA / CSSA
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