Tillage can reduce the radiocesium contamination of soybean after the Fukushima Dai-ichi nuclear power plant accident

• The effects of tillage and cover crop on radiocesium concentration were evaluated.
• Tillage systems alter surface soil movement and soil radiocesium distribution.
• Radiocesium in cover crops of rye were lower than in hairy vetch and weeds.
• Transfer factors for soybean were lower in cultivated treatments than in no tillage.
• Transfer factors in NT decreased each year after the nuclear accident.

A magnitude 9.0 earthquake and a subsequent large tsunami hit the northeastern coast of Japan on March 11, 2011. This resulted in serious damage to the reactors of the Fukushima Dai-ichi nuclear power plant (FDNPP), operated by the Tokyo Electric Power Company. Large amounts of radionuclides were released from the FDNPP, a proportion of which were deposited on the ground. In this study, we investigated soil radiocesium contamination of soybean fields in Ibaraki, approximately 170 km from the FDNPP. After the accident, we compared the radiocesium contamination in soybeans cultivated using different tillage systems and cover crop species. The different tillage systems were moldboard plow/rotary harrow (MP), rotary cultivation (RC), and no tillage (NT); the three types of winter cover crops were fallow weeds, rye, and hairy vetch; and for soybean production we used two rates of manure (0 and 1 Mg ha−1). MP and RC reduced the radiocesium contamination (134CS + 137CS) in the 0–2.5 cm soil layer (509.7 Bq kg−1 for MP and 782.7 Bq kg−1 for RC), although NT left a large amount of radiocesium on the soil surface (1324.8 Bq kg−1). The radiocesium concentration of the rye cover crop was significantly lower than hairy vetch and fallow during 3 years. In 2013, across the tillage system, radiocesium concentration of rye was only 3.4 Bq kg−1, although those values were 17.7 Bq kg−1 for hairy vetch and 56.4 Bq kg−1, for fallow. The radiocesium concentration in soybean grains was significantly lower in MP and RC than in NT from 2011 to 2013. In 2013, 3 years after the FDNPP accident, the radiocesium concentration of soybean grain was 14.8 Bq kg−1 for NT, although these values were 5.8 Bq kg−1 for MP and 6.5 Bq kg−1, for RC. The transfer factor for soybean grain was significantly lower in MP and RC than in NT, although the transfer factor in NT also decreased each sampling year from 0.12 to 0.08 after the FDNPP accident. We conclude that despite numerous benefits of the NT system for environmental conservation, soil inversion by tillage significantly decreased the radiocesium contamination of crops. Thus, tillage inversion would be appropriate to counter measures after the nuclear accident.