TOKYO/BEIJING (Reuters) – A year after spending $3.6 billion to buy grain trader Gavilon to expand in China, Japan’s Marubeni Corp has been shaken by defaults on soybean sales and faces an investigation into alleged tax evasion in the world’s top food consumer. The problems in China come as the Japanese trading house also faces more competition. Chinese state trader COFCO Corp has gone on its own spending spree and may soon rival Marubeni, the top grain exporter to China, as it builds its own trading house.