Question

main pollutant from rice field is

Answer 1

In case of rice, at least in China, the main pollutants are metals, like copper, zinc, lead and foremostly - cadmium. The last one is especially dangerous because it's easily absorbed by plants and can cause or contribute to cancer. 

This kind of soil pollution is mainly caused by unclean water supplies used for irrigation of fields. In case of China, water streams are polluted by waste coming from mines, situated upstream of the fields (which is quite natural, given that you mine in the mountainside and streams shoot out there as well).