Selenium (Se) is an essential trace element in humans and animals. Selenium content in the crust of the Earth is unevenly distributed, and it can be divided into available and unavailable Se. Selenium toxicity does, however, occur. In this regard, high Se content in Se-contaminated areas can lead to chronic poisoning diseases, such as alkaline disease and blindness disorder in livestock (Ohlendorf 1989). While low Se content in soil and plants can lead to white muscle disease in animals, Keshan and Kashin–Beck diseases in humans, and other Se deficiency disorders (Wang et al. 2013). Plants are an important source of Se for humans and animals, and soil is the ultimate source of Se for plants. From the perspective of environmental pathology, Tan (1982) pointed out that there is a low-selenium belt extending from North-eastern China to the southwest through the Loess Plateau and then to the southwest to the Tibetan Plateau with an average Se content of only 0.1 mg/kg. Heilongjiang in Northeastern China has abundant black soil resources and produces a major supply of commodity grains nationwide. Importantly it includes the region where the national low-Se belt starts. However, only a few studies have been conducted on the identification of different Se chemical forms in soil in this region.