Selenium distribution in cultivated Argosols and Gleyosols of dry and paddy lands: A case study in Sanjiang Plain, Northeast China

Distribution pattern of selenium (Se) fractions in soil could influence Se content in crops and thereby intake of Se in human body. In order to investigate the effects of soil types and farming conditions on Se distribution in small-scaled cultivated land developed under the same conditions of climate, topography and parent materials, two types of soils (i.e., Argosols and Gleyosols) from paddy and dry lands in the Sanjiang Plain of Northeast China were selected. Total Se (T-Se) content in Argosols was influenced by organic carbon (Org C) content and pH of bulk topsoil. In Gleyosols, it was mainly affected by Org C content in dry land and pH in paddy land, respectively. In rice root associated topsoil, organic matter associated Se (OM-Se) accounted for 70% of T-Se. Compared with pH (median 6.10) and OM weakly bound Se (OW-Se) (0.14 ± 0.04 mg kg−1) of Argosols, the higher pH (median 6.77) resulted in less OW-Se (0.10 ± 0.04 mg kg−1) of Gleyosols. Vertical distribution of Se in borehole cores within the depth of 0–900 cm was mainly affected by the soil type. Se accumulated mainly within 0–150 cm depth (horizon A, E and B) in Argosols and above 40 cm depth (horizon H), existing prominently as OM strongly bound Se (OS-Se), in Gleyosols. Within the depth of 0–150 cm, various Se fractions for both soils were probably controlled by reductive fixation and complexation of Org C; In the alkaline paddy land, DOM-complexed Se was the main composition of A-Se. The findings of this study could help in understanding the mechanisms of Se distribution and enrichment in soils developed under different formation processes and farming conditions.