Effects of long-term straw incorporation on cadmium speciation and bioavailability in paddy soils in Deyang Area

Long-term incorporation of crop straw into soil affects soil heavy metal speciation and bioavailability. In order to develop feasible control measures of cadmium pollution of the soil and food production, the effects of long-term incorporation of crop straw into soil on the enhancement or inhibition of cadmium contamination of paddy soils was investigated. The cadmium contaminated paddy soil was from Jinyang District, Deyang City, Sichuan Province. Field-investigated and field-sampled data were analyzed to determine the characteristics of cadmium fractions in straw-incorporated (returned rice straw into soil in autumn and returned wheat straw into soil in summer) paddy soils for 0, 1, 4 and 8 years. The results showed that the average content of total soil cadmium in the ploughing layer (020 cm soil layer) was 0.478 mg·kg^-1. The average content of total soil cadmium in plough pan (2040 cm soil layer) was 0.419 mg·kg^-1. The average contents of cadmium in both soil layers exceeded environmental quality standard for agricultural soils (farmland soil cadmium < 0.30 mg·kg^-1 of GB15618—1995). The distribution of cadmium speciation in the paddy soil was as follows: residual Cd > Fe-Mn oxide bonded Cd > organically bonded Cd > carbonate bonded Cd > exchangeable Cd. The incorporation of crop straw for over 4 years in paddy soils significantly improved soil organic matter content. It also obviously increased soil organically bonded Cd ions. The content of organically bonded Cd in soil with crop straw incorporation for 4 years was increased by 45%. The incorporation of crop straw in paddy soils slightly increased soil exchangeable Cd in the ploughing layer (020 cm soil layer). With increasing years of the incorporation of crop straw into soil, soil pH initially increased significantly (crop straw incorporation for 14 years) before decreasing slowly (crop straw incorporation for 48 years). This mainly affected the speciation of soil carbonate bonded Cd. The bioavailability of soil cadmium speciation under different years of crop straw incorporation was evaluate too. For short term crop straw incorporation (14 years), the bioavailability of cadmium in ploughing soil layer (020 cm) significantly decreased with increasing number of years. However, the bioavailability of soil cadmium fractions in the plough pan (2040 cm soil layer) did not significantly change with increasing number of years of crop straw incorporation. In conclusion therefore, short-term (14 years) incorporation of crop straw into paddy soils increased soil organic matter content. It also significantly enhanced soil organically bonded Cd and reduced soil availability of cadmium in the ploughing soil. This somehow reduced soil cadmium hazards and the risk of crop absorption and accumulation from soil cadmium. The findings of the study were beneficial for controlling heavy metal pollution in paddy soils induced by crop straw incorporation. It was also beneficial for designing more efficient and environmentally-friendly crop straw incorporation methods to prevent future soil cadmium pollution.