[Effects of Ferrous Sulfate and Ferric Nitrate on Cadmium Transportation in the Rhizosphere Soil-Rice System]

Abstract

Cadmium (Cd) contamination in the agricultural soils of China is a serious and growing environmental problem that urgently needs to be controlled and completely remediated. The biogeochemical cycles of nitrogen (N), sulfur (S), and iron (Fe), and the coupled cycles of Fe-N and Fe-S have been reported to control Cd transportation in the soil-rice system. Exploring practical remediation strategies for Cd from the perspective of the application of nutrients such as N, S, and Fe for rice growth is expected to obtain farm-specific and state-of-the-art technologies and products to reduce the accumulation of Cd in rice grains. Using our earlier study as a basis, the rhizosphere bag-pot experiment with ferrous sulfate (FeSO₄) and ferric nitrate[Fe(NO₃)₃] treatments was conducted to investigate Cd bioavailability in rhizosphere soil and Cd translocation in rice plants, and to highlight some possible factors and mechanisms controlling Cd accumulation in rice grains. The results showed that both FeSO₄ and Fe(NO₃)₃ treatments reduced the bioavailable Cd (NH₄Ac-Cd) content in rhizosphere soil, with the decreasing extent being significantly lower in the former (55.6%) than in the latter (76.0%). Both FeSO₄ and Fe(NO₃)₃ treatments changed the distribution characteristics of Cd in rice tissues, and the FeSO₄ treatment increased the Cd content in brown rice (0.6 mg·kg^-1), but the Fe(NO₃)₃ treatment decreased the Cd content in brown rice (0.1 mg·kg^-1). Adsorption or co-precipitation of Cd by iron plaque, increased accumulations of Cd in root, stem, and leaf, and enhanced translocations of Cd from root, stem, and nodule to brown rice occurred with the increased Cd content in brown rice of the FeSO₄ treatment. However, the decreased Cd content in brown rice with the Fe(NO₃)₃ treatment was ascribed to adsorption or co-precipitation of Cd by poorly crystalline Fe oxides and solid Fe sulfides, decreased accumulations of Cd in stem and nodule, and weakened translocations of Cd from root, leaf, and nodule to brown rice. These findings provide a scientific basis for the exploration and application of nutritive soil amendment, and will have significance in regards to the remediation of Cd-contaminated agricultural soils in China.

Keywords: Cd bioavailability; Cd translocation; brown rice; nutrient elements; rhizosphere soil-rice system.