Biochar application in a cadmium-contaminated paddy soil also reduces soil microelement zinc availability and its uptake by rice

Purpose Biochar has been considered as a good soil amendment for reducing soil cadmium (Cd) bioavailability and decreasing Cd uptake by rice. However, it is unclear if biochar can affect soil microelement zinc (Zn) bioavailability and its uptake by crops, particularly under field conditions. Materials and methods The experimental field was contaminated by Cd, located in Hunan Province. Biochar was added at the proportions of 0, 10, 20, 30, and 40 t ha(-1) on topsoil (0-17 cm) and then mixed before rice seedlings transplanted. Results Increasing application rate of biochar improved the rice plant height, biomass, and soil electrical conductivity (EC). The DTPA extracted Zn (DTPA-Zn) concentration in the 0-17 cm of biochar treatments was decreased by 12.3-45.6% as compared to control. The increased EC played an important role in soil Zn immobilization. No remarkable difference was shown for DTPA-Zn concentration in the subsoil (17-29 cm) among biochar treatments. Increasing amount of biochar reduced the concentration and ratio of acid extractable Zn fraction, while increased the reducible and oxidizable fractions in the 0-17 cm. The reduction of concentration of DTPA-Zn in < 0.25-mm soil aggregates in biochar treatments was 29.0-72.2% and 54.3-87.9% respectively relative to control. The Zn concentration in rice was declined from 4.1 to 14.0% after biochar addition. The reduction of soil Zn availability and the improvement of rice growth made a greater contribution for the low accumulation in rice tissues. Conclusions Biochar could immobilize more microelement Zn in soil and limit Zn uptake by rice. Biochar could limit the enrichment of DTPA-Zn in < 0.25-mm soil aggregates and enhance the transformation of soil acid extractable Zn into reducible and oxidizable fraction.