Straw incorporation increases solubility and uptake of cadmium by rice plants

Straw incorporation is a widespread agricultural practice to enhance soil fertility and to recycle crop residues. However, how straw incorporation affects mobility and plant uptake of heavy metals such as cadmium (Cd) still remains unclear. The effect of straw incorporation on Cd solubility and uptake by rice was investigated in a pot experiment. A Cd-contaminated paddy soil was incorporated with wheat straw at different rates (0, 0.25%, 0.5%, and 1% w/w). Cd and dissolved organic carbon (DOC) concentration in soil solution sampled during the experiment and Cd content in rice plant at maturity stage were assessed for each treatment. The results showed that the straw incorporation decreased pH and increased DOC and soluble Cd in the soil solutions, while rice growth was significantly inhibited by straw incorporation. Straw incorporation enhanced the Cd uptake by rice from plant tissue Cd concentration of 15.41 mg kg−1 (without straw) to 45.1 mg kg−1 (at 1% incorporation rate). Cd accumulated in the rice plant increased by 120% compared to the control. The Cd uptake by rice showed a plateau response, which cannot be explained by soluble Cd alone. Inhibited rice growth at high straw incorporation might decrease Cd uptake. Thus, we conclude that straw incorporation in heavy metal-contaminated rice paddy soil should not be recommended to avoid heavy metal contamination through food chain.