| Abstract:In China, elevated cadmium (Cd) levels in farmland soils are becoming a major environmental problem due to its great toxicity and high mobility from soil to plants and thereby into the food chain. At the same time, crop straws may accumulated much higher Cd than edible parts. However, there was little information about the agricultural utilization of the Cd-contaminated straws and the effects of Cd-contaminated straw returning on availability of Cd in soils. In that case, the Cd-contaminated rice straw was used and two incubation-pot experiments were conducted. The effects of Cd-contaminated rice straw incorporation directly on fractions transformation and phytoavailability of Cd in clean reddish yellow loamy paddy soil (pH=4.7) and Cd-contaminated tidal sandy soil (pH=7.9) were studies (Experiment I). Moreover, the effects of rice straw and its biochar on DTPA-Cd and phytoavailability of Cd in clean and Cd-contaminated reddish yellow loamy paddy soils (pH=4.7) were studied (Expeiment Ⅱ). The main results were as following:1. The results of the two incubation experiments indicated that under flooding conditions, the pH values of the three tested soils were changed to6.0-7.0and no significant effects of rice straw or rice straw biochar application on soil pH was observed. Further, the application of rice straw significantly increased soil dissolved organic carbon (DOC) and the effects were enhanced with the increase of rice straw applied. No significant effects of rice straw biochar on soil DOC was observed.2. After application of Cd-contaminated rice straw and its biochar, the contents of DTPA-Cd in soils significantly increased by23.5-225.0%. And the results of sequential extraction methods indicated that addition of Cd-contaminated rice straw significantly increased the proportions of acid extractable and reducible Cd, and relatively decreased the proportions of residual and oxidable Cd (Experiment1). These effects were enhanced with the increase of rice straw and its biochar application. After the application of Cd-contaminated rice straw in both Cd-contaminated soils, the DTPA-Cd firstly slightly increased and then significantly decreased by28.6～41.1%and7.4～39.6%in tidal sandy soil and reddish yellow loamy paddy soil, respectively. Further, the application of induced the transformation of acid extractable Cd to reducible and residual Cd (Experiment II). While, no significantly effects of the application of Cd-contaminated rice straw biochar on DTPA-Cd in contaminated reddish yellow loamy paddy soil was observed.3. In experiment I, after Cd-contaminated rice straw applied the soils were incubated for28d and then the rice seedlings were transplanted in clean reddish yellow loamy paddy soil. The application of Cd-contaminated rice straw slightly decreased the absorption of Cd by rice and significantly increases the uptake of Cd by Chinese cabbage by1.1to1.8times as compared with the control. While, the clean reddish yellow loamy paddy soil added Cd-contaminated rice straw were incubated for56d and then the rice seedlings were transplanted and the contents of Cd in rice straw, rice hull and brown rice were increased by1.8to3.9,1.7to3.9and3.0to6.8times, respectively, as compared with the control. For Cd-contaminated tidal sandy soil, application of Cd-contaminated rice straw significantly decreased the contents of Cd in rice grain and rice straw by41.1～69.0%and36.6～50.8%, respectively. Similarly, the uptake of Cd by Chinese cabbage were significantly decreased by1.0～48.3%after Cd-contaminated rice straw applied (Experiment I). However, the application of Cd-contaminated rice straw and its biochar significantly increased the contents of Cd in rice straw, rice hull and brown rice by4.1to7.9, 4.6to8.9and4.0to5.5times, respectively. And these increase effects were enhanced with the increase of rice straw and its biochar applied.4. In conclusion, the behaviors of Cd in soils after the application of Cd-contaminated rice straw were influenced by the soil pH and the concentrations of Cd in soil. The application of Cd-contaminated rice straw and its biochar may result in the risk of Cd pollution diffusion in clean reddish yellow loamy paddy soil. In Cd-contaminated tidal sandy soil, the application of Cd-contaminated rice straw significantly decreases the phytoavailability of Cd in soil. In Cd-contaminated reddish yellow loamy paddy soil, the application of Cd-contaminated rice straw significantly decreased the contents of DTPA-Cd in soil; however, the application of Cd-contaminated rice straw and its biochar significantly increase the uptake of Cd by rice. Consequently, for both clean and Cd-contaminated reddish yellow loamy paddy soil, the Cd-contaminated rice straw and it derived biochar were not suitable for incorporation in field, while, for Cd-contaminated tidal sandy soil the incorporation of Cd-contaminated rice may be useful in reducing the phytoavailability of Cd in soil, however, the effects of long-term application should to be studied. |
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