| Cadmium (Cd) is a toxic heavy metal which may be absorbed by rice seedlings, transported and accumulated in grains, thereby threatening the human health. In recent years, there are many studies on the effects of iron plaque on rice adsorption and uptake with different conclusions. On the other hand, under Cd stress, the influence of low molecular weight organic acid (LMWOA) on rice Cd adsorption, uptake and transportation was seldom reported so far. We applied the soil-vermiculite culture to study the migration of different Fe and Cd forms in the soil (nonrhizosphere)-vermiculite (rhizosphere) system and the influence of iron plaque and LMWOA on the adsorption and uptake Fe and Cd by rice under various soil redox conditions.1. Effects of transformations of Fe and Cd fractions on Cd adsorption and uptake by rice under different soil redox conditionsIn a pot experiment rice seedlings were planted in mesh bags that were filled with vermiculite to simulate rhizosphere and the bags were imbedded separately in submerged red, paddy and saline soils for cultivation for13days. Results show that during the rice cultivation period, pH in the red, paddy and saline soils varied in the range of6.05-6.78,6.47-7.33and6.47-7.44, respectively. In the OM (organic matter) treatments, all the soils, regardless of rhizospheric or nonrhizospheric, decreased in pH, except for rhizospheric red soil, which increased significantly. Eh in all the rhizospheric soils of the control treatments lingered within233～385mV, and treatment with OM reduced rhizospheric Eh, but increased nonrhizospheric Eh of the red and paddy soils. Oxide-Fe fraction accounted for more than90%of the reductively dissolved Fe and vermiculite-absorbed Fe, which was significantly correlated with soil solution Eh and pe+pH, indicating that Fe redox reactions occurred simultaneously on the surfaces of the soils and the vermiculites, but in opposite directions. The formation of iron plaque on root surface of the rice seedlings was related to redox conditions in rhizosphere. In the rhizospheres of CK (higher in Eh), Fe content on the root surface decreased with increasing pH, whereas in the rhizospheres of OM treatment (lower in Eh), Fe increased with increasing pH. In the red soil, iron plaques on the root surface retarded Fe absorption, while in the paddy and saline soils it acted reversely. Cd content on the root surface was significantly and positively related to Cd in root and in shoot. In the red soil iron plagued on the root surface retarded adsorption and uptake of Cd by rice, whereas in paddy and saline soils, it acted reversely too.2. Effects of root secreted of low molecular weight organic acids on Cd adsorption and uptake by riceIn a pot experiment rice seedlings were planted in mesh bags that were filled with vermiculite to simulate rhizosphere and the bags were imbedded separately in submerged paddy and saline soils for cultivation for10days. Results show that during the rice cultivation period, pH in the paddy and saline soils varied in the range of6.57-7.06and6.45-7.23, respectively. In two soils and treatments, secrection of LMWOA increased the the rhizospheric pH for0.10-0.29units, rather than acidification, compared soils and treatments, rhizospheric pH in paddy soil rose significantly. Eh in all the rhizospheric soils of the control treatments lingered within324-394mV and treatment with OM intensified redox in nonrhizosphere and Eh was lower than control treatments. Activiy of Cd in nonrhizosphere soils was restricted by Fe, the more dissolution of Carb-Fe^Oxide-Fe and Org-Fe entered to rhizosphere, the more Exc-Cd transformed to Oxide-Cd with low activity. Root surface was different from vermiculite surface:former was the interface with biological process, latter was with chemical precess. Cd content on the root surface was significantly and positively related to Cd in root and in shoot. Effects of serection of LMWOA on iron plaque were different in rhizospheres:in paddy soil, acetic promoted Fe adsorption; in saline soil, citric promoted Fe adsorption while oxalic and total organic acids inhibited Fe adsorpted by rice seedlings surface,, Cd contents in different parts of rice were specific with LMWOA types and acted variously:in rhizosphere of paddy soil, acetic acid promoted Cd accumulation in parts of rice; in rhizosphere of saline soil, tartaric acid promoted while malic acid and citric acid retarded Cd accumulation,,... |
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