Effects Of Different Sources Of Secondary, Micro-and Beneficial Elements On Availability Of Soil Cadmium And Its Uptake By Rice Under Waterlogged Condition

With rapid development in social and economics, the living environment of human being is facing the unprecedented pressures from various types of pollution. Among the different pollutants, cadmium (Cd) is typical for its widespread pollution areas and higher number of suffered agricultural produces (eg., rice and wheat) than the other risky elements. Once the soil is polluted by Cd, it is very difficult to remediate due to time-consuming and high cost of the recovering processes that are beyond imagination of the ordinary people. In the agricultural production, improper land use, crop cultivation, farmland management and sources of fertilizers and application methods in particular tend to significantly enhance availability of soil Cd and crop uptake, leading to higher probability of polluted agricultural produces. Therefore, it is of great importance to conduct research on the factors that can reduce quantity of soil available Cd and the measures that can control soil Cd availability to safeguard food safety and human health.Past research on some of the secondary, micro-and beneficial elements has been conducted both at home and abroad, however, there is lack of systematic studies with comparisons between different secondary, micro-and beneficial elements affecting the availability of soil Cd under the same conditions. Thus, the objectives of this study were to systematically investigate effects of secondary, micro-and beneficial elements on availability of soil Cd and Cd uptake by rice under waterlogged condition. The availability of soil Cd and Cd uptake by rice as affected by different secondary, micro-and beneficial elements were investigated by incubation studies and pot experiments under waterlogged condition as well, The availability of soil Cd and Cd uptake by rice as effects of water management methods and three secondary and microelements were investigated by incubation studies and pot experiments.On the basis of these studies, some optimized fertilizer practices that are effective in controlling Cd availability on the Cd polluted soil were concluded.1. The results of incubation experiments revealed that different fertilizers significantly affected soil pH and Cd availability. As the soil was flooded, soil pH rose rapidly from 6.4 to 7.0, maintained for a period of time and then dropped down to 6.7 at 60d, while the soil Cd availability underwent a reverse change. Ca2+, Mg2+, S, sodium silicate and calcium carbonate were highly effective in reducing amounts of Cd extracted, while the micro-elements of Zn2+, Cu2+ and Fe3+ significantly increased the extractable Cd. Thus, the authors suggest that when making fertilizer recommendations of secondary, micro-and beneficial elements to the Cd polluted soils in rice growing season, sodium silicate and S bearing materials are most preferred while use of Cu and Zn should be cautious by strictly controlling its rates following recommendations.2. The results of soil pot experiments revealed that folia application of sodium silicate significantly increased rice grain yield, while calcium carbonate, boric acid and sodium silicate incorporated into soil significantly reduced rice grain yield. The chloride forms of magnesium (Mg), zinc (Zn) and iron (Fe) were more favorable to enhance rice grain yield than the sulfate salts of the three elements, while the sulfate forms of calcium (Ca) and cupper (Cu) just behaved in an opposite manner. Among the three secondary elements of Ca, Mg and sulfur (S), Ca enhanced but Mg and S in particular reduced the concentrations and uptake of Cd in grain. In straw, the concentrations and uptake of Cd were minimized by magnesium chloride and S in particular. It was found that Mg retarded transferring Cd from straw to grain, with magnesium chloride more effective than the magnesium sulfate. Among the micro-elements studied, Zn was most effective in blocking Cd uptake by rice, and followed by Cu, while folia application of the beneficial element of Si as sodium silicate topped the all the treatments in concentrations and uptake of Cd in grain. The treatments of ferrous sulfate, magnesium chloride, cupper chloride, boric acid and borax effectively depressed but the treatments of Zn and folia application of sodium silicate promoted the transfer of Cd from straw to grain, indicating the mechanism of Si blocking Cd uptake by rice was most likely to occur in soil rather than within plant or in the aboveground portions of the plant. It implies that selecting appropriate forms of secondary, micro-and beneficial elements and using proper application methods could effectively reduce Cd uptake by and Cd content in grain in the Cd polluted soil.3. Effects of water management methods and S,Fe,Zn elements in a pot experiment revealed that among the three water management methods, water drained out at maximal tillering stage (drain-out) produced the highest rice yield, and followed by flooding through the rice growing season (flooding) and non-flooding at all (non-flooding). The flooding management significantly reduced Cd concentration and total content, especially in the Zn treatment, in grain compared to the non-flooding management, and effectively inhibited transfer of Cd from rice straw to grain. On contrary, the non-flooding management promoted Cd transfer. Thus, it is suggested that flooding water management should be used all time when possible for rice grown on a Cd polluted soil, the drain-out management can be adopted on soils with slight Cd pollution, and non-flooding management should avoided as possible. Use of Cd-resistant rice cultivars in combination with optimized nutrient and water management practices can produce rice product containing Cd below the permit levels by the state.