Effects Of Secondary, Micro-and Beneficial Elements On Availability Of Soil Cd And Its Uptake By Wheat

The heavy metal cadmium (Cd), one of the most toxic contaminants in the environment, can be accumulative in human body to an extent that becomes harmful to health through food chains. Among the different types of heavy metal pollution, the area of soil polluted by Cd is relatively larger than any other heavy metals in China, resulting in frequently reported Cd pollution of foodstuffs. Wheat is the crop that is prone to take up Cd from soil and susquently accumulate in the plant, and thus, lead to high Cd content in grains that surpasses the safe threshold set by the State. The quantity of Cd taken up by crops mainly depends on the quantity of soil available Cd and factors that promote transformation of non-available soil Cd to the available forms. 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.Calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), manganese (Mn),boron (B), molybdenum (Mo), and cupper (Cu) are the necessary nutrients of higher plants, and silicon (Si) and selenium (Se) are classified as beneficial elements of higher plants. Addition of these elements to soils tends to affect some of the soil physio-chemical properties, and further alter forms of soil Cd and plant uptake and accumulation as well. Past research on some of the above-mentioned elements has been conducted both at home and abroad, however, there is lack of systematic studies with comparisons between these 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 wheat. The availability of soil Cd and Cd uptake by wheat as affected by different secondary, micro-and beneficial elements were investigated by incubation studies and pot experiments as well. 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. Effects of different fertilizers of secondary, micro-and beneficial elements on soil pH and extractable Cd were examined in an incubation study using a Cd contaminated soil. Results revealed that these fertilizers had significant effects on soil pH and amounts of Cd extracted. The amounts of Cd extracted by the compounds of an element bearing Cl- or SO42- were not highly different. During the incubation at 65-100% field capacity it encountered a significant drop in soil pH and a rise in soil extractable Cd. Among the elements and materials studied, Ca, Mg, Cu, boric acid and elemental S significantly decreased soil pH, while calcium carbonate and sodium silicate significantly increased soil pH. Except Mg, the soil pH changes induced by addition of these elements or materials were negatively correlated with amounts of Cd extracted. An antagonistic effect was observed between Fe, Mn, Mg, Si, Se and soil extractable Cd, but a synergistic effect between Ca, Zn, Cu and soil extractable Cd. The results sμggest that when applying fertilizers of secondary, miro-and beneficial elements to a Cd polluted upland soil, use of elemental S,boric acid, Ca, Zn or Cu should be avoided, while Fe, Mn, Mg as well as borax, sodium silicate and sodium selenite are preferred.2. Effects of secondary, micro-and beneficial elements on wheat growth and cadmium (Cd) uptake were investigated in a pot experiment. The results showed that calcium sulfate, manganese sulfate and cupper chloride obviously increased wheat grain yield while magnesium chloride, elemental sulfur, cupper sulfate, ferrous sulfate and boric acid significantly enhanced wheat grain yield compared to the CK treatment. On the cadmium (Cd) polluted soil, application of secondary, micro-and beneficial elements played a regulatory role on soil Cd availability. Among the fertilizers and elements studied, Mg, Mn, Fe, borax, sodium silicate and sodium selenite reduced Cd uptake, while Ca as calcium chloride in particular, Cu, Zn and boric acid promoted Cd uptake by wheat. The chlorides of the secondary and micro-elements favored Cd uptake more than their counterpart sulfates. The Cd accumulated in wheat straw was far more than in grain. Thus, it is sμggested that on the Cd polluted upland soil it necessitates to avoid applying elemental sulfur, boric acid, Zn and Cu fertilizers but to use Fe, Mn, Mg, borax, sodium silicate and sodium selenite that favor lowering the activity of soil Cd and depress Cd uptake by wheat to a certain extent.3. On the soil that contains rather high amount of total Cd (0.92mg kg-1, much higher than the threshold value (0.3mg kg-1) of the soil environmental quality set by the State), thoμgh applying some of the secondary, micro-and beneficial elements could significantly reduce amounts of soil available Cd and Cd uptake by wheat, the reduction still failed to bring Cd content in wheat grains below the food safety threshold for wheat grains as set by the State. This further indicates that on the soil severely polluted by Cd, use of only one single measure appeared not effective enoμgh to reduce the Cd content in wheat grains below the food safety threshold.