Geochemical Characteristics Of Trace Elements Transfer In The Soil-wheat System From The Yangtze River Delta Area

Heavy metals contamination of soil and agro-products is an important issue with the increasing anthropogenic activities. There have been increasing studies on the heavy metals contamination of soil and plant. However, many questions about the geochemical behavior of trace elements in soil-plant are still unclear due to the complexity of the soil-plant system. Although many previous researches have studied the source of the contamination and the bio-availability of the heavy metals in soil with special experiments, the knowledge on the transfer and accumulation of fluorine, selenium, molybdenum, boron and other trace elements in the soil-plant and the influence factor is scant, so is the knowledge on the mobility of the trace isotopes transfer in the terrestrial system and in situ examination of trace elements in soil and plant at the micrometer level.The ecological environment of the Yangtze River delta area has been deteriorating with the accelerated economic develop in the past few decades. Although some researchers have investigated the environment pollution of heavy metals in this region, the study on the geochemical characteristics of trace elements in the soil-wheat system on a whole regional scale is lacking. What are characteristics of the concentrations (or accumulations) of trace elements and their spatial distributions? What do soil parameters effect the process of trace elements transfer from soil to wheat and which are the main influence factors? How do trace elements in the contaminated soil and wheat grain distribute at the micrometer scale? These questions need to be answered through the study of a representative area in the Yangtze River delta. The candidate selected the Yangtze River delta region as the study area and used the soil and wheat as the major study objects, and combined the field investigation, laboratory measurements and datum analyses, to characterize the geochemical behavior of trace elements in the soil-wheat system. The main results and conclusions are summarized below:(1) The topsoils of the Yangtze River delta region show relatively higher concentrations of Cu, Cd, Zn, Hg and F, similar concentrations of Pb, Ni, Cr, Se and B, and lower concentrations of As and Mo than the average level of Chinese topsoil. Comparing to average elemental concentration of the world soil, the soil in the study area has higher concentrations of Cu, Hg, As, B and F, similar concentrations of Zn and Cr, and lower concentrations of Cd, Pb, Ni, Se and Mo. The topsoil has accumulated a relatively higher amount of Cd, Cu, Zn, Cr, Ni and F during the past decades. The rate of heavy metals pollution is in the order of Cd> Hg> Cu=Zn> Cr> Ni> Pb> As. The spatial distributions of trace elements are not the same. In general, the topsoil from Suzhou and Hangzhou-Jiaxing area show apparent accumulation and pollution of heavy metals (except for As), F and B.(2) The accumulation (contamination) of Cd, Cu, Zn, Hg, Ni and Cr of the topsoil mainly came from metallurgy, electroplate and other industrial emission. The accumulation of F was a closely related to the stone mining and manufacturing and other relative anthropogenic activities. The accumulation of Mo, Se and As might be due to the exploration of local pedogenic parent rock and coal. And the accumulation of Pb may be a result of the increasing atmosphere deposition.(3) The wheat grain from the Yangtze River delta region shows general to severe F, Zn, Cd, Cu, Ni, Pb and Hg pollution, and the contamination is generally distributed in the south of Jiangsu and the north of Zhejiang. The contamination rates were in the order of F> Ni> Zn> Cd=Cu> Pb> Hg> As> Se> Cr. Different wheat tissues show different in elemental concentrations:As, Pb, Cd, Ni, Cr, F, Se and Fe concentrations follow root> straw> grain, Cu, Hg and B follow straw> root> grain, Zn and Mo follow grain> straw> root.(4) According to the difference of the transfer coefficient and the transfer model, all the elements can be divided into three groups. The first group includes Hg, Pb, Cu, As, Cr, Mo, F and B. Their transfer coefficient increases from the soil to the straw, then decreases from the straw to the grain. The whole transfer model shows "convex" pattern, indicating that wheat has a low tendency on absorbing these elements. This pattern mainly reflects the atmospheric component to the accumulation of Hg, Pb, Cu, As, Cr, Mo, F and B in wheat straw and grain. The second group includes Zn, Ni, Cd, Se and P, the element transfer coefficient increases step by step, i.e., increases from soil to wheat grain via root and straw. This pattern mainly reflects the strong transfer abilities of these elements.(5) Soil pH, Ca, Mg, TOC, carbonate and slowly available potassium hinder the heavy metals transfer from soil to wheat, whereas Ex-Ca, Ex-Mg, bio-available Fe, ammonium nitrogen and heavy metal themselves present the promote the transfer. Increasing the concentrations of Soil Se, Mo, bio-available Mo, S, oxalic acid extractable Al, carbonate, TOC, P and pH can enhance the accumulation of Se and Mo in the wheat. Boron absorption and accumulation in the wheat is mainly affected by soil B and bio-available B. Regression and partial correlation analysis show that besides the concentrations of heavy metal and its bio-availability and soil pH, soil ammonium nitrogen, slowly available potassium, carbonate, active bivalent cations and available phosphorus are the most important influence factors on the transfer of heavy metals, Mo and B from soil to wheat.(6) The effect of carbonate on the transfer of heavy metals from soil to wheat is different:Ni> Zn> Cd> Cu> Pb> Fe≈Cr≈Hg. Higher soil carbonate content will increase the absorption of Se and Mo in the wheat. Most of the topsoil samples from the Yangtze River delta area had lost carbonate, the carbon and oxygen isotopic analyses on the carbonate contained soil sample shows that the carbonate in the topsoil is mainly marine primary carbonate origin.(7) Pb isotopes have different mobility in different environmental matrixes. Pb isotopes in the geochemical cycle generally follow the equation of208Pb/206Pb=-1.157×206Pb/207Pb+3.46(r2=0.941). The anthropogenic Pb has the lower206Pb/207Pb (1.11-1.17)and relatively higher208Pb/206Pb (2.07-2.18), i.e., enriched in relatively heavy Pb isotopes, and the range of Pb isotopic ratios of anthropogenic Pb is similar to that of the local Pb ore. During the pedogenic process, the lighter Pb (206Pb) is more likely to transport into soil from the parent rock. Soil exchangeable Pb and carbonates phase Pb show the lower206Pb/207Pb relative to the total Pb, indicating that the anthropogenic Pb has an important effect on the existence of exchangeable Pb and carbonate phase Pb in the topsoil. In the pedosphere, the lighter Pb (206Pb) usually shows stronger mobility relative to the heavier Pb (208Pb), and is more likely to transfer into the soil exchangeable Pb fraction and move deeper, whereas heavier Pb has a priority to precipitate with carbonate in the deeper sediment horizon compared to the lighter Pb. The lighter Pb shows stronger transfer ability from soil to cereal grain via root compared to the heavier Pb. However, the cereal grains have lower206Pb/207Pb and higher208Pb/206Pb airborne Pb and anthropogenic Pb, implying that a considerable amount of Pb in cereal grains comes from the atmosphere. The model shows that16.7-52.6%(average:33.5%) of Pb in rice grain is the airborne Pb.(8) The spatial distributions of heavy metals at micrometer level are the most similar to that of Fe, next similar to that of Mg and Mn. This indicates that heavy metals have a similar distribution at the microcosmic level in the soil, mainly co-existing with Fe. Active bivalent heavy metal cations may mainly exist as isomorphs with Fe, Mg and other bivalent cations. They form into the compounds as the fine-grained clay-granule and been absorbed in the iron oxides, clays, and other soil solid. NanoSIMS images show that Si mainly distributes in the starch granules and cell wall of the aleurone. F generally distributes in the epidermis, aleurone and both starch granules and protein matrix of endosperm. Zn, Fe, Mg, Ca and K in the wheat grain mainly exist in the phytate granules of aleurone. The SIMS image of Zn is very similar to that of Fe, and Zn has a close relationship with P and O-H, implying that Zn, Fe, Mg, Ca and K in the wheat grain mainly coexist with phytic acid and polysaccharides.