| This Ph.D. Dissertation was a part of the research project "the laws of movement, transform and accumulation of typical heavy metals in the agricultural soil and their relationships with plant ecosystem and farm produce safety", a sub-project of the National Key Basic Research Program of China (973)"the temporal and spatial laws and adjusting and controlling principles of the environmental quality of soil and atmosphere in the areas of Yangtse Rive and Pearl Rive Deltas"(No. 2002CB410804)". According to the demands of the specific research content and aims of the national program, three typical agricultural soils, paddy field on redeposit of red soil (clayey kaolinitie thermic aeric endoaqualf), marine-fluvigenic yellow loamy paddy soil (loamy mixed active thermic typic endoaqualf) and blue clayey paddy soil (clayey illitic thermic typie epiaqualf), were selected in this study. Prior to planting rice and vegetable (Brassica chinensis), soils were treated by adding Cd and Pb with different levels. The soils were pre-incubated under water-logged conditions for 15 days before planting rice and were pre-incubated with keeping soil moisture of 60-70 % of water holding capacity for 60 days before planting vegetable crops. Taking the ecological chain in the soil-crop-soil microorganism ecosystem as a mail clew of research, this study investigated the chemical form distribution and bioavailability of Cd and Pb in the soils of both soil-rice and soil-vegetable systems, and the ecological effects of soil Cd and Pb on rice, vegetable and soil microorganism, and their interaction between chemical forms, bioavailability of the metals in the soils and the ecological effects. Main conclusions were as follows:1. In the soil-rice or soil-vegetable system, the chemical form distribution of Cd and Pb in soils and their bioavailability were affected by many factors, including concentrations of Cd or Pb in soils, soil types and soil properties and growing stages of rice and vegetable, and could comprehensively reflect interaction processes between soil and biology in the systems. General trends of the form distribution of soil Cd or Pb and their bioavailability in the ecosystem were: (1) The fractionation results of Cd and Pb using modified fractionation procedure developed by Tessier in 1979 indicated that Cd or Pb in soils were mainly presented as residue form, carbonate bound form (including acid dissolved form) and Fe-Mn oxide boundform, these three fractions accounted for 80~85%or more of their total. However, exchangeable form and organic bound form (including sulfide bound form) of the metals were low, ranging from 10 % to 20 %. (2) There were significant correlations between concentrations of Cd and Pb in plant tissue and available contents of Cd and Pb extracted with five extractants, including 0.1M HC1 and DTPA-TEA. Among the five extractants, 0.1M HC1 and Uvl NH4OAc were the best two extractants, which could efficiently reflect concentrations of Cd and Pb in plant tissue in soil-plant systems. (3) Because there were complex interactions among many factors in the soil-plant system, the form distribution of soil Cd or Pb and their bioavailability could vary greatly with soil types, soil properties, and growing stages of plants. For most cases, Cd was dominated by the carbonate bound and residue forms, and Pb was dominated by the Fe-Mn oxide bound and residue forms. When soils were water-logged and used for planting rice, carbonate bound and exchangeable forms of Cd or Pb in soils decreased, and the Fe-Mn oxide bound and residue forms of the metals increased with the development of rice. When soils were used for vegetable production with an aerobic soil condition, Cd in the marine-fluvigenic yellow loamy paddy soil was presented as carbonate bound form, the correlations between concentrations of Cd in plant tissue and soil extractable Cd decreased in the order of 0.1 M HC1 extractable > 0.1M CaCl2 extractable >1M NH4OAc extractable. However, Pb in paddy field on redeposit of red soil was presented as Fe-Mn oxide bound form, the correlations between concentrations of Pb in plant tissue and soil extractable Pb decreased in the order of 1M NH4OAc extractable > 0.1M HC1 extractable > 0.1M CaCl2 extractable. Understanding these complicated variation of soil Cd and Pb forms and their bioavailability in soils could aid scientific management of agricultural soils to reduce contamination risk of heavy metals.2. Chemical analysis of Cd and Pb contents of soils, root, stem and leaf of rice, and edible tissue (grain and unpolished rice) in soil-rice system and of soils, root and edible tissue of vegetable plant (stem and leaf) in soil-vegetable system at maturity stages of rice (90 day after rice transplanting) and vegetable (60 day growing vegetable) indicated that there were some laws of translocation of heavy metals between soils and plants. In the soil-rice system, the coefficients of enrichment of both Cd and Pb were roots > stem and leaf > grain. However, the proportion of Cd and Pb transferred from soil to roots, and then, to above-ground part decreased in the order of grain to stem and leaf > stem and leaf to roots > roots to soils. In soil-vegetable system, the coefficients of enrichment and proportion of Cd and Pb transferred from soil to roots, and then, to above-ground part were similar to those in soil-rice system. But, the values of the coefficients of enrichment and transferred proportion were much smaller in soil-vegetable systemthan soil-rice system. The coefficients of enrichment and transferred proportion of Cd and Pb among soil, and plant tissues varied with the level of Cd and Pb treatments, soil types and soil properties and crop species. For same soil, the coefficients of enrichment and transferred proportion of low Cd and Pb treatments were greater than those of high Cd and Pb treatments. When addition amounts of Cd or Pb were same, values of the coefficients of enrichment and transferred proportion were greater in marine-fiuvigenic yellow loamy paddy soil than those in paddy field on redeposit of red soil for soil-rice system, and greater in paddy field on redeposit of red soil than in marine-fiuvigenic yellow loamy paddy soil for soil-vegetable system. The coefficients of enrichment and transferred proportion were generally greater in soil-rice system than in soil-vegetable system. Knowledge of movement and redistribution law of heavy metals in systems were useful for recognizing the bioavailability of soil heavy metals and establishing the critical index of soil heavy metal contamination.3. There were several ecological effect characteristics of Cd and Pb in soil-rice system. (1) At tillering stage (30 days after rice transplanting), except for chlorophyll content which increased with the level of Cd and Pb treatments in low concentration and decreased then at the peak values of 3 mg/kg or 8 mg/kg Cd treatments and 300 mg/kg or 700 mg/kg Pb treatments, all measured physiological indices of rice, including rice biomass of ground part, proline content and peroxidase activity increased significantly with increasing level of Cd or Pb treatments. The increased soil Cd or Pb could elevate the respiration intensity and metabolic quotient of soil. The effects of soil Cd and Pb concentration on microbial biomass carbon, microbial biomass nitrogen, soil urease, acid phosphatase and dehydrogenase activities varied with the concentrations of the metals. Small addition of Cd and Pb to soil could promot above physiological indices, but excessive addition of Cd and Pb to soil declined the physiological indices. The peak values, turn point from positive effect to negative effect, varied with soil types and metal species, ranging from 1 mg/kg to 8 mg/kg for Cd and 100 mg/kg to 500 mg/kg for Pb. These results suggest that the soil microbial ecological indices underground were more sensitive than the physiological indices of rice aboveground for indicating effects of Cd and Pb. (2) The results of ecological effect of Cd and Pb measured in different growth stages of rice (15^ 30, 50, 70, 90 days after rice transplanting) indicated that all physiological and ecological indices including ground part of rice and underground part in soils fluctuated greatly with rice development. The difference in same index value measured in different growth stages of rice were noted. Most of the indices were lower in initial stages of rice, and then increased gradually to reach maximum in bloom stages of rice (30 to 50 days after transplanting). The indexes except microbial biomass and ureaseactivity became low again in maturity stages. The results suggest that rice growth had a key role controlling he ecological effects of Cd and Pb in the soil-rice system. As compared with time effect of rice growth, the concentration effect of Cd and Pb on rice physiological and ecological index was much smaller. In addition, effects of Cd and Pb concentration on rice physiological and ecological index were very complex, which may due to complicated action with many factors work together in the system. (3) Soil types also had effect on the ecological effect of Cd and Pb in soil-rice system. The content of chlorophyll and proline of rice at different levels of Cd and Pb treatment were higher in marine-fluvigenic yellow loamy paddy soil than in paddy field on redeposit of red soil. However, soil microbial biomass and enzymatic activities were higher in paddy field on redeposit of red soil than in marine-fluvigenic yellow loamy paddy soil. The peak values of some indices in different growth stages of rice varied with soils.4.The ecological effect of Cd or Pb in soil-vegetable (Brassica chinensis) system was similar to those in soil—rice system. In this experiment, most of the physiological indices of vegetable aboveground and the ecological indices of soil microbial biomass underground had same variation laws as soil—rice system. However, soil-vegetable system kept soil in oxidation environment, which was differed from soil-rice system with deoxidation environment and water-logged conditions, therefore, soil-vegetable system had its special characteristics. (1) At same concentration of Cd or Pb and for same soil, the soil microbial biomass carbon, respiration intensity, urease activity and dehydrogenase activity were lower and acid phosphatase activity was higher in soil-vegetable system than in soil-rice system. (2) The temporal variation characteristics of some soil microbial ecological indices in those two systems were quite different. The changing pattern in soil-rice system was relatively complex because of long growth period, while the changing pattern in soil-vegetable system was relatively simple due to its short growth period. At the 30th day (bloom stages) of vegetable {Brassica chinensis) growth, the soil microbial function and community structure diversity were analyzed using BIOLOG and Phosphor Lipid Fatty Acid (PLEA) methods, respectively. The results indicated that the average well color development (AWCD) on the ECO plate reduced with increasing level of Cd or Pb. There was a nonlinearly relationship between AWCD and incubation time, and the relationship followed growth dynamic model of microbial population, presenting "S" form growth model (Logistic curve). PLFA investigation indicated that the community structure of soil microbe was changed when Cd or Pb was added. The addition of Cd and Pb increased population of fungi and actinomycete with relatively higher endurance, but it reduced population of bacteria, as compared with control. The ratio of Gram-positive to Gram-negative also increased. Principalcomponents analysis (PC A) of the PLFA composition data also indicated that the characteristic fatty acids, including il5:0, al5:0, il6:0, i!7:0, 18:3 u 6c, and cyl9:0 increased, but concentrations of cyl7:0 andl0Mel7:0 decreased when Cd or Pb was added in soils. The results above further demonstrated that addition of heavy metals could change soil microbial community structure in the soil-vegetable system.
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