| A field experiment was conducted to investigate the effects of organic amendments such as rice straw (RS), green manure (GM) and pig manure (PM) on the immobilization of Cu and Cd, their binding by soil organic matter fractions, their accumulation in wheat (Triticum aestivum L.) and microbial properties in a heavy metal contaminated soil in Huang Shi City, Hubei Province, China. Addition of RS, GM and PM reduced significantly the concentrations of soluble/exchangeable Cu and Cd, but increased the amounts of organic-bound and inorganic precipitated Cu and Cd in the soil before the growth and after the harvest of wheat as compared with the control treatment (CK). The residual fractions of Cu and Cd decreased after the addition of RS, whereas they increased under GM and PM treatments before the growth of wheat. RS, GM and PM had no effects on the residual fractions of Cu and Cd after the harvest of wheat. The distribution of Cu and Cd in the soil strongly depended on multiple factors such as soil pH, electrical conductivity (EC), soil organic matter content (SOM) and available phosphorus (AP), which were significantly influenced by the organic amendments applications. Soil EC, pH, OM and AP correlated negatively with the soluble/exchangeable Cu and Cd before the growth and after the harvest of wheat. Among all examined soil properties, SOM was the most important soil chemical properties in decreasing the availability of Cu and Cd in the polluted soil. The role of SOM in decreasing the solubility of Cu and Cd before the growth of wheat was more significant than after the harvest of wheat. The best effective amendment in decreasing the availability of Cu and Cd was RS, especially at the highest applied amounts before the growth and after the harvest of wheat. The greatest amounts of organic-bound Cu and Cd were observed in RS treatments, whereas the least amounts were recorded in PM treatments. The highest concentrations of inorganic precipitated Cu and Cd were found in soil amended with PM, whereas the lowest concentrations were recorded in soil treated with RS. In general, the distribution of Cu in all treatments was in the following order: inorganic bound> organic bound> residual> soluble/exchangeable However, Cd was distributed descending as follows:inorganic bound> residual> organic bound> soluble/exchangeable. The results also showed that RS, GM and PM can be used as good and cheap amendments to remediate heavy metals contaminated soils. Addition of RS, GM and PM increased the binding of Cu and Cd by particulate organic matter fractions (POM), humic acid (HA), fulvic acid (FA) and humin (HU). The ability of the fine fraction of POM (200-50μm) in the fixation of Cu and Cd was higher than that of the coarse fraction (2000-200μm). The greatest amounts of Cu and Cd with POM, HA, FA and HU fractions were observed after the addition of RS, whereas the least amounts were noticed in the presence of PM. The increase of SOM and pH values had an important role in promoting the binding of Cu and Cd by POM, HA, FA and HU fractions. The retentions of Cu and Cd by POM, HA, FA and HU before the growth of wheat were higher than those after the harvest of wheat. In all the treatments, HA fraction had higher capacity for the fixation of Cu and Cd as compared with FA, HU and POM fractions. Therefore, HA fraction can play a more important role in controlling Cu and Cd pollution than FA, Hu and POM fractions in the agricultural and environmental systems.The application of RS, GM and PM in the studied polluted soil significantly decreased the amounts of Cu and Cd in different parts of wheat (roots, straw and grains) as compared with CK treatment. RS3 was responsible for the lowest accumulations of Cu and Cd in the roots (29.1 and 2.21 mg kg-1), shoots (21.4 and 0.69 mg kg-1) and grains (3.21 and 0.11 mg kg-1) of wheat. The grains and shoots yields of wheat also increased markedly under RS, GM and PM treatments. The highest grains and shoots yields of the wheat (5.07 and 9.50 t ha-1) were found in soil treated with RS3, whereas the lowest were recorded in soil amended with CK (2.15 and 2.41 t ha-1). Soil chemical properties such as pH, EC, SOM and AP correlated negatively with the uptake of Cu and Cd by grains of wheat. The highest correlations between SOM and the accumulation of Cu and Cd in roots, straw and grains of wheat indicated that SOM was the most important factor in controlling the accumulation of Cu and Cd in these parts of the wheat. The decrease of soluble/exchangeable Cu and Cd fractions in the contaminated soil after the addition of RS, GM and PM can be also considered as a vital factor in minimizing the uptake of Cu and Cd by wheat. The accumulations of Cu and Cd in the roots were higher than those in the shoots and grains of wheat. The bio-concentration factor (BCF) and the translocation factor (TF) of Cu and Cd decreased after the incorporation of RS, GM and PM. The lowest values of BCF and TF were observed in RS treatments. In contrast, the highest values of BCF and TF were recorded in PM treatments. The results of BCF and TF revealed that wheat can be healthy grown on soil contaminated Cu and Cd wheat and do not accumulate these metals in its different tissues (roots, shoots and grains), especiallyin the presence of highest amounts of RS, GM and PM. The results also highly suggested that RS and GM can be used as more effective and low-cost amendments than PM in decreasing the uptake of Cu and Cd by the different parts of wheatAll soil microbial properties such as enzymes activities (invertase, dehydrogenase, catalase, protease, urease, acid phosphatase, and reutral phosphatase), microbial biomass (C, N and P) and soil microbial counts (bacteria, fungi and actinomycetes) improved after the addition of RS, GM and PM as compared to CK treatment. The studied soil microbial properties increased with increasing RS, GM and PM application rates. The amelioration of the studied contaminated soil with RS led to the highest improvements in all soil microbial properties. Conversely, the lowest soil microbial properties were recorded in soil amended with PM and in CK. Soil pH had no correlation with all soil microbial properties except the activities of protease and neutral phosphatase before the growth of wheat. However, it had significant influences on soil microbial properties except acid phosphatase after the harvest of wheat. SOM had significant influences on all soil microbial properties before the growth and after the harvest of wheat The highest correlation values were noticed between SOM and all microbial properties. The values of all soil microbial properties before the growth of wheat were higher than those after the harvest of wheat. Soil soluble/exchangeable Cu and Cd fractions can be important factors in controlling the toxicity of Cu and Cd for soil microorganisms and their activities in the investigated contaminated soil. |
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