Chlorine weaken the immobilization of Cd in soil-rice systems by biochar.

Rice (Oryza sativa L.) was cultivated in a Cd-contaminated soils with rice straw biochar (BC) and water-washed rice straw biochar (W-BC) were applied to investigate the underlying mechanisms and possible reasons for biochar's weakening effects on the immobilization of Cd in soil-rice system. The results indicated that W-BC reduced the Cd concentration in pore water as well as in the roots and shoots of rice by 26.24%, 53.23% and 62.47% respectively. On the contrary, there was an increase in Cd contents by 50.27% in pore water, 2.32% in the roots, and 12.80% in the shoots of rice under BC treatment. Furthermore, Cd content in rice shoot was significantly and positively correlated with Cl- addition to the soil (P < 0.01). This phenomenon could be attributed to several combined effects: (1) the increase of Cl- in the soil decreased the soil pH, enhanced the dissolved organic carbon in soil pore water and increased the complexes of Cd2+ and Cl-, resulting in the release of Cd from solid phase into solution phase, (2) the chloride in the soil increased the uptake of CdCl+ instead of Cd2+ by the roots, thereby causing an increase of Cd in rice tissues. These results demonstrate for the first time that biochar with high chloride content could weaken its immobilization effects on soil Cd and even enhance Cd uptake by rice.

Effects and mechanisms on the reduction of lead accumulation in rice grains through lime amendment.

The influence of lime amendment on the absorption and accumulation of lead (Pb) in the grains of three rice cultivars ((Dong Lian 5 (DL-5, Indica), TeYou 009 (TY-009, Hybrid Indica), and YiYou 673 (YY-673, Hybrid Indica)) was investigated and the associated mechanisms were explored. Pot experiment was conducted in heavily Pb-polluted paddy soil. The transformation of Pb species in soil and roots under different levels of lime application was studied by Pb L3-edge X-ray absorption spectroscopy (Pb L3-XAS) and BCR sequential extraction. The results showed that lime amendment significantly increased soil pH and decreased DTPA-extractable Pb in soil. When the additional ratio of Liming was lower than 32 g kg-1, the Pb accumulation in rice plants, particularly in the grains was remarkably reduced. Moreover the biomass of rice was not significantly affected. Among the three cultivars, the lowest Pb content in rice grains occurred in YY-673. The cultivation of rice cultivar with low Pb accumulation will reduce the Pb content in the grains. Liming significantly inhibited the translocation of Pb from the roots to the aboveground parts, and from the stems and leaves to grains. Pb L3-XAS and BCR s sequential extraction method analysis revealed that lime amendment increased Pb2+ association with Fe oxides (Pb-Ferr), the precipitation of 2PbCO3·Pb(OH)2, the residual fraction of paddy soil, and also promoted the transformation of Pb(NO3)2 and Pb-pectin to 2PbCO3·Pb(OH)2 and Pb-Ferr in rice roots. The suitable lime addition (> 4 g kg-1, under this pot experiment) could effectively reduce Pb accumulation in rice grains.

Cadmium phytoextraction potential of king grass (Pennisetum sinese Roxb.) and responses of rhizosphere bacterial communities to a cadmium pollution gradient.

Screening for tolerant and high biomass producing plants is important for phytoextraction efforts in remediating agricultural soils contaminated by heavy metals. We carried out a greenhouse experiment involving a soil cadmium (Cd) concentration gradient (0.1, 0.5, 1, 2, 4, and 8 mg kg-1) to assess growth and phytoextraction capacity of king grass (Pennisetum sinese Roxb.) in soils contaminated by Cd and to explore changes in diversity and structure of rhizosphere soil bacterial communities in response to long-term Cd pollution. A significant positive relationship was observed between Cd concentrations in P. sinese stems, leaves, and roots and soil Cd concentration. The highest Cd concentrations in shoots and roots were 28.87 and 34.01 mg kg-1, respectively, at 8 mg kg-1of soil Cd supply. Total extraction amounts of Cd in P. sinese were 0.22-1.86 mg plant-1 corresponding to treatment with 0.5-8 mg kg-1 Cd. Most of the Cd was stored in shoots, and the largest accumulation was 1.56 mg plant-1 with 54.02 g dry shoot weight. After phytoextraction, changes in rhizobacterial community composition were found with different levels of Cd application, whereas there were no clear trends in diversity and richness. Results of this study show the feasibility of P. sinese in accumulating Cd and provide support for its application in remediation of soil moderately contaminated by Cd.

Effects and mechanisms of meta-sodium silicate amendments on lead uptake and accumulation by rice.

The objectives of this research were to study the effects of Na2SiO3 application on the uptake, translocation, and accumulation of Pb in rice and to investigate the mechanisms of Pb immobilization by Na2SiO3 in paddy rice soils and rice plants. Pot experiments were conducted using a Cd-Pb-Zn-polluted soil and Oryza sativa L. ssp. indica cv. Donglian 5. L3-edge X-ray absorption spectroscopy was used to identify Pb species in soils and roots. The results showed that the application of Na2SiO3 increased soil pH and available soil Si but decreased DTPA-extractable Pb in the soil. High dose of Na2SiO3 (12.5 g/kg) reduced the Pb level in brown rice as it inhibited Pb transfer from soil to rice grains, especially Pb transfer from the root to the stem. The Pb X-ray absorption near-edge spectroscopic analysis revealed that application of high dose of Na2SiO3 increased Pb-ferrihydrite and PbSiO3 precipitates in the soil and in the root while it reduced Pb-humic acids (Pb-HAs) in the soil and Pb-pectin in the root. The decrease in Pb availability in the soil can be partly attributed to increase the precipitation of PbSiO3 and the association of Pb2+ with Fe oxides in the soil. The inhibition of the root-to-stem translocation of Pb was partially due to the precipitation of PbSiO3 on the root surfaces or inside the roots.

Biochar amendment immobilizes lead in rice paddy soils and reduces its phytoavailability.

This study aimed to determine effects of rice straw biochar on Pb sequestration in a soil-rice system. Pot experiments were conducted with rice plants in Pb-contaminated paddy soils that had been amended with 0, 2.5, and 5% (w/w) biochar. Compared to the control treatment, amendment with 5% biochar resulted in 54 and 94% decreases in the acid soluble and CaCl2-extractable Pb, respectively, in soils containing rice plants at the maturity stage. The amount of Fe-plaque on root surfaces and the Pb concentrations of the Fe-plaque were also reduced in biochar amended soils. Furthermore, lead species in rice roots were determined using Pb L3-edge X-ray absorption near edge structure (XANES), and although Pb-ferrihydrite complexes dominated Pb inventories, increasing amounts of organic complexes like Pb-pectins and Pb-cysteine were found in roots from the 5% biochar treatments. Such organic complexes might impede Pb translocation from root to shoot and subsequently reduce Pb accumulation in rice with biochar amendment.

Effectiveness of lime and peat applications on cadmium availability in a paddy soil under various moisture regimes.

In paddy soils, amendments and moisture play important role in the immobilization of cadmium (Cd). The effects of applying lime, peat, and a combination of both on soil Eh, pH, and Cd availability in contaminated soils were investigated under wetted (80 ± 5 % of water holding capacity) and flooded (completely submerged) conditions. In wetted soils, there was little change in Eh, compared to flooded soils where Eh reduced rapidly. Amendments of lime only or in a mixture with peat increased soil pH to different degrees, depending on the lime application rate. However, peat addition only slightly affected soil pH. The decreased Cd availability in flooded soils was related to submergence duration and was significantly lower than that in wetted soils after 14 days. Liming wetted and flooded soils decreased exchangeable Cd and increased carbonates or Fe-Mn oxides bound fractions, while peat addition transformed Cd from carbonates to organic matter bound fractions. The combined application of peat and lime generally showed better inhibitory effects on the availability of Cd than separately application of lime or peat. Higher application rates of lime, peat, or their mixture were more effective at reducing Cd contamination in flooded soil. This indicates that application of peat and lime mixture under flooded conditions was most effective for in situ remediation of Cd-contaminated soils. Further studies are required to assess the long-term effectiveness of the peat and lime mixture on Cd availability in paddy soils.

Removal, redistribution, and potential risks of soil Cd, Pb, and Zn after washing with various extractants.

The effectiveness of four different washing extractants--HCl, FeCl3, citric acid, and EDTA--in removing Cd, Pb, and Zn from polluted soil was studied. The removal of these metals, their redistribution between fractions, and the potential risks posed by them, in soils washed with the tested extractants, were examined. Although all the rounds of washing removed Cd, Pb, and Zn from soil, the first round removed more metals than subsequent rounds. Each of the four extractants had different effects on the removal of the metals. At the end of the first round of washing, HCl, EDTA, and FeCl3 were the most effective in removing Zn, Pb, and Cd, respectively. Both the single round and five successive rounds of washing with various extractants resulted in significant increases in Pb in the exchangeable/acid extractable fraction. Washing with HCl, EDTA, and FeCl3 significantly reduced potential risks (calculated as the Potential Risk Index, PRI) posed by Cd in washed soil. The first round of washing, using all extractants, increased the risks posed by Pb and Zn. However, five successive rounds of washing with FeCl3 and EDTA reduced the risk posed by Pb, and washing with citric acid and FeCl3 increased the risks posed by Zn. EDTA and HCl were better for reducing Zn risks, and successive washing with EDTA and FeCl3 were more effective in reducing Pb risks than the other extractants. Finally, five successive rounds of washing, with all the extractants, effectively reduced the potential risks posed by Cd. Among the four reagents, EDTA was advised to be the alternative of the washing reagent by significantly reducing the PRI values of Cd, Pb, and Zn.