ABSTRACT:Various extraction procedures were employed for measuring extractable concentrations of potential toxic elements in soil. The extractability of Cd, Cu, Pb and Zn in four contaminated and four non-contaminated soils of Japan, was compared by single extraction (CaCl 2 , DTPA, NH 4 Cl, 0.1 M HCl and 1 M HCl ) and sequential extraction procedures [(six operationally defined chemical phases, viz. water soluble (F1), exchangeable (F2), carbonate (F3), oxide (F4), organic (F5) and residual (F6) fractions)]. Extractability of metals from soils samples varied depending on metals and/or extractants used. Among the extractants, 1 M HCl extracted the largest proportion of Cd (79 to 96% of total), Cu (61 to 83%), Pb (51 to 99%) and Zn (23 to 52%) from soils followed by 0.1 M HCl, NH 4 Cl, DTPA and CaCl 2 . In all the extractants, the proportion of extractability of metals was higher in the contaminated soils than the non-contaminated soils. Regardless of soils and extractants, relative extractability was higher for Cd as compared to other three metals. The use of 1 M HCl may be recommended for first-level screening of soil contamination with heavy metals. The other four weak extractants are believed to provide a better assessment of bioavailable/mobile metals content in soils than 1 M HCl extractant. However, 0.1 M HCl mobilized all four metals irrespective of soil types, therefore, might be the best choice if only one extractant is to be used. The sequential extraction procedures showed 22 to 64% of total Cd was in the mobile fraction (sum of F1 to F3), while the corresponding values for Cu, Pb and Zn in this fractions were 2 to 23% suggesting higher mobility of Cd than other three metals. The single extraction procedures are simple and easy to perform and obtained results are comparable with sequential extraction procedure.

A hydroponic experiment was conducted to observe the effect of arsenic (As) on a number of physiological and mineralogical properties of rice (Oryza sativa L. cv. Akihikari) seedlings. Seedlings were treated with 0, 6.7, 13.4 and 26.8 µmol L−1 As (0, 0.5, 1.0 and 2.0 mg As L−1) for 14 days in a greenhouse. Shoot dry matter yield decreased by 23, 56 and 64%; however, the values for roots were 15, 35 and 42% for the 6.7, 13.4 and 26.8 µmol L−1 As treatments, respectively. Shoot height decreased by 11, 35 and 43%, while that of the roots decreased by 6, 11 and 33%, respectively. These results indicated that the shoot was more sensitive to As than the root in rice. Leaf number and width of leaf blade also decreased with As toxicity. Arsenic toxicity induced chlorosis symptoms in the youngest leaves of rice seedlings by decreasing chlorophyll content. Concentrations and accumulations of K, Mg, Fe, Mn, Zn and Cu decreased significantly in shoots in the 26.8 µmol L−1 As treatment. However, the concentration of P increased in shoots at 6.7 and 13.4 µmol L−1 As levels, indicating a cooperative rather than antagonistic relationship. Arsenic and Fe concentration increased in roots at higher As treatments. Arsenic translocation (%) decreased in the 13.4 and 26.8 µmol L−1 As treatments compared with the 6.7 µmol L−1 As treatment. Arsenic and Fe were mostly concentrated in the roots of rice seedlings, assuming co‐existence of these two elements. Roots contained an almost 8–16‐fold higher As concentration than shoots in plants in the As treatments. Considering the concentration of Mn, Zn and Cu, it was suggested that chlorosis resulted from Fe deficiency induced by As and not heavy‐metal‐induced Fe deficiency.

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Physiological and mineralogical responses of sorghum (Sorghum bicolor L. cv. Fast sorghum) in hydroponic culture at elevated concentrations of arsenic (As) were evaluated. Seedlings were grown in the presence of 0, 6.7, 33.5 and 67 µM As levels (0, 0.5, 2.5 and 5 mg As l −1 ) up to 14 days after treatments (DAT). Shoot and root dry matter yield were repressed by higher As levels. At low As level (6.7 µM) shoot dry matter yield was enhanced by 2.3% but at 33.5 and 67 µM As levels, the yield decreased by 52 and 79%, respectively. The root growth was similarly enhanced (8%) at the lower As level while the growth decrement at the higher As levels were 33 and 68%, respectively for the two treatments. Considering 10% dry weight (DW) reduction, the critical toxicity level of As was calculated to be 11.7 µg g −1 DW for shoot and 367 µg g −1 DW for root, indicating that shoot was more sensitive to As-toxicity than root. A whitish chlorotic symptom was observed in the fully developed young leaves at the 67 µM As level. The lowest chlorophyll content was also observed at this As level. Arsenic concentration increased both in shoot and in root with increase in solution As concentration. The concentrations of As and Fe were about 16, 28 and 17 times; and 2, 25 and 144 times higher in root than shoot at 6.7, 33.5 and 67 µM As levels, respectively. The concentrations of K, Fe and Cu were significantly lower while Ca, Mg and Mn concentrations were higher in the shoot at the 67 µM As level compared to the control plants. On the other hand, Fe, Mn, Zn and Cu concentrations were higher in root at the 67 µM As level. In the shoot, accumulation and translocation of metal micronutrients, particularly that of Fe, decreased significantly because of the presence of As. The present observations suggested that As might induce a toxic effect on sorghum by hampering the translocation of the metal micronutrients. It is suggested that "As-induced FeWater Air Soil Pollut (deficiency" caused chlorotic symptoms in the hydroponically grown sorghum.

A hydroponic experiment with barley seedlings (Hordeum vulgare L. cv. Minorimugi) grown in iron (Fe)‐depleted medium in the presence of added arsenic (As) at the rates of 0, 0.67, 6.7 and 67 µmol L−1 (equivalent to 0, 0.05, 0.5 and 5 mg L−1 As, respectively) showed that increasing the As concentration in the medium lowered the release of phytosiderophores (PS) and their concentration in the roots. This Fe‐depleted experiment was conducted to clarify the effect of As on the release and concentration of PS in roots and on the phosphorus (P) and Fe concentrations in plants. The chlorophyll index increased substantially in the 67 µmol L−1 As treatment compared with the other treatments. This result indicated that higher concentrations of As might interrupt the appearance of Fe chlorosis in plants grown in Fe‐depleted medium. Arsenic at a level of 67 µmol L−1 increased the Fe concentration and accumulation in shoots disappearing the whitish chlorosis. An increased concentration of Fe in the shoot might also be responsible for lowering the release and concentration of PS in the roots. Increases in the concentration of Fe in the shoot most likely resulted from enhanced Fe translocation from the roots to the shoots. The physiological mechanism of the higher Fe translocation with As needs to be investigated. Arsenic lowered the concentrations of P, potassium (K), calcium (Ca) and magnesium (Mg) in the shoot at the 67 µmol L−1 level. A higher Fe concentration and higher ratio of Fe/P in the shoot might be responsible for the greening of the leaves in the 67 µmol L−1 As treatment. The concentrations of manganese (Mn), zinc (Zn) and copper (Cu) were reduced by the high As levels. There was a concomitant increase in the As contents in shoots with higher As levels in the growth medium. The relationship between the concentrations of As and Fe and between P and Fe in the shoots was the opposite. Thus, higher As level might play a role in increasing the mobility of root Fe in barley tissues grown in Fe‐depleted medium. Grasses grown under Fe‐deficient conditions might not show Fe chlorosis in the presence of high concentrations of As.