An understanding of the phosphate (P) dynamics in paddy rice fields is the basis for improving P fertilizer efficiency and reducing P loss from paddy fields. During the ripening stage of rice plants cultivated in pots, we identified vivianite on the roots. We placed 3 kg of air-dried soil in a pot with coated urea (1 g N), coated potassium sulfate (1 g K 2 O) and granular superphosphate (1 g P 2 O 5 ) as basal fertilizers. Three rice seedlings were transplanted into each pot and grown until the ripening stage under submergence outdoor conditions. The bulk soil showed a black color indicating the formation of amorphous FeS. According to the soil analysis data, the oxalate-extractable Fe content was much greater than the labile S and P contents, indicating that enough Fe(II) can be supplied to the S and P for the reaction. Bluish vivianite particles were observed on the roots using an optical microscope. Scanning electron microscopy revealed that the vivianite was an aggregate of platy crystals, and an energy dispersive X-ray analysis showed that Fe and P were the major elements in the crystal aggregates. The diffraction peak positions by the X-ray microdiffractometer were very close to the reported pattern for vivianite. Future research on the dynamics of P is expected based on vivianite formation in paddy field soils.
Athyrium yokoscense, a type of fern that grows vigorously in mining areas in Japan, is well known as a Cd hyperaccumulator as well as a Cu, Pb and Zn tolerant plant. However, no information is available on As accumulation of A. yokoscense, although it often grows on soils containing high levels of both heavy metals and As. In this study, young ferns collected from a mine area were grown in media containing As-spiked soils or mine soil in a greenhouse for 21 weeks. Athyrium yokosense was highly tolerant to arsenate and survived in soils containing up to 500 mg As (V) kg ) among As-spiked soils. Although the As concentration of the fern was lower than other As hyperaccumulators, such as Pteris vittata, A. yokoscense could hyperaccumulate As in mature and old fronds. Arsenic was accumulated most efficiently in old fronds (922 mg kg . Moreover, higher As accumulation was found in the roots of the ferns, with a range from 506 to 2,192 mg kg −1 . In addition, in the mine soil with elevated concentrations of As and heavy metals, A. yokoscense not only hyperaccumulated As (242 mg As kg −1 in old fronds), but also accumulated Cd, Pb, Cu and Zn at concentrations much higher than those reported for other terrestrial plants. Athyrium yokoscense accumulated Cd mostly in fronds in high concentrations, up to 1095 mg kg , respectively.
Populations of Athyrium yokoscense, a fern that hyperaccumulates Zn and Cd, have often been found at abandoned mining sites in Japan, and are often accompanied by another Zn and Cd hyperaccumulating plant, Arabis flagellosa. We compared the Zn and Cd uptake characteristics of At. yokoscense and Ar. flagellosa and examined the influences of community development on Zn and Cd uptake by each plant. A soil culture experiment was conducted using a rhizobox system with seven compartments, each filled with a metalliferous soil taken from Ikuno‐cho, Hyogo Prefecture, Japan. The treatments consisted of the following three planting schemes: two plants of At. yokoscense (AY), two plants of Ar. flagellosa (AF) and a mixed planting of two plants of each species (AY+AF). After 3 months of cultivation, the Zn and Cd concentrations in the shoots were approximately 4.0 and 0.9 g kg−1 for At. yokoscense in the AY treatment and 24 and 0.3 g kg−1 for Ar. flagellosa in the AF treatment, respectively. These results indicated that Ar. flagellosa was more efficient at accumulating Zn in the shoots than At. yokoscense. In the AY+AF treatment, the amounts of water‐soluble Zn and Cd in the soil of the central compartments were significantly higher than those in the AY and AF treatments. Despite this, the Zn and Cd concentrations in the shoots of Ar. flagellosa in the AY+AF treatment were not significantly different from those in the AF treatment. The Cd content in the shoots of At. yokoscense was higher in the AY+AF treatment than in the AY treatment, owing to a growth enhancement in the AY+AF treatment. Consistent with the results for the AY and AF treatments, Ar. flagellosa accumulated higher shoot concentrations of Zn than At. yokoscense and At. yokoscense accumulated higher concentrations of Cd than Ar. flagellosa in the AY+AF treatment. These results suggested that the levels of Zn and Cd accumulated by these two plants were not largely affected by their community development, and implied that communities of these plants do not develop as a result of mutual effects on metal uptake.
We examined possible adverse effects of heavy metals on microbial activity, biomass, and community composition using the simultaneously extracted metals (SEM)/acid-volatile sulfide (AVS)-based approach and measurements of exchangeable metal concentrations in three paddy soils (wastewater-contaminated soil, mine-contaminated soil, and noncontaminated soil) incubated for 60 days under flooded conditions. Incubation under flooding increased pH and decreased Eh in all samples. AVS increased when Eh decreased to approximately -200 mV for the mine-contaminated and noncontaminated soils, while the wastewater-contaminated soil originally had a high concentration of AVS despite its air-dried condition. Addition of rice straw or alkaline material containing calcium carbonate and gypsum increased AVS levels under flooded conditions. We observed no apparent relationship between soil enzyme activity (β-D-glucosidase and acid phosphatase) and concentrations of SEM, [∑SEM - AVS], and exchangeable metals. Bacterial and fungal community composition, assessed using polymerase chain reaction-denaturing gradient gel electrophoresis (DGGE) analysis targeting rRNA genes, was largely influenced by site of collection and incubation time, but metal contamination did not influence community composition. We observed significant negative correlations between biomass C and [∑SEM - AVS] and between biomass C and ∑SEM, suggesting that [∑SEM - AVS] and ∑SEM might reflect the bioavailability of organic matter to microorganisms in these soils.
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