Yuji Maejima

Heavy metal contamination of agricultural soil and countermeasures in Japan

Many heavy metals exist in minute amounts in natural agricultural soil. However, when their amounts exceed a certain level due to pollutants brought from outside, soil contamination occurs and agricultural products become contaminated. There have been many cases in Japan of heavy metal contamination originating from old mines and smelters, and soil contamination of agricultural land has become a social issue. In particular, cadmium (Cd) is one of the most harmful heavy metals. If agricultural products absorb an excessive amount of Cd, they may adversely affect people’s health, and therefore allowable concentrations are regulated by law. If agricultural land has become contaminated with Cd, measures for minimizing the absorption of Cd by agricultural crops are necessary; these include: (1) soil dressing, (2) water management (paddy field), (3) chemical cleaning of soil, (4) phytoextraction, and (5) use of different varieties and rootstock. Other heavy metals such as arsenic, lead, copper, zinc, and mercury also sometimes cause contamination of agricultural soil.

Arsenic Distribution and Speciation near Rice Roots Influenced by Iron Plaques and Redox Conditions of the Soil Matrix

Elevated arsenic (As) concentrations in rice and the soil solution result from changes in soil redox conditions, influenced by the water management practices during rice cultivation. Microscale changes in redox conditions from rhizosphere to soil matrix affect the As speciation and Fe plaque deposition. In order to focus on the rhizosphere environment, we observed microscale distribution and speciation of As around the rhizosphere of paddy rice with X-ray fluorescence mapping and X-ray absorption spectroscopy. When the soil matrix was anaerobic during rice growth, Fe-plaque did not cover the entire root, and As(III) was the dominant arsenic species in the soil matrix and rhizosphere. Draining before harvest led the conditions to shift to aerobic. Oxidation of As(III) to As(V) occurred faster in the Fe-plaque than the soil matrix. Arsenic was scavenged by iron mottles originating from Fe-plaque around the roots. The ratio of As(V) to As(III) decreased toward the outer-rim of the subsurface Fe mottles where the soil matrix was not completely aerated. These results provide direct evidence that speciation of As near rice roots depends on spatial and temporal redox variations in the soil matrix.

Arsenic speciation in rice and soil containing related compounds of chemical warfare agents.

Diphenylarsinic acid, phenylarsonic acid, methylphenylarsinic acid (MPAA), dimethylphenylarsine oxide (DMPAO), and methyldiphenylarsine oxide (MDPAO) in soil and rice were extracted, separated by reversed-phase chromatography, and quantified by ICPMS with a membrane desolvating system. For the extraction of arsenicals from rice grain and straw, 68% HNO3 provided better extraction efficiency than water, 50% methanol, or 2.0 mol L(-1) trifluoroacetic acid. For the extraction from soil, 68% HNO3 provided better extraction efficiency than H2O, 1 mol L(-1) H3PO4, or 1 mol L(-1) NaOH. The contaminated soil contained all five aromatic arsenicals along with inorganic arsenicals as main species (5.86 +/- 0.19 microg of As kg(-1): 60.8 +/- 2.0% of total extracted As). After pot experiments, rice straw contained mainly DMPAO (7.71 +/- 0.48 microg of As kg(-1): 60.5 +/- 3.7%), MDPAO (0.91 +/- 0.07 microg of As kg(-1): 7.2 +/- 0.5%), and inorganic As (2.85 +/- 0.20 microg of As kg(-1): 22.3 +/- 1.6%). On the other hand, rice grain contained mainly MPAA (1.17 +/- 0.04 microg of As kg(-1): 86.7 +/- 2.7%). The root uptake of each species from the soil and transport from straw to grains were significantly related to the calculated log K(ow) values.

Influence of soil-drying under field conditions on exchangeable manganese, cobalt, and copper contents

Abstract The changes in the exchangeable Mn (Ex-Mn), exchangeable Co (Ex-Co), and exchangeable Cu (Ex-Cu) contents by air-drying of soils were reported in our previous paper. The drying effect, however, was not analyzed in the field. The objective of this study was to investigate the influence of soil-drying under field conditions on Ex-Mn, Ex-Co, and Ex-Cu levels. Two paddy soils (Tsukuba and Yawara) were collected at four different soil depths (0∓1, 1∓5, 5∓10, and 10∓20 cm) under three different soil moisture conditions i.e., after harvest, after successive sunny days and after rainy days. After sunny days, Ex-Mn and Ex-Co contents in the surface soil layer markedly increased and drastically decreased after rainfall, while they remained almost unchanged in the lower soil layers. Ex-Cu contents slightly increased in the surface soil layer after sunny days. A similar tendency was observed in soil column experiments. The contents of Ex-Mn, Ex-Co, and Ex-Cu increased 12, 12, and 2 times in the upper layer of soil columns that had been sampled from the Yawara field and stored in a dark room for 8 d. The increase of the Ex-Mn and Ex-Co contents in the upper layer could not be explained by the capillary rise of soil water from the lower layers of the columns. In a soil-drying experiment under laboratory conditions, Ex-Mn and Ex-Co contents began to increase when the water content of the Yawara soil was lower than 0.09∓0.13 kg kg−1. The water content of the surface soil of the Yawara field after sunny days was 0.08 kg kg−1. Ex-Cu content did not change apprecially with the water content. It is concluded that the increase of the Ex-Mn and Ex-Co levels in field surface soils after sunny days can be ascribed to soil-drying. Some of the excess Ex-Mn and Ex-Co is expected to leach down into lower layers due to rain, which may contribute to plant uptake of Mn and Co.

14C ages and δ13C of sclerotium grains found in forest soils

Abstract 14C ages and δ13C were examined for sclerotium grains to elucidate the characteristics of these grains distributed in forest soils. The ages of the grains from surface A horizons and buried A horizons were ca 100–200 bp and ca 300–1,200 bp, respectively. In comparison with humic acid extracts, the 14C ages were in the increasing order: humic acid fraction < humic acid Pg fraction < sclerotium grains. The δ13C values for sclerotium grains in surface A horizons and buried A horizons were approximately −31‰ to −28‰, and these values were approximately 2–4‰ smaller than those of humic acids and soils. The C content of the grains had a tendency to decrease with increasing 14C ages, while the C content of humic acid was constant with age. The 14C ages of sclerotium grains indicate the individual age of grain formation, which are more likely to assign closer ages to the beginning of soil forming than the 14C ages of humic acid. The low δ13C values for sclerotium grains have presumably originated from characteristically biological organics, which may be protected from attack in soils because of their structure.

Reduction in uptake by rice and soybean of aromatic arsenicals from diphenylarsinic acid contaminated soil amended with activated charcoal

Activated charcoal (AC) amendment has been suggested as a promising method to immobilize organic contaminants in soil. We performed pot experiments with rice and soybean grown in agricultural soil polluted by aromatic arsenicals (AAs). The most abundant AA in rice grains and soybean seeds was methylphenylarsinic acid (MPAA). MPAA concentration in rice grains was significantly reduced to 2% and 3% in 0.2% AC treated soil compared to untreated soil in the first year of rice cultivation. In the second year, MPAA concentration in rice grains was significantly reduced to 15% in 0.2% AC treated soil compared to untreated soil. MPAA concentration in soybean seeds was significantly reduced to 44% in 0.2% AC treated soil compared to untreated soil. AC amendment was effective in reducing AAs in rice and soybean.

Bromine and iodine in Japanese soils determined with polarizing energy dispersive X-ray fluorescence spectrometry

Abstract The bromine (Br) and iodine (I) status of Japanese soils was investigated by analyzing around 900 soil samples and related materials with polarizing energy dispersive X-ray fluorescence spectrometry (EDXRF). The samples consisted of the following five groups: (1) 468 soil samples collected from 70 sites in the mainland of Japan (mainland soils); (2) 139 agricultural soils collected nationwide; (3) 93 samples taken from the southwestern island (island soils); (4) 52 coastal marine sediments; and (5) 137 tsunami deposits by the 2011 off the Pacific coast of Tohoku Earthquake. The geometric mean of Br was highest in marine sediments (30.6 mg kg−1), followed by that in island soils (24.8 mg kg−1), although the difference between these two groups was not significant. The values for the other three groups were significantly lower at p < 0.05 by Welch’s t test, and were in the following order: mainland soils (10.4 mg kg−1) > tsunami deposits (8.52 mg kg−1) > agricultural soils (7.86 mg kg−1). For I, the geometric mean of island soils (31.9 mg kg−1) was significantly higher than that of other soils at p < 0.05 by Welch’s t test, and was more than three times higher than that of mainland soils (9.11 mg kg−1), which was the second highest group. The values for the other three groups were in the following order: marine sediments (5.68 mg kg−1) > tsunami deposits (4.66 mg kg−1) > agricultural soils (3.50 mg kg−1). The contents of I were higher than those of Br for around two thirds of the island samples. As a result, the geometric mean of the Br/I ratio was less than 1.0 only in this group. The contents of both elements were significantly higher in upland soils than in paddy fields soils. These differences could be partly attributed to the higher ratio of Andosols containing higher amounts of Br and I in upland samples, in addition to the difference in the chemical forms of both elements in paddy field and upland samples. The correlation coefficients among rare earth elements, for example, were more than 0.9 for a considerable number of combinations, whereas more than 80% of the absolute values of correlation coefficients for Br and I against 60 other elements were less than 0.4. These results strongly suggest that the behavior of Br and I in the terrestrial environments differs considerably from that of most other metallic elements.

Spatial prediction of radioactive Cs concentration in agricultural soil in eastern Japan

Abstract As a result of the accident at the Fukushima Dai-ichi Nuclear Power Station (FDNPS) operated by the Tokyo Electric Power Company, radioactive cesium (Cs) was released into the surrounding environment. To determine the extent of decontamination required in agriculture fields and to consider management options, we surveyed and measured soil Cs concentrations in 3461 agricultural fields, and used these data to construct a distribution map of radioactive Cs concentration in agricultural soil in eastern Japan. Soil Cs concentration was positively correlated with radiation dose (r2 = 0.89, n = 2199). This linear correlation was affected to some extent by soil surface condition, soil group and land use type. Linear regression analysis was conducted by land surface condition, soil type and land use type. We constructed a soil Cs concentration map by regression and by a regression-kriging (RK) method that combines regression equations with ordinary kriging of the regression residuals. Prediction accuracy of the RK was higher than that of the regression; we therefore adopted the RK. Total radioactive Cs concentration in soil was highest in the 20-km evacuation zone surrounding FDNPS, and tended to be higher to the northwest of FDNPS than in other directions. About 5900 ha of paddy fields and 3000 ha of upland fields in Fukushima Prefecture were above contamination level 2 (> 5000 Bq kg−1), and were mostly distributed in the evacuation zone.

Effects of sterilization on the chemical forms of heavy metals in soils.

Abstract We used sequential extraction to investigate changes in the amounts of six chemical forms of manganese, cobalt and cadmium in soil samples after chloroform fumigation. The six forms were designated as follows: exchangeable, dilute-acid-soluble, manganese-oxide-occluded, organically bound, iron-oxide-occluded and residual. For all three metals, the decreases in the amounts of manganese-oxide-occluded forms were equivalent to the sum of the increases in the amounts of exchangeable and dilute-acid-soluble forms. The amounts of the other three forms did not change significantly after fumigation. These results indicate that some of the cobalt and cadmium in the manganese oxides was converted into exchangeable and dilute-acid-soluble forms, which suggests that soil sterilization may increase the availability of these heavy metals to plants.

The influence of aeolian dust in non-allophanic Andosols on Yakushima Island

The oxygen isotope (δ18O) composition of quartz and the d(060) values of clay minerals were determined from four pedons of non-allophanic Andosols derived mainly from the Holocene volcanic ash on Yakushima Island. These soils contained considerable amounts of aerosol-sized (1–10 µm) and coarse (>53µm) quartz. The δ18O values for the aerosol-sized quartz ranged from 14.7‰ to 17.4‰, which was comparable to or slightly lower than known values for loess-derived Red and Yellow soils on Tanegashima Island located approximately 20 km east of Yakushima Island. The abundance and δ18O values of the aerosol-sized quartz indicated that non-allophanic Andosols on Yakushima Island were strongly influenced by aeolian dust. However, the presence of coarse quartz implied that granite-derived materials were also incorporated into non-allophanic Andosols. X-ray diffraction patterns for most clay minerals showed two broad peaks around 0.154 and 0.150 nm, respectively. The d(060) values confirmed that 2:1–2:1:1 clay minerals consisted of dioctahedral and trioctahedral clay minerals. Since aeolian dust contains little or no coarse quartz and trioctahedral clay minerals, the abundance of coarse quartz and trioctahedral minerals confirmed that the occurrence of non-allophanic Andosols on Yakushima was influenced by biotite-granite, in addition to aeolian dust.