Ikuko Akahane

The effects of soil amendments on arsenic concentrations in soil solutions after long-term flooded incubation

Abstract We investigated the effects of various soil amendments on the concentration of arsenic (As) in soil solutions under flooded anaerobic conditions. Ten amendments (six As adsorbents and four general agricultural amendments) were used with two soil samples (Aquepts). The time-course changes in the concentrations of As, iron (Fe) and silicon (Si) in soil solutions with or without amendments were measured after incubations of approximately 30, 60 and 100 d. We conclude that a precipitate of polysilicate-iron solution (1.1 mol L‒1 Fe) shows promise as a novel As adsorbent for flooded anaerobic soils such as paddy soils. The precipitate is present in sludge from water purification plants and is therefore potentially cost effective. Moreover, it could increase the concentration of Si in soil solution, which could decrease As uptake by rice plants. Ferrihydrite and aluminum-substituted ferrihydrite were also effective for decreasing the dissolved As in both soils and nano-sized layered double hydroxides, and magnesium oxides worked in one soil. Poultry manure significantly increased dissolved As in both soils, but composted bark and silicate fertilizers did not change As in either soil. Some amendments had different effects in the two soil samples; further comprehensive studies will need to focus on how soil properties influence the effects of amendments.

Use of water-treatment residue containing polysilicate-iron to stabilize arsenic in flooded soils and attenuate arsenic uptake by rice (Oryza sativa L.) plants

ABSTRACT A pot experiment was conducted to examine how soil amendment with water-treatment residue (WTR) containing polysilicate-iron affected dissolved arsenic (As) in flooded soils and As uptake by rice plants (Oryza sativa L.). The WTR was applied at a rate of 0 (control), 5, 10 or 20 t ha−1. Simple linear regression analyses showed significant negative relationships between the concentrations of dissolved As in soil solution and WTR application rates at all sampling times, probably due to adsorption of As onto ferrihydrite in the WTR. Compared to As concentrations in rice plants grown on control soil, the concentrations in plants grown on WTR-treated soils decreased by 20.1–41.6% in straw (stems and leaves), 19.8–31.7% in husk and 18.6–21.0% in grain. The regression analyses demonstrated that the concentration and content of As in rice are negatively correlated with WTR application rate. Total As content was 16.5–32.0% lower in rice shoots grown on WTR-treated soils than on control soil. The percentage of each As species in grain decreased in the following order: As(III) » dimethylarsinic acid » As(V). The application of WTR did not change the As speciation in grain. Silicon contents in shoot significantly increased with application of WTR, indicating the potency of WTR as a silicate fertilizer. Taken together, our results indicate that WTR containing polysilicate-iron promises to be a practical amendment for stabilizing As and attenuating As uptake by rice plants.

Simultaneous decrease of arsenic and cadmium in rice (Oryza sativa L.) plants cultivated under submerged field conditions by the application of iron-bearing materials

ABSTRACT The Codex Alimentarius Commission has recently adopted maximum levels for inorganic arsenic (As; in 2014) and total cadmium (Cd; in 2006) in polished rice grains to maintain food safety and to decrease the risk to human health. As rice is a staple crop in Japan and monsoon Asian countries, reducing concentrations of As and Cd in rice is an urgent matter. In flooded conditions, Cd concentration in soil solution decreases whereas As concentration increases. Therefore, we aimed to evaluate the efficiency of iron-bearing materials to decrease As concentration in soil solution and rice (Oryza sativa L.) grain under submerged cultivation, while also considering Cd concentration. In experiments conducted in paddy fields in six regions, As concentrations in the soil solution during the cultivation period decreased in the following order: control (REF) > steel converter furnace slag (SCS) > non-crystalline iron hydroxide (FH) > zero-valent iron (ZVI). The concentrations of As in brown rice were in the same order, with ZVI achieving particularly strong reduction. Cadmium concentrations were low, probably owing to submerged cultivation conditions. Application of iron-bearing materials slightly and insignificantly reduced the yields of brown rice and straw. Application of these materials did not have a significant negative impact on the quality of rice. Our data indicate that the application of iron-bearing materials effectively reduces As concentrations in soil solution and rice grains without negative effects on yield and quality, with a particularly powerful effect of ZVI which is possibly explained by arsenic sulfide formation.

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.

Relationship between vertical distributions of Bray II P content and organic P content in the Ap horizon of paddy rice fields

Abstract We examined the vertical distributions of total, Bray II and organic phosphorus (P) in the Ap horizons of five paddy rice fields including no tillage treatment. Soil samples were collected from the Ap horizons as nine thin layers of 0–1, 1–2, 2–3, 3–4, 4–6, 6–8, 8–10, 10–15 and 15–20 cm from the soil surface after harvest. In three lowland paddy soils of no-tillage Hachirogata (HA-NT), conventional-tillage Hachirogata (HA-T) and conventional-tillage Furukawa (FU-T), the total P (TP) content was relatively low. In these soils, the modified Bray II P (soil : solution = 1:20; shaking time 60s) content decreased and the organic P (OP) content increased at a depth of 0–1 cm compared with the underlying layers, possibly because of high microbial activities as suggested from the increased total C and N contents in this layer. In contrast, the decrease in Bray II P content and the increase in the OP content of the surface layer were not evident in two Andosols of the conventional-tillage Utsunomiya (UM-T) and conventional-tillage Kawatabi (KA-T). Even TP content fluctuated in the thin surface layers of 0–3 cm depending on the sampling site. Accordingly, percentages of Bray II P and OP to TP were calculated to compare the vertical distribution curves between Bray II P and OP. The percentage of Bray II P to TP of the 0–1 cm layer was lower and the percentage of OP to TP in the layer was higher than the underlying layers for HA-T, HA-NT and FU-T. The decrease in Bray II P was nearly compatible with the increase in OP for the surface of the 0–1 cm layers of HA-NT, HA-T and FU-T. This result suggests that a part of Bray II P was converted to OP in the thin surface layer of the Ap horizons in these paddy soils. This change in chemical form of P was more pronounced in the HA-NT by accumulating the effect of the microbial activities over a longer period of time because of no tillage compared with HA-T and FU-T. These findings add a new dimension to P behavior in submerged rice soils under field conditions, particularly near the soil surface. Moreover, we must be careful in soil sampling for P availability evaluation, and soil samples should represent the vertical distribution of P in the whole Ap horizon of a paddy field. Changes in OP with depth in UM-T and KA-T may have been masked by the high OP content as well as by the high TP content in these Andosols.

Significance of hydrous iron oxides in enhancing P sorption of soils with the addition of sodium hydrosulfite (Na2S2O4)

Abstract Phosphorus sorption (Psor) of soils is affected by redox conditions. It has been shown that Psor of lowland soils at a pH value of about 4.3 increases when a small amount of sodium hydrosulfite (Na2S2O4) is added and decreases when an excess amount of (Na2S2O4) is added to the mixture of a soil and P solution. Hydrous Fe-Al oxides, manganese dioxide (MnO2) exchangeable Ca, models of reactive components with P in soils, were examined to identify the factors responsible for the increase of Psor in lowland soils when a small amount of Na2S2O4 was added. For clarifying the contribution of the hydrous Fe-Al oxides, goethite and 7 hydrous Fe-Al oxides (Fe/Al atomic ratio: 1/0, 5/1, 2/1, 1/1, 1/2, 1/5, and 0/1) were used. The Psor of all these materials increased when they were treated with a small amount of Na2S2O4 although the increase was the smallest for the hydrous Al oxide among the 7 oxides. Thus, the hydrous Fe oxides, and Al oxide to a smaller extent, play an important role in the increase in Psor of the lowland soils treated with a small amount of Na2S2O4 The Psor of the hydrous Fe oxides was not appreciably affected by the addition of MnO2. The increase in Psor of the Ca-saturated Hachirogata soil was almost the same as that of the Nasaturated Hachirogata soil, indicating that exchangeable Ca did not affect appreciably the increase of Psor in reduced soils at a pH value of about 4.3.

Effect of air temperature after heading of rice on the arsenic concentration of grain

ABSTRACT High air temperatures during the grain-filling stage of rice (Oryza sativa L.) decrease grain yield and quality. Temperatures above the optimal growth temperature impair dry matter production due to reduced grain size. Since the arsenic (As) concentration in grain is determined by the As accumulation and the grain weight, we focused on the effect of air temperature before and after rice heading on the As concentration in grain. Rice plants have been planted in the same field every year since 1974 with a similar set-up and water management. We analyzed the As concentrations in stored rice grains produced in 1995–2014 and evaluated the possible relationships with the meteorological data for the associated production year. The average daily mean air temperatures (DMTs) from 2 weeks after the heading day to 4 weeks after the heading day were significantly correlated with the inorganic As concentrations in the grains, whereas there was no significant correlation between the grain As concentration and the average DMT during 1 week after the heading day or 3 weeks before the heading day. These findings indicate that high air temperature is more effective at increasing As in grain in the late ripening stage of rice than before the ripening stage or in the early ripening stage. Further experiments are necessary to clarify why As concentrations in grain increase when high air temperatures occur in the late ripening stage of rice.