Dar-Yuan Lee

Geostatistical analysis of soil properties of mid-west Taiwan soils

Analysis and interpretation of soil survey data are very important for effective management of agricultural fields. In this study, kriging and cokriging methods were applied to estimate the spatial distribution of soil properties from available large-scale survey data of Taiwan. The data were derived from soils in a 10-km 2 area divided into 250 m X 250 m node intervals. The soil properties examined included the extractable P, Ca, Mg, and Fe contents, the sum of exchangeable bases (SEB), %sand, %silt, and %clay. The sum of exchangeable bases and particle-size distribution were regarded as the primary and auxiliary variables, respectively, in the cokriging procedure. The ratio of nugget to total variation was about 57 to 80%, indicating that the spatial correlation of the tested soil properties at the large scale was moderately (cross-)dependent.The estimated spatial distributions of the soil properties by kriging, under decreasing sampling densities, all correlated significantly (P < 0.1%) with those obtained from original data. Furthermore, with the over-sampled particle-size distribution, the overall estimation of SEB quality by cokriging was superior to that by kriging. The results suggested that by kriging and cokriging, the existing sampling density could be decreased under the large-scale sampling interval by nearly half and that sufficient spatial information about the soil properties could still be retained. The information obtained could be used to improve the long-term sampling designs of soil surveys in Taiwan. It also may be useful for identifying the appropriate sampling densities for these scales of soil surveys.

Arsenic accumulation and speciation in rice grains influenced by arsenic phytotoxicity and rice genotypes grown in arsenic-elevated paddy soils

Rice consumption is a major route of As exposure to human for the population of worldwide. This study investigates the effect of phytotoxicity and rice genotypes on the content and speciation of As in rice grains grown in different levels of As-elevated paddy soils from Taiwan. Three levels of As-elevated soils and six rice genotypes commonly planted in Taiwan were used for this study. The results indicate that As contents in grains of rice is not proportional to soil As concentrations and they were equal or higher in indica genotypes than japonica genotypes used in this study. It was also found that the As phytotoxicity not only reducing the grain yields but also the As concentrations in grain of rice. The predominant As species found in rice grains were dimethylarsinic acid (DMA) and arsenite. The concentrations of DMA increased with total As concentrations, wherggeas the arsenite remained in a narrow range from 0.1 to 0.3 mg kg(-1). Because of the lower toxicity of DMA than inorganic As species, the health risks may not be increased through consumption of rice even when total As content in the grains is increased.

Arsenic sequestration in iron plaque and its effect on As uptake by rice plants grown in paddy soils with high contents of As, iron oxides, and organic matter

Abstract The aim of this study is to investigate the effects of the rice (Oryza sativa L.) roots iron (Fe) plaques on arsenic (As) uptake by rice plants grown in As-contaminated soil with high contents of iron oxides and organic matter, from the Guandu Plain of northern Taiwan. The results of the soil incubation study show that both As and Fe concentrations in the soil solutions increased with flooding time due to reductive dissolution of iron oxides induced by the soils high contents of organic matter. Large Fe plaques were deposited on rice roots and substantial amounts of As were sequestrated in these Fe plaques. About 73.8 to 90.4 % of the total As released from soils was distributed in the iron plaques and only a small proportion was distributed in the rice plants. This study provides evidence that iron plaques are the main controlling factor in limiting the uptake of As into the rice plants grown in the tested soils, and that iron plaque formation enhancement could be used to reduce As uptake by paddy rice grown in As-contaminated soils, thus reducing As toxicity.

The effectiveness of ferrous iron and sodium dithionite for decreasing resin-extractable Cr(VI) in Cr(VI)-spiked alkaline soils

Ferrous iron, Na(2)S(2)O(4), and a mixture of Fe(II) and Na(2)S(2)O(4) (4:1 mol/mol) were tested for their effectiveness for decreasing resin-extractable Cr(VI) in alkaline Cr(VI)-spiked soils. The results indicated that adding those reductants greatly decreased the amount of resin-extractable Cr(VI) when the application rate of reductants equaled the number of equivalents of dichromate added to the Cr(VI)-spiked soils. This was mainly as a result of the Cr(VI) reduction into Cr(III), as supported by the XANES spectra. Among the tested reductants, a mixture of Fe(II) and Na(2)S(2)O(4) was the most effective to decrease resin-extractable Cr(VI). The extent to which resin-extractable Cr(VI) and soil pH were decreased was affected by the pH of the reductants. Among the tested reductants at various pH, FeSO(4) at pH below 1 was the most effective in decreasing resin-extractable Cr(VI) in alkaline soils. However, the soil pH was the most decreased as well. On the other hand, the mixtures of ferrous iron and dithionite at a wide range of pH were all efficient (>70% efficiency) in decreasing resin-extractable Cr(VI). Moreover, the extent of the decrease in soil pH was much smaller than that by FeSO(4) (pH<1) alone, and thus the possibility of the Cr(III) hazard can be avoided.

The effectiveness of four organic matter amendments for decreasing resin-extractable Cr(VI) in Cr(VI)-contaminated soils.

This paper compared the effectiveness of four organic materials for decreasing the amounts of soil extractable Cr(VI) in Cr(VI)-contaminated soils using the DOWEX M4195 resin-extraction method. Organic matters were added into Cr(VI)-spiked soils [500 mg Cr(VI)(kgsoil)(-1)] in the form of sugarcane dregs compost (SCDC), cattle-dung compost (CDC), soybean meal (SBM) and rice bran (RB), in the amounts of 0, 1%, and 2% by dry weight, respectively. The results indicated that adding only 1% organic matter to the studied soils could effectively decrease the amount of soil resin-extractable Cr(VI) after 12 days of incubation. The decrease of resin-extractable Cr(VI) by organic materials was mainly the result from the reduction of Cr(VI) to Cr(III) supported by the XANES spectroscopy. Among the four tested organic materials, SBM and RB had higher effectiveness in decreasing soil resin-extractable Cr(VI) than CDC and SCDC. This result may be due to the fact that SBM and RB have more dissolved organic carbon (DOC) and protein than CDC and SCDC. Therefore, it was concluded that the contents of DOC and protein are the main factors that determine the effectiveness of organic materials for decreasing the amounts of soil available Cr(VI) in Cr(VI)-contaminated soils.

Increase of As release and phytotoxicity to rice seedlings in As-contaminated paddy soils by Si fertilizer application.

Silicon (Si) was shown to be able to reduce arsenic (As) uptake by rice in hydroponic culture or in low As soils using high Si application rates. However, the effect of Si application on As uptake of rice grown in As-contaminated soils using Si fertilizer recommendation rate has not been investigated. In this study, the effect of Si application using Si fertilizer recommendation rate on As release and phytotoxicity in soils with different properties and contents of As was examined. The results show that the concentrations of As in soil solutions increased after Si applications due to competitive adsorption between As and Si on soil solids and the Si concentrations in soil solutions were also elevated to beneficial levels for rice growth. The rice seedlings accumulated more As and its growth was inhibited by Si application in As contaminated/spiked soils. The results indicate that there is an initial aggravation in As toxicity before the beneficial effects of Si fertilizing to rice were revealed when Si application based on fertilizer recommendation rate to As-contaminated paddy soils. Therefore, for As-contaminated paddy soils with high levels of As, the application of Si fertilizer could result in increasing As phytotoxicity and uptake by rice.

Simultaneous extraction of soil phytoavailable cadmium, copper, and lead by chelating resin membrane

Cadmium, copper, and lead were extracted from suspensions of contaminated soils using metal chelating exchange resin membranes. Nine soils with widely varying properties and Cd, Cu and Pb levels were tested. Soil suspensions made up with 4 g in 40 mL deionized water were equilibrated with 5 cm2 Ca-saturated Chelex exchange resin membrane which was retained inside a polypropylene bag and shaken at 150 rpm for 24 hrs. Resin membrane extractable Cd, Cu and Pb of the soils were correlated with Cd, Cu, and Pb uptake by young wheat seedlings grown in these soils and compared with soil Cd, Cu, and Pb extracted by 0.1 M HCl, 0.01 M CaCl2, and 0.005 M Diethylenetriamine pentaacetic acid (DTPA). The amounts of Cd, Cu and Pb extracted by the Ca-saturated Chelex membrane from all tested soils correlated well with those absorbed by young wheat seedlings. The Ca-saturated Chelex membrane extractable Cd, Cu and Pb of the soil had the strongest correlation with plant uptake Cd, Cu and Pb among the extraction methods we tested. It was demonstrated that the Ca-saturated Chelex membrane extraction is an appropriate method in simultaneously estimating Cd, Cu and Pb phytoavailability of soil and is applicable to a wide range of soils.

Assessment of Phytotoxicity of Chromium in Flooded Soils using Embedded Selective Ion Exchange Resin Method

Chromium present in the forms of Cr(VI) or Cr(III) in soils. Since the toxicity and mobility of Cr(VI) are higher than those of Cr(III), it would be important to estimate soil Cr(VI) accurately in order to assess the phytotoxicity of Cr. Soil redox potential can influence the distribution of Cr between Cr(VI) and Cr(III) forms, and thus an in situ method which is not affected by the soil redox condition is needed for determining Cr(VI) availability in paddy fields. In this study, the Cu-saturated selective ion exchange resin (DOWEX M4159), serving as an infinite sink, was embedded in soils to extract available Cr(VI) from three representative saturated soils with different amounts of Cr(VI). The results suggested that Cr(VI) reduction occurred in the flooded soils, and the acid environment favored the adsorption and reduction of Cr(VI). There was a significant dose-response relationship between the soil resin-extractable Cr(VI) and the plant height of rice seedlings for test soils. The experimental results suggested that the embedded selective ion exchange resin method could be a suitable in situ method for assessing the phytotoxicity of Cr in flooded soils.

Determination of bioavailable cadmium in paddy fields by chelating resin membrane embedded in soils

Diffusion is an important mechanism in determining the bioavailability of heavy metals in cropland soils because it controls the rate of movement of heavy metal ions in soil solution to the root surface. Methods currently used for estimating the bioavailability do not take this process into account. In this study a chelating resin membrane was embedded in saturated soils for determining bioavailability of cadmium in situ.Nine Taiwanese soils, including five major agricultural soils which were treated with various levels of Cd and four heavy metal contaminated soils, were used in this study. One piece of Ca-Chelex resin membrane (4 cm × 4 cm) was embedded horizontally in saturated soil pastes and recovered after a fixed time interval. From the amounts of Cd accumulated on the resin membrane, the Cd bioavailability index (C2D) was calculated, where C is the labile Cd concentration and D is the effective diffusion coefficient. This index provides an integrated measure of Cd ion diffusion, sorption-desorption rates, and solution concentration. The results showed that the index was approximately constant and did not vary with the length of time embedded in soil (from 24 to 96 hrs). The index decreased as soil pH increased. When soils were flooded for 7, 15, and 30 days respectively before the resin membrane was embedded in soil for 1 day, the resulting index decreased as the length of flooding time increased. This suggests that the bioavailability index reflects the dynamic change of Cd bioavailability in saturated soils. The indices were then correlated with total uptake of Cd by rice seedlings. The Cd bioavailability index correlated significantly with plant uptake of Cd (p < 0.1%) for all the flooding time period treatments. The above results indicated that the embedded chelating resin membrane method is suitable for determining bioavailable Cd of saturated soils across a wide range of soil properties and Cd concentrations. In addition, the method is nondestructive, and therefore, feasible for in situ determination of the bioavailable Cd of paddy soils.

Effects of foliar and soil application of sodium silicate on arsenic toxicity and accumulation in rice (Oryza sativa L.) seedlings grown in As-contaminated paddy soils

ABSTRACT Silicon (Si) and arsenite (iAsIII) share the same pathway of uptake and translocation in rice (Oryza sativa L.), and it has been reported that Si can decrease arsenic (As) uptake by rice in hydroponic culture. Due to the competitive sorption between Si and As for the soil minerals, different extents of effects on As toxicity and uptake by rice as influenced by soil Si application were found. In order to avoid the competitive sorption between Si and As for the soil minerals, foliar applications of Si might be a more efficient alternative to reduce As uptake by rice. Therefore, the aim of this study was to investigate the effects of Si foliar and soil applications on the growth and As accumulation in rice seedlings. In this study, three application rates of Si (as sodium silicate) were applied to (1) the foliage and (2) the soil, of rice grown in two soils with different Si retention capacities and As contents. The results showed that there were no significant differences in Si concentrations in shoots among different Si foliar applications, and it had no significant effect on the growth and As accumulation in rice seedlings. In contrast, soil applications of Si caused a decrease in As accumulation in shoots grown in the tested soils, resulting from a high Si/As ratio in the soil pore water allowing enough Si to out-compete the As for uptake by roots. These results were further supported by the significant negative correlation found between Si and As in the shoots. However, a growth inhibition of rice plants was also observed due to As toxicity from the addition of excessive treatments of Si into As-contaminated soils. Therefore, the results of this study suggest that foliar applications of Si are not able to decrease As accumulation in rice seedlings, but that the application of moderate amounts of Si into As-contaminated soils could effectively reduce As uptake by rice seedlings.