Exchangeable Fraction Of Soil Research Articles

Tracing changes in base cation sources for Arctic tundra vegetation upon permafrost thaw

Upon permafrost thaw, the volume of soil accessible to plant roots increases which modifies the acquisition of plant-available resources. Tundra vegetation is actively responding to the changing environment with two major directions for vegetation shift across the Arctic: the expansion of deep-rooted sedges and the widespread increase in shallow-rooted shrubs. Changes in vegetation composition, density and distribution have large implications on the Arctic warming and permafrost stability by influencing the albedo, the snow accumulation and the litter decomposition rate. A better understanding of these cumulated effects of changing vegetation on warming and permafrost requires assessing the changes in plant nutrient sources upon permafrost thaw, nutrient access being a limiting factor for the Arctic tundra vegetation development. In this study, we determined the influence of permafrost degradation on the base cation sources for plant uptake by using the radiogenic Sr isotope ratio as a tracer of source, along a permafrost thaw gradient at Eight Mile Lake in Interior Alaska (USA). As plants take up Sr from the exchangeable soil fraction with no measurable fractionation, we determined the differences in 87Sr/86Sr ratio of the exchangeable Sr between shallow and deeper soil horizons, and we compared the 87Sr/86Sr ratio of foliar samples for three Arctic tundra species with contrasted rooting depths (Betula nana, Vaccinium vitis-idaea, and Eriophorum vaginatum) upon different permafrost thaw conditions. The higher foliar 87Sr/86Sr ratios of shallow-rooted Arctic tundra shrubs (B. nana, V. vitis-idaea) was consistent with a shallow source of soil exchangeable Sr from surface soil horizons, whereas the lower foliar 87Sr/86Sr ratios of deep-rooted Arctic tundra sedges (E. vaginatum) reflected a source of Sr from deeper soil horizons. There is a shift between poorly and highly thawed soil profiles towards lower foliar 87Sr/86Sr ratios in both deep- and shallow-rooted plant species. This shift supports that micro-landscape variability in the exchangeable base cation reserve with soil depth represents a key source of readily available nutrients for both shallow- and deep-rooted plant species upon permafrost thaw. This study highlights a key change in plant nutrient source to consider upon thaw. This finding lies beyond the common view that nutrient release at the permafrost thaw front preferentially benefits deep-rooted plant species.

Estimation of the rate of 137Cs root uptake into stemwood of Japanese cedar using an isotopic approach

Japanese cedar (Cryptomeria japonica) is the main timber species in Japan. The prediction of the temporal changes in the 137Cs concentration in the stemwood of Japanese cedar after the Fukushima nuclear accident is essential for optimizing forest management in contaminated areas. However, it is difficult to estimate the respective contributions of root and foliar uptake to 137Cs accumulation in stemwood from simple field measurements, especially in trees that contain the residue of initially-deposited 137Cs. In this study, we devised a method for estimating the rate of 137Cs root uptake into stemwood using the 133Cs content in stemwood and the 137Cs/133Cs ratio in the exchangeable fraction of soil. As a trial, the method was applied to a cedar stand in Fukushima Prefecture, using available monitoring data from prior studies over 5 years from August 2011 to August 2016. The mean annual rate of 137Cs root uptake into stemwood over this period was estimated as 53 ± 20 Bq m−2 yr−1. We note that our method likely provided a maximum estimate, because it is based on the assumptions that 133Cs in wood is exclusively supplied by root uptake, and that Cs isotopes are taken up by roots in the top 5 cm of mineral soil. Moreover, the mean annual increase of the 137Cs inventory in stemwood during the study period was measured as 108 Bq m−2 yr−1, although this value was associated with considerable uncertainty (95% confidence interval from −109 to 324 Bq m−2 yr−1). As a result, the maximum estimated rate of 137Cs root uptake into stemwood accounted for around half of the measured rate of 137Cs accumulation in stemwood. Our results show that the Cs isotopic approach has potential to distinguish the main pathway of stemwood contamination (i.e., root vs. foliar uptake) following radioactive fallout.

Soil washing with biodegradable chelating agents and EDTA: Technological feasibility, remediation efficiency and environmental sustainability

In washing soils contaminated with toxic metals, the replacement of recalcitrant EDTA with biodegradable chelators has gained high expectations. Herein we investigated the feasibility of using EDTA and biodegradable GLDA, EDDS and IDS under conditions pertinent to operational remediation technology, in a pilot-scale experiment. GLDA and IDS did not precipitate from process solutions, which lessened their recyclability. In other process parameters, chelator supplement, Na-saturation of process solutions and processing time, EDTA outperformed biodegradable chelators. Treatment with EDTA was also the most effective in total Pb and Zn removal and least impacted soil properties. GLDA was slightly better in Cd removal. EDDS and IDS were inefficient. All chelators effectively removed easily-available Pb, Zn and Cd from the exchangeable soil fraction. EDTA was the most efficient chelator in reducing the bioaccessibility of Pb and GLDA in reducing the bioaccessibility of Cd from simulated human gastrointestinal tract. Treatment with GLDA had an edge in reducing plant bioaccessibility of toxic metals, but induced worrying leachability of Pb. This was 8.3-times higher than with the process with EDTA and 3.4-times higher than in original soil. In general, our results demonstrate the advantage of EDTA over tested biodegradable chelators in process and remediation efficiency and environmental safety.

Distribution and migration of heavy metals in soil and crops affected by acid mine drainage: Public health implications in Guangdong Province, China

Acid mine drainages (AMD) contain high concentrations of heavy metals, and their discharges into streams and rivers constitute serious environmental problems. This article examines the effects of AMD on soil, plant and human health at Dabaoshan mine in Guangdong Province, China. Although the large scale mining was stopped in 2011, the heavy metal pollution in soil continues to endanger crops and human health in that region. The objectives of this study were to elucidate distribution and migration of Cd, Cu, Zn, As and Pb and associated health implications to local inhabitants. We collected and analyzed 74 crop samples including 28 sugarcane, 30 vegetables, 16 paddy rice and the corresponding soil samples, used correlation and linear relationship for transformation process analysis, and applied carcinogenic and non-carcinogenic risk for hazard evaluation. Results showed that the local soils were heavily polluted with Cd, Cu and As (especially for Cd) and the mean Igeo value was as high as 3.77. Cadmium, Cu, and Zn in rice and vegetables were comparable with those found four years ago, while As and Pb in edible parts were 2 to 5 times lower than before. The root uptake of Cd and Zn contributed mainly to their high concentrations in crops due to high exchangeable fraction of soil, while leafy vegetables accumulated elevated As and Pb contents mainly due to the atmospheric deposition. Metal concentrations in sugarcane roots were higher than those in rice and vegetable roots. The risk assessment for crops consumption showed that the hazard quotients values were of 21 to 25 times higher than the threshold level for vegetables and rice, indicating a potential non-carcinogenic risk to the consumers. The estimated mean total cancer risk value of 0.0516 more than 100 times exceeded the USEPA accepted risk level of 1×10−4, indicating unsuitability of the soil for cultivating the food crops. Therefore, the local agricultural and the land-use policies need to be reevaluated.

Determination of the source of bioavailable Sr using ⁸⁷Sr/⁸⁶Sr tracers: a case study of hot pepper and rice.

The geographical origin of agricultural products has been intensively studied, but links between agricultural products and the environments are poorly established. Soils, water (streamwater and groundwater), and plants (hot pepper, Capsicum annuum; and rice, Oryza sativa) were collected from all regions of South Korea and measured Sr isotope ratios ((87)Sr/(86)Sr). Sequential leaching of soil showed that Sr in the exchangeable and carbonate fractions (bioavailable) had a lower (87)Sr/(86)Sr ratio than that in the silicate fraction, consistent with a low (87)Sr/(86)Sr ratio in the plant. Although the bedrock-soil-water-plant system is closely linked, statistical analysis indicated that (87)Sr/(86)Sr ratios of the plant showed the greatest agreement with those of water and the exchangeable fraction of soil. This study is the first report of (87)Sr/(86)Sr isoscapes in South Korea and first demonstrates that the agricultural product is strongly linked with the exchangeable fraction of soil and water.

Biomineralization based remediation of As(III) contaminated soil by Sporosarcina ginsengisoli

Arsenic is a highly toxic metalloid and has posed high risk to the environment. As(III) is highly mobile in soil and leached easily into groundwater. The current remediation techniques are not sufficient to immobilize this toxic element. In the present study, an As(III) tolerant bacterium Sporosarcina ginsengisoli CR5 was isolated from As contaminated soil of Urumqi, China. We investigated the role of microbial calcite precipitated by this bacterium to remediate soil contaminated with As(III). The bacterium was able to grow at high As(III) concentration of 50 mM. In order to obtain arsenic distribution pattern, five stage soil sequential extraction was carried out. Arsenic mobility was found to significantly decrease in the exchangeable fraction of soil and subsequently the arsenic concentration was markedly increased in carbonated fraction after bioremediation. Microbially induced calcite precipitation (MICP) process in bioremediation was further confirmed by ATR-FTIR and XRD analyses. XRD spectra showed presence of various biomineralization products such as calcite, gwihabaite, aragonite and vaterite in bioremediated soil samples. The results from this study have implications that MICP based bioremediation by S. ginsengisoli is a viable, environmental friendly technology for remediation of the arsenic contaminated sites.

Changes in the speciation, partitioning and phytoavailability of chromium induced by organic soil amendments

This study investigated the effect of two organic amendments (compost of cattle ruminai content and Sphagnum-moss peat) on the reduction of hexavalent chromium and the distribution of this metal among the main solid phases of a soil with low organic matter content treated with different levels of Cr(VI) (0–2000 mg Cr kg−1 soil). At the same level of added organic carbon, the peat reduced Cr(VI) added to the soil from 250 to 2000 mg kg−1, with 100% efficiency. The reduction efficiency of the compost, however, decreased with the increasing dose of Cr(VI) soil. The distribution of Cr between the different soil components was evaluated by a sequential chemical extraction procedure. The concentration of water-soluble and exchangeable Cr decreased with the addition of organic amendments to the soil, whereas Cr increased in the organic fraction. The effect of added organic material on the Cr absorption was examined with two ornamental plants (Melissa officinalis and Begonia semperflorens). The increased Cr(VI) in the soil increased the Cr concentration in plant tissues. The addition of organic matter produced a greater aerial biomass for each level of added Cr in comparison with unamended soil. Sphagnum moss peat was more effective than the compost to decrease the total Cr and the Cr(VI) concentration in the water-soluble and exchangeable fraction of soil, thereby reducing the Cr accumulation in plants tissues and phytotoxic symptoms.

Enhanced heavy metal phytoextraction by Echinochloa crus-galli using root exudates

Heavy metal uptake and growth by Echinochloa crus-galli were investigated to determine if the use of root exudates enhanced phytoextraction. E. crus-galli were planted in soils contaminated with 600 mg kg(-1) Pb, 40 mg kg(-1) Cd and 100 mg kg(-1) Cu. E. crus-galli were then cultivated with and without root exudates from Belamcanda chinensis for 4 weeks. The growth of E. crus-galli in metal-contaminated soils that contained root exudates showed increased roots and shoots when compared to E. crus-galli grown without root exudates (p<0.05). In addition, the accumulation of metal in E. crus-galli that was cultivated with the root exudates was two- to fourfold higher than in plants that were cultivated without the root exudates. The exchangeable soil fraction in the rhizosphere of E. crus-galli grown with root exudates was greater than when E. crus-galli was grown without root exudates. Finally, the BCF and TF values of Cd, Cu and Pb were greater when the root exudates were added (p<0.05). Taken together, these results indicate that root exudates can be used as a natural chelating agent to enhance phytoextraction.

Metal uptake by woodlice in urban soils

This study evaluated the uptake of bioavailable metal (Cd, Cr, Cu, Fe, Pb and Zn) by woodlice (Isopoda) collected from public open spaces in urban areas of Renfrewshire, Central West Scotland, UK. The species Oniscus asellus and Porcellio scaber were collected at 13 different locations together with associated surface soil samples. Soils were subject to sequential extraction to evaluate metal availability and analyzed by ICP-AES and flame AAS for Cd, Cr, Cu, Fe, Pb and Zn concentrations. The soil samples had metal concentrations typical of general urban environments and the potentially toxic elements were well below published guideline values for contaminated sites. The metal concentration showed differing inputs of natural and anthropogenic sources. Metals were bioconcentrated in the order Cu>Cd>Pb>Cr>Zn>Fe for O. asellus and Cu>Zn>Cd>Cr>Fe for P. scaber. Principal Component Analysis of soil geochemical properties and Isopoda metal concentration identified metal to metal variation in uptake. Multiple linear regression analysis was applied in order to investigate the metal uptake in relation to soil properties (total metal concentration, pH and organic matter (OM)). The results showed that factors affecting metal concentration were both species and site specific. The most available forms of metals were generally poorly related to metal accumulation by woodlice, with the only exception being for Cu, which was related to the exchangeable soil fraction. Soil conditions e.g. pH and OM, influenced metal association within the soil and OM played a significant role in restricting uptake of Cr and Pb in particular. For most of the metals studied, despite differences in the environmental availability of the metals, accumulation from ambient soil concentration is controlled by ecological and physiological factors influencing metal assimilation, storage and excretion and that the two biological species vary considerably in their regulation of individual metals.

Selected bioavailability assays to test the efficacy of amendment-induced immobilization of lead in soils

Lead immobilization in 10 soils contaminated with Pb from different origin was examined using lime (CaCO3), a mix of cyclonic ash and steelshots (CA+ST), and a North Carolina phosphate rock. The immobilization efficacy of the three amendments was evaluated using single (CaCl2solution) and sequential (BCR method) chemical extractions in tandem with a microbiological Pb biosensor (BIOMET), a Pb phytotoxicity test, Pb plant uptake, and a Physiologically Based Extraction Test (PBET) mimicking Pb bioavailability in the human gastro-intestinal tract. The results demonstrated the necessity of using a diverse suite of bioavailability methodology when in situ metal immobilization is assessed. Sequential (BCR) extractions and PBET analysis indicated that PR was the most effective amendment. PR however, proved ineffective in totally preventing Pb phytotoxicity and Pb uptake on all soils tested. On the contrary, CA+ST and lime decreased BIOMET Pb, Pb phytotoxicity, and Pb uptake to a far greater extent than did PR. BIOMET detectable Pb and Pb uptake, however, were strongly related to Pb in soluble or exchangeable soil fractions (i.e., CaCl2 extractable). By combining these fractions with the acid-extractable Pb, accomplished by using acetic acid extractant, the recently developed BCR sequential extraction scheme appeared to have lost some valuable information on judging Pb bioavailability. The data show that different amendments do not behave consistently across different soils with different sources of contamination. Different indices for measuring Pb bioavailability are also not necessarily consistent within particular soil and amendment combinations.

Radiochemical assessment of the readsorption and redistribution of lead in the SM&T sequential extraction procedure

The objective of this study was to employ radiochemical methods to assess the importance and extent of the readsorption and redistribution of lead in the standards, measurement and testing programme’s (SM&T) sequential extraction procedure. Step-wise tests indicated that 60% to over 90% of the lead extracted from the exchangeable fraction was readsorbed. Approximately 35% to over 85% of the lead extracted from the reducible fraction was readsorbed, while negligible readsorption was detected for the oxidisable fraction. Statistically significant correlations were observed between readsorption from the first two fractions and both the total iron content and the organic carbon matter even at low organic contents (i.e. less than 6 wt.%). By following the radiotracer activity through the first two steps of the method, the reducible fraction was shown to play an important role in scavenging lead released from the exchangeable soil fraction. The radiotracer partitioning results were employed to “correct” for the redistribution effects yielding the “true” fractionation of the Pb in the samples.

Current interlaboratory precision of exchangeable soil fraction measurements

In addition to conventional aqua regia and EDTA extracts for monitoring trace metals in soils, the technique of examining exchangeable soil fractions has been suggested to estimate soil contamination and trace metal availability to plants. In order to establish a useful method for soil monitoring, interlaboratory precision as a primary selection criterion has been investigated. In order to assess the quality of data provided by laboratories participating in the organization of the Austrian Governmental Agricultural Research Institutes (ALVA), three soil samples have been analysed in a common ring test, annually, for the last 20 years. In addition to the annual list of parameters used for soil monitoring, within ALVA two NH4-acetate extracts were run in 1994, three NH4-acetate and NH4NO3 extracts in 1995 and three LiCl extractions in 1998. The procedures were tested for analytical precision and environmental indications in up to 12 laboratories, with respect to Cd, Cr, Cu, Ni, Pb and Zn. Due to the low extraction efficiency, for determinations in the resultant solutions, graphite furnace AAS was preferably selected, except for Zn and Cu. Flame-AAS and ICP-OES were not sensitive enough for non-contaminated sites. Interlaboratory precision of the data was in the range 10–65% coeff.var., and thus within the range of data given in the appendix of DIN 19730 (NH4NO3), as well as in a previous BCR report. Indications from exchangeable fractions seemed to be good for Zn and Cu, whereas they were impossible for Cr.

Environmental contamination and seasonal variation of metals in soils, plants and waters in the paddy fields around a PbZn mine in Korea

The objective of this study is to investigate the extent and degree of heavy metal contamination of paddy fields influenced by metalliferous mining activity. Paddy soils, rice plants and irrigation waters were sampled along six traverse lines in the vicinity of the mine and nearby control site. Soil samples were taken 30, 80 and 150 days after rice transplanting, to study seasonal variation of their chemical properties and heavy metal concentrations. Sampling of rice plants and irrigation waters was also undertaken with seasons. The analysis of the samples were carried out using ICP-AES for 25 elements including Cd, Cu, Pb and Zn. Physical and chemical properties of soils (pH, loss-on-ignition, cation exchange capacity and texture) and waters (pH, Eh and temperature) were also measured. The properties of soils were similar to the average Korean soils, with the exception of some samples taken in the vicinity of the mine. Concentrations of Cd, Cu, Pb and Zn in paddy soils, rice plants and irrigation waters sampled in the immediate vicinity of the mine were relatively high due to the seepage of metals from mining dump sites. Although there was variation between sampling sites, soil pH values under reducing conditions were on average higher than those under oxidising conditions. Relatively low content of organic matter and low cation exchange capacity of soils were found at 80 days after rice transplanting ( P < 0.05). No seasonal variations in metal concentrations were found in paddy soils throughout the period of the rice growing, in which soils ranged from flooded reducing conditions through most of the growing season to drained oxidising conditions before and at harvest. Relatively high metal concentrations were found in the rice stalks and leaves under oxidising conditions. The sequential extraction analysis of selected soil samples confirmed that high proportions of exchangeable fractions of the metals were found under oxidising conditions. It was shown that Cd and Zn concentrations in rice leaves and stalks and rice grain increased with increasing metal concentrations in paddy soils to a greater extent than for Cu and Pb. This difference in uptake is in agreement with the greater proportions of Cd and Zn, compared with Cu and Pb, in the exchangeable soil fraction extracted with MgCl 2. Average daily intake from locally grown rice by the residents was estimated to be 121 μg Cd and 126 μg Pb. Thus, long-term metal exposure by regular consumption of the rice poses potential health problems to residents in the vicinity of the mine, although no adverse health effects have as yet been observed.