Geochemistry of magnetite and maghemite in soils in European Russia

A method is proposed for determining the proportions of soluble Fe oxides (magnetite, FeOFe₂O₃, and maghemite, γ-Fe₂O₃) based on the measured magnetic susceptibility before and after treatment of soil with the Tamm or Mehra-Jackson (DCB) reagents. The development of hydromorphism in steppe soils in Ciscausiaia is associated with an increase in the magnetite fraction and, consequently, the average magnetite: maghemite ratio increases from 0.8–0.9 to 1.1. In these soils, smectites facilitate magnetite oxidation to maghemite. Soddy-podzolic and dark humic soils in the Cis-Ural region are noted for low values of the magnetite: maghemite ratio (0.5 on average) due to maghemite predominance. Soils in the Cis-Ural region on cover red-earth clays inherit lithogenic Fe oxides: hematite and maghemite. Hydromorphism in humid environments in northern taiga is accompanied by a significant increase in the magnetite: maghemite ratio to 4–9. Some issues of Fe geochemistry in magnetite and maghemite are considered.     

Magnetic stability of power-plant fly ash in different soil solutions

Temporal stability of magnetic parameters of anthropogenic ferrimagnetics in model soil conditions was studied. Fly ash from a power plant was used as original contaminant material, while soil reactive medium was modeled by different soil solutions. Changes in mass-specific magnetic susceptibility (χ), percentage frequency dependent magnetic susceptibility (χ_FD%), hysteresis parameters and thermomagnetic curves were measured for samples which underwent different leaching time periods. Virtually constant magnetic susceptibility values were obtained, independently on the soil medium (acid, neutral). On the other hand, the original highly non-stoichiometric maghemite phase rapidly transformed to stable magnetite. In some cases (model of free-draining soils) further decrease of Curie temperature (T_c) to 550–560°C is observed.     

钢厂周边污染土壤的电性与磁性特征及其环境意义

利用直流电测深、环境磁学以及矿物学方法,开展对一钢铁厂周边土壤的污染评价研究,获得了土壤垂向电阻率与磁化率分布特征。研究表明,随着污染土剖面的深度由底部至地表,土壤电阻率逐渐减小,磁化率和重金属含量反而增高;该变化特征反映了土壤受污染的程度与钢铁公司历年粗钢产量及武汉市历年汽车保有量的变化程度相对应。通过对污染样品的磁滞回线、热磁曲线和SEM/EDX等矿物学分析,污染样品的矿物成分以磁铁矿和赤铁矿为主,以准单畴(PSD)颗粒存在,形貌特征和物质成分与成土过程中形成的磁性颗粒明显不同。由于土壤的电性与磁性都具有良好的污染程度指示作用,所以土壤的电性与磁性研究可以应用于土壤污染程度的评价,两者联合运用可大大提高土壤污染程度评价的分辨率。     

Magnetic susceptibility and electrical conductivity as a proxy for evaluating soil contaminated with arsenic,cadmium and lead in a metallurgical area in the San Luis Potosi State,Mexico

A total of 113 samples of waste and soil were collected from a site in the state of San Luis Potosi, Mexico, that was occupied for several years by the metallurgical industry. Specific magnetic susceptibility (MS), electrical conductivity (EC) and pH were determined, as well as the total and available concentrations of potentially toxic elements (PTEs) such as As, Cd, Cu, Fe, Pb and Zn, which may cause a health risk for humans, animals and ecosystems, and the concentrations of major ions in aqueous extracts of soils and wastes. The solid phases of the samples were also characterized. The results revealed that the soils and wastes exhibited elevated values of PTEs, MS and EC. For soils these values decreased with increasing distance from the waste storage sites. The MS values were elevated primarily due to the presence of Fe-oxyhydroxides, such as magnetite, hematite and goethite, which contain PTEs in their structure leading to a high correlation between the value of MS and the As, Cd, Fe and Pb contents (r = 0.57–0.91) as well as between the PTEs values (r = 0.68–0.92). The elevated EC values measured in the metallurgical wastes were the result of presence of the sulfate minerals of Ca, Mg and Fe. The pollution index, which indicates the levels of simultaneous toxicity from elements such as As, Cd and Pb, was determined, with extreme hazard zones corresponding to areas that exhibit high MS values (0.91 correlation). In conclusion, MS measurements can be used as an indirect indicator to evaluate the PTE contamination in metallurgical areas, and EC measurements can aid in the identification of pollution sources.     

Iron speciation and isotope fractionation during silicate weathering and soil formation in an alpine glacier forefield chronosequence

The chemical weathering of primary Fe-bearing minerals, such as biotite and chlorite, is a key step of soil formation and an important nutrient source for the establishment of plant and microbial life. The understanding of the relevant processes and the associated Fe isotope fractionation is therefore of major importance for the further development of stable Fe isotopes as a tracer of the biogeochemical Fe cycle in terrestrial environments. We investigated the Fe mineral transformations and associated Fe isotope fractionation in a soil chronosequence of the Swiss Alps covering 150 years of soil formation on granite. For this purpose, we combined for the first time stable Fe isotope analyses with synchrotron-based Fe-EXAFS spectroscopy, which allowed us to interpret changes in Fe isotopic composition of bulk soils, size fractions, and chemically separated Fe pools over time in terms of weathering processes. Bulk soils and rocks exhibited constant isotopic compositions along the chronosequence, whereas soil Fe pools in grain size fractions spanned a range of 0.4‰ in δ⁵⁶Fe. The clay fractions (<2 μm), in which newly formed Fe(III)-(hydr)oxides contributed up to 50% of the total Fe, were significantly enriched in light Fe isotopes, whereas the isotopic composition of silt and sand fractions, containing most of the soil Fe, remained in the range described by biotite/chlorite samples and bulk soils. Iron pools separated by a sequential extraction procedure covered a range of 0.8‰ in δ⁵⁶Fe. For all soils the lightest isotopic composition was observed in a 1 M NH₂OH-HCl-25% acetic acid extract, targeting poorly-crystalline Fe(III)-(hydr)oxides, compared with easily leachable Fe in primary phyllosilicates (0.5 M HCl extract) and Fe in residual silicates. The combination of the Fe isotope measurements with the speciation data obtained by Fe-EXAFS spectroscopy permitted to quantitatively relate the different isotope pools forming in the soils to the mineral weathering reactions which have taken place at the field site. A kinetic isotope effect during the Fe detachment from the phyllosilicates was identified as the dominant fractionation mechanism in young weathering environments, controlling not only the light isotope signature of secondary Fe(III)-(hydr)oxides but also significantly contributing to the isotope signature of plants. The present study further revealed that this kinetic fractionation effect can persist over considerable reaction advance during chemical weathering in field systems and is not only an initial transient phenomenon.     

典型石漠化区不同种植年限桃树下土壤微量元素变化特征

为明确石漠化地区不同种植年限果树根系表层土壤微量元素的变化特征,选择桂林市恭城县大岭山村同一农户的不同种植年限桃树（2 a、10 a、20 a）各5棵,以撂荒地（0 a）作为对照,对土壤微量元素铁、铜、锰、锌、硼的全量及有效态变化规律进行研究,并探讨理化性质与微量元素有效态的相关性。结果表明:（1）研究区除全铁的变异系数小于10%,铜、锰、锌、硼的全量及有效态均为中等变异;（2）土壤铁、铜、锌、硼全量种植0~2 a间均增加（P<0.05）,种植2~10 a则下降（P<0.05）,种植10~20 a间铁元素随年限增加而下降（P<0.05）,其余研究元素全量均表现为增加趋势;（3）有效铁、锌、硼在种植0~10 a先增加（P<0.05）,种植10~20 a均有明显下降趋势,有效锰在0~20 a间趋势则与其他元素相反,并且与全量锰保持一致性,因此种植10 a是果树土壤微量元素变化的转折点;（4）不同种植年限果树根系土壤性质的改变（pH值、有机质、全磷的变化）是微量元素有效态含量随时间变化的主要内在原因,而人为活动如施加有机肥是外在原因。在种植10 a后,应重视微肥的施加。      

Effect of treated municipal wastewater on bean growth, soil chemical properties, and chemical fractions of zinc and copper

The current study was carried out in order to investigate the short-term effect of different dilutions of wastewater on soil chemical properties, chemical fractions of zinc (Zn) and copper (Cu), and to assess the chemical buildup of heavy metal on two bean species. The experiment was performed as factorial based on completely randomized design with different dilutions of wastewater on two bean cultivars in two soil textures. The treatments consisted of irrigation with treated wastewater over all growing season, irrigation with wastewater and freshwater in equal proportions, and irrigation with well water only as control. The result showed that soil parameters are significantly affected by application of wastewater irrigation. Irrigation with wastewater increased the concentrations of organic matter, electrical conductivity, N, K, P, Fe, Cu, Zn, Mn, and Ni in soils compared to the control treatment. However, their values were all below international standards. Application of wastewater decreased soil pH and calcium carbonate equivalent. Plant tissue analysis showed increases in N, P, K, Zn, Cu, Zn, and Mn concentrations in grain and frond of beans in wastewater treatment as compared to the control. The concentrations of all elements in plants were lower than the toxic threshold. Chemical fractionations of Zn and Cu indicated that chemical forms of these metals were affected by irrigation with wastewater. Irrigating with wastewater resulted in the movement of Zn from the labile fractions towards the nonlabile fractions. In turn, mobility factor of Cu increased with application of wastewater. Maximum fresh and dry yields of beans were obtained from wastewater treatment compared to the control treatment. This study indicated that wastewater irrigation improves soil properties, plant growth, and yield without any contamination in soil and toxicity in plants.     

Dependence of microbial magnetite formation on humic substance and ferrihydrite concentrations

Iron mineral (trans)formation during microbial Fe(III) reduction is of environmental relevance as it can influence the fate of pollutants such as toxic metal ions or hydrocarbons. Magnetite is an important biomineralization product of microbial iron reduction and influences soil magnetic properties that are used for paleoclimate reconstruction and were suggested to assist in the localization of organic and inorganic pollutants. However, it is not well understood how different concentrations of Fe(III) minerals and humic substances (HS) affect magnetite formation during microbial Fe(III) reduction. We therefore used wet-chemical extractions, magnetic susceptibility measurements and X-ray diffraction analyses to determine systematically how (i) different initial ferrihydrite (FH) concentrations and (ii) different concentrations of HS (i.e. the presence of either only adsorbed HS or adsorbed and dissolved HS) affect magnetite formation during FH reduction by Shewanella oneidensis MR-1. In our experiments magnetite formation did not occur at FH concentrations lower than 5 mM, even though rapid iron reduction took place. At higher FH concentrations a minimum fraction of Fe(II) of 25-30% of the total iron present was necessary to initiate magnetite formation. The Fe(II) fraction at which magnetite formation started decreased with increasing FH concentration, which might be due to aggregation of the FH particles reducing the FH surface area at higher FH concentrations. HS concentrations of 215-393 mg HS/g FH slowed down (at partial FH surface coverage with sorbed HS) or even completely inhibited (at complete FH surface coverage with sorbed HS) magnetite formation due to blocking of surface sites by adsorbed HS. These results indicate the requirement of Fe(II) adsorption to, and subsequent interaction with, the FH surface for the transformation of FH into magnetite. Additionally, we found that the microbially formed magnetite was further reduced by strain MR-1 leading to the formation of either dissolved Fe(II), i.e. Fe²⁺, in HEPES buffered medium or Fe(II) carbonate (siderite) in bicarbonate buffered medium. Besides the different identity of the Fe(II) compound formed at the end of Fe(III) reduction, there was no difference in the maximum rate and extent of microbial iron reduction and magnetite formation during FH reduction in the two buffer systems used. Our findings indicate that microbial magnetite formation during iron reduction depends on the geochemical conditions and can be of minor importance at low FH concentrations or be inhibited by adsorption of HS to the FH surface. Such scenarios could occur in soils with low iron mineral or high organic matter content.     

Heavy metal contamination of soils and tea plants in the eastern Black Sea region,NE Turkey

The study area covers two geologically different regions which have intensively been carpeted by tea plants in the eastern Black Sea. The rocks exposed in the region contain considerable amount of trace metals due to Upper Cretaceous massive sulfide formations and tertiary epithermal mineralizations. Tea plants that grow in soils derived from such mineralized rocks contain different concentrations of Cu, Pb, Zn, Fe, Cd, P, Al, Na, K, and S. The content ratios of most of the analyzed elements except Al are higher in basaltic and sedimentary rocks. To describe the transfer of metals from soil to tea leaf, the Freundlich-type model (log c _plant = ac _soil + log b) is used. The metal concentrations in leaves of tea plant in the studied soils are ranked as Zn > Cu > Pb > Al > Cd > Fe. The capacity of the plant to affect the metal accumulation decreased as follows: Fe > Cd > Pb > Cu > Zn > Al. Negative correlations were found between pH and availability of Cu, Pb, Zn, Mn, and Al elements by the tea plant. Experimental applications indicated that tea plant leaves growing on soils with high metal contents show some signs of toxicity. In soils where, particularly, ammonium sulfate fertilizer is used, metal uptake by the tea plant was found to be significantly higher as a result of extremely acidic character of the soil.     

Trace elements in sub-alpine forest soils on the eastern edge of the Tibetan Plateau,China

Industrial development has increased fast in China during the last decades. This has led to a range of environmental problems. Deposition of trace elements to forest ecosystems via the atmosphere is one potential problem. In this paper, we report the results from a pilot study where the trace element levels of the sub-alpine forest soils on the eastern edge of the Tibetan Plateau have been measured. Possible relationships between soil properties and trace element concentrations have also been investigated. The obtained concentrations (mg kg⁻¹) were boron (B) 48.06–53.70, molybdenum (Mo) 1.53–2.26, zinc (Zn) 68.18–79.53, copper (Cu) 36.81–42.44, selenium (Se) 0.33–0.49, cadmium (Cd) 0.16–0.29, lead (Pb) 25.80–30.71, chromium (Cr) 96.10–110.08, nickel (Ni) 30.16–45.60, mercury (Hg) 0.05–0.11, and arsenic (As) 3.09–4.17. With a few exceptions, the element concentration can be characterized as low in the investigated sub-alpine forest soils. No clear differences in trace element levels were found between topsoil and subsoil samples, indicating that the atmospheric deposition of trace element has been low. The soil parent material plays a key role to determine trace element levels. Soil properties, including pH_w, organic carbon (OC), clay fraction, cation-exchange capacity (CEC), total iron (Fe), and total aluminum (Al) concentrations were related to trace element concentration using correlation analysis. Total Fe and Al showed the strongest relationships with concentrations of most trace elements in the sub-alpine forest soils. PCA analyses indicated that a significant increase in the number of cars with the fast development of local tourism may result in higher Pb concentration in the future.     

Growing up green on serpentine soils: Biogeochemistry of serpentine vegetation in the Central Coast Range of California

Serpentine soils derived from the weathering of ultramafic rocks and their metamorphic derivatives (serpentinites) are chemically prohibitive for vegetative growth. Evaluating how serpentine vegetation is able to persist under these chemical conditions is difficult to ascertain due to the numerous factors (climate, relief, time, water availability, etc.) controlling and affecting plant growth. Here, the uptake, incorporation, and distribution of a wide variety of elements into the biomass of serpentine vegetation has been investigated relative to vegetation growing on an adjacent chert-derived soil. Soil pH, electrical conductivity, organic C, total N, soil extractable elements, total soil elemental compositions and plant digestions in conjunction with spider diagrams are utilized to determine the chemical relationships of these soil and plant systems. Plant available Mg and Ca in serpentine soils exceed values assessed in chert soils. Magnesium is nearly 3 times more abundant than Ca in the serpentine soils; however, the serpentine soils are not Ca deficient with Ca concentrations as high as 2235 mg kg⁻¹. Calcium to Mg ratios (Ca:Mg) in both serpentine and chert vegetation are greater than one in both below and above ground tissues. Soil and plant chemistry analyses support that Ca is not a limiting factor for plant growth and that serpentine vegetation is actively moderating Mg uptake as well as tolerating elevated concentrations of bioavailable Mg. Additionally, results demonstrate that serpentine vegetation suppresses the uptake of Fe, Cr, Ni, Mn and Co into its biomass. The suppressed uptake of these metals mainly occurs in the plants’ roots as evident by the comparatively lower metal concentrations present in above ground tissues (twigs, leaves and shoots). This research supports earlier studies that have suggested that ion uptake discrimination and ion suppression in the roots are major mechanisms for serpentine vegetation to tolerate the chemistry of serpentine soils.     

三峡库区消落带土壤磁性变化规律及成因探讨

为动态跟踪三峡库区消落带土壤物性在水位上涨后所受影响,选择重庆市忠县石宝寨镇长江左岸一带作为研究区,以消落带外同类型土壤作对照,从分析不同水位土壤的磁学性质入手,探究三峡库区蓄水前后土壤性质对环境变化的响应。库区蓄水后,不仅导致了消落带内土壤理化性质变化,还使土壤磁性空间分布在不同水位这一维度上发生显著改变。消落带监测断面数据显示,土壤磁性由蓄水前垂直方向上较稳定的分布,变为目前整体上随淹没水深增加而增高的特征;169~175 m高程附近,土壤磁性接近当地同类土壤背景值,而低于此高度尤其是低于157 m后,磁化率值迅速增高。磁性的变化是库区蓄水后土壤磁性矿物组成发生改变造成的,由热磁曲线可知,次生磁铁矿的加入是首要原因。扫描电镜下观测出的浑圆状磁铁矿的最可能来源是上游燃烧化石燃料排放的磁性外来污染物,其随江水输运至此并沉积吸附在土壤颗粒上。研究结果为间接监测污染物的迁移提供了依据。     

Heavy metal pollution and soil magnetic susceptibility in urban soil of Beni Mellal City (Morocco)

The assessment of anthropogenic impact in the urban environment can be evaluated according to heavy metal contents of soils such as Pb, Cu, Zn, Cd and Fe. These elements have more affinity to establish metallic bond with ferrous material leading to enhancement of soil magnetic susceptibility. The objective of this study was to undertake joint magnetic and geochemical investigations of road-side urban soil materials to address the environmental pollution of Beni Mellal city that has been subjected to environmental stress, due to population overpressure and related urbanization. Twenty three soils magnetic susceptibility profiles were made along 5 km peripheral national road (N8) in Beni Mellal. The magnetic survey reported here for the first time on this City’s topsoils tries to establish the link between magnetic properties and the content of heavy metals. High magnetic susceptibility values and high contents of heavy metals were found near the paved edge of the road and within the place reserved as large engine park. Magnetic extracts of highly polluted areas and unpolluted soil (olive plantation) were analyzed by SEM coupled with RDX in order to discriminate anthropogenic magnetic spherules and pedo-lithogenic magnetite-like minerals. Magnetic mineralogy determined by Mössbauer spectroscopy suggests the presence of hematite, magnetite and goethite in highly polluted areas. The iron oxides and especially goethite are efficient in incorporating and/or adsorbing foreign ions.     

Trace Element Geochemistry of Magnetite from the Fe(-Cu) Deposits in the Hami Region, Eastern Tianshan Orogenic Belt, NW China

Laser ablation–inductively coupled plasma–mass spectrometry(LA–ICP–MS) was used to determine the trace element concentrations of magnetite from the Heifengshan, Shuangfengshan, and Shaquanzi Fe(–Cu) deposits in the Eastern Tianshan Orogenic Belt. The magnetite from these deposits typically contains detectable Mg, Al, Ti, V, Cr, Mn, Co, Ni, Zn and Ga. The trace element contents in magnetite generally vary less than one order of magnitude. The subtle variations of trace element concentrations within a magnetite grain and between the magnetite grains in the same sample probably indicate local inhomogeneity of ore–forming fluids. The variations of Co in magnetite between samples are probably due to the mineral proportion of magnetite and pyrite. Factor analysis has discriminated three types of magnetite: Ni–Mn–V–Ti(Factor 1), Mg–Al–Zn(Factor 2), and Ga– Co(Factor 3) magnetite. Magnetite from the Heifengshan and Shuangfengshan Fe deposits has similar normalized trace element spider patterns and cannot be discriminated according to these factors. However, magnetite from the Shaquanzi Fe–Cu deposit has affinity to Factor 2 with lower Mg and Al but higher Zn concentrations, indicating that the ore–forming fluids responsible for the Fe–Cu deposit are different from those for Fe deposits. Chemical composition of magnetite indicates that magnetite from these Fe(–Cu) deposits was formed by hydrothermal processes rather than magmatic differentiation. The formation of these Fe(–Cu) deposits may be related to felsic magmatism.     

Major and trace elements in plants and soils in Horton Plains National Park,Sri Lanka: an approach to explain forest die back

Forest die back has been observed from 1980s in the montane moist forest of Horton Plains in the Central Sri Lanka for which the aetiology appears to be uncertain. The concentration levels of Na, K, Ca, Mg, Fe, Mn, Cu, Ni, Zn and Pb in canopy leaves, bark and roots, which were collected from dying and healthy plants of three different endemic species, Calophyllum walkeri, Syzygium rotundifolium and Cinnamomum ovalifolium, from three different die back sites were studied. Soils underlying the plants were also analyzed for their extractable trace metals and total contents of major oxides. Analysis of dead and healthy plants does not show any remarkable differences in the concentrations of studied trace elements. The results show that there is a low status of pollution based on the concentrations of chemical elements of environmental concern. Extractable and total trace element analysis indicates a low content of Ca in soils due to high soil acidity that probably leads to Mg and Al toxicity to certain plants. Relatively high Al levels in the soil would affect the root system and hamper the uptake and transport of essential cations to the plant. It therefore seems that the forest declining appears to be a natural phenomenon, which occurs due to the imbalance of macro and micronutrients in the natural forest due to excessive weathering and the continuous leaching of essential elements.     

Contaminant dispersion at the rehabilitated Mary Kathleen uranium mine, Australia

This study reports on the transfer of contaminants from waste rock dumps and mineralised ground into soils, sediments, waters and plants at the rehabilitated Mary Kathleen uranium mine in semi-arid northwest Queensland. Numerous waste rock dumps were partly covered with benign soil and the open pit mine was allowed to flood. The mineralised and waste calc-silicate rock in the open pit and dumps has major (>1 wt%) Ca, Fe and Mg, minor (>1,000 ppm) Ce, La, Mn, P and S, subminor (>100 ppm) Ba, Cu, Th and U, and trace (<100 ppm) As, Ni, Pb, Y and Zn values. Consequently, chemical and physical weathering processes have acted on waste rock and on rock faces within the open pit, mobilising many elements and leading to their dispersion into soils, stream sediments, pit water and several plant species. Chemical dispersion is initiated by sulfide mineral breakdown, generation of sulfuric acid and formation of several soluble, transient sulfate minerals as evaporative efflorescent precipitates. Radiation doses associated with the open pit average 5.65 mSv year⁻¹; waste dumps commonly have lower values, especially where soil-covered. Surface pit water is slightly acid, with high sulfate values accompanied by levels of U, Cu and Ni close to or above Australian water guideline values for livestock. Dispersion of U and related elements into soils and stream sediments occurs by physical (erosional) processes and from chemical precipitation. Plants growing in the mine void, on waste dumps and contaminated soil display evidence of biological uptake of U, LREE, Cu and Th and to a lesser degree of As, Ni, Pb, Y and Zn, with values being up to 1–2 orders of magnitude above background sites for the same species. Although rehabilitation procedures have been partly successful in reducing dispersion of U and related elements into the surrounding environment, it is apparent that 20 years after rehabilitation, there is significant physical and chemical mobility, including transfer into plants.     

梵净山西麓古茶树立地岩-土系统地球化学特征分析

以梵净山西麓印江县紫薇镇古茶树立地剖面上的岩石和土壤为研究对象,使用ICP-MS和ICP-AES测定21种元素的含量,分析古茶树立地岩石-土壤系统的地球化学特征,以期为印江县古茶树保护及开发提供科学依据。结果表明：古茶树生长地岩石-土壤系统的主量、微量和重金属元素富集程度低（EF <3）,受人为因素影响程度小,可为古茶树提供安全的生长环境。其中主量元素K和Al在岩石-土壤系统中含量丰富,Ca、Mg、Na元素含量较少,符合茶树喜钾嫌钙的生理特征;微量元素在岩石-土壤系统中含量变化均匀,但与地球化学丰度值和中国土壤背景值相比,其含量较少;重金属元素含量均低于无公害农产品种植业产地环境条件限定值。同时研究发现该岩土系统中Se元素含量丰富,可为当地发展富硒茶提供参考。     

地层岩性对植物群落分布特征的影响

地层岩性是生态地质环境条件的重要组成部分,查明不同岩性对生长在其上植物群落的影响,对生态保护和生态修复具有重要意义。以江西赣州市相同地貌条件和气候条件下的岩石、土壤、植物群落为研究对象,通过对植被和土壤开展全面调查与采样分析,探索了岩石-土壤-植物群落间的对应关系。分析了不同成土母岩衍生的土壤在粒度组成、化学成分、物理性质等方面的特征,揭示了地质背景条件与植物群落分布的关系。结果表明: ①成土母岩岩性、土壤质地、土壤pH值、土壤养分、土壤中主量和微量元素含量的差异是影响植物群落分布的重要因素; ②研究区各岩性区的土壤养分含量从高到低为变余杂砂岩区土壤>粉砂岩区土壤>砂砾岩区土壤>长石石英砂岩区土壤>花岗岩区土壤。     

Soil and plant composition in the Noun river catchment basin,Western Cameroon: a contribution to the development of a biogeochemical baseline

Soils and selected edible plants of the Noun river catchment basin of western Cameroon were sampled to investigate the distribution of trace elements, based on the preliminary idea of unusual anomalies. Analytical techniques for trace elements included ICP-AES, GF-AAS, and ICP-MS. Further soil analyses comprised the mineralogy and contents of the biogenic elements carbon, nitrogen and sulphur (CNS). The trace element concentrations in the soils reflect those of the lithogeochemical background of the pluto-volcanic rocks of the region. This is consistent with the results from the mineralogical analyses and physicochemical parameters such as pH, taken in the field, which also do not suggest any geochemical anomaly. Most trace elements analyzed in the plants showed concentrations that reflect those of the soils (Al, Fe, Ti, and Rb). However, some trace elements were enriched in the plants as compared to the soils, such as Zn, Cu, Cd, Mo (excluding yam), Ni (peanut), Ba (peanut), Sr (peanut, bean), and B. Trace elements such as As, Cr, V, and Se were not bioavailable for all the analyzed plants. Besides, trace elements such as Cu, Zn, Mo, Fe, Al, Ni, B, Ti, Rb, Cs, and Ba were in the range of phytotoxicity and reached or exceeded human food tolerance level (Cu). The plants with seeds showed a higher absorption of trace elements compared to plants with tubercles.     

Distribution and speciation of arsenic around roots in a contaminated riparian floodplain soil: Micro-XRF element mapping and EXAFS spectroscopy

Riparian soils are periodically flooded, leading to temporarily reducing conditions. Diffusion of O₂ through plants into the rhizosphere maintains oxic conditions around roots, thereby promoting trace element fractionation along a redox gradient from the reduced soil matrix towards the oxic rhizosphere. The aim of this study was to determine the distribution and speciation of arsenic around plant roots in a contaminated (170-280 mg/kg As) riparian floodplain soil (gleyic Fluvisol). The analysis of soil thin sections by synchrotron micro-X-ray fluorescence (μ-XRF) spectrometry showed that As and Fe were enriched around roots and that As was closely correlated with Fe. Arsenic contents of three manually separated rhizosphere soil samples from the subsoil were 5-9 times higher than respective bulk As contents. This corresponds to the accumulation of about half of the total As in the subsoil in Fe-enrichments around roots. The speciation of As in the soil was assessed by oxalate extractions at pH 3.0 as well as by X-ray absorption near edge structure (XANES) and extended X-ray fine structure (EXAFS) spectroscopy. More than 77% of the total As was oxalate extractable in all samples. XANES and EXAFS spectra demonstrated that As was predominantly As(V). For the accurate analysis of the EXAFS data with respect to the bonding of As(V) to the Fe- or Al-octahedra of (hydr)oxides and clays, all 3-leg and 4-leg multiple scattering paths within the As^(V)O₄-tetrahedron were considered in a fully constrained fitting scheme. We found that As(V) was predominantly associated with Fe-(hydr)oxides, and that sorption to Al- and Mn-hydroxides was negligible. The accumulation of As in the rhizosphere may affect As uptake by plants. Regarding the mobility of As, our results suggest that by oxygenation of the rhizosphere, plants attenuate the leaching of As from riparian floodplain soils during periods of high groundwater levels or flooding.