Cadmium availability at different soil pH to transgenic tobacco overexpressing ferritin

Knowledge on physiological mechanisms and plant metabolism can be used to enhance metal uptake. The capacity to uptake metals of transgenic tobaccos overexpressing ferritin in plastids (P6) or in cytoplasm (C5) and a control tobacco (A) is assessed in three polluted soils from the same soil series, with a similar Cd content, but displaying pH from 5.8 to 7 (8b2, 8b3, S11). Differences in dry leave weight were not significant between the three tobaccos growing on each soil. Iron concentration in ferritin overexpression either in P6 or in C5 tobaccos increased only on the S11 soil, which had a soil pH 7, in comparison to A tobacco. In both 8b2 and 8b3 soils at pH lower than 7, the leaf Fe content was not different between the three tobaccos. Only the P6 tobacco growing in the S11 soil accumulated more Cd, Zn and Mn compared with the A and C5 tobaccos. This increase represents, compared to A tobacco, 88% for Fe, 120% for Mn, 30% for Cd, 80% for Zn. In soils with a pH higher than 7, the ferritin synthesis is over-expressed in the P6 tobacco cells inducing a iron deficiency. These metal accumulations were increased, owing to a rhizosphere modification. The A and P6 tobaccos uptake Cd in the same soil pool. The Cd totality in the 8b2 soil is labile and in the S11 soil, the labile Cd represents 86–88% of the total Cd.     

施用污泥对油菜根际养分和不同种类重金属的影响

根际是控制植物养分动态的重要因素,养分动态也影响着根际土壤环境。当土壤被污水污泥改良后,根际土壤中的养分和重金属性质也会发生变化。目前很少有人研究施用污泥的土壤中植物根系对根际重金属有效性和分布的影响。采用根垫—冰冻薄层切片法对施用污泥后土壤中油菜根际的养分和重金属分布情况进行研究,以期探明污泥改良土壤中根际重金属的活化特征。当土壤施用污泥后,根际土壤中DTPA提取态Zn,Cd,Ni,Mn,有效磷,有效钾和铵态氮被显著消耗,而根际土壤中DTPA提取态Cu没有明显的消耗或积累。当土壤中施用大量污泥时,根际土壤的pH值随着离根表面距离的增加而增加。无论土壤是否用污泥处理,油菜根际土壤中可交换态Cu都显著减少。当土壤被50%污泥改良时,在距离根表面0—2 mm处的油菜根际土壤中碳酸盐结合态,铁锰氧化物结合态,有机物结合态,残渣态的Cu和Zn都被消耗较多。污泥的施用对油菜的生长有促进作用。随着污泥施用量的增加,油菜地上部分Cu和Zn的含量没有显著变化。施用污泥量小于25%的土壤中,污泥没有增加重金属的可利用性和移动性。除了Cu,油菜根际土壤中DTPA提取态Zn,Cd,Ni的减少表明施用污泥的土壤中重金属的活化是非常有限的。     

Mobilisation of Cu,Mn and Zn in the soil solutions of barley rhizospheres

Summary We report data showing changes during the growing season in the concentrations of manganese, zinc and copper in the rhizosphere in soil solutions of barley plants. Substantial mobilisation of these trace elements occurred during the early stages of rhizosphere development so that soil solution concentrations increased three fold for the copper and zinc and fifteen fold for manganese.     

Potential errors caused by roots in analyses of rhizosphere soil

Roots contain high concentrations of many elements, and have the potential to interfere with measurements of chemical change in rhizosphere soil. To assess potential interferences, maize (Zea mays L.) roots (free of soil) and soil (free of roots) were extracted separately with several extractants commonly used to assess the status of soil nutrients. The maize roots were grown within filter envelopes which prevented direct contact with soil, but permitted passage of mineral nutrients and water from the adjacent soil. Water, ammonium acetate (pH 7), DTPA (pH 7.3), Morgan's solution (pH 4.8), and dilute HCl were used as extractants. Most elements were released readily into soluble forms from roots killed by freezing to lyse the cells. Significantly lower amounts were extracted from fresh roots, with the greatest differences between fresh and killed roots for the extractants H₂O and DTPA, which were the mildest in terms of acidity and salt concentration. Extraction of P from the fresh roots by H₂O and HCL was particularly low. Contamination of rhizosphere samples with root materials would almost certainly prevent the accurate measurement of water-soluble P, K, Mn, Zn, Cu, and Na in the slightly alkaline soil used in this experiment. Large errors would be likely also for P, Mn, and Cu extracted by ammonium acetate. The DTPA extractant is normally used only for micronutrient metals or heavy metals, and the small amounts of these elements released by roots should not contribute to significant error. With Morgan's solution, errors would likely be large only for P. Dilute HCl is a reasonably strong extractant for many elements in soil, and major errors from roots contained in rhizosphere samples are unlikely. The relatively high probability of errors in extractions of soluble elements from rhizosphere soil is unfortunate, because these elements are among the most readily available to plants and the most likely to be altered by the normal activities of roots.     

超积累植物东南景天根际可溶性有机质对土壤锌吸附解吸的影响

采用盆栽和模拟试验研究了锌超积累植物东南景天根际土壤可溶性有机质(DOM)对土壤锌吸附解吸的影响.结果表明: 采用矿山土壤种植锌超积累植物东南景天后,根际土壤水溶性锌和乙酸铵提取态锌含量显著降低,而非根际土壤有效锌无明显变化.与不种植东南景天的对照土壤相比,根际土壤pH值降低了0.3个单位,有机质和DOM含量分别提高13.6%和20.9%.东南景天根际DOM对土壤锌吸附解吸的影响方式和程度与土壤类型有关.与对照DOM相比,在矿山土、黄筋泥和黄松土中,添加根际DOM使锌的最大吸附量(X_m)分别降低了17.8%、21.9%和27.7%,而非根际DOM对锌的最大吸附量没有影响.在3种供试土壤中添加根际DOM促进了锌的解吸,其中以黄松土最明显.东南景天根际DOM能降低土壤锌吸附容量,减少土壤对锌的吸附,促进吸附态锌的解吸,从而提高了根际锌的生物有效性.     

Effect of rhizosphere pH on the availability and uptake of Fe,Mn and Zn

In pot experiments the relationships between rhizosphere pH, the extractable levels of Fe, Mn and Zn in the soil and their uptake into the roots and shoots of dwarf French beans have been studied. Variations in rhizosphere pH were induced by applying three different sources of nitrogencholine phosphate, ammonium phosphate and calcium nitrate-to an initially homogenised soil (preadjusted to either pH 7 or 8). The rhizosphere pH was found to be significantly lower following the application of either ammonium or choline phosphates and to be increased by calcium nitrate treatment.The Fe and Zn contents of both shoot and root were inversely proportional to rhizosphere pH. The Mn contents also increased with decreasing pH but a sharp increase was apparent below pH 5.5. The shoot Fe, Zn and Mn content were significantly correlated with the extractable levels determined in the rhizosphere and non-rhizosphere soil.This paper forms part of the Ph. D. thesis submitted by A.N.S. to the University of Wales in 1977.     

Production of organic acids by soil micro-organisms

Summary Micro-organisms of the rhizosphere soils in respect of four plant speciesviz sesbania, cowpea, sorghum, and finger millet and those of the non-rhizosphere soil (control) maintained under pot culture conditions were incubated for a fortnight in liquid media containing glucose and the relative amounts and nature of the organic acids produced were investigated by pH changes and chromatographic studies respectively. Micro-organisms with the rhizosphere of all the four plant species produced greater amounts of acids than those of the non-rhizosphere soil. Rhizosphere micro-organisms with legumes produced greater amounts of acids than those with non-legumes. However no qualitative differences were observed in the organic acids produced by the rhizosphere micro-organisms of the four plant species investigated as well as those of non-rhizosphere soil.     

黄土丘陵区不同植被根际土壤微量元素含量特征

为了解黄土高原不同植被土壤微量元素的根际效应,分析了该地区柠条、沙棘、沙打旺、柳枝稷、阿尔泰狗娃花和茵陈蒿6种植被根际与非根际土壤中有机碳、全氮、Mn、Cu、Fe、Zn含量.结果表明： 6种植被中,柠条、阿尔泰狗娃花和茵陈蒿根际土壤的有机碳、全氮含量高于非根际土壤;除柠条和沙棘外,其余4种植被非根际土壤pH值均显著高于根际土壤.6种植被根际土壤有效Mn含量均低于非根际;柠条、沙打旺和柳枝稷根际有效Cu含量显著高于非根际,表现出强烈根际富集现象.除沙打旺外,其他5种植被根际有效Fe含量均略高于非根际.沙打旺、柳枝稷、茵陈蒿和阿尔泰狗娃花表现出强烈的有效Zn根际富集现象.根际与非根际土壤有机碳、全氮与有效Mn、有效Zn,以及有效Mn与有效Zn呈极显著正相关.根际土壤pH值与有效Mn和有效Zn呈显著负相关.由于不同植物根系的生长特征、根际pH值及微生物种类等的差异,不同植被根际的微量元素含量不同,茵陈蒿根际4种微量元素含量高于其他植被.     

施用韩国假单胞菌(Pseudomonas koreensis)CLP-7对连作烟田土壤质量及微生物群落功能多样性的影响

针对目前连作导至植烟土壤酸化和根茎病害发生重的问题,田间条件下,研究了嗜酸性韩国假单胞Pseudomonas koreensis CLP-7对酸化植烟土壤pH、养分、酶活性及微生物群落功能多样性的影响。结果表明:CLP-7可以提高酸化植烟土壤pH;同时,CLP-7显著提高根际土壤速效钾、有效磷、铵态氮、硝态氮和有机质含量,其中有机质含量增加最显著;根际土壤脲酶、蔗糖酶活性呈上升趋势且均高于未施菌土壤,过氧化氢酶活性变化不明显。CLP-7处理不同时间的土壤微生物代谢多样性指数表现出明显的差异,施菌30 d时Shannon指数、Simpson指数,Richness及McIntosh指数均达到最高;所有处理的Pielou指数变化较小。微生物碳代谢主成分分析结果显示,施菌30 d时,根际土壤微生物对碳源利用与其他处理差异显著,主要为羧酸类和糖类碳源;随着施菌时间增加,微生物对氨基酸类、羧酸类、双亲化合物类、聚合物类和糖类利用率明显提高,说明CLP-7有利于提高连作烟田根际土壤微生物对碳源的利用能力。综上所述,P.koreensis CLP-7能够明显提高土壤pH、土壤酶活性,增加土壤养分含量和土壤微生物群落功能多样性,进而改善酸化植烟土壤质量,在微生物修复酸化土壤和减轻根茎病害发生的烟草绿色防控中具有较大的应用潜力。     

Development of specific rhizosphere bacterial communities in relation to plant species, nutrition and soil type

Rhizosphere microbial communities are important for plant nutrition and plant health. Using the culture-independent method of PCR-DGGE of 16S rDNA for community analyses, we conducted several experiments to investigate the importance of pH, soil type, soil amendment, nutritional status of the plant, plant species and plant age on the structure of the bacterial community in the rhizosphere. At the same time, we assessed the spatial variability of bacterial communities in different root zone locations. Our results showed that the bacterial community structure is influenced by soil pH and type of P fertilization. In a short-term experiment (15–22 days) with cucumber and barley growing in a N deficient or a P deficient soil, the bacterial community structure in the rhizosphere was affected by soil type and fertilization but not by plant species. In a 7.5-week experiment with three plant species (chickpea, canola, Sudan grass) growing in three different soils (a sand, a loam and a clay), the complex interactions between soil and plant effects on the rhizosphere community were apparent. In the sand and the loam, the three plant species had distinct rhizosphere communities while in the clay soil the rhizosphere community structures of canola and Sudan grass were similar and differed from those of chickpea. In all soils, the rhizosphere community structures of the root tip were different from those in the mature root zone. In white lupin, the bacterial community structure of the non-cluster roots differed from those of the cluster roots. As plants matured, different cluster root age classes (young, mature, old) had distinct rhizosphere communities. We conclude that many different factors will contribute to shaping the species composition in the rhizosphere, but that the plant itself exerts a highly selective effect that is at least as great as that of the soil. Root exudate amount and composition are the key drivers for the differences in community structure observed in this study.     

Studies on soil rhizosphere: speciation and availability of Cd

ABSTRACT

The rhizosphere soils of two durum wheat (Triticum turgidum var. durum L.) cultivars Kyle and Areola grown in two selected soils of southern Saskatchewan were collected both at 2-week and 7-week plant growth stages. The cadmium availability index (CAI), determined as M NH₄CI-extractable Cd, pH and the distribution of the particulate- bound Cd species of the soils were carried out and the data were discussed in comparison with those of the corresponding bulk soil. At the 2-week growth stage, the pH of the rhizosphere soil was less than that of the corresponding bulk soil and the CAI values were higher in the rhizosphere soil, indicating that more Cd was complexed with the low-molecular-weight organic acids (LMWOAs) at the soil-root interface and was extractable by M NH₄CI. Compared with the bulk soils, the CAI values were 2–9 times higher in the soil rhizosphere of the plots fertilized with Idaho monoammonium phosphate fertilizer at 2-week growth stage, which is attributed to the combined effects of the Cd introduced into the soil rhizosphere from the fertilizer (Cd content of the fertilizer was 144 mg kg^?1) and complexation reactions of phosphate and LMWOAs with soil Cd. At 7-week plant growth stage, such differences were not observed. The increased amounts of carbonate-bound and metal-organic complex-bound Cd species of the rhizosphere soils are due to the increased amounts of carbonate, a product of plant respiration, and the LMWOAs at the soil-root interface, respectively. Simple correlation analysis of the data showed that the CAI of the rhizosphere soils of the control plots correlated at least two orders of magnitude better with the metal-organic complex-bound Cd whereas the CAI of the rhizosphere soils treated with Idaho phosphate correlated better with carbonate-bound Cd species in comparison to other species.     

Phytoextraction of cadmium and zinc by Salix from soil historically amended with sewage sludge

Short rotation coppice (SRC) such as Salix spp. can be grown as an energy crop and offers some potential for economic and practical phytoextraction of marginally contaminated arable soil. This study tested various soil amendments intended to increase soil metal availability to Salix, investigated the distribution of metal between different tree fractions and assessed the viability of phytoextraction using SRC on arable soils. Several Salix genotypes were grown in field trials over 4 years. Cd and Zn concentrations were generally ranked in the order leaves > bark > wood. Metal concentrations in wood increased towards the top of the willow stems, whereas concentrations in leaves showed the opposite trend. None of the amendments significantly increased uptake of Zn by willow. However, in response to a range of soil HCl treatments, mean Cd concentrations in stems and leaves were 112% and 130% of control values. Data from the current experiment, and previous studies, were combined to develop a predictive model of Cd and Zn stem uptake by Salix. The minimum biological concentration factor (BCF) required to achieve a prescribed soil metal target was also calculated based on typical proportions of bioavailable Cd in sludge-amended soils for a 25-year Salix rotation. The best Salix genotypes investigated achieved less than 20% of the uptake rate required to remove one third of the soil Cd content (equivalent to the average isotopically exchangeable Cd fraction in soils at the study site).     

Root traits as tools for creating phosphorus efficient crop varieties

This paper provides a brief assessment of the genetic variation in root properties (root morphology, including root hairs), mycorrhizal symbiosis, uptake kinetics parameters and root-induced changes (pH, organic acids and acid phosphatase) in the rhizosphere of various crop species and their genotypes and then briefly discusses the opportunities and challenges of using such knowledge for enhancing P efficiency of future crop genotypes by genetic means. Wide genotypic variation and heritability of root morphology, root hair length and density and thereby P acquisition provide opportunities for selection and breeding for root characteristics for increasing P acquisition. The progress is challenged by the concerns of high carbon cost of larger root systems and by the lack of cost effective methods to determine root length of a large number of genotypes under field conditions. The carbon cost of root hairs is low. Furthermore, low cost methods now exist to compare root hair formation of field grown genotypes. The development and application of sophisticated methods has advanced our knowledge on the role of mycorrhizal symbiosis in P acquisition and also on the molecular basis of fungi and plant interactions. However, extensive studies to explore genotypic variation in mycorrhizal responsiveness are rare, which makes it difficult to assess, how mycorrhizal symbiosis can be manipulated through breeding efforts. The promising variation found in P uptake kinetics parameters of crop genotypes in few studies indicates that more genotypes may be screened by relatively simple nutrient solution culture techniques. The genetic manipulation of the overall differences in cation-anion uptake, which is the main cause of rhizosphere pH change, may be difficult. For manipulation of rhizosphere pH, agronomic measures such as applications of ammonium or nitrate fertilisers may be more useful than breeding approaches. Also it seems difficult to assess what kind of genetic analysis should be performed to support the breeding efforts. Phosphorus mobilisation effect of pH depends on soil P compounds, therefore will differ with soil type. Both the enhanced release of organic acids and higher acid phosphatase activity in the rhizosphere may be useful for increasing P acquisition from inorganic and organic P pools, respectively. Modification of these traits by genetic means should be considered. For successful breeding programmes, the role of various root traits needs to be targeted in an integrated manner and then methods need to be developed for studying their importance under natural soil conditions, so that the genotypic variation can be explored and their ecological significance in P acquisition can be established.     

A new approach for rhizosphere research by X-ray microanalysis of microliter soil solutions

Lolium perenne growing with high root density on a fine nylon mesh (Kuchenbuch and Jungk, 1982) caused the development of element gradients in the rhizosphere below the mesh. Micro-liter soil solutions from 2-mg soil samples were sprayed onto Formvar-coated grids and analyzed by X-ray microanalysis in a transmission electron microscope. The results were comparable to those obtained by flame photometry and atomic absorption spectrometry (AAS) of conventional soil solutions from 1 g soil. X-ray microanalysis of micro-soil solutions allows the application of different extraction procedures to even small amounts of soil usually available from rhizosphere experiments. Information about soil buffering characteristics in the rhizosphere can thus be obtained. Aluminum accumulation in the rhizosphere of small segments of single Picea abies fine roots grown in undisturbed natural forest soil could be detected with this technique.     

Cd、Zn、Pb及其相互作用对烟草、小麦的影响

烟草对Cd、Zn、Pb是一种敏感性作物,Cd、Zn、Pb对烟草的影响比对小安、水稻都大。 Cd、Zn、Pb在烟草各器官中的累积随土壤Cd、Zn、Pb浓度的增高而增大。 Cd、Zn、Pb在烟草各器官中含量的次序为:茎叶>根>籽粒。它们在根中受阻,而较易转移到茎叶和籽粒中。 烟草对Pb的吸收比对Zn和Cd明显地低。Pb仍然是一种低吸收性元素。 土壤Zn增加,减低了烟草对Cd的吸收,而土壤Pb的增加,则促进了烟草对Cd的吸收。     

根际酸化作用对杨树无性系磷营养效率的影响

应用土培盆栽技术,研究了杨树P营养效率的无性系差异与根际酸化作用的关系。P处理设4个水平,分别为0、40、80和120mg·kg^-1(P2O5),每个处理重复3次。各用土40kg,随机排列．结果表明,在缺P胁迫下,P营养高效型无性系S17、S19和105的根际pH值明显降低,最大为1．32个单位。降幅均在10％以上;而P营养低效型无性系106、797、I-69、1388和3244的根际pH值的降低则很小。最大的仅为0．21个单位,降幅均在2．5％以下。高效型无性系的根际pH值。随缺P胁迫的增加而逐渐降低。一旦缺P胁迫缓和,根际pH值随之增高,低效型无性系则不具有这种反应机理。S17、S19和105在缺P胁迫环境下,根际有效磷分别为2．64、3．27和3．28mg·kg^-1,根际有效磷的积累率均在60％以上;而无性系106、797、I-69、1388和3244的根际有效磷均不足2．00mg·kg^-1,根际有效磷的积累率不足10％。高效型无性系在缺P胁迫环境下对P的吸收量显著高于低效型无性系,统计分析也证明,缺P胁迫条件下。无性系根际有效磷的积累与根际pH值降低呈显著相关。说明根际酸化作用是根际有效磷增加的原因。     

Effect of ferritin overexpression in tobacco on the structure of bacterial and pseudomonad communities associated with the roots

The genetic structures of total bacterial and pseudomonad communities were characterized in rhizosphere soil and rhizoplane+root tissues of tobacco wild type and a ferritin overexpressor transgenic line (P6) by a cultivation-independent method using directly extracted DNA at the end of three consecutive plant cultures. The structure of total bacterial communities was characterized by automated ribosomal intergenic spacer analysis (A-RISA), and that of pseudomonad communities was characterized by PCR-restriction fragment length polymorphism (PCR-RFLP) from DNA amplified with specific primers. The structure of total bacterial communities was significantly modified in the rhizosphere soil by the overaccumulation of iron in the tobacco transgenic P6 line at the first culture, to a lesser extent at the second culture, and not at all at the third culture. No significant difference was recorded between the total communities associated with the roots (rhizoplane+root tissues) of the two plant genotypes in any of the cultures. In contrast, the difference in pseudomonad structure between the two plant genotypes increased with successive culture at the root level, but was not detected at a significant level in the rhizosphere soil. The impact of iron overaccumulation by the tobacco transgenic P6 line on pseudomonads supports previous findings on the importance of iron competition among fluorescent pseudomonads.     

Variation in acquisition of soil phosphorus among wheat and barley genotypes

To assess the extent of variation in phosphorus acquisition efficiency of some winter wheat (Triticum aestivum L.), winter and spring barley (Hordeum vulgare L.) genotypes, depletion of inorganic phosphorus (P) extractable with 0.5 M NaHCO₃ (NaHCO₃-P_i) from the rhizosphere soil was studied. Nutrients supply, rhizosphere soil pH and soil water content was kept equal for all the genotypes with the aim to reduce the confounding variation due to these factors. The experimental set up implied that no difference in the relative growth rates, nitrogen, potassium and calcium content of shoot dry matter occurred among the genotypes.The winter wheat, winter barley and spring barley genotypes differed significantly (p>0.05) in their efficiency to acquire NaHCO₃-P_i from the rhizosphere soil. The efficiency of the winter wheat genotypes to acquire NaHCO₃-P_i from rhizosphere soil ranked Kraka > Gawain > Foreman > Sleipner = Obelisk > Kosack > Pepital > Arum. Winter wheat genotypes differed in extent of P depletion profiles in the rhizosphere, indicating variation in root hair length. The winter barley and spring barley genotypes also showed significant differences in their P depletion profiles near roots. The efficiency of the winter barley genotypes to acquire soil P in the rhizosphere ranked Hamu > Frost > Marinka > Astrid > Clarine = Angora. The efficiency of spring barley genotypes to acquire NaHCO₃-P_i in the rhizosphere ranked Canut > Etna Riga > Digger > Peel > Semal > Alexis. The rhizosphere pH remained unchanged, suggesting that additional mechanisms such as root hair formation and root exudates play a significant role in causing variation in P acquisition among the genotypes.     

Plant genotype, micronutrient fertilization and take-all infection influence bacterial populations in the rhizosphere of wheat

The relationship between micronutrient efficiency of four wheat (Triticum aestivum L.) genotypes, tolerance to take-all disease (caused by Gaeumannomyces graminis (Sacc.) Arx and Olivier var. tritici Walker), and bacterial populations in the rhizosphere was tested in soil fertilized differentially with Zn and Mn. Plant growth was reduced by Mn or Zn deficiency and also by take-all. There was an inverse relationship between micronutrient efficiency of wheat genotypes when grown in deficient soils and the length of take-all lesions on roots (efficient genotypes had shorter lesions than inefficient ones). In comparison to the rhizosphere of control plants of genotypes Aroona and C8MM receiving sufficient Mn and Zn, the total numbers of bacterial cfu (colony forming units) were greater in the rhizosphere of Zn-efficient genotype Aroona under Zn deficiency and in Mn-efficient genotype C8MM under Mn deficiency. These effects were not observed in other genotypes. Take-all decreased the number of bacterial cfu in the rhizosphere of fully-fertilized plants but not of those subjected to either Mn or Zn deficiency. In contrast, the Zn deficiency treatment acted synergistically with take-all to increase the number of fluorescent pseudomonads in the rhizosphere. Although numbers of Mn-oxidising and Mn-reducing bacteria were generally low, take-all disease increased the number of Mn reducers in the rhizosphere of Mn-efficient genotypes Aroona and C8MM. Under Mn-deficiency conditions, the number of Mn reducers in the rhizosphere increased in Aroona but not in C8MM wheat. The results suggest that bacterial microflora may play a role in the expression of Mn and Zn efficiency and tolerance to take-all in some wheat genotypes.     

Arsenic in the rhizosphere soil solution of ferns

The aim of this study was to explore the evidence of arsenic hyperaccumulation in plant rhizosphere solutions. Six common fern plants were selected and grown in three types of substrate: arsenic (As) -tailings, As-spiked soil, and soil-As-tailing composites. A rhizobox was designed with an in-situ collection of soil solutions to analyze changes in the As concentration and valence as well as the pH, dissolved organic carbon (DOC) and total nitrogen (TN). Arsenite composed less than 20% of the total As, and As depletion was consistent with N depletion in the rhizosphere solutions of the various treatments. The As concentrations in the rhizosphere and non-rhizosphere solutions in the presence of plants were lower than in the respective controls without plants, except for in the As-spiked soils. The DOC concentrations were invariably higher in the rhizosphere versus non-rhizosphere solutions from the various plants; however, no significant increase in the DOC content was observed in Pteris vittata, in which only a slight decrease in pH appeared in the rhizosphere compared to non-rhizosphere solutions. The results showed that As reduction by plant roots was limited, acidification-induced solubilization was not the mechanism for As hyperaccumulation.