堆肥缓解土壤镉的植物毒性: 对白菜生长的影响

The growth performance of pakchoi (Brassica chinensis L.) in relation to soil cadmium (Cd) fractionations was investigated to evaluate the remediating effect of poultry manure compost on Cd-contaminated soil. A yellow-brown soil (Alfisol) treated with various levels of Cd (0–50 mg Cd kg-1 soil) was amended with increasing amounts of compost from 0 to 120 g kg-1 . Compost application transformed 47.8%–69.8% of soluble/exchangeable Cd to the organic-bound fraction, and consequently decreased Cd uptake of pakchoi by 56.2%–62.5% as compared with unamended soil. Alleviation of Cd bioavailability by compost was attributed primarily to the increase of soil pH and complexation of Cd by organic matter including dissolved organic matter. In general, the improvement of pakchoi performance was more pronounced in higher Cd-contaminated soil. Addition of large amount of compost also favored the anti-oxidative capability of pakchoi against Cd toxicity. This low cost remediation method seems to be very effective in the restoration of Cd-contaminated soils.     

基于小白菜Cd吸收推算土壤Cd安全阈值

Cadmium(Cd), a common toxic heavy metal in soil, has relatively high bioavailability, which seriously threatens agricultural products. In this study, 8 different soils with contrasting soil properties were collected from different regions in China to investigate the Cd transfer coefficient from soil to Chinese cabbage(Brassica chinensis L.) and the threshold levels of Cd in soils for production of Chinese cabbage according to the food safety standard for Cd. Exogenous Cd(0–4 mg kg~(-1)) was added to the soils and equilibrated for 2 weeks before Chinese cabbage was grown under greenhouse conditions. The influence of soil properties on the relationship between soil and cabbage Cd concentrations was investigated. The results showed that Cd concentration in the edible part of Chinese cabbage increased linearly with soil Cd concentration in 5 soils, but showed a curvilinear pattern with a plateau at the highest dose of exogenous Cd in the other 3 soils. The Cd transfer coefficient from soil to plant varied significantly among the different soils and decreased with increasing soil p H from 4.7 to 7.5. However, further increase in soil pH to 8.0 resulted in a significant decrease in the Cd transfer coefficient. According to the measured Cd transfer coefficient and by reference to the National Food Safety Standards of China, the safety threshold of Cd concentration in soil was predicted to be between 0.12 and 1.7 mg kg~(-1) for the tested soils. The predicted threshold values were higher than the current soil quality standard for Cd in 5 soils, but lower than the standard in the other 3 soils. Regression analysis showed a significant positive relationship between the predicted soil Cd safety threshold value and soil p H in combination with soil organic matter or clay content.     

石灰与磷肥可以降低华南5种常见蔬菜对镉的吸收量

A pot experiment was conducted in artificially Cd-contaminated (5 mg Cd kg-1) soils to investigate the feasibility of using lime (3 g kg-1) or phosphate (80 mg P kg-1) to mitigate uptake of Cd by vegetables.Five common vegetables in South China,including lettuce (Lactuca sativa L.var.ramosa Hort.),Chinese cabbage [Brassica rapa L.subsp.Chinensis (L.) var.parachinensis (L.H.Bailey) Hanect],Chinese broccoli (Brassica oleracea L.var.albiflora Kuntze),white amaranth (Amaranthus tricolor L.) and purslane (Amaranthus viridis L.),were grown in the soils and harvested after 60 d.The results showed that liming significantly reduced Cd uptake by most vegetables by 40%-50% (or a maximum of 70%),mainly due to immobilization of soil Cd.Increased availability of Ca in the soil might also contribute to the Cd uptake reduction as a result of absorption competition between Ca and Cd.Liming caused biomass reduction in white amaranth and purslane,but did not influence growth of the other vegetables.Phosphate decreased Cd uptake by vegetables by 12%-23%.Compared with lime,phosphate decreased,to a smaller extent,the bioavailability of Cd in most cases.Phosphate markedly promoted growth of vegetables.Changes in soil chemistry by adding lime or phosphate did not markedly influence nutrient uptake of vegetables except that lime increased Ca content and phosphate increased P content in shoots of the vegetables.The results suggested that a proper application of lime could be effective in reducing Cd uptake of vegetables,and phosphate could promote growth of the vegetables as well as alleviate the toxicity of Cd.     

镉修复土壤中磷酸盐对镉的形态转化和植物吸收的影响

Phosphate was found to have neither influence on Cd transformation nor effect on plant Cd uptake in three Cd-amended upland soils.However,on submerged red earth,high phosphate dressing inhibited the transformation of Cd from exchangeable fraction to other lower-available ones.Cadmium uptake by rice plants increased simultaneously with increasing phosphate supply though plant resistance to Cd also increased at high phosphate level.Application of phosphate as an amendment for Cd-contaminated soil was therefore not recommended in view of the increasing influx of Cd into food chain especially on flooded soils.     

作物品种和化学固定剂对玉米谷物中镉和锌积累的影响

A field experiment was conducted to investigate the effects of soil amendments(lime,nano-Si foliar solution and used diatomite) on the growth and metal uptake of three maize(Zea mays L.) cultivars grown in a Cd and Zn-contaminated acidic soil.The addition of lime significantly increased the maize grain yields and decreased the concentrations of Zn and Cd in the grains and shoots of maize when compared with the control.Among the three maize cultivars,Yunshi-5 accumulated the lowest amounts of Cd and Zn in the grain.The concentrations of Zn and Cd in the grain of Yunshi-5 conformed to the Chinese feed standards.These data revealed that a combination of low metal-accumulating maize and chemical fixation could effectively provide a barrier to prevent metals from entering the human food chain.     

土壤－水分－植物体系中铅和镉的交互作用及其机制: Ⅱ. 根际中Pb-Cd的交互作用

The interaction of Pb-Cd can be observed not only in the uptake process of elements by plants and in their influence on the growth,but also in rhizosphere.The changes in extractable Cd and Pb concentrations in the rhizosphere soil of rice plants ,root exudates from wheat and wheat plant and their complexing capacity,with Pa and Cd were investigated under different Pb and Cd treatments.Results showed that the concentration of extractable Cd in the rhizosphere of rice in red soil was markedly increased by Pb-Cd interaction,It increased by 56% in the treatment with Pb and Cd added against that in the treatment with only Cd added in soil . The considerable differences in both composition and amount of root exudate from wheat and rice were found among different treatments.Pb and Cd might be complexed by root exudates ,The concentrations of free Pb and Cd in the solution were increased markedly by adding root exudate from wheat and decreased by that from rice due to Pd-Cd interaction.The distribution patterns of Pb and Cd in roots were affected by Pb-Cd interaction,which accelerated transport of Pb into internal tissue and retarded accumulation of Cd in external tissue.     

植物汁液化学组成及其与菜心抗性和Cd累积的关系

Sap mixtures of the xylem, phloem, and vacuoles from low and high Cd accumulator varieties of Brassica parachinensis L. H. Bailey were analyzed under Cd stress to understand the biochemical mechanisms of Cd accumulation in plants. Low Cd accumulator （‘Teqing-60＇） and high Cd accumulator （‘Chixin-2＇） plants were grown in Cd-treated soil in pots in a greenhouse. Percentage of cell wall-bound Cd was estimated, pH level and the concentrations of amino acids, organic acids, anions, and cations in both stem and root saps were determined for the calculation of Cd speciations using the computer program GEOCHEM. The results showed that ‘Teqing-60＇ had a significantly higher （P ≤ 0.05） percentage of Cd bound to cell walls in roots and a significantly higher （P ≤ 0.05） pH in the root sap. ‘Teqing-60＇ also contained a higher concentration of total amino acids in both roots and stems compared with the high Cd accumulator variety ‘Chixin- 2＇. However, between the two accumulators, for stems and for roots, there were no significant differences in non-amino organic acids. GEOCHEM calculations showed that Cd in the root sap of ‘Teqing-60＇ mainly combined with amino acids, especially alanine. Compared with ‘Chixin-2＇, in the root sap of ‘Teqing-60＇, much lower levels of Cd as free ions or bound to simple ligands were found, indicating that less ‘Teqing-60＇ is transferred to stems and leaves. Cadmium activity in the shoot sap of ‘Teqing-60＇ was much lower than that in ‘Chixin-2＇; therefore, ‘Teqing-60＇ exhibited higher Cd resistance. However, direct determination of the Cd complexes from xylem and phloem sap is needed to verify these results.     

根瘤菌存在下土壤胶体和矿物对镉的吸附

Experiments were conducted to study the adsorption of Cd on two soil collids(red soil and yellowbrown soil) and three variable-charge minerals (goethite,noncrystalline Fe oxide and kaolin) in the absence and presence of rhizobia.The tested strain Rhizobium fredii C6,tolerant to 0.8 mmol L^-1 Cd,was selected from 30 rhizobial strains.Results showed that the isotherms for the adsorption of Cd by examined soil colloids and minerals in the presence of rhizobia could be described by Langmuir equation.Within the range of the numbers of rhizobial cells studied,the amount of Cd adsorbed by each system increased with increasing rhizobial cells,Greater increases for the adsorption of Cd were found in red soil and kaolin systems.Rhizobia influence on the adsorption of Cd by examined soil colloids and minerals was different from that on the adsorption of Cu.The presence of rhizobia increased the adsorption affinity of soil colloids and minerals for Cd,particularly for the goethite and kaolin systems.The discrepancies in the influence of rhizobia on the adsorbability and affinity of selected soil colloids and minerals for Cd suggesed the different interactions of rhizobia with various soil components.It is assumed that bacterial biomass plays an important role in controlling the mobility and bioavailability of Cd in soils with kaolinite and goethite as the major colloidal compnents,such as in variable-charge soil.     

三叶草对污染土壤中芘的去除研究

Phytoremediation has been used as an emerging technology for remediation of soil contamination with polycyclic aromatic hydrocarbons (PAHs), ubiquitous persistent environmental pollutants derived from natural and anthropogenic processes, in the last decade. In this study, a pot experiment was conducted to investigate the potential of phytoremediation of pyrene from spiked soils planted with white clover (Trifolium repens) in the greenhouse with a series of pyrene concentrations ranging from 4.22 to 365.38 mg kg^-1. The results showed that growth of white clover on pyrenecontaminated soils was not affected. The removal of pyrene from the spiked soils planted with white clover was obviously higher than that from the unplanted soils. At the end of the experiment (60 d), the average removal ratio of pyrene in the spiked soils with white clover was 77%, which was 31% and 57% higher than those of the controls with or without micobes, respectively. Both roots and shoots of white clover took up pyrene from the spiked soils and pyrene uptake increased with the soil pyrene concentration. However, the plant-enhanced dissipation of soil pyrene may be the result of plant-promoted microbial degradation and direct uptake and accumulation of pyrene by white clover were only a small part of the pyrene dissipation. Bioconcentration factors of pyrene (BCFs, ratio of pyrene, on a dry weight basis, in the plant to that in the soil) tended to decrease with increase in the residual soil pyrene concentration. Therefore, removal of pyrene in the contaminated soils was feasible using white clove.     

间作两个生态气候区的龙葵和少花龙葵促进茄子幼苗生长且降低镉吸收

Soil contamination by heavy metals is a serious environmental problem worldwide,and reduction of heavy metal accumulation in vegetables grown on contaminated land is a matter of urgency.A pot experiment was conducted to study the effects of intercropping with the Cd hyperaccumulators Solanum nigrum and Solanum photeinocarpum from two ecoclimatic regions,Ya'an and Chengdu,Sichuan Province,China,on the growth and cadmium (Cd) uptake of eggplant (Solanum melongena L.).The biomass,photosynthetic pigment contents,and activities of antioxidant enzymes of eggplant were enhanced by intercropping.The biomass of eggplant was the highest after intercropping with S.photeinocarpum from Ya'an,but did not differ significantly from that after intercropping with S.nigrum from Chengdu.The shoot Cd content of eggplant was significantly reduced by intercropping with the hyperaccumulators,which ranked as follows:S.nigrum from Chengdu ＞ S.nigrum from Ya'an ＞ S.photeinocarpum from Chengdu ＞ S.photeinocarpum from Ya'an,with the decreases being 19.60％,14.36％,9.66％,and 6.42％,respectively,as compared with the control.The lowest shoot Cd content and translocation factor of eggplant were attained after intercropping with S.nigrum from Chengdu.Therefore,it was feasible to intercrop eggplant with S.nigrum and S.photeinocarpum on Cd-contaminated soil.     

太湖地区不同土壤-森林生态系统中林木生长对微量元素的反应及反馈效应

Vector analysis technique and ecological sequential comparison methods were adopted to study tree growth response to the micronutrients,B,Mo,Cu,Zn,Fe,and Mn,in soils derived from various parent materials in the forest area of Tailhu Lake region in southeast China,The results showed that the dry weight of individual current-year needle of Chinese fir(Cunninghamia lanceolata) grown on the soil derived from granite parent rock was increased by 8% and 13% in comparison with that grown on the soils derived from sandstone and ash-tuff parent rock,respectively.And such dry weight of loblolly pine (Pinus taeda) grown on the soil derived from sandstone parent rock was increased by 21% in comparison with that on the soil derived from ash-tuff parent rock.One of the reasons for those results was that micronutrients content in the soil derived from ash-tuff parent rock were not sufficient to meet the requirement of the growth of Chinese fir and loblolly pine ,i.e.,micronutrients in soil were deficient and/or induced defiient.The amounts of Cu,Zn,Fe,and Mn uptake by Chinses fir and loblolly pine were in agreement with the contents of available micronutrients in soil respectively,except for B and Mo.Meanwhile,there might exist and “antagonism“ between the uptake of B versus Mo by trees,although more studies are needed to confirm it .Regression analysis indicated that amount of a soil available micronutrient was correlated to the type of parent material and its total amount in the forest floor, except for B.The F test identified that the correlation of each equation reached the significant level to different extents,respectively,The t test confirmed that amount of available forms was mainly depended on the type of parent material for Mo,Cu,Zn and Mn but on the forest floor for ,Fe,There was a feedback effect of forest stand on the amount of soil available micronutrients.The ability of accumulating available micronutrients in soil was better by the sawtooth oak(Quercus acutissima) stand than by the Chinese fir stand (except for B).The ability of accumulating available Zn,Fe,Mn and Mo in soil was better by the Chinese fir stand than by the loblolly pine stand ,while as for available B and Cu,by the latter was better than by the former,When discussing the efect of forest stand on the amount of soil available micronutrients,not only the amount of micronutrient in the forest floor and the parent materials but also the amount of micronutrient taken up by current-year needles have to be considered.     

镉在中性和碱性土柱中不同深度处的运移模拟

Human health has been potentially threatened by cadmium (Cd) contained in sewage irrigation water.Previous studies of Cd transport in soils were mainly conducted using small soil cores with pH values less than 6.The objectives of this study were to determine the parameters of the convection-dispersion equation (CDE) for Cd transport in relatively larger columns with neutral and alkaline soils,and to investigate the parameters' variability with depth.The soil columns were 50 cm in length and 12.5 cm in diameter.Ceramic suction lysimeters were buried at depths of 2.5,7.5,17.5,27.5,and 37.5 cm to abstract soil solution.Cd concentration in the soil solution samples were subsequently analyzed to obtain breakthrough curves (BTCs).Equilibrium and nonequilibrium models in CXTFIT program were used to estimate parameters of the CDE.The results suggested that both equilibrium and non-equilibrium models performed well in modeling Cd transport.The hydrodynamic dispersion coefficient (D) ranged from 0.18 to 10.70 cm 2 h 1,showing large differences among different depths.The retardation factor (R d) ranged from 25.4 to 54.7 and the standard deviation of R d value was lower than 30% of the mean value.Precipitation coefficient (R p) decreased consistently with increasing depth,varying from 1.000 × 10 10 to 0.661 h 1.Sensitivity tests showed that D was less sensitive than R d.These results would be helpful in understanding the transport and retention of Cd in non-acidic soils.     

外源铬对小白菜(Brassica chinensis L.)根际土壤微生物生物量和磷脂脂肪酸的剖面分布特征的影响

The effects of root activity on microbial response to cadmium （Cd） loading in the rhizosphere are not well understood. A pot experiment in greenhouse was conducted to investigate the effects of low Cd loading and root activity on microbial biomass and community structure in the rhizosphere of pakchoi （Brassica chinensis L.） on silty clay loam and silt loamy soil. Cd was added into soil as Cd（NO3）2 to reach concentrations ranging from 0.00 to 7.00 mg kg-1. The microbial biomass carbon （MBC） and community structure were affected by Cd concentration, root activity, and soil type. Lower Cd loading rates （〈 1.00 mg kg-1） stimulated the growth of pakchoi and microorganisms, but higher Cd concentrations inhibited the growth of microorganisms. The content of phospholipid fatty acids （PLFAs） was sensitive to increased Cd levels. MBC was linearly correlated with the total PLFAs. The content of general PLFAs in the fungi was positively correlated with the available Cd in the soil, whereas those in the bacteria and actinomycetes were negatively correlated with the available Cd in the soil. These results indicated that fungi were more resistant to Cd stress than bacteria or actinomycetes, and the latter was the most sensitive to Cd stress. Microbial biomass was more abundant in the rhizosphere than in the bulk soil. Root activity enhanced the growth of microorganisms and stabilized the microbial community structure in the rhizosphere. PLFA analysis was proven to be sensitive in detecting changes in the soil microbial community in response to Cd stress and root activity.     

上海郊区蔬菜田氮素流失的研

Nitrogen （N） leaching in vegetable fields from December 2002 to May 2003 with equal dressings of total N for a sequential rotation of Chinese flat cabbage （Brassica chinensis L. var. rosularis） and lettuce （Lactuca sativa L.） in a suburban major vegetable production base of Shanghai were examined using the lysimeter method to provide a scientific basis for rational utilization of nitrogen fertilizers so as to prevent nitrogen pollution of water resources. Results showed that leached N consisted mainly of nitrate N, which accounted for up to more than 90% of the total N loss and could contribute to groundwater pollution. Data also showed that by partly substituting chemical N （30%） in a basal dressing with equivalent N of refined organic fertilizer in the Chinese flat cabbage field, 64.5% of the leached nitrate N was reduced, while in the lettuce （Lactuca sativa L.） field, substituting 1/2 of the chemical N in a basal dressing and 1/3 of the chemical N in a top dressing with equivalent N of refined organic fertilizers reduced 46.6% of the leached nitrate N. In the twoyear sequential rotation system of Chinese flat cabbage and lettuce, nitrate-N leaching in the treatment with the highest amount of chemical fertilizer was up to 46.55 kg ha^-1, while treatment plots with the highest amount of organic fertilizer had only 17.58 kg ha^-1. Thus, partly substituting refined organic fertilizer for chemical nitrogen in the first two seasons has a great advantage of reducing nitrate-N leaching.     

草坪缓冲带限制斜坡上葡萄园敌草隆向地表水和地下水迁移效果的研究

Trichloroethylene (TCE), as one of the most common chlorinated organic compounds in soils and aquifers at many industrial sites, is carcinogenic and often recalcitrant in environment. TCE degradation in artificially contaminated soil samples was conducted using Fenton-like processes, i.e., by addition of excess hydrogen peroxide (H2O2 ). H 2 O 2 could directly oxidize TCE without addition of ferrous iron in contaminated soil. Under the optimal condition (H2O2 concentration of 300 mg kg 1 , pH at 5.0, and reaction time of 30 min), the removal efficiency of TCE in the soil was up to 92.3%. When the initial TCE concentration increased from 30 to 480 mg kg 1 in soil, the TCE removal rates varied from 89.2% to 86.6%; while the residual TCE in soil ranged from 2.28 to 47.57 mg kg 1 . Results from successive oxidations showed that the TCE removal rate with the TCE concentration of 180 mg kg 1 increased slightly from 91.6% to 96.2% as the number of successive oxidation cycle increased from one to four. Therefore, increasing the frequency of H2O2 oxidation was perhaps a feasible way to increase TCE removal rate for TCE-contaminated soil.     

Root cell walls and phytochelatins in low-cadmium cultivar of Brassica parachinensis

‘Lvbao-701’ is a cultivar of Chinese flowering cabbage(Brassica parachinensis) that exhibits low cadmium(Cd) accumulation and high Cd tolerance.In this study, this cultivar was compared with a high-Cd accumulating cultivar, ‘Chixin-4’, to characterize the mechanisms influencing Cd accumulation in B. parachinensis. Root cell walls were isolated by dissolving the cytoplasm with an organic solvent, and root Cd and phytochelatin(PC) contents were analyzed. In addition, a PC synthase gene fragment was cloned and expressed under Cd stress conditions. The proportions of Cd bound to root cell walls were higher in the ‘Lvbao-701’ plants(68.32%–76.80%) than in the ‘Chixin-4’ plants(35.36%–54.18%) after exposure to Cd stress. The proportions of Cd bound to root cell walls measured using cell walls isolated with a non-destructive method were higher than those obtained using a conventional method that required grinding and centrifugation. Exposure to Cd stress induced the PC production and resulted in higher PC contents in the ‘Lvbao-701’ plants than in the ‘Chixin-4’ plants. Cloning and expression analysis of a B. parachinensis PC synthase cDNA fragment indicated that PC synthase gene expression was induced by Cd and occurred mainly in the roots of both ‘Lvbao-701’ and ‘Chixin-4’ plants. However, the PC synthase gene expression level was higher in the‘Lvbao-701’ roots than in the ‘Chixin-4’ roots. Therefore, a higher abundance of Cd in the root cell walls of ‘Lvbao-701’ and up-regulated PC production in the roots are probably the main reasons why ‘Lvbao-701’ exhibits lower Cd translocation to the shoots and higher tolerance to Cd than ‘Chixin-4’.     

两个小麦品种对根际土壤中磷的吸收

Inorganic soil phosphorus extractable with sodium bicarbonate(NaHCO3-Pi),soil pH and root hairs length and density in the rhizosphere of two winter wheat cultivars (Tritium aestivum L.cv.Shichum,Sleipner)grown on a high pH Chinese silt loam(52.7 mg NaHCO3-Pikg^-1) and a Danish sandy loam(43.4mg NaHCO3-Pi kg^-1)wer studied to assess how these wheat cultivars differed in phosphorus uptake.The rhizosphere soil pH of two wheat cultivars grown on the two soils were fairly unchanged with increasing distrance from the roo surface.However the root hairs of Shichun were 2.1 times longer than those of Sleipner,Root surface area(RSA) of Shichun increased by 192% due to root hairs whereas root hairs of Sleipner increased RSA by 68% only.Hence the root system of Shichun was in contact with more soil than that of Sleipner,even though Sleipner had a longer root,Grown at the lower pH and level of NaHCO3-Pi in the Danish soil Shichun absorbed more inorganic phosphorus than Sleipner whereas at the higher pH and level of NaHCO3-Pi in the Chinese soil there was no phosphorus uptake difference between the two wheat culivars.     

接种菌根真菌影响青菜中铅镉的累积和迁移

Heavy metal(HM) contamination in soils is an environmental issue worldwide that threatens the quality and safety of crops and human health. A greenhouse experiment was carried out to investigate the growth, mycorrhizal colonization, and Pb and Cd accumulation of pakchoi(Brassica chinensis L. cv. Suzhou) in response to inoculation with three arbuscular mycorrhizal(AM) fungi(AMF), Funneliformis mosseae, Glomus versiforme, and Rhizophagus intraradices, aimed at exploring how AMF inoculation affected safe crop production by altering plant-soil interaction. The symbiotic relationship was well established between pakchoi and three AMF inocula even under Pb or Cd stress, where the colonization rates in the roots ranged from 24.5% to 38.5%. Compared with the non-inoculated plants, the shoot biomass of the inoculated plants increased by 8.7%–22.1% and 9.2%–24.3% in Pb and Cd addition treatments, respectively. Both glomalin-related soil protein(GRSP) and polyphosphate concentrations reduced as Pb or Cd concentration increased. Arbuscular mycorrhizal fungi inoculation significantly enhanced total absorbed Pb and Cd(except for a few samples) and increased the distribution ratio(root/shoot) in pakchoi at each Pb or Cd addition level. However, the three inocula significantly decreased Pb concentration in pakchoi shoots by 20.6%–67.5% in Pb addition treatments, and significantly reduced Cd concentration in the shoots of pakchoi in the Cd addition treatments(14.3%–54.1%), compared to the non-inoculated plants.Concentrations of Pb and Cd in the shoots of inoculated pakchois were all below the allowable limits of Chinese Food Safety Standard.The translocation factor of Pb or Cd increased significantly with increasing Pb or Cd addition levels, while there was no significant difference among the three AMF inocula at each metal addition level. Meanwhile, compared with the non-inoculated plants, AMF inocula significantly increased soil p H, electrical conductivity, and Pb or Cd concentrations in soil organic matter in the soils at the highest Pb or Cd dose after harvest of pakchoi, whereas the proportion of bioavailable Pb or Cd fraction declined in the AMF inoculated soil. Our study provided the first evidence that AM fungi colonized the roots of pakchoi and indicated the potential application of AMF in the safe production of vegetables in Pb or Cd contaminated soils.     

外生菌根诱导油松根际土壤重金属形态变化

To understand the role of ectomycorrhizas in improving the tolerance of its host to excessive heavy metals in soil, this study was conducted to exam the patterns of four fractions （the exchangeable, the carbonate-bound, the Fe-Mn oxide- bound and the organically bound） of both Cu and Cd in the rhizosphere of Chinese pine （Pinus tabulaeformis） seedlings grown in excessive Cu and Cd environment. The results showed that the speciation of Cu and Cd in the rhizosphere was significantly influenced by inoculation of ectomycorrhizal fungus Boletus edulis. Compared to the rhizosphere, the content of exchangeable Cu slightly decreased in the mycorrhizosphere of the seedlings grown in 166 and 400 mg kg^-1 Cu contaminated soil, whereas the exchangeable Cd in the mycorrhizosphere decreased remarkably to only 33% and to 60% that of the rhizosphere at 0.75 and 1.50 mg kg^-1 Cd levels, respectively. These indicate the potential capacity of mycorrhizas to alleviate the damage of heavy metals to the host plants by reducing the bioavailability of heavy metals in soil. Distribution of the 4 tested fractions of Cu and Cd at different contamination levels showed that there was a strong tendency of changing from loosely associated fractions to strongly associated fractions in the mycorrhizosphere. The most stable Cd fraction, organically bound Cd, was significantly larger in the mycorrhizosphere than in the rhizosphere at different Cd contamination levels. This phenomenon was also observed for Cu but the difference was not statistically significant.     

A conceptual framework and an empirical test of complementarity and facilitation with respect to phosphorus uptake by plant species mixtures

Plant species have different traits for mobilizing sparingly soluble phosphorus (P) resources,which could potentially lead to overyielding in P uptake by plant species mixtures compared to monocultures due to higher P uptake as a result of resource (P) partitioning and facilitation.However,there is circumstantial evidence at best for overyielding as a result of these mechanisms.Overyielding (the outcome) is easily confused with underlying mechanisms because of unclear definitions.We aimed to define a conceptual framework to separate outcome from underlying mechanisms and test it for facilitation and complementarity with respect to P acquisition by three plant species combinations grown on four soils.Our conceptual framework describes both mechanisms of complementarity and facilitation and outcomes (overyielding of mixtures or no overyielding) depending on the competitive ability of the species to uptake the mobilized P.Millet/chickpea mixtures were grown in pots on two calcareous soils mixed with calcium-bound P (CaP) and phytate P (PhyP).Cabbage/faba bean mixtures were grown on both acid and neutral soils mixed with P-coated iron (hydr)oxide (FeP) and PhyP.Wheat/maize mixtures were grown on all four soils.Rhizosphere carboxylate concentration and acid phosphatase activity (mechanisms) as well as plant P uptake and biomass (outcome) were determined for monocultures rhizosphere and species mixtures.Facilitation of P uptake occurred in millet/chickpea mixtures on one calcareous soil.We found no indications for P acquisition from different P sources,neither in millet/chickpea,nor in cabbage/faba bean mixtures.Cabbage and faba bean on the neutral soil differed in rhizosphere acid phosphatase activity and carboxylate concentration,but showed no overyielding.Wheat and maize,with similar root exudates,showed overyielding (the observed P uptake being 22%higher than the expected P uptake) on one calcareous soil.We concluded that although differences in plant physiological traits (root exudates) provide necessary conditions for complementarity and facilitation with respect to P uptake from different P sources,they do not necessarily result in increased P uptake by species mixtures,because of the relative competitive ability of the mixed species.