Human and ecological risks of metals in soils under different land-use types in an urban environment of Bangladesh

Trace metal contamination in soil is of great concern owing to its long persistence in the environment and toxicity to humans and other organisms.Concentrations of six potentially toxic trace metals,Cr,Ni,Cu,As,Cd,and Pb,in urban soils were measured in Dhaka City,Bangladesh.Soils from different land-use types,namely,agricultural field,park,playground,petrol station,metal workshop,brick field,burning sites,disposal sites of household waste,garment waste,electronic waste,and tannery wast,and construction waste demolishing sites,were investigated.The concentration ranges of Cr,Ni,Cu,As,Pb,and Cd in soils were 2.4–1258,8.3–1044,9.7–823,8.7–277,1.8–80,and 13–842 mg kg^-1,respectively.The concentrations of metals were subsequently used to establish hazard quotients(HQs)for the adult population.The metal HQs decreased in the order of As>Cr>Pb>Cd>Ni>Cu.Ingestion was the most vital exposure pathway of studied metals from soils followed by dermal contact and inhalation.The range of pollution load index(PLI)was 0.96–17,indicating severe contamination of soil by trace metals.Considering the comprehensive potential ecological risk(PER),soils from all land-use types showed considerable to very high ecological risks.The findings of this study revealed that in the urban area studied,soils of some land-use types were severely contaminated with trace metals.Thus,it is suggested that more attention should be paid to the potential health risks to the local inhabitants and ecological risk to the surrounding ecosystems.     

陇海铁路郑州—圃田段铁路旁土壤重金属污染

The pollution status and horizontal distribution of heavy metals (Ni, Pb, Cr, Zn, Cu, and Cd) in the soil on railroad side along the Zhengzhou-Putian section of Longxi-Haizhou Railroad were studied by collecting soil samples along a sampling section perpendicular to the railroad at the distances of 0, 10, 20, 30, 50, 100, 200, 300, and 500 m from the railroad edge. The concentrations of heavy metals in the sampling soils were higher than those of the control site. The concentrations of Pb, Zn, and Cd were found to be the highest in the soils at the railroad edge, and then decreased with increasing distance from the railroad. The highest concentrations of Ni, Cr, and Cu in soils were located at about 10-30 m from the railroad. Compared with the single factor pollution index (SFPI) of heavy metals calculated for the control site, the average SFPI from the sampling sites decreased in the order of Cr > Cd > Pb > Zn > Ni > Cu. There were notable negative correlations between the integral pollution index (IPI) of soil heavy metals at all sampling sites and the distances from the railroad. According to three IPIs calculated from the background values of heavy metals in och-aquic Cambisols, the heavy metal concentrations in the control soil, and the 2nd levels for soil heavy metals in GB15618-1995, the study area could be divided, based on the distances from the railroad, into four pollution zones: heavy pollution zone (0-10 m), medium pollution zone (10-50 m), slight pollution zone (50-100 m), and warning zone (100-500 m), respectively.     

土法炼锌区大气沉降Pb、Zn、Cd及其对土壤质量的影响

Dust emissions from smelters, as a major contributor to heavy metal contamination in soils, could severely influence soil quality. Downwind surface soils within 1.5 km of a zinc smelter, which was active for 10 years but ceased in 2000, in Magu Town, Guizhou Province, China were selected to examine Pb, Zn, and Cd concentrations and their fractionation along a distance gradient from a zinc smelter, and to study the possible effects of Pb, Zn, and Cd accumulation on soil microorganisms by comparing with a reference soil located at a downwind distance of 10 km from the zinc smelter. Soils within 1.5 km of the zinc smelter accumulated high levels of heavy metals Zn (508 mg kg-1), Pb (95.6 mg kg-1), and Cd (5.98 mg kg-1) with low ratios of Zn/Cd (59.1--115) and Pb/Cd (12.4--23.4). Composite pollution indices (CPIs) of surface soils (2.52--15.2) were 3 to 13 times higher than the reference soils. In metal accumulated soils, exchangeable plus carbonate-bound fractions accounted for more than 10% of the total Zn, Pb, and Cd. The saturation degree of metals (SDM) in soils within 1.5 km of the smelter (averaging 1.25) was six times higher than that of the reference soils (0.209). A smaller soil microbial biomass was found more frequently in metal accumulated soils (85.1--438 μg C g-1) than in reference soils (497 μg C g-1), and a negative correlation (P < 0.01) of soil microbial biomass carbon to organic carbon ratio (Cmic/Corg) with SDM was observed. Microbial consumption of carbon sources was more rapid in contaminated soils than in reference soils, and a shift in the substrate utilization pattern was apparent and was negatively correlated with SDM (R = -0.773, P < 0.01). Consequently, dust deposited Pb, Zn, and Cd in soils from zinc smelting were readily mobilized, and weredetrimental to soil quality mainly in respect of microbial biomass.     

施用液态家畜废物的水稻田土壤中硝酸盐异化还原成铵过程的研究

Most studies on dissimilatory nitrate reduction to ammonium (DNRA) in paddy soils were conducted in the laboratory and in situ studies are in need for better understanding of the DNRA process.In this study,in situ incubations of soil DNRA using 15 N tracer were carried out in paddy fields under conventional water (CW) and low water (LW) managements to explore the potential of soil DNRA after liquid cattle waste (LCW) application and to investigate the impacts of soil redox potential (Eh) and labile carbon on DNRA.DNRA rates ranged from 3.06 to 10.40 mg N kg 1 dry soil d 1,which accounted for 8.55%-12.36% and 3.88%-25.44% of consumption of added NO 3-15 N when Eh at 5 cm soil depth ranged from 230 to 414 mV and 225 to 65 mV,respectively.DNRA rates showed no significant difference in paddy soils under two water managements although soil Eh and/or dissolved organic carbon (DOC) were more favorable for DNRA in the paddy soil under CW management 1 d before,or 5 and 7 d after LCW application.Soil DNRA rates were negatively correlated with soil Eh (P < 0.05,n=5) but positively correlated with soil DOC (P < 0.05,n=5) in the paddy soil under LW management,while no significant correlations were shown in the paddy soil under CW management.The potential of DNRA measured in situ was consistent with previous laboratory studies;and the controlling factors of DNRA in paddy soils might be different under different water managements,probably due to the presence of different microfloras of DNRA.     

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.     

根迹土壤根诱导的化学变化对植物吸收重金属的影响

It is increasingly recognized that metal bioavailability is a better indicator of the potential for phytoremediation than the total metal concentration in soils; therefore, an understanding of the inffuence of phytoremediation plants on metal dynamics at the soil-root interface is increasingly vital for the successful implementation of this remediation technique. In this study, we investigated the heavy metal and soil solution chemical changes at field moisture, after growth of either Indian mustard (Brassica juncea) or sunffower (Helianthus annuus L.), in long-term contaminated soils and the subsequent metal uptake by the selected plants. In addition, the fractions of free metal ions in soil solution were determined using the Donnan membrane technique. After plant growth soil solution pH increased by 0.2-1.4 units and dissolved organic carbon (DOC) increased by 1-99 mg L^-1 in all soils examined. Soluble Cd and Zn decreased after Indian mustard growth in all soils examined, and this was attributed to increases in soil solution pH (by 0.9 units) after plant growth. Concentrations of soluble Cu and Pb decreased in acidic soils but increased in alkaline soils. This discrepancy was likely due to a competitive effect between plant-induced pH and DOC changes on the magnitude of metal solubility. The fractions of free Cd and Zn ranged from 7.2% to 32% and 6.4% to 73%, respectively, and they generally decreased as pH and DOC increased after plant growth. Metal uptake by plants was dependant on the soil solution metal concentration, which was governed by changes in pH and DOC induced by plant exudates, rather than on the total metal concentrations. Although plant uptake also varied with metal and soil types, overall soluble metal concentrations in the rhizosphere were mainly inffuenced by root-induced changes in pH and DOC which subsequently affected the metal uptake by plants.     

长期使用含铜杀真菌剂对苹果园土壤中自由态铜离子、固相铜形态分级以及微生物活性的影响

Soil samples were collected from apple orchards 5,15,20,30,and 45 years old,and one adjacent forest soil was used as reference to investigate the free Cu2+ion activity in soil solution and the soil Cu fractionation in the solid phase following long-term application of copper fungicide,Bordeaux mixture,in apple orchards and to investigate the relationships among soil free Cu2+ions,Cu fractionation and soil microbial parameters.The total Cu concentration in the orchard soils varied from 21.8 to 141 mg kg-1,increasing with the orchard age,and the value for the reference soil was 12.5 mg kg-1.The free Cu2+ion concentrations in the soil solutions extracted by 0.01 mol L-1 KNO3 ranged from 3.13×10-8(reference)to 4.08×10-6 mol L-1(45 years-old orchard).The concentration of Cu complexed in the fulvic fraction increased with orchard age from 5.16 to 52.5 mg kg-1.This was also the case for other soil Cu fractions except the residual one.The residual soil Cu remained practically constant,ranging from 4.28 to 5.66 mg kg-1,suggesting that anthropogenic soil Cu mainly existed in the more labile active fractions.Regression analyses revealed that both the free Cu2+ions in the soil solution and the humic acid-complexed Cu fraction in the solid phase were strongly related with soil microbial parameters.     

植物修复金属污染土壤上丛枝菌根真菌与植物之间的交互作用

Metal contamination in the environment is a global concern due to its high toxicity to living organisms and its worldwide distribution.The principal goal of this review is to examine the current strategies and technologies for the remediation of metalcontaminated soils by metal-accumulating plants and assess the roles of arbuscular mycorrhizal(AM)fungi in remediation of soils under hyperaccumulator or non-accumulator plants.The use of plants to remove metals from the environment or reduce the toxicity,known as phytoremediation,is an environmentally sustainable and low cost remediation technology.The mechanisms of the use of hyperaccumulator plants for phytoremediation included solubilization of the metal in the soil matrix,the plant uptake of the metal,detoxification/chelation and sequestration,and volatilization.Recently,some ecologists have found that phytoremediation with the aids of mycorrhizae can enhance efciency in the removal of toxic metals.AM fungi can facilitate the survival of their host plants growing on metal-contaminated land by enhancing their nutrient acquisition,protecting them from the metal toxicity,absorbing metals,and also enhancing phytostabilization and phytoextraction.Such information may be useful for developing phytoremediation program at metal-contaminated sites.     

阴离子对加入到土壤中的铅吸附与提取的影响

The effect of accompanying anions on the adsorbability and extractability of Pb added in soils was studied by using bach equilibria and incubation-extraction mothods.Pb concentration ranged between 0.48-2.41 mM in the adsorption experiment,and Pb added to soil from 2.14 to 19.31mM Pb/kg with an incubation time of 2-16 weeks in the incubation-extraction test in which 1M NH4OAc(pH 7) was taken as an extraction agent,and the Pb compounds used are PbCl2,Pb(NO3)2 and Pb(OAc)2.The values of pH50 for Pb retention and the parameters,b and k values,of Langmuir equation were used to distinguish the difference of adsorbability and extractability of Pb affected by anions,respectively.Judging from the results of either pH50 value or b value,the influences of Cl^- and NO3^- on the adsorbability and extractability of Pb in soils were similar but quite different from those of OAc^-.The relative loading capacities of soils to Pb calculated according to the extraction rate showed that the values of Pb(OAc)2 were higher than those of PbCl2 and Pb(NO3)2.     

Toxicity of lead pollution to the collembolan Folsomia candida in Ferri-Udic Cambosols

Toxic effect of lead(Pb) pollution on Collembola in soils has seldom been studied in depth, which is especially true for growth responses since the juveniles are very small and numerous. A single species test was conducted using the collembolan Folsomia candida as the indicator species, and soil Pb pollution(at Pb concentrations of 0, 300, 600, 1 200, 2 400, and 4 800 mg kg~(-1)) was simulated by mixing Pb(NO_3)_2 with a natural unpolluted soil. Adult survival, reproduction, body length, and body Pb concentration were determined. Adult survival and reproduction started to decrease at soil Pb concentrations of 1 200 and 600 mg kg~(-1), respectively. Lead concentrations in adult and juvenile bodies increased with increasing Pb concentration in soil. The percentage and individual abundances based on body length analysis showed that the larger adults(2.1–2.6 mm, P 0.01) were more sensitive to soil Pb concentration than the intermediate(1.9–2.1 mm, P 0.001) and smaller(1.7–1.9 mm, P 0.001) adults. Similarly, the larger juveniles(1.0–1.4 mm, P 0.001) were more sensitive to soil Pb concentration than the intermediate(0.6–1.0 mm, P 0.001) and smaller(0.3–0.6 mm, P 0.001) juveniles. For both adults or juveniles, the abundance and percentage of larger individuals declined significantly with increasing soil Pb concentration, suggesting that larger individuals were more sensitive to soil Pb pollution. Therefore, body length features, especially the percentage and abundance of larger individuals, would be potential bio-indicators of soil Pb pollution and possibly other types of soil pollution.     

Long-Term Effects of Different Passages of Vehicular Traffic on Soil Properties and Carbon Storage of a Crosby Silt Loam in USA

The passage of vehicles with heavy axle loads causes soil compaction, and this adversely affects soil properties and crop yield.The adverse effects can persist for several years due to significant changes in key soil properties. However, the mechanisms of the aforementioned effects are not well understood for conservation agriculture(CA)(e.g., no-till(NT)) wherein the use of heavy machinery is considerably common. Therefore, known compaction forces(0 Mg load for compaction(NT-0, control), two passages of 2.5 Mg water wagon axle load(NT-2), and four passages of 2.5 Mg water wagon axle load(NT-4)) were applied to all the plots annually for 20 consecutive years. The experiment was established in 1997 at the Waterman Agricultural and Natural Resources Laboratory(WANRL), Ohio State University, Columbus, Ohio. Each treatment was replicated thrice. Soil samples were obtained in November2016 to determine the effects of variations in the axle load and vehicular passages on carbon(C) and nitrogen(N) storage and selected soil properties of a Crosby silt loam soil under NT-based corn-soybean rotation with residue retention in Central Ohio, USA. Three locations were also randomly selected in an adjoining natural woodlot(WL) soil plot and sampled(30 m away from the compaction field) to compare the effects of vehicular traffic on soil under NT with WL soil. Results revealed that soil bulk density(ρb) and total porosity at 0–10 and 10–20 cm depths were not affected by the passages of vehicular traffic for 20 years under the NT system.The penetration resistance(PR)(1.86 and 2.03 MPa at 0–10 and 10–20 cm soil depths, respectively) was significantly higher under NT-4 compared with that under other treatments. Saturated hydraulic conductivity at 0–10 and 10–20 cm soil depths ranged from19.7 to 31.4 and 18.5 to 29.5 mm d~(-1), respectively, across all the treatments. The proportion of macroaggregates( 0.25 mm) and microaggregates( 0.25 mm), mean weight diameter and geometric mean diameter of aggregates, pH, electrical conductivity, and C and N contents and storage did not differ significantly between the treatments at either of the sampling soil depths. The data indicated that 2 to 4 passages of vehicles with 2.5 Mg of axle load did not cause significant compaction of the Crosby silt loam under NT compared with that under natural WL. Therefore, the continuous cultivation of row crops with NT and residue retention is feasible with passages of vehicular traffic for well-drained soils in Central Ohio.     

在积累镉油菜后茬种植白菜可否减少其对镉的吸收?

Rapeseed (Brassica campestris L.), a Cd-accumulating crop, is effective in reducing plant-available Cd from soil. A pot experiment was conducted to test the hypothesis that the Cd uptake by Chinese cabbage (Brassica pekinensis Rupr.) grown in rotation with rapeseed would be reduced due to the efficient removal of bioavailable Cd from soil solution by the rapeseed crops. The Cd removal by shoot for the rapeseed cultivar Zhucang Huazi ranged from 0.24% to 0.99% in natural Cd-contaminated soil (soil A) and from 0.63% to 1.23% in artificial Cd-contaminated soil (soil B) during the growth period of 4–7 weeks; whereas that for the rapeseed cultivar Chuanyou II-93 ranged from 0.25% to 0.66% and from 0.38% to 1.02%, respectively. Chinese cabbage grown in the pots with soil A for five weeks after harvesting rapeseed exhibited a significantly lower Cd concentration compared to that grown in the control pots (uncropped with rapeseed). However, the rotation of rapeseed did not lower the Cd concentration of Chinese cabbage on soil B. Although an increase of Cd uptake was observed in the rapeseed cultivars with growth time, and the decrease of soil NH 4 OAc-extractable Cd was found only in the natural rather than artificial Cd-contaminated soil.     

氧化节杆菌生物降解苯并芘

A pot experiment was conducted with multi-metal (Pb, Cd, Cu, and Zn) contaminated acidic soil to investigate changes in available metal burden resulting from the application of industrial wastes (fly ash and steel slag). The efficiency of amendments-induced metal stabilization was evaluated by diffusive gradients in thin films (DGT), sequential extraction, and plant uptake. The stability of remediation was assessed by an acidification test and by chemical equilibrium modeling. Addition of fly ash (20 g kg-1 ) and steel slag (3 g kg-1 ) resulted in similar increase in soil pH. Both amendments significantly decreased the concentrations of metals measured with DGT (C DGT) and the metal uptake by Oryza sativa L. Significant correlations were found between C DGT and the concentration of a combination of metal fractions (exchangeable, bound to carbonates, and bound to Fe/Mn oxides), unraveling the labile species that participate in the flux of metal resupply. The capability of metal resupply, as reflected by the R (ratio of C DGT to pore water metal concentration) values, significantly decreased in the amended soils. The C DGT correlated well with the plant uptake, suggesting that DGT is a good indicator for bioavailability. Acidification raised the extractable metal concentration in amended soil but the concentration did not return to the pre-amendment level. Equilibrium modeling indicated that the soil amendments induced the precipitation of several Fe, Al and Ca minerals, which may play a positive role in metal stabilization. Chemical stabilization with alkaline amendments could be an effective and stable soil remediation strategy for attenuating metal bioavailability and reducing plant metal uptake.     

Effect of Silicon on Growth, Physiology, and Cadmium Translocation of Tobacco (Nicotiana tabacum L.) in Cadmium-Contaminated Soil

Silicon(Si) offers beneficial effect on plants under cadmium(Cd) stress such as promoting plant growth and increasing resistance to heavy metal toxicity. In this study, a pot experiment was performed to study the role of Si in alleviating Cd toxicity in tobacco(Nicotiana tabacum L.) plants on a yellow soil taken from Guiyang, China. Nine treatments consisting of three concentrations of Cd(0, 1, and 5 mg kg~(-1)) together with three Si levels(0, 1, and 4 g kg~(-1)) were established. Plant growth parameters, Cd concentration,and the malondialdehyde(MDA), chlorophyll, and carotenoid contents were determined 100 d after transplanting of tobacco seedlings.Application of exogenous Si enhanced the growth of tobacco plants under Cd stress. When 5 mg kg~(-1) Cd was added, Si addition at 1 and 4 g kg~(-1) increased root, stem, and leaf biomass by 26.1%–43.3%, 33.7%–43.8%, and 50.8%–69.9%, respectively, compared to Si addition at 0 g kg~(-1). With Si application, the transfer factor of Cd in tobacco from root to shoot under both 1 and 5 mg kg~(-1) Cd treatments decreased by 21%. The MDA contents in the Si-treated tobacco plants declined by 5.5%–17.1% compared to those in the non-Si-treated plants, indicating a higher Cd tolerance. Silicon application also increased the chlorophyll and carotenoid contents by 33.9%–41% and 25.8%–47.3% compared to the Cd only treatments. Therefore, it could be concluded that Si application can alleviate Cd toxicity to tobacco by decreasing Cd partitioning in the shoots and MDA levels and by increasing chlorophyll and carotenoid contents, thereby contributing to lowering the potential health risks of Cd contamination.     

热带土壤上连续9年施入污水污泥后重金属Zn, Cd和Pb的分离研究

A long-term field experiment was carried out in the experiment farm of the Sao Paulo State University, Brazil, to evaluate the phytoavailability of Zn, Cd and Pb in a Typic Eutrorthox soil treated with sewage sludge for nine consecutive years, using the sequential extraction and organic matter fractionation methods. During 2005-2006, maize (Zea mays L.) was used as test plants and the experimental design was in randomized complete blocks with four treatments and five replicates. The treatments consisted of four sewage sludge rates (in a dry basis) : 0.0 (control, with mineral fertilization), 45.0, 90.0 and 127.5 t ha^-1, annually for nine years. Before maize sowing, the sewage sludge was manually applied to the soil and incorporated at 10 cm depth. Soil samples (0-20 cm layer) for Zn, Cd and Pb analysis were collected 60 days after sowing. The successive applications of sewage sludge to the soil did not affect heavy metal (Cd and Pb) fractions in the soil, with exception of Zn fractions. The Zn, Cd and Pb distributions in the soil were strongly associated with humin and residual fractions, which are characterized by stable chemical bonds. Zinc, Cd and Pb in the soil showed low phytoavailability after nine-year successive applications of sewage sludge to the soil.     

磷改良剂对中度和高度含碳酸盐的菜园土壤中Cd和Pb环境有效性和植物有效性的影响

The behaviour of metals mainly depends on soil p H, carbonate contents and contamination level, which should be considered for the management of contaminated soils. In this study, kitchen garden topsoils(0–25 cm) were sampled from the area around three smelters in France, with different Cd and Pb contamination levels. Effect of a phosphate amendment(a mixture of diammonium phosphate and hydroxyapatite) on the environmental availability and phytoavailability of Cd and Pb was evaluated by different chemical extractions and cultivating lettuce(Lactuca sativa L.), respectively. Changes in the distribution of Cd and Pb were found in most contaminated soils after phosphate amendment. An increase of Cd and Pb in the residual phase was highlighted in almost all carbonated contaminated soils, whereas a decrease of Pb in the exchangeable, water and acid-soluble phase was observed in most contaminated soils with the lowest carbonate contents. The concentrations of extractable Cd and Pb using calcium chloride and acetic and citric acids generally decreased after the soil amendment. Lettuces grown on amended soils were acceptable for human consumption as regard to Pb concentration. In contrast, some lettuces were unacceptable for human consumption, since the concentrations of Cd in the leaves were higher than the European legislation limit. Surprisingly, in carbonated soils with very low concentration of Cd, the Cd concentrations in lettuce reached up to the European legislation limit, making the lettuce unacceptable for human consumption.Our study highlighted the fact that the total metal concentration in soils does not always allow to predict the metal accumulation in the edible parts of vegetables in order to make a judgement about their acceptability or unacceptability for human consumption.     

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.     

Fate of Basal N Under Split Fertilization in Rice with 15N Isotope Tracer

Split fertilization strategy is popularly adopted in rice to synchronize soil nitrogen(N) supply and crop N demand. Attention has been paid more on mid-season topdressing N, but limited on basal N. A clearer understanding of the basal N fate under split fertilization is crucial for determining rational basal N split ratio to improve the yield and reduce the loss to environment. A two-year field experiment with two N rates of 150 and 300 kg Nha~(-1), two split ratios of basal N, 40% and 25%, and two rice varieties,Wuyunjing 23(japonica) and Y-liangyou 2(super hybrid indica), was conducted. Labelled ~(15) N urea was supplied in micro-plots as basal fertilizer to determine the plant uptake, translocation, soil residual, and loss of basal N fertilizer. The results showed that basal N absorbed by rice was only 1.6%–11.5% before tillering fertilization(8–10 d after transplanting), 6.5%–21.4% from tillering fertilization to panicle fertilization, and little(0.1%–4.4%) after panicle fertilization. The recovery efficiency of basal N for the entire rice growth stage was low and ranged from 18.7% to 24.8%, not significantly affected by cultivars or N treatments. Soil residual basal N accounted for 10.3%–36.4% and decreased with increasing total N rate and basal N ratio, regardless of variety and year. 43.8%–70.4% of basal N was lost into the environment based on the N balance. Basal N loss was significantly linearly positive related with the basal N rate and obviously enhanced by the increasing basal N ratio for both varieties in both 2012 and 2013. The N use efficiency and yield was significantly improved when decreasing the basal N ratio from 40% to 25%. The results indicated that the basal N ratio should be reduced, especially with limited N inputs, to improve the yield and reduce the N loss to the environment.     

苏南地区土壤重金属向蔬菜的迁移研究

Vegetable fields in peri-urban areas receive large amounts of extraneous heavy metals because of rapid urbanization and industrialization in China. The concentrations of Cu, Zn, and Pb in 30 soil samples and 32 vegetable samples, collected from 30 different sites in southern Jiangsu Province of China, were measured and their transfer from soil to vegetable was determined. The results showed that the soil samples had wide ranges of pH (4.25-7.85) and electrical conductivity (EC) (0.24-3.42 dS m^-1). Among the soil samples, there were four soil samples containing higher Cu and two soil samples containing higher Zn concentrations than those specified in the Chinese Soil Environmental Quality Standard II. However, no vegetable sample was found to contain a high level of Cu or Zn. In contrast, one vegetable sample contained 0.243 mg Pb kg^-1 FW, which was above the Chinese Food Hygiene Standard, whereas the corresponding soil Pb concentration was lower than the Chinese Soil Environmental Quality Standard II. The transfer coefficients of Cu of all vegetable samples exceeded the suggested coefficient range, implying that extraneous Cu had high mobility and bioavailability to vegetables. There was no significant correlation between extractable soil heavy metal concentrations with four kinds of extractants and soil pH, EC, heavy metal concentrations in vegetables and soils, except that soil pH correlated well with the extractable soil Cu, Zn, and Pb concentrations with 1.0 mol L^-1 NH₄NO₃. Moreover, diethylenetriamine pentaacetic acid (DTPA) extraction method was a more effcient method of extracting heavy metals from the soils independent of soil pH and EC than other three methods used.