Mercury (II) Adsorption on Three Contrasting Chinese Soils Treated with Two Sources of Dissolved Organic Matter: I. Langmuir and Freundlich Isotherm Evaluation

The behavior and fate of mercury (Hg) in soil is mainly controlled by adsorption and desorption processes with various adsorbents, particularly dissolved organic matter (DOM). This study was conducted to assess the effect of DOM from wheat straw (DOMw) and swine manure (DOMs) on Hg (II) adsorption of black, red, and fluvo-aquic soils in China. Results showed that the Hg (II) adsorption isotherms fitted well with Langmuir and Freundlich equations. The maximum Hg (II) potential adsorption capacity by the three soils followed this trend: black soil > red soil > fluvo-aquic soil. The amount of Hg (II) adsorbed on the soils significantly decreased when DOMw and DOMs were added to the soil samples. Furthermore, the extent by which DOMw affected the Hg (II) adsorption of the three soils was higher than the effect of equivalent amounts of DOMs. Therefore, DOM is important to determine the fate of Hg (II) and control Hg (II) pollution in the environment.     

土壤中六氯苯的老化特征及其在蚯蚓体内的富集规律

采用室内模拟试验,研究了不同土壤中六氯苯的老化特征及其在蚯蚓体内的生物富集规律. 结果表明：六氯苯在不同土壤中的老化过程呈现出先快后慢的二阶段特征,即在初始的60 d内,六氯苯老化速率较快, 随后老化趋势减缓.六氯苯在土壤中的老化作用主要发生在与土壤初始接触的60 d内,其老化速率呈现水稻土>红壤>潮土的规律.六氯苯在蚯蚓体内的生物富集量和生物富集系数也随着老化时间的延长而逐渐降低. 虽然六氯苯在土壤中的可提取态含量随着老化时间延长而下降,但在蚯蚓体内有较高的生物富集量(457.6～984.3 ng·g^-1)和一定的生物富集能力(生物富集系数BAF在3.74～6.35),仍会对土壤生态安全构成潜在威胁.     

黄土高原生物结皮覆盖对风沙土和黄绵土溶质运移的影响

干旱和半干旱地区生物结皮的普遍发育显著改变了表层土壤的结构与养分富集特征,但其对土壤养分迁移和淋失的影响目前尚不明确。本研究针对黄土高原风沙土和黄绵土上发育的藓类生物结皮,以Ca²⁺和Cl^-为示踪离子开展溶质穿透试验,对有无生物结皮层及其覆盖下不同深度土壤的溶质运移特征进行了研究。结果表明: 在0～5 cm土层,生物结皮覆盖延缓了风沙土和黄绵土的溶质穿透过程,其Cl^-的穿透时间比无结皮延长了3.83(风沙土)和2.09倍(黄绵土),而Ca²⁺则分别延长了2.50和2.73倍。生物结皮覆盖条件下,表层0～5 cm土壤溶质完全穿透所对应的孔隙体积数比下层5～10 cm土壤更高,且其穿透历时更长;其中,Cl^-的穿透时间分别增加了67.3%(风沙土)和51.8%(黄绵土),Ca²⁺的穿透时间分别增加了8.0%和33.7%。生物结皮覆盖降低了土壤孔隙水流速(37.5%～70.2%);除风沙土的5～10 cm土层外,生物结皮使溶质弥散系数提高了1.73～6.29倍,使溶质弥散度提高了2.77～20.95倍。置换液完全穿透土壤后,风沙土和黄绵土的生物结皮层Ca²⁺含量显著高于无结皮,其分别比无结皮高4.14和2.58倍。研究证实,生物结皮覆盖能够提高表层土壤对养分的吸附与固持能力,从而减少土壤表聚养分的深层渗漏和流失,对干旱和半干旱地区退化土壤的肥力提升与植被恢复具有重要意义。     

豫南黏板土壤分层酸化和耕层速效磷分布特征

黄淮海平原南部以黄褐土和砂姜黑土等为代表的耕地土壤酸化趋势明显.为深入认识该类型黏板土壤垂直剖面上pH值和耕层养分的空间变异情况,以豫南西平县为例,对县域范围内63个耕地样点进行pH值和速效磷测定,运用地统计学方法和ArcGIS技术分析不同深度土壤pH值和耕层土壤(0～20 cm)速效磷的空间分布状况,并分析了pH值与...     

不同老化时间对土壤中外源Zn的形态转化及生态毒性阈值(ECx)的影响

选取了6种不同性质土壤, 添加7个浓度水平的Zn, 研究了不同老化时间(14、90、180、360和540 d)对土壤中外源性Zn有效形态及毒性阈值(EC_x)的影响, 并对Zn老化过程的主要影响因子进行分析. 结果表明: 土壤中0.01 mol·L^-1CaCl₂提取态Zn含量随着老化时间的增加先快速下降, 随后变化减缓, 到540 d时基本达到平衡.随着老化时间的增加, 土壤中Zn对小白菜生长的毒性阈值(EC_x, x=10、50)逐渐增加,毒性显著降低(P<0.05).Zn老化因子(AF)AF₁₀和AF₅₀分别为1.077~1.743和1.174~1.441, 老化因子随老化时间增加而增大.土壤中Zn的平衡浓度(C_∞)与土壤pH、阳离子交换量(CEC)、有机碳含量呈显著负相关关系, 其中pH是决定Zn老化速率最重要的因素, 其次是CEC和有机碳含量,高pH下土壤中Zn达到平衡所需的时间较短. 基于土壤老化因子与主控因子建立土壤中Zn的毒性阈值预测模型,所得预测值与实测值之间有较好的相关性.研究结果将为不同土壤中外源性Zn毒性阈值的归一化处理及生态风险基准值的制定提供理论依据.     

长期施肥下三类典型农田土壤小麦磷肥利用效率的差异

对我国北方长期施肥下三类典型农田土壤（塿土、潮土和褐潮土）的小麦产量、小麦磷肥农学利用效率、小麦磷肥利用率进行了研究.结果表明：长期施用磷肥处理（氮磷化肥配合施用、氮磷钾化肥配合施用、氮磷钾化肥和秸秆配合施用、氮磷钾化肥和有机肥配合施用）的小麦产量为2914～6219 kg·hm^-2,较不施磷肥处理（不施肥对照、单施化肥氮、氮钾化肥配合施用）提高了2～4倍,各施磷肥处理之间无显著差异.试验起始年施用氮磷钾化肥处理的塿土、潮土和褐潮土上的小麦磷肥农学利用效率分别为17.0、20.3和13.3 kg·kg^-1,小麦磷肥利用率分别为15.3%、31.2%和23.8%;施肥15年后,小麦磷肥农学利用效率每年分别增加3.9、2.5和2.8 kg·kg^-1,小麦磷肥利用率每年分别增加1.3%、0.9%和1.0%.同一类型土壤不同施磷处理间的磷肥农学利用效率和利用率差异不显著.在我国北方地区,长期施用磷肥可以显著提高小麦产量和磷肥利用效率;氮磷钾化肥和有机肥配施处理下,塿土平均每年增长的小麦磷肥农学利用效率和磷肥利用率较潮土和褐潮土高.     

Assessing the in situ bioavailability of trace elements to snails using accumulation kinetics

The bioavailability of trace elements in soils is conditioned by both physico-chemical and biological parameters. In this study, the accumulation kinetics of cadmium (Cd), lead (Pb), arsenic (As) and antimony (Sb) were determined for 3 industrially impacted sites to assess the bioavailability of these contaminants to the garden snail (Cantareus aspersus). Mono and multivariate regressions allowed the identification of cation exchange capacity (CEC), silts and organic carbon content as the soil parameters modulating the in situ bioavailability of Cd and Pb. For all elements, the total concentrations in the soils were not good predictor (not significant correlation) of the bioavailability to snails. The Cd, As and Sb assimilation fluxes were correlated with the calcium chloride (CaCl₂) extract concentrations, but this correlation was not observed with Pb. The total soil concentration coupled with soil properties best explained the variation in Pb assimilation, whereas their influences on Cd bioavailability were lower, signifying that other parameters such as contamination sources may modulate Cd bioavailability. Here, the As and Sb in situ accumulation kinetics are described for the first time and highlighted a slight bioavailability to snails at the studied sites. The absence of a correlation between the As or Sb assimilation fluxes and total metals in the soil coupled with the absence of influence of soil properties on their bioavailability may result from the speciation of these metalloids, which are known to modulate their mobility in soils. This study highlights the need to consider both physico-chemical and biological aspects of metal and metalloid bioavailability to assess the risk of metal transfer from soil to organisms.     

Effect of vanadium on the growth of soybean seedlings

Pot experiments were conducted in a glasshouse to investigate the effects of vanadium (V) on the growth of soybean seedlings in two soils. As the concentration of V added to the fluvo-aquic soil (Fluvaqents) exceeded 30 mg V kg^-1 soil, the dry matter yields of shoots and roots were significantly decreased (>1%LSD), and the leaves of soybean seedlings turned yellow and withered and the roots were short and beginning to rot. In the red earth (Oxisols), no marked stunting was observed (<5% LSD), even when the concentration of V added to the soil was as high as 75 mg V kg^-1. As the concentrations of vanadium in soybean seedling were closely related to the concentration of soluble vanadium in soil solutions at pH 5–9 in the soil equilibrium solution, the fluvo-aquic soil had lower adsorption capacity for V than the red earth, there was much higher concentration of soluble V in the soil solution, so the symptom of V toxicity appearing in the plants grown on fluvo-aquic soil was easily observed. In addition, the ratio of the total Mo to the total V in shoots decreased slightly with increase of concentration of V added to soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

Dermal Bioavailability of Toluene Aged in Soil

When organic chemicals age in soil, they can become more sequestered with time and less bioavailable. The impact of aging in soil was assessed on the dermal bioavailability of toluene. Comparisons were made between the dermal bioavailability of toluene aged in two soils (Atsion and Keyport) for 3 months, toluene in freshly spiked soil, and toluene without soil (pure chemical). In vitro flow-through diffusion cell methodology measured the amount of radioactive chemical that penetrated dermatomed male pig skin into receptor fluid, and that was detected in skin following soap and water decontamination. Volatilization reduced the amount of toluene that was available for skin penetration. Therefore, relative to the available dose, the total penetration of pure toluene (sum of the dose in the receptor fluid and skin) was 93%. The majority of pure toluene was found in skin (82%). The dermal bioavailability of non-aged and aged toluene was decreased to 4–6% and 3–4% of the available dose, respectively. The data suggest that there would be a lower potential risk to human health from dermal exposure to toluene in soil. However, lower soil loading and longer aging time are needed to quantitate the risk.     

The long-term effect of sludge application on Cu, Zn, and Mo behavior in soils and accumulation in soybean seeds

A long-term greenhouse column experiment using two soils of different textures amended with dewatered, composted and alkaline-stabilized sludges (biosolids) tested the effect of aging on trace metal solubility, mobility and crop uptake over 15 cropping cycles. Specifically, soil chemical properties and extractability of Cu, Zn and Mo were measured after each cropping cycle, and soybeans (Glycine max (L.) Merr.) grown as the final crop were analyzed for those metal concentrations in the seeds. Significant Cu loss from the surface soil through leaching, and increased Zn extractability resulting from soil acidification were evident in the early cropping cycles shortly after sludge application, with the degree of Cu mobilization and soil acidification strongly dependent on the type of soil and sludge. Liming to counter acidification in later cycles enhanced Mo extractability and bioavailability substantially, with some sludge treatments producing soybean seeds with Mo concentrations up to 5 times greater than the control. Aging effects were difficult to discern for trace metals in this long-term study, since soil pH changes caused by sludge and liming amendments dominated metal solubility and crop uptake.     

The effect of regeneration burns on the growth,nutrient acquisition and mycorrhizae of Eucalyptus regnans F. Muell. (mountain ash) seedlings

This study was conducted to compare the effects on the growth of Eucalyptus regnans seedlings of unheated soil and soil heated to different extents (as indicated by soil colour–bright red or black) in burnt logging coupes, and to separate the effects of heating of the soil on direct nutrient availability and on morphotypes and effectiveness of ectomycorrhizae. Burnt soils were collected from three logging coupes burnt 2, 14 and 25 months previously and unbumt soil from adjacent regrowth forests. Compared to unburnt soil, the early seedling growth was stimulated in black burnt soil from all coupes (burnt 2, 14 and 25 months previously). Seedling growth was generally poor in red burnt soil, especially in soil collected 2 months after burning. However, the concentration of extractable P was extremely high in red burnt soil, especially in soil collected 2 months after burning. In black burnt soil, extractable P was increased in soil 2 months after burning, but not in the soils collected 14 or 25 months after burning. However, both total P content and concentration in seedlings were increased in all collections of black burnt soil. Frequency of ectomycorrhizae was high in seedlings grown in all black burnt soils, but the mycorrhizal mantles were poorly developed in seedlings in black burnt soil collected 2 months after burning. Seedlings were also ectomycorrhizal in red burnt soil, except in soil collected 2 months after burning. Fine root inocula from seedlings grown in black burnt soils collected 14 and 25 months after burning significantly stimulated both seedling growth and P uptake compared with the uninoculated control, whereas the fine root inocula from the seedlings grown in all the other soils did not. These results suggest that, in black burnt soil, both direct nutritional changes and changes in the ectomycorrhizae may contribute to seedling growth promotion after regeneration burns. The generally poor seedling growth in red burnt soils is likely to have been due to N deficiency as the seedlings in these soils were yellow-green and the tissue concentrations of N were significantly lower than in other treatments. This revised version was published online in June 2006 with corrections to the Cover Date.     

Estimation of Cd,Pb, and Zn Bioavailability in Smelter-Contaminated Soils by a Sequential Extraction Procedure

Chemical fractionation methods may be capable of providing an inexpensive estimate of contaminant bioavailability and risk in smelter-contaminated soil. In this study, the relationship between metal fractionation and methods used to estimate bioavailability of these metal contaminants in soil was evaluated. The Potentially BioAvailable Sequential Extraction (PBASE) was used for Cd, Pb, and Zn fractionation in 12 soils contaminated from Pb and Zn mining and smelting activities. The PBASE procedure is a four-step sequential extraction: extraction 1 (E1) is 0.5 M Ca(NO₃)₂, E2 is 1.0 M NaOAc, E3 is 0.1 M Na₂EDTA, and E4 is 4 M HNO₃. Metal bioavailability for two human exposure pathways, plant uptake (phytoavailability) and incidental ingestion (gastrointestinal, Gl, availability), was estimated using a lettuce (Lactuca sativa L.) bioassay and the in vitro-Gl Physiologically Based Extraction Test(PBET). Metal in the PBASE E1 fraction was correlated with lettuce Cd (P < 0.001) and Zn (P < 0.05) and was the best predictor of Cd and Zn phytoavailability. Only total metal content or the sum of all PBASE fractions, ΣE_1–4, were correlated (P < 0.001) with PBET gastric phase for Pb. The sum of the first two PBASE fractions, ΣE_1–2, was strongly correlated (P < 0.001) with Pb extracted by the PBET intestinal phase. The PBASE extraction method can provide information on Cd and Zn phytoavailability and Gl availability of Pb in smelter-contaminated soils.     

Plant-Soil-Contaminant Specificity Affects Phytoremediation of Organic Contaminants

The objective of this study was the evaluation of seven forage and conservation crop species for phytoremediation of trinitrotoluene (TNT) and pyrene-contaminated soils. TNT and pyrene were added to soil at 100 mg kg^-1. Crop species screening studies were conducted in a greenhouse and growth chambers on two soil types with different organic matter contents. Under high soil organic matter conditions, adsorption or covalent binding to the soil organic matter appeared to be a dominant force of removal limiting TNT and pyrene availability. In both soil types, pyrene dissipation could not be attributed to the presence of plants. However, in soils with lower organic matter content, all of the plant species treatments showed a significantly higher degree of TNT transformation compared with the unplanted control. Statistically significant differences in TNT transformation were observed among crop species grown in the low OM soil. Reed canary grass (Phalaris arundinacea L.) and switchgrass (Panicum virgatum L.) were the most effective species in enhancing TNT transformation. Our data indicated that use of plants was effective for phytoremediation of TNT-contaminated low OM content soils, but did not have any significant effect on pyrene dissipation. Based on these observations, it appears that plant-soil-contaminant interactions are very specific, and this specificity determines the effectiveness of phytoremediation schemes.     

Field Released Transgenic Papaya Affects Microbial Communities and Enzyme Activities in Soil

Soil properties, microbial communities, and enzyme activities were studied in soil planted with transgenic or nontransgenic papaya under field conditions. The transgenic papaya contained a replicase (RP) mutant gene of the papaya ringspot virus (PRSV), which conferred resistance to the virus, the neomycin phosphotransferase II (NPT II) marker gene, which conferred Km resistance, and a cauliflower mosaic virus 35S promoter (CaMV 35S). There were significant differences (P < 0.05) in the total number of colony forming units (CFUs) of bacteria, actinomycetes, and fungi between soils planted with RP-transgenic and nontransgenic plants; total CFUs of bacteria, actinomycetes, and fungi in soil planted with transgenic papaya were significantly higher by 0.43, 0.80, and 0.46 times, respectively. Significantly higher (P < 0.05) CFUs of bacteria, actinomycetes, and fungi resistant to kanamycin (Km) were present in soils planted with the transgenic papaya than in those planted with nontransgenic papaya. Resistance quotients (CFU in the presence of a chemical relative to that without) of Km-resistant bacteria, actinomycetes and fungi were higher in soil planted with transgenic papaya, and the resistance quotients of Km-resistant bacteria, actinomycetes, and fungi in soils planted with transgenic papaya increased statistically significantly (P<0.05) from 1.5 to 2.5, from 1.2 to 2.6, and from 0.9 to 2.8 times, respectively. Soils planted with transgenic papaya had significantly higher enzyme activities of arylsulfatases (+5.4 times), alkaline phosphatases (+0.5 time), invertase (+0.5 time) and phosphodiesterases (+0.2 time), but lower enzyme activities of proteases (−2.1 times), polyphenol oxidases (−1.4 times), urease (−0.2 time) than the soils planted with nontransgenic papaya. Our results suggest that transgenic papaya could alter chemical properties, enzyme activities, and microbial communities in soil.     

Effect of 2,4,6-trinitrotoluene on soil bacterial communities

To gain insight into the impact of 2,4,6-trinitrotoluene (TNT) on soil microbial communities, we characterized the bacterial community of several TNT-contaminated soils from two sites with different histories of contamination and concentrations of TNT. The amount of extracted DNA, the total cell counts and the number of CFU were lower in the TNT-contaminated soils. Analysis of soil bacterial diversity by DGGE showed a predominance of Pseudomonadaceae and Xanthomonadaceae in the TNT-contaminated soils, as well as the presence of Caulobacteraceae. CFU from TNT-contaminated soils were identified as Pseudomonadaceae, and, to a lesser extent, Caulobacteraceae. Finally, a pristine soil was spiked with different concentrations of TNT and the soil microcosms were incubated for 4 months. The amount of extracted DNA decreased in the microcosms with a high TNT concentration [1.4 and 28.5 g TNT/kg (dry wt) of soil] over the incubation period. After 7 days of incubation of these soil microcosms, there was already a clear shift of their original flora towards a community dominated by Pseudomonadaceae, Xanthomonadaceae, Comamonadaceae and Caulobacteraceae. These results indicate that TNT affects soil bacterial diversity by selecting a narrow range of bacterial species that belong mostly to Pseudomonadaceae and Xanthomonadaceae.     

Accelerated Transformation and Binding of 2,4,6-Trinitrotoluene in Rhizosphere Soil

Enhanced microbial activity and xenobiotic transformations take place in the rhizosphere. Degradation and binding of 2,4,6-trinitrotoluene (TNT) were determined in two rhizosphere soils (RS) and compared to respective unplanted control soils (CS). The rhizosphere soils were obtained after growing corn for 70 d in soils containing 2.8% (Soil A) or 5.9% (Soil B) organic matter. Aerobically agitated soil slurries (3:1, solution/soil) were prepared from RS and CS and amended with 75 mg TNT L^-1 (¹⁴C-labeled). TNT degraded more rapidly and formed more un-extractable bound residue in RS than in CS. In Soil A, total extractable TNT decreased from 225 to 1.0 mg kg^-1 in RS, whereas 11 mg kg^-1 remained in CS after 15 d. Unextractable bound ¹⁴C residues accounted for 40% of the added ¹⁴C-TNT in RS and 28% in CS. The smaller differences in Soil B were attributed partially to the higher organic matter content. The predominant TNT degradation products were monoaminodinitrotoluenes (ADNT), which accumulated and disappeared more rapidly in RS than in CS, and hydroxylaminodinitrotoluenes (HADNT). When sterilized by γ-irradiation, no significant differences between RS and CS were observed in TNT loss or bound residue formation. More rapid TNT degradation and enhanced bound residue formation in the unsterilized RS was attributed to micro-bial-facilitated production and transformation of HADNT and ADNT, which are potential precursors to bound residue formation. If plants can be established on TNT-contaminated soil, these results indicate that the rhizosphere can accelerate reductive transformation of TNT and promote bound residue formation.     

Effects of Sterilization and Temperature on the Decrease Kinetic of Lead Bioavailability in Two Different Soil Groups

Sorption of metal ions by soil and clay minerals is a complex process involving different mechanisms, and controlled by different variables that can interact. The impacts of sterilization and incubation temperature on the decrease kinetic of Pb bioavailability in two different groups of soils were studied. Surface soils were sampled from Guilan and Hamadan provinces in the north and northwest of Iran with temperate and semiarid climates. The decrease kinetic of Pb bioavailability in the Pb(NO₃)₂ treated (400 μg Pb g^?1) soils has been studied in solid state incubation in sterile and unsterile conditions at 15, 27 and 37°C. The decrease of DTPA-extractable Pb in both groups of soils is often characterized by an initial rapid step followed by a slow step. The temperate soil with high affinity surface sites for Pb sorption compared to semiarid soils had a lower DTPA-extractable Pb in each time of extraction. Sterilization and soil incubation at lower temperature decreased the rate of Pb sorption/precipitation processes. Among the kinetic models the second order model and Elovich kinetic equation were the better choice to express the decrease kinetic of Pb bioavailability according to higher determined coefficient and the small standard error of the estimate. The determination coefficients of the mass transfer equation were increased and the standard errors of the estimates were decreased in sterile and unsterile conditions by increasing incubation temperature from 15 to 37°C.     

Changes in carbon, nutrients and stoichiometric relations under different soil depths, plant tissues and ages in black locust plantations

To investigate influences of forest plantations on soil nutrient properties, biomass accumulation, major nutrient elements (NPK) and their stoichiometric couplings in different tissues and aged plants, and correlations between major nutrient contents in soils and in foliage of plants, 5-, 10-, 15- and 20-year-old plantations of black locust (Robinia pseudoacacia L.) and farmland were selected. Black locust plantations increased soil organic carbon (SOC) and N stocks by 23–327 and 23–119 %, respectively, in the 0–10 cm top soil layer compared to those in farmland. Soil C:N, C:P, C:K, N:P, N:K and P:K ratios were 10.1, 22.9, 0.7, 2.2, 0.7 and 0.03, respectively. These ratios were higher in the 0–10 cm soil layer than those in the 10–20 cm soil layer and increased under older plantations. Higher C contents in stem, N contents in leaf, the largest C pools in stem and N pools in root in 20-year-old plantation were observed. Correspondingly, the highest C:N, C:P and C:K and the lowest N:P and N:K ratios in stem, decreased C:N and C:P ratios in older trees were found. No strong correlations were observed between element contents in soils and in leaves of black locust trees. These results suggest that black locust plantations can increase soil nutrient concentrations, SOC and N stocks resulting in changes in element stoichiometric relations. CNPK contents and their stoichiometries vary with tissues and tree ages of black locust. No strong coupling relations exist between major nutrient element contents in the top soil and in foliage of black locust.     

Long-Term Explosive Contamination in Soil: Effects on Soil Microbial Community and Bioremediation

Explosive contamination in soil is a great concern for environmental health. Following 50 years of munitions manufacturing and loading, soils from two different sites contained ≥ 6,435 mg 2,4,6-trinitrotoluene (TNT), 2,933 mg hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and 2,135 mg octahydrol-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) kg^{? 1} soil. Extractable nitrate-N was as high as 315 and ammonium-N reached 150 mg N kg^{? 1} soil. Water leachates in the highly contaminated soils showed near saturation levels of TNT and RDX, suggesting great risk to water quality. The long-term contamination resulted in undetectable fungal populations and as low as 180 bacterial colony forming units (CFU) g^–1 soil. In the most severely contaminated soil, dehydrogenase activity was undetectable and microbial biomass carbon was very low (< 3.4 mg C _mic kg^–1 soil). The diminished biological activity was a consequence of long-term contamination because short-term (14 d) contamination of TNT at up to 5000 mg TNT kg^–1 soil did not cause a decline in the culturable bacterial population. Natural attenuation may not be a feasible remediation strategy in soils with long-term contamination by high concentrations of explosives.     

An Investigation of the Soil Property Changes and Heavy Metal Accumulation in Relation to Long-term Wastewater Irrigation in the Semi-arid Region of Iran

Concentrations and spatial distribution of Zn, Cu, Cd, and Pb along two landscapes including a wastewater-irrigated area and a control area were determined to assess the impact of long-term wastewater irrigation and landscape properties on heavy metal contamination. Some disturbed and undisturbed soil samples were taken from soil trenches and soil cores, located on three main landscape positions (upper slope, midslope, and lower slope) in northwestern Iran. The investigation showed that the mean concentration of the heavy metals followed the order Zn > Cu > Pb > Cd in the wastewater-irrigated soil and Pb > Zn > Cu > Cd in the control soils. On average, compared to similar positions in the control region, the wastewater-irrigated regions contained 3.0 (midslope) to 4.9 (lower slope), 2.7 (midslope) to 4.6 (lower slope), 3.3 (upper slope) to 4.1 (lower slope), and 1.7 (upper slope) to 2.6 (lower slope) times higher amounts of Zn, Cu, Cd, and Pb, respectively. Significant positive relationships (P < 0.05) were recorded between the heavy metals concentration with <0.002 mm particle-size fraction and organic matter content, the fractions linked to runoff and soil erosion. It is believed that the two soil fractions play a crucial role in the distribution of the metals along the wastewater-irrigated landscape. Despite the significant increase of heavy metals (P < 0.05) in the wastewater-irrigated soils compared with control soils, the concentration of all evaluated metals was below the maximum accepted limits (Zn < 300 mg/kg, Cu < 100 mg/kg, Cd < 5 mg/kg, and Pb < 100 mg/kg), and grouped as “not-enriched” to “moderately-enriched” categories regarding the topsoil enrichment index. Overall, the lower slope was shown to be more contaminated with the heavy metals compared to the other positions.