Sequestration Enhancement of Metals in Soils by Addition of Iron Oxides Recovered from Coal Mine Drainage Sites

Iron oxides recovered from abandoned coal mine drainage (AMD) sites (Lowber, Scrubgrass, and Horner) as a soil amendment were investigated in this laboratory study for their effectiveness in the stabilization of cadmium, copper, and zinc in two metal-contaminated soils. The adsorption experimental results demonstrated that all three AMD iron oxides possess significant capacity for adsorption of Cd(II), Cu(II), and Zn(II). Horner iron oxide exhibited the highest adsorption capacity. Both the adsorption and the extraction experimental results showed metal sequestration enhancement through addition of Horner iron oxide to soil (5% to 50% by weight). With soil pH of 4.5 to neutral range, AMD iron oxide addition worked best for strongly adsorbed metals such as Cu, not so well for more weakly adsorbed metals such as Cd and Zn. The more AMD iron oxide amendment added, the less the mobility of the cationic target metals. Addition of AMD iron oxide for metal sequestration was more effective for the contaminated soils with low organic content.     

Effect of Metal Oxide Nanoparticles on Microbial Community Structure and Function in Two Different Soil Types

Increased availability of nanoparticle-based products will, inevitably, expose the environment to these materials. Engineered nanoparticles (ENPs) may thus find their way into the soil environment via wastewater, dumpsters and other anthropogenic sources; metallic oxide nanoparticles comprise one group of ENPs that could potentially be hazardous for the environment. Because the soil bacterial community is a major service provider for the ecosystem and humankind, it is critical to study the effects of ENP exposure on soil bacteria. These effects were evaluated by measuring bacterial community activity, composition and size following exposure to copper oxide (CuO) and magnetite (Fe₃O₄) nanosized (<50 nm) particles. Two different soil types were examined: a sandy loam (Bet-Dagan) and a sandy clay loam (Yatir), under two ENP concentrations (1%, 0.1%). Results indicate that the bacterial community in Bet-Dagan soil was more susceptible to change due to exposure to these ENPs, relative to Yatir soil. More specifically, CuO had a strong effect on bacterial hydrolytic activity, oxidative potential, community composition and size in Bet-Dagan soil. Few effects were noted in the Yatir soil, although 1% CuO exposure did cause a significant decreased oxidative potential and changes to community composition. Fe₃O₄ changed the hydrolytic activity and bacterial community composition in Bet-Dagan soil but did not affect the Yatir soil bacterial community. Furthermore, in Bet-Dagan soil, abundance of bacteria annotated to OTUs from the Bacilli class decreased after addition of 0.1% CuO but increased with 1% CuO, while in Yatir soil their abundance was reduced with 1% CuO. Other important soil bacterial groups, including Rhizobiales and Sphingobacteriaceae, were negatively affected by CuO addition to soil. These results indicate that both ENPs are potentially harmful to soil environments. Furthermore, it is suggested that the clay fraction and organic matter in different soils interact with the ENPs and reduce their toxicity.     

Rhizosphere bacterial populations of metallophyte plants in heavy metal-contaminated soils from mining areas in semiarid climate

Rhizosphere bacterial populations associated with four metallophyte plants in one of major polymetallic (Pb–Zn–Cu) semiarid Moroccan Hercynian province (Draâ Sfar, Marrakech, Morocco) presenting long-term contamination mainly with Zn and Pb were analysed and compared to selected control soils. In the highly Zn-, Cu-, Pb- and Cd- contaminated soils, the total number of culturable heterotrophic bacteria were found in low proportions (< 2.6 × 10² – 1.6 × 10⁴ g^–1soil). This bacterial content was slightly similar to that found in moderately polluted and controls soils (6.7 × 10⁴ – 5.8 × 10⁶). However, the bacterial diversity and the rhizosphere/soil ratio, which compares the bacterial content (or bacterial charge) around the metallophyte plants with that in non-rhizosphere soil, were the bacteriological parameters mostly affected by heavy metal contamination. The chronic Zinc-stress results in an increase of tolerance to this metal of both the rhizosphere and non-rhizosphere bacterial communities. However, in general, the rhizosphere bacterial populations exhibited less tolerance to Zn toxicity than the bacterial population of non-rhizosphere soils. This result suggests that toxic effects of Zn decrease in the rhizosphere soils of the metallophyte plants.     

Transfer of selected mineral nutrients and trace elements in the host–hemiparasite association, Cistus–Odontites lutea, growing on and off metal-polluted sites

The role of a hemiparasitic life-style in plant resistance to toxic trace elements in polluted soils is unclear. Restriction of metal uptake by the host, restriction of metal transfer from host to parasite, or transformation of metals into a less toxic form may play a role. This study analysed the transfer of selected mineral elements from soil to host ( Cistus spp . ) and from host to hemiparasite ( Odontites lutea) at locations with different metal burdens: a Cu-rich serpentine site, Pb–Ba mine spoil and an unpolluted soil. Highest soil-to-host transfer factors for K, Mg, Ca, Zn, Cu and Pb were observed on the unpolluted soil. Statistically significant differences among locations of host-to-parasite transfer factors were only found for Ca and Pb. Restriction of transfer of unfavourable Ca/Mg ratios, characteristic at the serpentine site, and of high Pb and Zn concentrations at the Pb–Ba mine occurred mainly at the soil–host, and not at the host–parasite, level. Odontites lutea was able to withstand enhanced Zn and Pb concentrations and low Fe/Cu ratios in shoot tissue without developing toxicity symptoms. This could be caused by specific metal resistance mechanisms in this hemiparasite and/or the transformation and transfer of these metals into a less toxic form by the metal-tolerant host.     

Behavior of Cu and Zn under combined pollution of 2,4-dichlorophenol in the planted soil

Behavior of heavy metals under combined pollution of 2,4-dichlorophenol (2,4-DCP) was investigated using metal contaminated soil which was sampled from the heavily industrialized areas, Fuyang county, Zhejiang Province, P.R. China and pretreated with 100 μg g^?1 2,4-DCP for 1 month. Metal complexes were the predominant species for Cu and Zn in the soil solution. The treatment of 2,4-DCP had limited effect on the dissolution of Cu and Zn in the soil without plant root growth. But the metal species might be changed due to the addition of organic pollutant. Planting with rye grass for 1 month, greatly increased both water soluble Cu and Zn. The increase of water soluble Cu and Zn in the presence of 2,4-DCP was much more than that in the absence of 2,4-DCP, which suggested more attention should be paid to the behavior of heavy metals under combined pollution of organic pollutants in the planted soil. The results also indicated that in comparison to Cu, soil planted with ryegrass was more effective in activating Zn from soils, which was consistent with its relative weak chemisorptions on clays, oxides and humus of soils.     

Assessment of a heavy metals‐contaminated site using sequential extraction,TCLP, and risk assessment techniques

The mobility of selected heavy metals in contaminated soil at a previous industrial site in Brisbane, Australia, was assessed using a sequential extraction technique. Copper, Pb, Zn, Cr, Fe, and Mn were extracted from the soil solution/exchangeable, carbonate, Fe and Mn oxides, and organic matter fractions. The amounts of metals adsorbed by these fractions were used as an indicator of each metal's mobility in the soil. Copper and Pb were largely adsorbed by the organic and oxide fractions, while a significant amount of Zn was extracted from the carbonate fraction. The potential mobility and biological availability of the metals in these soils is Zn > Cr = Cu ≈ Pb. Soils were also analyzed using the toxicity characteristic leaching procedure (TCLP) to determine whether the contaminated soil could be disposed of by landfilling. The leachability of all metals from the soils was very low, with metal concentrations below the allowable limits. The TCLP also showed that Zn was the most mobile metal in these soils. An environmental and health risk assessment was undertaken, and it was concluded that the site did not represent a risk despite the “total”; concentrations of some metals being up to 40 times the investigation threshold value adopted in Australia.     

根瘤菌对土壤铜、锌和镉形态分配的影响

以湖南郴州红壤和河北巩义褐土为供试土壤。制备Cu、Zn、Cd污染土壤。接种大豆根瘤菌(Rhi-zobium fredii)HN01,用连续提取法浸提土壤中不同形态的重金属．结果表明。褐土接种根瘤菌后固相结合态Zn总量降低10％。专性吸附态、氧化锰结合态和有机结合态Zn减少达9％～26％．红壤中结合态Zn的总量变化不显著,但专性吸附态和氧化锰结合态Zn含量显著减少。交换态Zn含量显著增加．褐土中接种根瘤菌抑制了Cu向土壤溶液的释放,固相结合态Cu总量增加18％,可交换态、专性吸附态、氧化锰结合态和有机结合态的Cu增加20％～54％．接种根瘤菌对土壤中Cd的溶解没有明显的抑制或促进作用,但改变了红壤中各形态Cd的含量高低顺序．Cd污染红壤中可交换态和有机结合态Cd含量分别增加22％和11％,专性吸附态和氧化锰结合态Cd分别减少14％和29％．根瘤菌对不同类型重金属及不同土壤中重金属形态影响的差异主要与土壤pH降低有关．     

芜湖钢铁厂周边土壤及油菜籽中镉、铜、锌、铅含量和形态分布研究

研究了污染土壤、油菜籽中Cd、Cu、Zn、Pb含量、形态分布特征和重金属富集状况及可能存在的生物毒性.结果表明,土壤中Cd、Zn、Pb以铁锰氧化物结合态、Cu以残留态占5种形态最高比例,分别为31.1%、39.3%、53.79%、46.24%;Cd、Pb交换态比例较高,为23.47%、16.32%,Cu、Zn的交换态比例较小,为3.14%、0.54%;土壤中不同重金属与各重金属形态相关关系有差别,5种重金属形态转化为有效态重金属难易程度不同;油菜籽和油菜籽壳中不同重金属累积趋势有差异,Cu易在油菜籽壳中累积,Cd、Zn、Pb易在油菜籽中累积;油菜籽中不同重金属累积率不同,Cd累积率最高,为0.56.油菜籽中重金属累积率与土壤中重金属总量呈显著负相关关系(P＜0.05),土壤中重金属的形态、转化差异是此种负相关关系的主要原因;油菜籽中Cd、Cu、Pb以氯化钠态为主,分别为32.50%、22.94%、34.69%,Zn以EDTA态为主,为45.97%.油菜籽中重金属形态可能影响其毒性,但其毒性的人类膳食风险还需进一步研究证实.油菜籽中重金属形态与油菜中重金属总量相关性不好.     

Development of Metal Tolerance in Soil Bacterial Communities Exposed to Experimentally Increased Metal Levels

The development of metal tolerance in soil bacterial communities exposed to different heavy metals was examined under laboratory conditions. An agricultural soil amended with different Zn concentrations was studied most intensively, and measurements were made over a 28-month incubation period by means of the thymidine incorporation technique. Tolerance levels were not affected by metal concentrations lower than 2 mmol of Zn kg (dry weight) of soil(sup-1), but above this value, the level of Zn tolerance increased exponentially with the logarithm of the soil Zn concentration. An increased metal tolerance was detected after only 2 days of Zn exposure. Thereafter, stable tolerance values were observed at different sampling times for bacterial communities exposed to up to 8 mmol of Zn kg (dry weight)(sup-1), indicating no changes in tolerance with time. The tolerance of bacterial communities exposed to 32 mmol of Zn kg (dry weight)(sup-1) increased rapidly within the second week of incubation, but then the values remained unchanged until the end of the experiment. Bacterial communities from soil contaminated with 16 mmol of Zn kg (dry weight)(sup-1) showed an increase of the same magnitude, but the increase started later, after 4 months of incubation, and took place for a much longer period (more than 1 year). Cd, Cu, and Ni addition also resulted in metal-tolerant communities, and the level of tolerance increased with prolonged incubations of the soils. The bacterial community at the end of the incubation period also exhibited a lower pH optimum and an increased tolerance to low osmotic potential. The results suggest that the increase in metal tolerance of the community after adding metals can be attributed to an immediate effect due to the death of sensitive species and a later effect due to different competitive abilities and adaptation of surviving bacteria.     

Phospholipid Fatty Acid Composition and Heavy Metal Tolerance of Soil Microbial Communities along Two Heavy Metal-Polluted Gradients in Coniferous Forests

The effects of long-term heavy metal deposition on microbial community structure and the level of bacterial community tolerance were studied along two different gradients in Scandinavian coniferous forest soils. One was near the Harjavalta smelter in Finland, and one was at Ronnskar in Sweden. Phospholipid fatty acid (PLFA) analysis revealed a gradual change in soil microbial communities along both pollution gradients, and most of the individual PLFAs changed similarly to metal pollution at both sites. The relative quantities of the PLFAs br18:0, br17:0, i16:0, and i16:1 increased with increasing heavy metal concentration, while those of 20:4 and 18:2(omega)6, which is a predominant PLFA in many fungi, decreased. The fungal part of the microbial biomass was found to be more sensitive to heavy metals. This resulted in a decreased fungal/bacterial biomass ratio along the pollution gradient towards the smelters. The thymidine incorporation technique was used to study the heavy metal tolerance of the bacteria. The bacterial community at the Harjavalta smelter, exposed mainly to Cu deposition, exhibited an increased tolerance to Cu but not to Cd, Ni, and Zn. At the Ronnskar smelter the deposition consisting of a mixture of metals increased the bacterial community tolerance to all tested metals. Both the PLFA pattern and the bacterial community tolerance were affected at lower soil metal concentrations than were bacterial counts and bacterial activities. At Harjavalta the increased Cu tolerance of the bacteria and the change in the PLFA pattern of the microbial community were found at the same soil Cu concentrations. This indicated that the altered PLFA pattern was at least partly due to an altered, more metal-tolerant bacterial community. At Ronnskar, where the PLFA data varied more, a correlation between bacterial community tolerance and an altered PLFA pattern was found up to 10 to 15 km from the smelter. Farther away changes in the PLFA pattern could not be explained by an increased community tolerance to metals.     

重金属对土壤中萝卜种子发芽与根伸长抑制的生态毒性

高等植物是生态系统中的基本组成部分。一个平衡、稳定的生态系统生产健康、优良的高等植物。反之 ,一个不稳定或受到外来污染的生态系统 ,对高等植物的生长可带来不利和可见的负面影响。因此 ,利用高等植物的生长状况监测土壤污染程度 ,是从生态学角度衡量土壤健康状况 ,评价土壤质量的重要方法之一[4 ,6] 1) 。土壤生态毒理学评价方法是对化学分析方法的重要补充。目前已建立的高等植物毒理试验有三种方法 ,即 1根伸长试验 ;2种子发芽试验 ;3早期植物幼苗生长试验[3 ,5,6,10 ] 。最初 ,这类试验主要用于纯化学品的毒性检验 ,但随着对土壤…     

菌根植物根际环境对污染土壤中Cu、Zn、Pb、Cd形态的影响

采用根垫法和连续形态分析技术,分析了生长在污灌土壤中菌根小麦和无菌根小麦根际Ｃｕ、Ｚｎ、Ｐｂ、Ｃｄ的形态分布和变化趋势。结果表明,下对照土壤相比,菌根际土壤中交换态Ｃｕ含量显著增加,交换态Ｃｄ呈减少的趋势;与非菌根际相比,Ｃｕ、Ｚｎ、Ｐｂ的有机结合态在菌根根际中显著增加,而４种测定金属２的碳酸盐态和铁锰氧化态都没有显著改变,该结果表明,植物根系能影响根际中金属形态的变化,且菌根比无菌根的影响程度大     

Copper mobility and toxicity of soil percolation water to bacteria in a metal polluted forest soil

In order to assess the success of in situ remediation of coniferous forest soil polluted by a Cu–Ni smelter, the total Cu concentration in soil percolation water, the fluxes of Cu down through the soil profile, and the toxicity of soil percolation water to soil bacteria were studied. Total Cu in percolation water was also fractionated into free ionic and complexed forms. The toxicity of the percolation water was measured by the [³H]-thymidine incorporation method, which measures bacterial growth rates. Soil percolation water was collected during one growing season by zero tension lysimeters inserted at depths of 0.2 and 0.4 m in the soil. The treatments consisted of a control, mulch application to the forest floor (M) and mulch application after removing the polluted organic soil layer (MR). The mulch consisted of a mixture of compost and woodchips (1/1; vol/vol). Analysis of Cu species and dissolved organic carbon (DOC) indicated that DOC leached from the mulch and complexed Cu into forms that were less toxic to soil bacteria. At 0.2 m depth percolation water toxicity was 19% lower in the M and 42% lower in the MR treatment than in the control. Toxicity correlated with the Cu²⁺ concentration, which was 61 and 84% lower in the M and MR treatments, respectively, compared to the control. However, there were signs that total Cu had leached down through the soil profile, the leaching being more pronounced in the MR treatment.     

Zinc and copper effects on metal-tolerant and non-tolerant clones ofAgrostis tenuis (Poaceae)

The toxicity of various Cu and Zn concentrations, as well as their mutual influence upon tolerant and non-tolerant clones ofAgrostis tenuis originating from soils of different metal content has been studied. Specific resistance has been clearly established. Co-existence of toxic metals causes an increase in toxicity. Uptake of metals (Cu and Zn) obviously occurs rather independently, and the toxic activity of the one is not affected competitively by the presence of the other. Above normal concentrations of Cu and Zn negatively affect cell division and growth in roots of non-tolerant genotypes. There is less ability to store metals than in tolerant genotypes.     

Effect of amendments on phytoavailability and fractionation of copper and zinc in a contaminated soil

The ability of amendments to modify the soil properties and influence plants to immobilise Cu and Zn was studied in a naturally contaminated, additionally spiked podzolic soil. Lolium perenne L (perennial rye grass), Festuca rubra L (creeping red fescue) and Poa pratensis L (Kentucky blue grass) were tested in a pot study in the presence of soil amendments (lime, phosphate, and compost, individually and in combination) to assess the effect of soil-plant-amendment interaction on phytostabilisation. The ability of treatments to stabilize metals was assessed on the basis of metal fractionation in soil, partitioning of metals in plants, and metal uptake by the plants. Significant partitioning of Cu into immobile forms occurred as a result of the growth of Festuca rubra, and of Zn by the growth of Poa pratensis. Application of lime significantly reduced the exchangeable fraction of Zn, whereas phosphate application had an accelerating effect on exchangeable Cu. With combined application of amendments, the plant metal concentration decreased by more than 40% for Cu and 70% for Zn, compared to soils receiving no amendments. Combined application of amendments, in conjunction with growth of Festuca and Poa, can be recommended for phytostabilising of Cu and Zn in moderately contaminated acid soils of southwest British Columbia.     

Solid-Phase Chemical Fractionation of Selected Trace Metals in Some Northern Kentucky Soils

The fractionation and distribution with depth of Cd, Cr, Cu, Ni, Pb, and Zn in 26 soils of Northern Kentucky were determined through a sequential extraction procedure in response to environmental concerns about increasing anthropogenic inputs in a fast-paced, urbanizing area. The selected sites have not received any biosolid- or industrial-waste applications. Average total concentrations per metal in soil profiles derived from alluvial, glacial till, and residual materials ranged from 0.43 to 56.00 mg kg^?1 in the sequence Zn > Ni > Pb > Cr > Cu > Cd, suggesting relatively small anthropogenic inputs. The distribution of Cu, Cr, Ni, and Zn increased with soil depth, whereas Cd and Pb remained stable, indicating a strong geological or pedogenic influence. Residual forms were most important for the retention of Cu, Zn, and Ni. Cadmium and Pb exhibited a strong affinity for the Fe-Mn oxide fraction, while Cr showed the strongest association with the organic fraction. In terms of metal mobility and toxicity potential inferred from metal concentrations in labile fractions, Cd posed the greatest risk, followed by Cr ～ Pb > Ni > Zn > Cu. Soil pH, OM, and clay content were the most important parameters explaining the partitioning of metals in labile and residual fractions, emphasizing the importance of metal fractionation in soil management decisions. Alluvial soils generally contained the highest total and labile metal concentrations, suggesting potential metal enrichment through anthropogenic additions and depositional processes. These environments exhibit the highest risk for metal mobilization due to drastic changes in redox conditions, which can destabilize existing metal retention pools.     

Concentrations and chemical forms of heavy metals in agricultural soil near the world’s largest and oldest tungsten mine located in China

Abstract

Tungsten (W) mining has taken place in Ganzhou in China for about 100 years. Such long-term W mining may release large amount of metals to soils and waters around these mines. Twenty soil samples were taken from the area around the W mines and 10 soil samples from an area much farther away. These soil samples were analysed for physicochemical properties, heavy metal content and their chemical forms. Results show that long-term W mining significantly increased both total and labile contents of Cd, Cu, Pb and Zn, but did not, or only slightly, increased the total content of Co, Cr and Ni in the soil near the mine. Average enrichment factor (EF) in the agricultural soils was 4.0, 2.4, 2.2, and 2.0 for Cd, Cu, Pb and Zn, respectively. The labile fraction was dominated by the carbonate-bound fraction for Cd (54.5%) and organic matter-bound fraction for Cu (37.9%), while the major labile fractions for Pb and Zn were associated with carbonates (30.2% and 6.4%), oxides (17.9% and 10.6%) and organic matter (9.2% and 18.8%). Consequently, there is a need to be cautious about Cd in the soils contaminated by W mining.     

Effect of Metal-Rich Sludge Amendments on the Soil Microbial Community

The effects of heavy-metal-containing sewage sludge on the soil microbial community were studied in two agricultural soils of different textures, which had been contaminated separately with three predominantly single metals (Cu, Zn, and Ni) at two different levels more than 20 years ago. We compared three community-based microbiological measurements, namely, phospholipid fatty acid (PLFA) analysis to reveal changes in species composition, the Biolog system to indicate metabolic fingerprints of microbial communities, and the thymidine incorporation technique to measure bacterial community tolerance. In the Luddington soil, bacterial community tolerance increased in all metal treatments compared to an unpolluted-sludge-treated control soil. Community tolerance to specific metals increased the most when the same metal was added to the soil; for example, tolerance to Cu increased most in Cu-polluted treatments. A dose-response effect was also evident. There were also indications of cotolerance to metals whose concentration had not been elevated by the sludge treatment. The PLFA pattern changed in all metal treatments, but the interpretation was complicated by the soil moisture content, which also affected the results. The Biolog measurements indicated similar effects of metals and moisture to the PLFA measurements, but due to high variation between replicates, no significant differences compared to the uncontaminated control were found. In the Lee Valley soil, significant increases in community tolerance were found for the high levels of Cu and Zn, while the PLFA pattern was significantly altered for the soils with high levels of Cu, Ni, and Zn. No effects on the Biolog measurements were found in this soil.     

Copper,Lead, Cadmium,and Zinc Sorption By Waterlogged and Air-Dry Soil

Competitive sorption of copper (Cu), lead (Pb), cadmium (Cd), and zinc (Zn) was studied in three soils of contrasting chemical and physical properties under air-dry and waterlogged conditions. Competitive sorption was determined using the standard batch technique using six solutions, each with Cu, Pb, Cd, and Zn concentrations of approximately 0, 2.5, 5, 10, 20, and 50?mg L^?1Waterlogged soils tended to sorb higher amounts of added Cu, Pb, Zn and Cd relative to soils in the air-dry condition; however, this increase in sorption was generally not statistically (p<0.05) significant. The magnitude of sorption under both waterlogged and air-dry conditions was affected by the type and amount of soil materials involved in metal sorption processes, and competition between other metals for the sorption sites. Metal sorption was closely correlated with soil properties such as cation exchange capacity, organic carbon, and Fe and Mn hydrous oxides. Exchangeable Al may have markedly reduced metal sorption due to its strong affinity for the sorption sites, while increases in exchangeable Mn may have enhanced Zn and Cd sorption. Heavy metal sorption was best described as a combination of both specific and nonspecific interactions. The extractability of Cu, Pb, Cd, and Zn under waterlogged and air-dry conditions was also studied. Three solutions containing these metals were mixed with each soil to achieve a final concentration of 0, 50, and 500?mg kg^?1. Each soil was extracted every 7 days using 1?M MgCl₂ (pH 7) to determine metal extractability. Metal extractability initially decreased then increased due to waterlogging. The increased extractability of added metals was closely related to increased solubility of Fe and Mn suggesting that dissolution of Fe and Mn, oxides under reducing conditions caused a release of previously sorbed Cu, Pb, Cd, and Zn.     

Identification of Soil Bacteria Susceptible to TiO2 and ZnO Nanoparticles

Because soil is expected to be a major sink for engineered nanoparticles (ENPs) released to the environment, the effects of ENPs on soil processes and the organisms that carry them out should be understood. DNA-based fingerprinting analyses have shown that ENPs alter soil bacterial communities, but specific taxon changes remain unknown. We used bar-coded pyrosequencing to explore the responses of diverse bacterial taxa to two widely used ENPs, nano-TiO(2) and nano-ZnO, at various doses (0, 0.5, 1.0, and 2.0 mg g(-1) soil for TiO(2); 0.05, 0.1, and 0.5 mg g(-1) soil for ZnO) in incubated soil microcosms. These ENPs significantly altered the bacterial communities in a dose-dependent manner, with some taxa increasing as a proportion of the community, but more taxa decreasing, indicating that effects mostly reduced diversity. Some of the declining taxa are known to be associated with nitrogen fixation (Rhizobiales, Bradyrhizobiaceae, and Bradyrhizobium) and methane oxidation (Methylobacteriaceae), while some positively impacted taxa are known to be associated with the decomposition of recalcitrant organic pollutants (Sphingomonadaceae) and biopolymers including protein (Streptomycetaceae and Streptomyces), indicating potential consequences to ecosystem-scale processes. The latter was suggested by a positive correlation between protease activity and the relative abundance of Streptomycetaceae (R = 0.49, P = 0.000) and Streptomyces (R = 0.47, P = 0.000). Our results demonstrate that some metal oxide nanoparticles could affect soil bacterial communities and associated processes through effects on susceptible, narrow-function bacterial taxa.