Biodegradation of the herbicide mecoprop-p with soil depth and its relationship with class III tfdA genes

Mecoprop-p [(R)-2-(4-chloro-2-methylphenoxy) propanoic acid) is widely used in agriculture and poses an environmental concern because of its susceptibility to leach from soil to water. We investigated the effect of soil depth on mecoprop-p biodegradation and its relationship with the number and diversity of tfdA related genes, which are the most widely known genes involved in degradation of the phenoxyalkanoic acid group of herbicides by bacteria. Mecoprop-p half-life (DT₅₀) was approximately 12 days in soil sampled from <30 cm depth, and increased progressively with soil depth, reaching over 84 days at 70-80 cm. In sub-soil there was a lag period of between 23 and 34 days prior to a phase of rapid degradation. No lag phase occurred in top-soil samples prior to the onset of degradation. The maximum degradation rate was the same in top-soil and sub-soil samples. Although diverse tfdAα and tfdA genes were present prior to mecoprop-p degradation, real time PCR revealed that degradation was associated with proliferation of tfdA genes. The number of tfdA genes and the most probable number of mecoprop-p degrading organisms in soil prior to mecoprop-p addition were below the limit of quantification and detection respectively. Melting curves from the real time PCR analysis showed that prior to mecoprop-p degradation both class I and class III tfdA genes were present in top- and sub-soil samples. However at all soil depths only tfdA class III genes proliferated during degradation. Denaturing gradient gel electrophoresis confirmed that class III tfdA genes were associated with mecoprop-p degradation. Degradation was not associated with the induction of novel tfdA genes in top- or sub-soil samples, and there were no apparent differences in tfdA gene diversity with soil depth prior to or following degradation.     

Novel genes continue to enhance population growth in adders (Vipera berus)

In 1999, we published a report on how an introduction of 20 males into a severely inbred and isolated population of adders (Vipera berus) halted its decline towards extinction, induced a profound change in population genetic variability, resulted in a dramatic increase in offspring viability and thus in a rapid increase in numbers. Since the publication of our paper we have received numerous inquiries as to whether we have continued to monitor this population, as this is the case we herein present population demographic data gathered between 1981 and 2003. The population has continued to increase in numbers and in 2003, we collected 39 adult male adders, more than at any time over the 23-year study. Our continued work, hence, lends strong support to the importance of novel genes in enhancing the viability of inbred populations and supports the importance of preserving genetic variability in order to maintain viable wild populations.     

Growth and development of Cirsium arvense in relation to herbicide dose,timing of herbicide application and crop presence

The aim of this study is to assess control effects of 2-methyl-4-chlorophenoxyacetic acid (MCPA) dosage, application timing and crop presence on the weed Cirsium arvense. Swedish farmers are recommended to control C. arvense chemically when most shoots are 10-20 cm tall, and mechanically at the compensation point (CP). Recent studies have shown that the CP occurs before shoots reach the three-leaf stage. We hypothesised that (i) herbicide application near the three-leaf stage gives the strongest control, (ii) crop presence increases herbicide effects, and (iii) a 50% herbicide dose gives the same effect as 100%. Treatments of the pot experiment consisted of MCPA 750; 0%, 50% and 100% of the recommended dose applied at leaf stages 3-8, with and without barley. The strongest control was obtained at four leaves and a maximum shoot height of 13 cm, using the recommended dose and with spring barley. In a field population, a maximum shoot height of 13 cm corresponded to a medium height of 6 cm. The 50% dose gave poorer control. Spraying with the recommended dose at the four-leaf stage reduced the development of C. arvense most effectively. Based on this, we recommend that herbicide spraying should be performed at earlier leaf stages/median heights than previously recommended.     

Distribution of Pseudomonas populations harboring phlD or hcnAB biocontrol genes is related to depth in vineyard soils

The abundance and population structure of pseudomonads in soils collected from long-(1006 years) and short-(54 years) term grapevine monocultures in Switzerland were examined across five soil horizons within the 1.20-1.35 m range. Soil samples were baited with grapevine, and rhizosphere pseudomonads containing the biocontrol genes phlD (2,4-diacetylphloroglucinol synthesis) and/or hcnAB (hydrogen cyanide synthesis) were analyzed by MPN-PCR. The numbers of total, phlD⁺ and hcnAB⁺ pseudomonads decreased with depth by 1.5-2 log (short-term monoculture) and 3-3.5 log (long-term monoculture). In addition, the percentages of phlD⁺ (except in short-term monoculture) and hcnAB⁺ pseudomonads were also lower in deeper horizons. RFLP-profiling of phlD⁺ and hcnAB⁺ pseudomonads revealed three phlD and twelve hcnAB alleles overall, but the number of alleles for both decreased in relation to depth. The only phlD allele found in deeper horizons was also found in topsoil, whereas one hcnAB allele (k) found in deeper horizons in long-term monoculture was absent in the topsoil. This suggests that certain Pseudomonas ecotypes are adapted to specific depths. Four hcnAB alleles enabled discrimination between monocultures. We conclude that soil depth is a factor selecting phlD and hcnAB genotypes, and that the allelic diversity of the two biocontrol genes decreases with depth.     

Stimulative effects of elemental sulfur in the presence of ammonium on chile and broccoli growth in calcareous soils

The chile pepper plant seldom responds to N and P fertilizers on fertile soils. Surplus industrial H₂SO₄ and elemental S have created interest in “mining”; calcareous soils for additional supplies of P, Ca, Mg and micronutrients. The effect of variable S, on the growth of chile and broccoli was evaluated holding other nutrients constant. Growth of chile and broccoli plants was significantly increased in the greenhouse and chile yield increased in the field. Incremental S additions increased the water extractable and desorbable Ca + Mg and P contents of soil. The total N and K content of chile plant grown in the greenhouse increased, and then decreased, P decreased, as S rates increased. Yield of dry red chile with constant N peaked at 16.5 g S m^‐2 and then decreased with increasing S in the field. Rroccoli responded more to S application than to directly applied foliar micronutrient solutions (Fe and 7n), and responded much better to (NH₄)₂SO₄ + S than to Ca(NO₃)₂ at equivalent N rates. Increased soluble Ca + Mg content of the soil in the presence of S was thought to influence plant absorption of NH₄ and/or K.     

Field-scale study of the variability in pesticide biodegradation with soil depth and its relationship with soil characteristics

The extent of within-field spatial variability of pesticide degradation was characterised in topsoil and subsoil, using the compounds isoproturon, bentazone and mecoprop, which are major contaminants of groundwater and surface freshwater in Europe. Twenty topsoil samples from 0 to 15 cm depth and twenty subsoil samples from 50 to 60 cm depth were collected from a single agricultural field within a 160×90 grid. It was shown that degradation rates of all compounds declined with soil depth. Variability of pesticide degradation rates, pesticide sorption and formation of non-extractable pesticide residues was higher in subsoil relative to topsoil. Furthermore, in the subsoil, there was variation in large scale soil physicochemical composition, which did not occur in topsoil. The greater variability in pesticide degradation rates in subsoil relative to topsoil could be the result of a greater range of degradation kinetics, which could reflect greater spatial variability in the distribution and/or activities of pesticide metabolising communities.     

钝齿棒杆菌精氨酸生物合成相关基因簇argCJBDFR的扩增及序列分析

采用PCR方法扩增钝齿棒杆菌精氨酸生物合成基因簇argCJBDFR,其序列长为6080bp,其中含有argJ、argB、argD、argF和argR5个完整的开放阅读框。序列和结构分析表明这5个结构基因编码的氨基酸序列与谷氨酸棒杆菌的相应基因编码的氨基酸高度同源,同源性分别为92%、95%、96%、97.5%和100%。     

Bacterial community structure of nirK-bearing denitrifiers and the development of properties of soils in created mitigation wetlands

We investigated the abundance and genetic heterogeneity of bacterial nitrite reductase genes (nir) and soil structural properties in created and natural freshwater wetlands in the Virginia piedmont. Soil attributes included soil organic matter (SOM), total organic carbon (TOC), total nitrogen (TN), pH, gravimetric soil moisture (GSM), and bulk density (D_b). A subset of soil attributes were analyzed across the sites, using euclidean cluster analysis, resulting in three soil condition (SC) groups of increasing wetland soil development (i.e., SC1 < SC2 < SC3; less to more developed or matured) as measured by accumulation of TOC, TN, the increase of GSM, and the decrease of D_b. There were no difference found in the bacterial community diversity between the groups (p = 0.4). NirK gene copies detected ranged between 3.6 × 10⁴ and 3.4 × 10⁷ copies g⁻¹ soil and were significantly higher in the most developed soil group, SC3, than in the least developed soil group, SC1. However, the gene copies were lowest in SC2 that had a significantly higher soil pH (~6.6) than the other two SC groups (~5.3). The same pattern was found in denitrifying enzyme activity (DEA) on a companion study where DEA was found negatively correlated with soil pH. Gene fragments were amplified and products were screened by terminal restriction fragment length polymorphism (T-RFLP) analysis. Among 146 different T-RFs identified, fourteen were dominant and together made up more than 65% of all detected fragments. While SC groups did not relate to whole nirK communities, most soil properties that identified SC groups did significantly correlate to dominant members of the community.     

Effects of amino-acid chemistry and soil properties on the behavior of free amino acids in acidic forest soils

Free amino acids (FAAs) in soil solution are increasingly recognized as a potentially important source of nitrogen (N) for plants, yet we are just beginning to understand the behavior of FAAs in soil. I investigated the effects of amino-acid chemistry and soil properties on mineralization, microbial assimilation and sorption of amino-acid N in soils from three ecosystems representing the two endpoints and mid point of a temperate forest fertility gradient ranging from low mineral N availability/high FAA oak forests to high mineral N availability/low FAA maple-basswood forests. Soils were amended with six ¹⁵N-labeled amino-acid substrates that ranged widely in chemical properties, including molecular weight, C:N ratio, average net charge, hydrophobicity, and polarity: Arginine (Arg), Glutamine (Gln), Glutamate (Glu), Serine (Ser), Glycine (Gly) and Leucine (Leu). Mineralization of amino-acid N accounted for 7-45% (18% avg.) of the added label and was most strongly affected by soil characteristics, with mineralization increasing with increasing soil fertility. Mineralization of amino-acid N was unrelated to amino-acid C:N ratio, rather, I observed greater N mineralization from polar FAAs compared to non-polar ones. Assimilation of amino-acid N into microbial biomass accounted for 6-48% (29% avg.) of the added label, and was poorly predicted by either intrinsic amino-acid properties or soil properties, but instead appeared to be explicable in terms of compound-specific demand by soil micoorganisms. Sorption of amino-acid N to soil solids accounted for 4-15% (7% avg.) of the added label and was largely controlled by charge characteristics of individual amino acids. The fact that both positively- and negatively-charged amino acids were more strongly sorbed than neutral ones suggests that cation and anion exchange sites are an important factor controlling sorption of FAAs in these acid forest soils. Together, the findings from this study suggest that there may be important differences in the behavior of free amino acids in sandy, acidic forest soils compared to generalizations drawn from finer-textured grassland soils, which, in turn, might affect the availability of some FAAs in soil solution.     

Magnetic properties of the urban soils in Shanghai and their environmental implications

To study the feasibility of using magnetic techniques for monitoring soil pollution in Shanghai, magnetic properties and heavy metals in the topsoils in an urban site (Songnan Town) and a less-urbanized agricultural site (Luojing Town) in Baoshan District, Shanghai, were studied. Compared with the background, magnetic signals of the urban topsoils are extremely enhanced with magnetic susceptibility (χ_lf) from 127.3–1959 × 10^− 8 m³ kg^− 1; while those of the agricultural topsoils are only slightly increased. However, both the urban and agricultural topsoils contain few pedogenic SP grains, as indicated by their low χ_fd% (< 3.6%). Ratios of χ_arm/SIRM, χ_arm/χ_lf and SIRM/χ_lf indicate that the grain size of magnetic minerals in the urban topsoils is significantly coarser than that in the background and the agricultural topsoils. Furthermore, the urban topsoils show low coercivity and magnetic soft behaviors, as indicated by higher SOFT%, lower HARD%, higher IRM_300 mT/SIRM (close to 1) and lower IRM_− 200 mT/SIRM (close to − 1). It suggests that the urban topsoils have received some coarse ferrimagnetic particles. Heavy metals are highly enriched in the magnetic fractions of the topsoils. Geochemical properties of the magnetic fraction of the urban topsoils are significantly different from those of the agricultural topsoils, further indicating that the extra magnetic minerals accumulated in the urban topsoils are neither inherited from soil parent materials nor from pedogenic processes, but originate from anthropogenic activities. The significant correlations between heavy metals and χ_lf, χ_arm, SIRM, SOFT and HIRM of the topsoils in the district indicate that the magnetic techniques can be used for monitoring soil pollution in Shanghai. The soils with χ_lf from 39–50 × 10^− 8 m³ kg^− 1 in the district are tentatively defined as “slightly polluted soils”; those with χ_lf > 50 × 10^− 8 m³ kg^− 1 are defined as “polluted soils”.     

Effects of long-term manure applications on the occurrence of antibiotics and antibiotic resistance genes (ARGs) in paddy soils: Evidence from four field experiments in south of China

Most studies of the effects of manure amendment on the occurrence of antibiotics and antibiotic resistance genes (ARGs) in soil employ the investigation of grab samples or short-term laboratory studies. However, the effects of long-term manure applications on antibiotics, ARGs and their vertical distribution in paddy soil in field experiments are lacking. We assessed the concentrations of antibiotics, ARGs and their vertical distribution in paddy soil receiving long-term manure applications in four field experiments. High concentrations of tetracyclines were detected in most manured soils, while sulfonamides were not detectable. Long-term manure amendments generally increased the antibiotic concentrations and ARGs abundances in the paddy soil over decades. However, in some sites such significant trends of ARGs could not be observed. The abundance of ARGs was statistically correlated with antibiotics and soil properties including pH and soil organic matter (SOM), indicating their importance in the selection of resistance genes. Tetracyclines could be detected in soil at different depths and the concentrations of tetracyclines and abundance of ARGs generally decreased with increasing soil depths.     

Immobilization and species of heavy metals in soils in the absence and presence of rhizobia

Abstract

The effect of bacterial inoculation of Rhizobium fredii HN01 on the immobilization and speciation of Cu, Zn, and Cd was studied in Red and Cinnamon soil which are typical Chinese soils. The soil was mixed with bacterial suspension for one week followed by an immobilization of each heavy metal for another week. The total binding and fractionation of heavy metals in soils were analyzed. As compared with the control, the retention of total Cu, Zn, and Cd in Red soil increased by 28, 16, and 28%, respectively, in the presence of rhizobia. The amount of exchangeable, NH₄OAc-extractable, Mn oxides-bound and organic matter-bound Cu increased by 23–123%. There were significant decrease of exchangeable Cu and marked increases of NH₄OAc-extractable and Mn oxide-bound Cu in Cinnamon soil with the presence of rhizobial cells, although no changes for the total retention of Cu were observed. The amount of exchangeable Zn in Red soil-rhizobia composite was 20% greater than that of the no-rhizobia soil. Addition of rhizobia also increased exchangeable Cd and specifically-adsorbed Cd by 25 and 93%, respectively, in Red soil. No considerable differences were found for the total immobilization of Zn and Cd as well as their distribution in various solid fractions of Cinnamon soil in the absence and presence of rhizobial cells. In terms of soil components, it is assumed that bacterial biomass had a relatively less impact on the species of heavy metals bound with Fe oxides. Results suggested that the retention and speciation of heavy metals in soil are governed largely by the interactions of bacteria with various inorganic and organic soil constituents. The data are useful in understanding the impact of microorganisms on the behavior, mobility and transformation of heavy metals in soil environments.     

土壤压实指标在城市土壤评价中的应用与比较

通过测定南京市不同土地利用下的52个样点的紧实度、容重和孔隙度3个压实指标来反映城市土壤的压实程度。结果表明,南京市大多数土壤存在不同程度的压实,部分压实严重,可能限制植物的生长。不同压实指标在反映土壤压实程度上基本一致,它们之间具有极显著的相关性,可以相互转换。但紧实度指标受到土壤含水量的显著影响。在同一质地或质地相近的土壤,容重和孔隙度可以很好地反映土壤的压实程度。与总孔隙度和毛管孔隙度相比,通气孔隙度在反映土壤压实时更为敏感。所以在进行城市土壤压实状况评价时,可以选择不同的土壤压实程度指标,但就方法的实用性和可靠性来说,容重比紧实度和孔隙度指标一般更方便可靠。     

Net nitrification rate and presence of Nitrosospira cluster 2 in acid coniferous forest soils appear to be tree species specific

The impact of four coniferous tree species and their corresponding soil factors on N transformation rates and presence of ammonia-oxidising bacteria (AOB) was studied in an acid pine forest soil (Appelscha, The Netherlands). Pine soil had a relatively low net nitrification rate, while spruce, fir and larch soils showed high net nitrification rates. 16S rRNA and amoA sequences were only found in soils with high nitrification rates and belonged solely to Nitrosospira cluster 2. We conclude that tree species, possibly through their effects on soil C/N ratios, determines the presence of Nitrosospira cluster 2. Whenever AOB are present, however, the AOB community composition appears to be similar.     

Influence of agricultural soils on the growth and reproduction of the bio-indicator Folsomia candida

Soil organisms have been recommended as bio-indicators of soil quality due to their sensitivity to anthropogenic influences and their high degree of site-specificity. The objectives of this study were to determine if the relationship between the soil Collembola, Folsomia candida, and forest soil could be extended to other soils and, if not, to determine the relationship between its life parameters and soils from the different phases of an agricultural rotation sequence. A comparison of growth, reproduction and survival of 1 day old neonates and 10 day old juveniles subjected to a series of different soils (composted manure, pasture soil, forest soil, sand alone, and sand supplemented with yeast) established that the association between F. candida and forest soil quality cannot be automatically transferred to soils from other ecosystems. The test also showed that neonate (1 d) individuals were more sensitive to the different soil treatments than 10 d old juveniles and should be used in future tests. On this basis, neonate F. candida individuals were used to determine if the species could be employed to characterize the agricultural soils from the different phases of an organic or conventional rotation sequence. Results demonstrated that changes in body growth and reproduction constituted suitable criteria to characterize the soil quality of these different phases. The research required to develop these results as a standard bio-indicator test of agricultural soil quality is discussed.     

Phylogeny and nodulation signal molecule of rhizobial populations able to nodulate common beans—other than the predominant species Rhizobium etli—present in soils from the northwest of Argentina

We examined the bean rhizobia community other than the predominant species Rhizobium etli present in soils of a region that is part of the range occupied by the host in Northwest Argentina, which showed Rep and 16S rDNA RFLP polymorphism. Two populations represented by isolates T29N3L and T44N22P were found to be distinct chromosomal genotypes and closely related to species Rhizobium tropici and Agrobacterium rhizogenes. Their symbiotic genes were analyzed and found to cluster with those from R. tropici as well as with rhizobia isolated from leguminous trees. Three nodulation metabolites produced by T44N22P were detected which are tetra- and pentameric chitocompounds, N-methylated, O-carbamoylated, and N-substituted either by a C_18:0 or C_18:1 acyl chain at their non-reducing end, and all them sulphated at the reducing end. Isolates T29N3L and T44N22P exhibited broad host range but unlike T29N3L, only T44N22P was able to efficiently nodulate Medicago truncatula.     

Ecogeographic adaptation and genetic variation in wild barley, Application of molecular markers targeted to environmentally regulated genes

Long primer PCR (LP-PCR) markers specifically targeted to sequences involved in the response to abiotic stress were utilised to analyse genetic variation within and among wild barley populations from Israel.Populations were sampled in locations reflecting a wide range of habitats and the potential correlation between genetic variation and ecogeographic parameters was examined. A high level of polymorphism was observed. Most of the variability was due to differences within populations, however a substantial portion of genetic variation (36%) could be attributed to differences among populations, indicating local adaptation of genotypes to microclimatic conditions. Moreover, clustering and ordination techniques showed that genotypes grouped together according to their area of origin and were separated on thebasis of ecogeography, chiefly by a combination of temperature, altitude and rainfall variables. Finally, 28% of the polymorphic PCR fragments were significantly correlated with ecogeographic parameters. Our results demonstrate the effectiveness of PCR-based molecular markers targeted to environmentally regulated genes in detecting useful variation andthus in monitoring the impact exerted by adaptation to the environment ongenetic differentiation.     

Contribution of technic materials to the mobile fraction of metals in urban soils in Marrakech (Morocco)

Background, Aim and Scope  In urban areas, soils are often dramatically altered by anthropogenic activity and these modifications distinguish these soils (Anthrosols, Technosols) from those in natural systems. In urban environments, they receive considerable pollution from industry, traffic and refuse. Since contaminated soil particles can be easily inhaled or ingested, there is a potential transfer of toxic pollutants to humans. Risk assessment is essentially based on the determination of the total or mobile contents of pollutants in soils using chemical extractions. This approach could be improved by taking into consideration the bioavailable fractions of these toxic elements as measured by biotests. The coarse soil fraction usually neglected in analyses can nevertheless have an effect on the concentration of metals in the soil solution. This coarse fraction is made up of the natural materials and of technic materials constituting anthropogenic soils (plastic, paper, fabric, wood, bones, metallic elements and building materials). These materials have variable capacities to release or adsorb trace elements. Samples representative of different technic fraction components of Marrakech urban soils permit one to quantify their contribution to the enrichment of the soluble metal concentrations. Works are carried out to achieve partial extractions of metals from the three fractions (less than 2 mm, coarse natural and coarse technic) of selected urban soils in order to determine their contribution to the metal contamination of soils. Materials and Methods  Selected soils were collected from 9 sites according to a gradient of increasing anthropogenic influence from suburban to urban zones. Soils were air-dried, homogenized, and sieved (2 mm). The coarse fraction was sorted to separate the different technic materials and natural materials. Water extractions were run, on the natural, coarse fraction, on the complete technic fraction of the 9 soils and on average samples made of technic materials sorted out of 58 topsoils sampled from different sites in the city of Marrakech. Results  Results show that the percentage of the technic fraction increases while approaching the historic city center. It represented about 14% in the most anthropogenically disturbed soils. Along this gradient, soils changed progressively from Anthrosols to Technosols according to the WRB classification of urban and industrial soils. Analyses of metal contents showed that the fine fraction (<2 mm) mainly contributed to the metallic contamination of the water soluble fraction. The natural coarse fraction had the highest contribution to the copper release and was responsible for the release of all water-extractable copper in some soils. Concerning the technic fraction, it has a significant contribution essentially in the most anthropogenically disturbed soils as characterized by an elevated percentage of anthropogenic elements. The water extractable metal contents of average samples of these anthropogenic elements shows that elevated metal concentrations were released by bones, wood, plastic and fabric/paper. Discussion  This study concerns soils in urban areas, which are strongly impacted by human activities. Part of the soils can be classified as Anthrosols, profoundly impacted through the addition of organic materials from household wastes, irrigation, or cultivation. Other soils strongly impacted by human activities are Technosols dominated or strongly influenced by man-made materials. Technosols appear mostly in urban and industrial areas and are more likely to be contaminated than Anthrosols. The composition and heterogeneity of urban soils lead to modifications of the mobility and availability of pollutants depending on successive land-uses and on the composition of technic materials. The fine fraction offers a high transferring surface capacity, leading to a high mobilization of metals. The technic fraction contributes significantly to the metal release in the Technosols. This property can be explained by a reversible adsorption of metals on the organic matter. Conclusions  Results confirm that anthropogenic activity causes a wide spatial diversity of soil quality in the urban and suburban area. It introduces large amounts of technic materials in soils that could have an impact on the metal availability. It therefore acts on the metal bioavailability in the urban Technosols. Recommendations and Perspectives  These results show that it is necessary, in addition to the characterization of the fine particles, to take into account the contribution of the coarse fraction of the Technosols in the evaluation of risks of transfer of metals to the food chain.     

Retention and extractability of phenol,cresol, and dichlorophenol exposed to two surface soils in the presence of horseradish peroxidase enzyme

The retention of phenol, o-cresol, 2,4-dichlorophenol (DCP), and their peroxidase-catalyzed polymerization products was evaluated on two surface soils. The extractability of the parent solutes and their polymerization products was also investigated. (14)C-Labeled radioisotopes were used to quantify the contaminant retained on soil as water-extractable, methanol-extractable, humic/fulvic (HA/FA) acid-bound, and soil/humin bound. Between 2 and 20% of the solute retained on soil after a 7-day contact period remained bound to the HA/FA and soil/humin components in unamended soils; in the presence of peroxidase this amount was as high as 40-75%. The alkali-extractable HA/FA component contained the largest fraction of radioactivity in peroxidase-amended soils. Whereas the soil organic matter content was the predominant factor controlling the extent of sorption of the parent phenols, the clay content and particle surface area appeared to contribute to the retention of the polymerization products. High molecular weight oligomers produced during peroxidase-mediated polymerization of phenols associate strongly with soil components and are likely incorporated into the soil organic matter via oxidative coupling reactions.     

Diversity and Characterization of Potential H2-Dependent Fe(Ⅲ)-Reducing Bacteria in Paddy Soils

Microbial ferric iron reduction, with organic carbon or hydrogen as the electron donor, is one of the most important biogeochemical processes in anoxic paddy soils; however, the diversity and community structure of hydrogen-dependent dissimilatory iron-reducers remain unknown. Potential H2-dependent Fe(III)-reducing bacteria in paddy soils were explored using enrichment cultures with ferrihydrite or goethite as the electron acceptor and hydrogen as the electron donor. Terminal restriction fragment length polymorphism (T-RFLP) analysis and cloning/sequencing were conducted to reveal bacterial community structure. Results showed that Geobacter and Clostridium were the dominant bacteria in the enrichment cultures. Fe(III) oxide mineral phases showed a strong effect on the community structure; Geobacter and Clostridium were dominant in the ferrihydrite treatment, while Clostridium spp. were dominant in the goethite treatment. These suggested that H2-dependent Fe(III)-reducing bacteria might be widely distributed in paddy soils and that besides Geobacter, Clostridium spp. might also be an important group of H2-dependent Fe(III)-reducing microorganisms.