Diversity of rhizobia isolated from an agricultural soil in Argentina based on carbon utilization and effects of herbicides on growth

Seventy-six rhizobial isolates belonging to four different genera were obtained from the root nodules of several legumes (Vicia sativa, Vicia faba, Medicago sativa, Melilotus sp., Glycine max and Lotus corniculatus). The action of five commonly used herbicides [2,4-dichlorophenoxyacetic acid (2,4-D), glyphosate (GF), dicamba, atrazine and metsulfuron-methyl] on the growth of rhizobial strains was assessed. Subsequently, GF and 2,4-D were tested in a minimum broth as C and energy sources for 20 tolerant strains. The ability of these strains to metabolize different carbon sources was studied in order to detect further differences among them. Tolerance of the bacteria to agrochemicals varied; 2,4-D and GF in solid medium inhibited and diminished growth, respectively, in slow-growing rhizobial strains. Among slow-growing strains we detected Bradyrhizobium sp. SJ140 that grew well in broth + GF as the sole C and energy source. No strain was found which could use 2,4-D as sole C source. The 20 strains studied exhibited different patterns of C sources utilization. Cluster analysis revealed three groups, corresponding to four genera of rhizobia: Rhizobium (group I), Sinorhizobium (group II) and Mesorhizobium–Bradyrhizobium (group III). On the basis of the results obtained on responses to herbicides and C sources utilization by the isolates investigated, it was possible to differentiate them at the level of strains. These results evidenced a considerable diversity in rhizobial populations that had not been previously described for Argentinean soils, and suggested a physiological potential to use natural and xenobiotic C sources.     

Chemical characteristics and potential source of fulvic acids leached from the plow layer of paddy soil

To understand better the chemical characteristics and source of dissolved organic matter (DOM) leached from the plow layer of rice paddies, a lysimeter study was conducted, which simulated submerged paddy topsoil during rice growth. The fulvic acid (FA) fraction in the percolation water from the lysimeter was collected by adsorption onto insoluble polyvinylpyrrolidone (PVP), and the temporal variations in its elemental composition, molecular size distribution, IR spectra, ¹³C CPMAS NMR spectra, and δ¹³C values were investigated. The proportion of the FA fraction to bulk DOM varied greatly, but the chemical characteristics of the FA fraction did not change appreciably during the experimental period. Thus, it is considered that the changes in the DOM composition in percolation water were mainly due to the differing contributions of the FA fraction. Further, to investigate the source of the FA fraction in the leachate, the chemical characteristics of the FA fraction in the leachate were compared with those extracted from the plow layer soil. A sequential extraction of the FA fraction was conducted using a sequence of water, 0.25 M Na₂SO₄, 0.1 M Na₄P₂O₇ (pH 7.0), 0.1 M Na₄P₂O₇ (pH 10.5), and NaBH₄+0.1 M Na₄P₂O₇ (pH 10.5). It was found that the water- and 0.25 M Na₂SO₄-extractable fractions, which were most mobile, were not the only source of the FA fraction in the leachate. The small molecular size sub-fraction of the NaBH₄+0.1 M Na₄P₂O₇ (pH 10.5)-extractable FAs, most of which are probably bound mainly to iron oxides, are considered to be another source of the FA fraction leached from the plow layer of paddy fields.     

The Application of Multivariate Statistical Methods for the Evaluation of Soil Profiles (8 pp)

Background, Aim and Scope   Contamination of soils does not only occur on their surface over large areas, but also in depth. Therefore a characterization of soil state after pollution demands a three-dimensional soil sampling, by what a large number of samples has to be analyzed. Analytical results could be evaluated by multivariate statistical methods, which have already been used for the evaluation of data sets containing results from soil sampling of two dimensions like areas or single profiles. In this case study, multivariate statistical methods were applied to investigate structure and interactions between features in a data set containing results of three-dimensional soil sampling. The investigated soil profiles were contaminated by emissions of a former cement and phosphate fertilizer plant. The aim of this study was to determine the remaining extent of contamination and to analyze whether pollutants are mobilized and vertically transported within the profiles. Materials and Methods: Three soil profiles were sampled in the surroundings of the plant. Grain size, organic and carbonatic bonded carbon, pH value, and the total contents of Ca, Cd, Co, Cu, F, Fe, K, Mn, Mg, Na, Ni, P, Pb, and Zn were determined. The resulting data set was evaluated by cluster analysis, linear discriminant analysis, and principal components analysis. The sequential extraction procedure according to Zeien and Brümmer was applied to analyze the binding properties of Ca, Cd, Cu, Na, Pb, and Zn from selected samples. Results: Cd was identified as contaminant of the top soils. The pH values of the bottom soils were determined to be in alkaline range, which is unnaturally high. Variables were clustered according to enrichment of variables in top soils. The samples were classified regarding their pollution state and their substrate by cluster analysis, which was confirmed by linear discriminant analysis. Geogenic and anthropogenic sources of variables as well as relationships between variables like the binding of heavy metals at organic matter were detected by using principal components analysis. The binding of heavy metals at organic matter in the top soils was confirmed by the results of the applied sequential extraction. A vertically altered distribution of Na binding was determined. Discussion: According to the current soil conditions, the uptake of heavy metals had probably occurred by the over ground part of plants during the deposition. The distribution of Na should likely result from the vertical transport of Na, which would also explain the high pH values of the bottom soils by ion exchange. Altogether, the main amount of deposited Ca, F, Na, P, and heavy metals is likely nearly insoluble bound in the top soils. Conclusions: Ten years after the end of production, the pollution of top soils in the surroundings of the former plant is still high. However, regarding the ecotoxicological relevance the now explored interactions between several soil features and elements strongly indicate that there is no short-term to medium-term risk of a mobilization of the deposited elements with the exception of Na. Recommendations and Perspectives: The results of this case study prove that multivariate statistical methods are powerful tools to explore interactions of variables and relationships in a data set derived from three dimensional soil sampling. The methods applied in this work can be highly recommended for evaluations of large data sets resulting from two- or three-dimensional samplings. Multivariate statistical methods enable the characterization of soils and their pollution state in a simple and economic way.     

Reciprocal transfer effects on denitrifying community composition and activity at forest and meadow sites in the Cascade Mountains of Oregon

In order to examine the effects of disturbance, vegetation type, and microclimate on denitrification and denitrifier community composition, experimental plots were established at the H. J. Andrews Experimental Forest in the Cascade Mountains of Oregon. Soil cores were reciprocally transplanted between meadow and forest and samples were collected after 1 and 2 years. Denitrifying enzyme activity (DEA) was measured using the acetylene block assay and terminal restriction length polymorphism profiles were generated with nosZ primers that target the gene coding for nitrous oxide reductase. Nitrate concentrations, C mineralization, and water content were measured to gain additional insights into soil properties controlling DEA. Meadow soils were significantly higher in DEA than forest soils, and the highest DEA levels were observed in cores transferred from the meadow into the forest. Nitrate concentrations were also different between forest and meadow soils, but did not correlate to DEA. DEA was higher in open versus closed cores, suggesting an association between denitrification and the rhizosphere. Denitrifier communities of undisturbed forest and meadow soils shifted through a 4-year period but remained distinct from each other. Similarly, denitrifier communities clustered by vegetation type of origin regardless of manipulation, suggesting that the overall denitrifier communities are well buffered against environmental changes.     

Multi-element analysis of soils and sediments by wavelength-dispersive X-ray fluorescence spectrometry

Soil and sediment reference materials were used to calibrate and evaluate an analytical method for the determination of major (Si, Al, Fe, Mg, Ca, Na, K, Mn, P, Ti) and trace elements (As, Ba, Cd, Co, Cr, Cu, Ga, Mo, Mb, Ni, Pb, Rb, S, Sb, Sn, Sr, Th, U, V, Y, Zn, Zr) by sequential wavelength X-ray fluorescence spectrometry. Samples were prepared as pressed pellets and analysis was done with a total measuring time of thirty minutes per sample. Special attention was given to the selection of the thirty reference materials used for calibration of the spectrometer. Another set of eleven RM (reference materials) was analyzed for the evaluation of accuracy. Detection limits for trace elements (1-2 mg kg^-1) are adequate both for geochemical and environmental purposes, except for Cd and Sb. Accuracy for trace elements falls within the expected interval of certified or recommended values in most cases, but for some major elements, like SiO₂, some results showed discrepancies, evidencing difficulties associated with the determination of light elements in complex matrices. But when quality criteria proposed by mapping programs are applied to the results, their requirements are fulfilled. Both instrumental precision, obtained by twelve sample replicate analyses, and analytical precision, considering also sub-sampling and pellet preparation, lie between the limits of the Horwitz expression, except at concentrations close to the detection limits.     

不同种植模式下紫色土养分流失影响因子研究

通过建立5个不同种植模式的径流小区，对每个小区径流泥沙中养分的含量进行研究．探讨机械组成、微团聚体含量、粘粒团聚度等因子对养分流失影响的程度，结果发现：15～30cm土层中〈0．01mm的粘粒含量与径流泥沙中全钾含量和速效钾含量有显著的相关性。0～15cm层中的〈0．01mm和〈0．001mm微团聚体含量都与径流泥沙中有机质、速效钾和全氮的含量有显著的相关性。同层中〈0．01mm粘粒团聚度与径流泥沙中全氮含量和速效钾含量有显著的相关性，而同层〈0．001mm粘粒团聚度与径流泥沙中全磷含量、速效钾含量和速效磷含量有显著的正相关。     

污染土壤的微生物修复原理与技术进展

微生物修复是土壤环境生物修复技术的重要组成部分,是最具发展和应用前景的生物学环保新技术.本文以土壤中重金属及典型有机污染物为对象,从土壤微生物与污染物质的相互作用入手,较为系统地综合评述国内外污染土壤的微生物修复原理与技术,并结合当前土壤污染的新特点,提出了需进一步认识和解决微生物修复过程中出现的新现象和新问题,以不断丰富和拓展土壤环境微生物修复范畴与内涵.     

The effects of crop husbandry and nitrogen fertilizer on nitrate leaching from a shallow limestone soil growing a five course combinable crop rotation

Abstract. Nitrogen (N) leaching losses from a shallow limestone soil growing a five course combinable croprotation (oilseed rape, wheat, peas, wheat, barley) were measured from 1990 until 1995 using porous ceramic cups, at 60 cm depth, and drainage estimates. The crops were grown with three husbandry systems and two levels of N fertilizer. The husbandry systems were designed to reflect local practice (Standard), the best possible techniques to reduce N loss (Protective) and an Intermediate system which was a compromise between the two. Nitrogen was applied at full and half recommended rates. Drainage started during September in four years and November in one year, with above average drainage in three years. Losses of N were largest after peas (58 kg/ha) and oilseed rape (42 kg/ha), and least (17 kg/ha) before peas sown in spring after a cover crop. Over five years, the Protective management system, which used early sowing and shallow cultivation wherever possible, lost least N (31 kg/ha/y) and the Standard system, with conventional drilling dates and ploughing as the primary cultivation, lost most (49 kg/ha/y). Halving the N fertilizer decreased N loss by 11 kg/ha/y, averaged over the rotation. None of the treatments gave mean drainage water nitrate concentrations of less than 50 mg/l, averaged over the five years. Changes to arable cropping alone will not eliminate the need for other measures to control nitrate concen-trations in public drinking water supplies.     

水稻土和湿地土壤有机碳测定的CNS元素分析仪法与湿消化容量法之比较

土壤固碳研究中需要精确的(有机)C计量,而常规的湿氧化法与CNS元素分析仪法测定结果的吻合性足C计量中的问题.国外对旱地土壤的研究表明,这两种方法的结果基本可以对比,但是否同样对于湿地土壤也适用还不清楚.采用CNS元素分析仪(仪器法)和重铬酸钾外加热湿氧化法(容量法)对20个水稻土样品,26个淡水湿地土壤样品和20个沿海湿地土壤样品进行了总有机C(TOC)的对比测定.结果表明,无论是土壤的表土样品还是剖面样品,仪器法测定结果约低于容量法10%以下.两种方法对淡水湿地土壤和水稻土的有机C含量的测定可以对比,但是重铬酸钾氧化容量法对于含氯化物较高的沿海湿地土壤有机C的测定可能不精确.CNS元素分析仪测定的精密度和准确度较高,可以用于各种湿地土壤的C计量.     

Ion Transport Dynamics in Acid Variable Charge Subsoils

This is a mini-review of the research work conducted by the authors with the objective of studying ion transport in variable charge subsoils collected from different areas around the world. An attempt is made in these studies to relate the unique behavior manifested during ionic transport in these subsoils with their mineralogical, physical and chemical properties, which are markedly different from those in soils from temperate regions. The variable charge subsoils have a relatively high salt sorption capacity and anion exchange capacity (AEC) that retards anions downward movement. The AEC correlates closely with the anion retardation coefficients. Ca²⁺ applied with gypsum in topsoil may be transported to the subsoil and may improve the subsoil chemical properties. These results may help in developing appropriate management strategies under a range of mineralogical, physical, and chemical conditions.