The effect of DD nematocide on soil micro-organisms

The application of DD, a 1∶1 mixture of 1,3-dichloropropene and 1,2-dichloropropane, at field rate, to soil had neither stimulatory nor suppressive effects on bacteria, actinomycetes, fungi and cellulose decomposing bacteria. The toxic effect of the nematocide onAzotobacter and nitrogen-fixing clostridia was slight, the organisms recovering in a very short time. In fumigated soils the counts ofNitrosomonas andNitrobacter were markedly reduced and remained lower than the counts in the untreated soils for the 30 days duration of the experiment. The inhibitory effects of DD were to some extent influenced by soil type, being most marked in sandy and loam and least in calcareous soils.     

Influence of root-induced pH on the solubility of soil aluminium in the rhizosphere

Increase in solubility of soil aluminium (Al) as a result of root-induced decrease of soil pH was studied. Soil samples of known distances from the roots of NH₄-N fertilized Ryegrass were analyzed for pH and aluminium extractable with 0.01 M CaCl₂. Results showed that though no Al was found in bulk soil (pH 6.8), its concentration in the vicinity of roots increased to 0.023 mM with a concomitant decrease of soil pH from 6.8 to 4.4.     

Effect of antibiotics on ethylene production by soil micro-organisms

Summary Treatment of arable soils with antibacterial (novobiocin) and antifungal (cycloheximide) agents had no effect on ethylene accumulation. Novobiocin had a very small effect on the growth ofMucor hiemalis and cycloheximide decreased its maximum specific growth rate but neither agent impaired the capacity of the fungal culture to produce ethylene. Caution must therefore be exercised when using antibiotics to establish the source of soil metabolic processes.     

Molecular and functional diversity in soil micro-organisms

Traditional approaches to the study of microbial diversity have relied on laboratory cultivation of isolates from natural environments and identification by classical techniques, including analysis of morphology, physiological characteristics and biochemical properties. These approaches provide information on fine-scale diversity but suffer from bias, resulting from the media and cultivation conditions employed, and from the inability to grow and isolate significant proportions of natural communities in the laboratory. An alternative approach is the amplification of ribosomal RNA and functional genes from nucleic acids extracted directly from environmental samples, with subsequent analysis by `fingerprinting' methods or by sequencing and phylogenetic analysis. This approach avoids the need for laboratory cultivation and has provided major insights into species and functional diversity of bacterial and archaeal populations. This article reviews molecular approaches to the characterisation of prokaryote diversity in natural environments, their more recent application to fungal diversity and the advantages and limitations of molecular analyses.     

Effect of copper on the degradation of phenanthrene by soil micro-organisms

AIMS: The effect of copper on the degradation by soil micro-organisms of phenanthrene, a polycyclic aromatic hydrocarbon, was investigated. METHODS AND RESULTS: Inert nylon filters were incubated in the soil for 28 days at 25 degrees C. Each filter was inoculated with a soil suspension, phenanthrene (400 ppm), copper (0, 70, 700 or 7000 ppm) and nitrogen/phosphorus sources. The filters were assessed for phenanthrene degradation, microbial respiration and colonization. Phenanthrene degradation proceeded even at toxic copper levels (700/7000 ppm), indicating the presence of phenanthrene-degrading, copper-resistant and/or -tolerant microbes. However, copper at these high levels reduced microbial activity (CO2 evolution). CONCLUSION: High levels of copper caused an incomplete mineralization of phenanthrene and possible accumulation of its metabolites. SIGNIFICANCE AND IMPACT OF THE STUDY: The presence of heavy metals in soils could seriously affect the bioremediation of PAH-polluted environments.     

Biodegradation of wheat stubbles by soil micro-organisms and role of the products on soil fertility

Summary Soil microorganisms caused considerable degradation of wheat stubbles under laboratory conditions. Mixtures of different organisms proved to be more efficient than individual organisms in degrading lignin, holocellulose and nitrogenous substances which constitute the major components in stubble. Lignin and holocellulose content in fresh stubble was about 25% and 60% of dry weight respectively. Degradation of these compounds caused an increase in simpler components like soluble carbohydrates. Degradation of nitrogenous substances (represented by 0.7% total nitrogen in fresh stubbes) caused accretion in the soluble nitrogen of the degraded product. In most cases, these products proved to be important in improving fertility of the soil thereby causing significant promotion of growth of crops like maize and rice.     

An effect of conifers on available soil nitrogen

Summary Comparison of the soil and herbaceous vegetation beneath young conifers and adjacent open fields identical in prior treatment shows that conifers increase nitrogen availability above their own net uptake. The effect extends a short distance beyond their crowns. The presumed explanation is greater mineralization of soil nitrogen rather than fixation.     

The use of thin soil sections for the study of soil micro-organisms

Summary Modifications of Alexander and Jackson's¹ technique for the preparation of thin soil sections are described.The distribution of bacterial colonies within soil aggregates has been examined using this technique and it has been demonstrated that distinct distribution patterns may occur.     

Influence of organic residues and soil incorporation on temporal measures of microbial biomass and plant available nitrogen

Aims

Despite our current understanding of plant nitrogen (N) uptake and soil N dynamics in arable systems, the supply and demand of N are infrequently matched as a result of variable seasonal and soil conditions. Consequently, inefficiencies in N utilisation often lead to constrained production and can contribute to potential environmental impacts. The aim of this study was to examine the influence of plant residue quality (C/N ratio) and extent of residue incorporation into soil on temporal changes in soil mineral N and the associated plant N uptake by wheat in the semi-arid agricultural production zone of Western Australia.

Methods

Oat (Avena sativa); lupin (Lupinus angustifolius) and field pea (Pisum sativum) were incorporated into a Red-Brown Earth using varying degrees of mechanical disturbance (0 to 100% residue incorporated). Soil samples for inorganic N (NO ₃ ^? and NH ₄ ⁺ ) profiles (0?C50?cm), microbial biomass-C (0?C50?cm) and plant N uptake were taken throughout the growing season of the subsequent wheat (Triticum aestivum) crop. Grain yield and yield components were determined at harvest.

Results

Despite observed treatment effects for plant residue type and soil disturbance, fluctuations in inorganic N were more readily influenced by seasonal variability associated with wet-dry cycles. Treatment effects resulting from residue management and extent of soil disturbance were also more readily distinguished in the NO ₃ ^? pool. The release of N from crop residues significantly increased (p?=?0.05) with greater soil-residue contact which related to the method of incorporation; the greater the extent of soil disturbance, the greater the net supply of inorganic N. Differences in microbial biomass-C were primarily associated with the type of plant residue incorporated, with higher microbial biomass generally associated with legume crops. No effect of residue incorporation method was noted for microbial biomass suggesting little effect of soil disturbance on the microbial population in this soil.

Conclusions

Despite differences in the magnitude of N release, neither crop type nor incorporation method significantly altered the timing or pattern of N release. As such asynchrony of N supply was not improved through residue or soil management, or through increased microbial biomass in this semi-arid environment. N fluxes were primarily controlled by abiotic factors (e.g. climate), which in this study dominated over imposed agricultural management practices associated with residue management.     

The influence of soil micro-organisms on the growth and chemical composition of soybean

Summary A plastic film isolator was adapted for the growth of soybeans in sterile soil, sterile reinoculated soil, and unsterilized soil. The presence of micro-organisms significantly increased the mass of tops and roots as well as the plant content of Ca, Mg, Fe, Al, Cu, Zn, and Mo. Plant content of N, P, and K was not affected by microbial activity, while manganese, made available by steam-sterilization, was oxidized to unavailable forms by the soil microflora. The mixed microflora influenced the quantity and distribution of free amino acids within the plant but did not affect the distribution of amino acids within the crude protein fraction.     

Biologically available versus exchangeable magnesium in soil

Summary Equal weight of dry mycelium of the fungusAspergillus niger was found equivalent with equal magnesium uptake from synthetic nutrient (as standard series) and from solutions with soil as magnesium source. The quantities of magnesium remaining in the nutrient solutions after growth were also the same in cultures with or without addition of soil. It may be concluded that the method employed will give a correct picture of the ability of the fungus to release magnesium from the soil.During growth the fungus releases magnesium in excess of the maximum content of magnesium exchangeable with ammonium ions. This excess was found to be derived mainly from the clay fraction of the soil, although the silt fraction gives a considerable supplement. This is emphasized by the results from up to five repeated cultivations of the fungus in the same soil or its clay-/silt-fractions.Microbiologically available magnesium was determined after removal of the exchangeable fraction in 13 soils after three years of exhaustion by crop plants. The results showed a marked correlation (r=0.90) between the magnesium uptake by the fungus and the higher plant, and in addition the actual figures for magnesium uptake were on a comparable magnitude. Analysis of the soils after the third year of experiment showed that the fraction of magnesium available toAspergillus niger was not exhausted but only reduced to approximately one-half of the original content.     

土壤速效养分空间变异的尺度效应

在GIS技术支持下,运用经典统计学和地统计学的方法,从经典统计分析、变异函数和Kriging插值图等方面探讨了禹城市耕地土壤速效磷(AP)、速效钾(AK)在县级和镇级两个不同采样尺度下的空间变异特征.结果表明:在两个采样尺度下,AP、AK都服从对数正态分布,它们的变异系数范围为26.5%～36.6%,均属中等变异强度,随着采样尺度的缩小,土壤AP、AK的变异系数都增大.两个采样尺度下,土壤AP和AK均在一定范围内存在空间相关关系,县级采样尺度下土壤AP和AK的空间自相关距离较大,分别为9.0 km和26.5 km,镇级采样尺度下土壤AP和AK的空间自相关距离明显变小,分别为1.7 km和2.8 km.两个采样尺度下的土壤AP和AK受结构性因素和随机性因素的影响,表现出明显不同的分布规律.     

Microbial diversity in soil: effect of releasing genetically engineered micro-organisms

This review examines the potential for change in microbial diversity, with the emphasis on bacteria, in soil resulting from the introduction of genetically engineered microorganisms (GEMs). With the advent of GEMs came the impetus for new technologies to recover these micro-organisms from soil and to assess their effects on microbial diversity. This review also presents general aspects of and genetic approaches to accessing bacterial diversity in the environment.