Expansion and intensification of cultivation are among the predominant global changes of this century. Intensification of agriculture by use of high-yielding crop varieties, fertilization, irrigation, and pesticides has contributed substantially to the tremendous increases in food production over the past 50 years. Land conversion and intensification, however, also alter the biotic interactions and patterns of resource availability in ecosystems and can have serious local, regional, and global environmental consequences. The use of ecologically based management strategies can increase the sustainability of agricultural production while reducing off-site consequences.

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Agricultural Intensification and Ecosystem Properties

Preface.- Contributors.- List of Abbreviations.- Section 1: Basic Principles: Introduction.-Sources of Heavy Metals and Metalloids in Soils.- Chemistry of Heavy Metals and Metalloids in Soils.- Methods for the Determination of Heavy Metals and Metalloids in Soils.- Effects of Heavy Metals and Metalloids on Soil Organisms.- Soil-Plant Relationships of Heavy Metals and Metalloids.- Heavy Metals and Metalloids as Micronutrients for Plants and Animals.-Critical Loads of Heavy Metals for Soils.- Section 2: Key Heavy Metals And Metalloids: Arsenic.- Cadmium.- Chromium and Nickel.- Cobalt and Manganese.- Copper.-Lead.- Mercury.- Selenium.- Zinc.- Section 3: Other Heavy Metals And Metalloids Of Potential Environmental Significance: Antimony.- Barium.- Gold.- Molybdenum.- Silver.- Thallium.- Tin.- Tungsten.- Uranium.- Vanadium.- Glossary of Specialized Terms.- Index.

Heavy metals in soils : trace metals and metalloids in soils and their bioavailability

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Myths and Dogmas of Biocontrol Changes in Perceptions Derived from Research on Trichoderma harzinum T-22.

We investigated the capability of the plant-growth-promoting and biocontrol fungus Trichoderma harzianum Rifai 1295-22 (T-22) to solubilize in vitro some insoluble or sparingly soluble minerals via three possible mechanisms: acidification of the medium, production of chelating metabolites, and redox activity. T-22 was able to solubilize MnO2, metallic zinc, and rock phosphate (mostly calcium phosphate) in a liquid sucrose-yeast extract medium, as determined by inductively coupled plasma emission spectroscopy. Acidification was not the major mechanism of solubilization since the pH of cultures never fell below 5.0 and in cultures containing MnO2 the pH rose from 6.8 to 7.4. Organic acids were not detected by high-performance thin-layer chromatography in the culture filtrates. Fe2O3, MnO2, Zn, and rock phosphate were also solubilized by cell-free culture filtrates. The chelating activity of T-22 culture filtrates was determined by a method based on measurement of the equilibrium concentration of the chrome azurol S complex in the presence of other chelating substances. A size exclusion chromatographic separation of the components of the culture filtrates indicated the presence of a complexed form of Fe but no chelation of Mn. In liquid culture, T. harzianum T-22 also produced diffusible metabolites capable of reducing Fe(III) and Cu(II), as determined by the formation of Fe(II)-Na2-bathophenanthrolinedisulfonic acid and Cu(I)-Na2-2, 9-dimethyl-4,7-diphenyl-1,10-phenanthrolinedisulfonic acid complexes. This is the first report of the ability of a Trichoderma strain to solubilize insoluble or sparingly soluble minerals. This activity may explain, at least partially, the ability of T-22 to increase plant growth. Solubilization of metal oxides by Trichoderma involves both chelation and reduction. Both of these mechanisms also play a role in biocontrol of plant pathogens, and they may be part of a multiple-component action exerted by T-22 to achieve effective biocontrol under a variety of environmental conditions.

Solubilization of phosphates and micronutrients by the plant-growth-promoting and biocontrol fungus trichoderma harzianum rifai 1295-22

The essential micronutrients for field crops are B, Cu, Fe, Mn, Mo, and Zn. Other mineral nutrients at low concentrations considered essential to growth of some plants are Ni and Co. The incidence of micronutrient deficiencies in crops has increased markedly in recent years due to intensive cropping, loss of top soil by erosion, losses of micronutrients through leaching, liming of acid soils, decreased proportions of farmyard manure compared to chemical fertilizers, increased purity of chemical fertilizers, and use of marginal lands for crop production. Micronutrient deficiency problems are also aggravated by the high demand of modern crop cultivars. Increases in crop yields from application of micronutrients have been reported in many parts of the world. Factors such as pH, redox potential, biological activity, SOM, cation-exchange capacity, and clay contents are important in determining the availability of micronutrients in soils. Plant factors such as root and root hair morphology (length, density, surface area), root-induced changes (secretion of H + , OH − , HCO 3 − ), root exudation of organic acids (citric, malic, tartaric, oxalic, phenolic), sugars, and nonproteinogenic amino acids (phytosiderophores), secretion of enzymes (phosphatases), plant demand, plant species/cultivars, and microbial associations (enhanced CO 2 production, rhizobia, mycorrhizae, rhizobacteria) have profound influences on plant ability to absorb and utilize micronutrients from soil. The objectives of this article are to report advances in research on the micronutrient availability and requirements for crops, in correcting deficiencies and toxicities in soils and plants, and in increasing the ability of plants to acquire needed amounts of micronutrient elements.

Micronutrients in Crop Production

Nitrogen form and plant disease.

Mineral nutrition and plant disease.

A multiple component analysis of the take-all disease of cereals

Plant nutrition, although frequently unrecognized, has always been an important component of disease control. The effects on disease of crop rotation, crop sequence, liming for pH adjustment, irrigation, and organic amendments such as manure are frequently through nutritional interactions as much as other factors. These practices supply nutrients directly, or render them more or less readily available for plant uptake through altered solubility or modified microbial activity. The effect of microbial activity and environment also may obscure the role of a specific element. The generally adequate availability of most nutrients for plant growth has induced an apathetic attitude toward the vital role of minerals in disease resistance. This is especially true for Mn which, although recognized as an essential element, has received most attention because of its potential toxicity.

The Role of Manganese in Resistance to Plant Diseases

Manganese redox chemistry plays an important role in Mn uptake by plants, ecotoxicology of trace elements associated with Mn oxides, and the etiology of some soil-borne plant fungal diseases. Manganese solubility is closely related to oxidation state, but direct measurement of Mn oxidation state has been difficult. We used x-ray absorption near-edge structure (XANES) spectroscopy to determine the relative proportions of Mn(II), Mn(III), and Mn(IV) in four soils, and correlated this with Mn solubility obtained from a sequential extraction procedure that consisted of: (i) 1 M pH 7 ammonium acetate (Ac), (ii) 0.5 M pH 2.9 CuSO 4 or 1 M pH 3 NH 4 OAc, (iii) 0.018 M quinol in 1 M pH 7 NH 4 OAc, and (iv) dithionite-citrate-bicarbonate (DCB). In moist aerated soil, most of the Mn was present as Mn(IV) and XANES spectroscopy tracked its progressive removal with increasingly aggressive extractants. Lowering the pH from 7 to 3 resulted in solid state reduction of Mn(IV) to Mn(III) within the soil Mn minerals without release of Mn to solution. After a 7-d microbial reduction treatment, most of the Mn occurred as Mn(II), 2/3 of which was extracted by pH 7 NH 4 Ac, while the remaining 1/3 was extracted by pH 3 NH 4 Ac. The XANES spectra suggest that this acid soluble Mn(II) phase is not rhodocrosite (MnCO 3 ), but its exact form (solid solution in calcite or MnPO 4 ) still remains to be determined. In both aerated and reduced soils, a considerable fraction of the total Mn occurs as Mn(III), presumably incorporated into the structure of Mn and Fe oxides. Rapid air drying of a microbially reduced soil prevented reoxidation of the reduced Mn and preserved the distribution of Mn in the different extractable Mn pools.

D. M. Huber

Papers

Nitrogen form and plant disease.

Mineral nutrition and plant disease.

A multiple component analysis of the take-all disease of cereals

The Role of Manganese in Resistance to Plant Diseases

Correlating Manganese X-Ray Absorption Near-Edge Structure Spectra with Extractable Soil Manganese