Molybdenum in plants and soils

Molybdenum is essential to plant growth as a component of the enzymes nitrate reductase and nitrogenase. Legumes need more molybdenum than other crops, such as grass or corn, because the symbiotic bacteria living in the root nodules of legumes require molybdenum for the fixation of atmospheric nitrogen. If insufficient molybdenum is available nodulation will be retarded and the amount of nitrogen fixed by the plant will be limited. If other factors are not limiting the amount of molybdenum will determine the amount of nitrogen fixed by the plant. Increasingly vigorous plant growth, higher protein contents and greater buildup of nitrogen in the plant and soil accompany nodulation and symbiotic microbial activity.

Albrigo, L. G., Szafranck, R. C.and Childers, N. F., The Role of Molybdenum in Plants and Soils, Climax Molybdenum Co., Supplemental volume, 1966.
Childers, N. F. and Borys, M. W., The Role of Molybdenum in Plants and Soils, Climax Molybdenum Co., 1962.
Ivanova, N. N., Agrochimica, 1972 (Publ. 1973), 17, 96.
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Nitrogen fixation

The mechanism of nitrogen fixation in enzymes and in model systems in vitro has been extensively investigated.

Burris, R. H. and Roberts, G. P., Ann. Rev. Nutrition, 1993, 13, 317.
Sellman, D., Agnew. Chem. Int. Ed., 1993, 105, 64.

The microorganisms which fix molecular nitrogen fall into two classes: (a) the symbiotic microorganisms which fix nitrogen in association with plants, e.g., Rhizobium; (b) asymbiotic microorganisms which are free-living and include Azotobacter vinelandii and Clostridium Pasteurianum . From cell-free extracts of C. Pasteurianum, two metalloproteins have been obtained. The hydrogen donating system, azoferredoxin, which contains iron and sulfide and the nitrogen activating system, molybdoferredoxin, which contains molybdenum, iron, and sulfide. The structure of the active centre has been shown by X-ray crystallography to be a Fe-Mo-S cluster.

Chen, J., Christiansen, J., Campbasso, N., Bolin, J. T., Tittsworth, B. C., Hales, B. J., Rehr, J. J. and Cramer, S. P., Angew. Chem. Int. Edn. ,1993, 32,1592.
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Rudolf, M. and Kroneck, P. M. H., The nitrogen cycle: Its biology, Biogeochemical Cycles of Elements, 2005, 43, 75-103.

Nitrate reduction

In plants and some animals the first stage in the reduction of nitrate is to nitrite. The reduction is catalysed by nitrate reductase, a flavoprotein enzyme which has molybdenum as the only metal requirement. Molybdenum acts as an electron acceptor from reduced FAD in the enzyme. The molybdenum cofactor is an oxomolybdenum sulfur species with a pterin ligand [Berks et al., 1995; Campbell, 1996; Collison et al., 1996].

Berks, B. C., Ferguson, S. J., Moir, J. W. B. and Richardson, D. J., Biochim. Biophys. Acta - Bioenergetics, 1995, 1232, 97.
Campbell, W. H., Plant Physiology, 1996, 111, 355.
Collison, D., Garner, C. D. and Joule, J. A., Chem. Soc. Rev., 1996, 25, 25.

Concentrations in normal herbage may range from 0.1 to 1.5 ppm on a dry matter basis, see Natural occurrence of molybdenum: molybdenum in plants. The molybdenum may be present as soluble ammonium molybdate, insoluble molybdenum trioxide, calcium molybdate and molybdenum disulfide. In areas of high industrial activity herbage values of up to 231 ppm have been found. The differential in the molybdenum concentration of soils due to pH could result in differing levels due to the consumption of herbage grown in soils of regional type.

Gardner, A. W. and Hall-Patch, P. K., J. Nutr. , 1962, 84 , 31.

Some soils may require supplemental molybdenum. These include soils low in organic matter, severely eroded or heavily weathered soils, soils low in total molybdenum, sandy soils, soils high in iron, and acid soils (pH <6.3). Molybdenum is not readily absorbed by plants from acid soils and liming or addition of molybdenum is required to increase the molybdenum concentration in pasture. Some plants exhibit visual symptoms of molybdenum deficiency, e.g., the classic whiptail in cauliflower and yellow spot in citrus, but often visual symptoms of molybdenum deficiency are not present or appear as symptoms of nitrogen deficiency. A typical supplemental molybdenum addition for legumes is approximately 0.25 kg molybdenum per acre. Molybdenum can be applied in fertilisers, by seed treatment or foliar sprays.

Some pastures have exceptionally high concentrations of molybdenum (generally associated with alkaline soils) and may give rise to symptoms of molybdenum toxicity in sheep and cattle. Guideline values of up to 50 mg/kg dry weight have been fixed for molybdenum concentrations in agricultural soils [Hornick et al., 1977]. A higher incidence of uratic diathesis was reported from a locality in Armenia where the soil was found to contain 77 mg/kg of molybdenum and 39 mg/kg of copper. The total daily intake of molybdenum and copper in the adults of this area were estimated to be 10 times that of an adult in a control area [Kovalskij et al., 1961; ILO Geneva, 1980].

Hornick, S. B., Baker, D. E. and Guss, S. B., Molybdenum in the Environment , 1977, 2, Marcel Dekker, New York.
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ILO, Occupational Exposure Limits, 2nd (revised) Ed., Occupational Safety and Health Series, 1980, 37, ILO Geneva.

A clay loam topsoil that tends to form surface crusts was mixed with unweathered fly ash from a western Canada coal burning power plant in mixtures ranging from 0 to 100% fly ash (v/v). Fly ash increased plant Mo concentrations to alter Cu/Mo such that it could be a concern for ruminant diets.

Sale, L.Y., Naeth, M.A., Chanasyk, D.S., Plant And Environment Interactions - Growth-Response Of Barley On Unweathered Fly Ash-Amended Soil, Journal Of Environmental Quality, 1996, 25, 684-691.