Cadmium availability to plants in relation to major long-term changes in agronomy systems
Introduction
Soil quality is a complex subject because soil has a variety of functions (Barth and L'Hermite, 1987). For rural land the most obvious are: the filtering of surface and groundwater, crop production affecting both yield and food quality, and an ecosystem function serving as a matrix for numerous living organisms and biological processes. Several abiotic and biotic processes can cause soil degradation; e.g., water and wind erosion, salinization, accumulation of chemical contaminants, physical degradation, increase in weeds and pests. All are serious problems. A report on soil quality stated that primary soil contaminants display the following: high persistence in the environment, high toxicity and bioaccumulation, relatively high mobility, and presence in significant quantities (de Haan et al., 1989). Accordingly, Cd is of special interest in assessments of soil quality.
To protect human health, the concentration of contaminants in food products must be controlled. In many countries, maximum permissible concentrations (MPC) for Cd have been set by national health authorities (Ewers, 1991; Tiller et al., 1997). Soil Cd concentrations in food products may be controlled slightly by reducing metal inputs in agrosystems. However, soil Cd is available to plants and the management of its residual fraction in soil must be considered. Resulting changes in soil properties (e.g., pH, cation exchange capacity, organic matter, redox potential, oxide content, and microbial biomass) because of alterations to agricultural systems may produce changes in the mobility and plant availability of chemicals such as Cd. These long lasting effects are not clearly understood because the results are frequently contradictory. This paper focuses on the interactions between major agricultural practices, i.e., crop rotation, fertilization, tillage method, and stubble treatments, and Cd availability to plants, by summarizing results from long-term field experiments. Attention is mostly restricted to Cd concentrations in potatoes and cereal grains that are the major plant-derived contributions to the European diet. Relevant data on grassland is also considered.
Section snippets
What risks?
The main inputs of trace elements to agricultural soils are: atmospheric deposition, fertilizers such as phosphates, pesticides, and animal manures (Merian, 1991; Adriano, 1992; Alloway, 1995). Minor sources of contamination such as sewage sludge, municipal solid wastes, and industrial wastes are important because of their regional or local impacts. In France, 70% of industrial facilities are located in rural areas. So, sometimes, the input of trace elements from a local source cannot be
Impact of changes in agricultural systems on Cd content in edible plant parts
The importance of soil factors such as pH, soil texture, organic matter, type of soil colloids, and plant factors such as species, cultivar, and rhizosphere on the transfer of cadmium from soil to crop is well recognized (Jackson and Alloway, 1992; Alloway, 1995; Tiller et al., 1997). Changes in agricultural systems may significantly affect the buffering of a contaminant by soil. Cd adsorption by clay, iron and manganese oxides, and organic matter decreases with decreasing pH (Alloway, 1995).
Conclusions
Studies in Germany and in the United Kingdom on grain samples from earlier years reveal no increase in the cadmium concentration, neither during the recent period (Chaudri et al., 1995) nor when very old wheat samples were compared with those of recent years (Jones and Johnston, 1989; Weigert, 1991). However, some agricultural products already have Cd concentrations over MPC in foods established in several states (Jackson and Alloway, 1992; Chaudri et al., 1995). To protect the food chain in
Acknowledgements
The author is grateful to the late Dr. K. Tiller, CSIRO, Division of Soils, Australia, Prof. Dr. S.P. McGrath, IACR, Rothamsted, Harpenden, UK, Drs. S. Pellerin and B. Mocquot, INRA Agronomy Unit, Villenave d'Ornon, France, for their help in gathering data and reprints. Thanks are expressed to Dr. K. Goulding, IACR, Rothamsted for comments. Special thanks go to the Joint Research Centre, and especially to Dr. G. Bidoglio, Commission of the European Communities, Ispra, Italy.
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