Cadmium availability to plants in relation to major long-term changes in agronomy systems

Abstract

Cd concentrations in food products may be controlled to some extent by reducing metal inputs in agrosystems. However, Cd is available to plants, so the management of its residual fraction in soil must be considered. Differential Cd uptake among crop species and cultivars is well known, but the long lasting effects of other agricultural practices are less clearly understood. Generally, cropping systems affect the physico-chemical properties of soil. These may produce subsequent changes in metal mobility and bioavailability. The effects of agricultural practices, e.g. crop rotation, fertilization, tillage method, and stubble treatments, were therefore examined. Attention was mostly focused on Cd concentrations in potato tubers and cereal grains that are the major plant-derived contributions to the European diet. Results from long-term field experiments at well separated locations indicate that: Cd concentration in grain is highest in wheat grown after a legume such as lupins, and lowest in wheat grown after a cereal; Cd in wheat grain and potato tubers can increase with increasing rates of nitrogen irrespective of the crop rotation; Cd in wheat grain can be influenced by Zn supply to the plant; a higher concentration of Cd is found in wheat grain in continuous wheat under direct drilling, compared to reduced till or conventional cultivation; high Cd can be measured in potato tubers growing on neutral or alkaline soils that have relatively low Cd concentration, and so the practice of adding lime to decrease Cd in tubers is questionable; the effects of stubble management and fallow in crop rotation are too inconsistent to allow conclusions to be drawn. Maximum increase in Cd concentration resulting from changes in the cropping system could be 0.04 mg kg⁻¹ FW in wheat grain and 0.03 mg kg⁻¹ FW in potato tubers.

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.