Cadmium and zinc interactions and their transfer in soil-crop system under actual field conditions

Abstract

The transfer of Cd and Zn from calcareous soils nearby a non-ferrous mining and smelting bases to the spring wheat (Triticum aestivum L.) and corn (Zea mays L.) tissues and the interactions between the two metals concerned were investigated under actual field conditions. Samples of soils and entire crops were randomly collected during harvest time in 1998 in the Baiyin region. The soil metal contents showed that the furrows had been polluted (mean values: 3.16 mg kg⁻¹ for Cd; 146.78 mg kg⁻¹ for Zn) and the significant spatial variation of the soil contamination existed here (ranges, Cd: 0.14–19.3 mg kg⁻¹; Zn: 43.5–565.0 mg kg⁻¹). The translocation ratios of the two metals from soil to crop parts in the region studied were relatively lower and the order of the element transfer in different plant tissues was root>stem>grain. The transfer ratio of element Cd was lower than that of element Zn. Cd and Zn uptake by the crop structures could be best described by four models (P<0.01): linear; exponential; quadratic; and cubic. Apart from a linear relationship between the element Cd in the corn grains and soils, models were generally non-linear. An analysis of CdZn interaction mechanism led to the conclusion that the effects of the two metals were synergistic to each other under field conditions, in which increasing Cd and Zn contents in soils could increase the accumulations of Zn or Cd in the two crops.

Introduction

Toxic heavy metal contamination in soils and crop plants is of major importance due to their health effects on humans and other animals (Farmer and Farmer, 2000, Pichtel et al., 2000). Cadmium (Cd) is a toxic element and zinc (Zn) is toxic in high concentrations, and numerous investigations showed that the pronounced amounts of Cd and Zn were often found in arable soils adjacent to a non-ferrous metal production bases (Pierzynski and Schwab, 1993, Dudka et al., 1996, Dahmani-Muller et al., 2000). The processing and subsequent release of zinc to the environment is normally accompanied by cadmium environmental pollution because of zinc ores (ZnS) generally containing 0.1–5% and sometimes even higher cadmium (Adriano, 1986). The trace elements concerned entered the soils mainly from sludge-borne heavy metal applications and industrial wastewater irrigation, and partially from aerial deposition and the usage of fertilizers and pesticides (Mench et al., 1994, Ullrich et al., 1999, Nan and Zhao, 2000, Sterckeman et al., 2000).

Heavy metal uptake by roots depends on both soil and crop plant factors (e.g. source and chemical form of elements in soil, pH, CEC, organic material, plant species and tissues, plant age, etc.). Interactions among the co-exist elements occurring at root surface and within the plant also affect uptake and translocation. Cadmium, unessential to plants, and zinc, essential to plants, are elements having similar geochemical and environmental properties. This association of cadmium and zinc in the environment and their chemical similarity can lead to interaction between cadmium and zinc during plant uptake, transport from roots to aboveground parts, or accumulation in edible parts (Das et al., 1997).

Accumulation of heavy metals in crop tissues and transfer of them in soil crop system had well been documented (Dudka et al., 1996, Barman et al., 2000, Kisku et al., 2000). Interactions of Cd and Zn and their accumulation in plant parts in solution culture or in pot experiment had been reported (Coughtrey and Martin, 1979, Smilde et al., 1992, Moraghan, 1993, Mckenna et al., 1993, Dudka et al., 1994, Zhou et al., 1994). However, to our knowledge, there is little information about the fates and behaviors of the metals in crop tissues in fields. Therefore, the present investigation was undertaken to examine the interactions between cadmium and zinc in arable soils and their effects on the accumulation of both metals in crop structures. Spring wheat (Triticum aestivum L.) and corn (Zea mays L.) were chosen because they are staples in the diet of the local people in the northern China, and both of the crops readily absorb Cd and Zn from ZnCd contaminated soils (Hinsely et al., 1984, Dudka et al., 1994). The goals of our study were to examine in these two crops: (1) distribution patterns of cadmium and zinc contents in different parts of the two crops grown in the field influenced by non-ferrous metal mining and smelting operation; (2) transfer ratios and relationship models of the two metals concerned from soils to plant parts; and (3) effects of CdZn interactions on the concentrations of both metals in crop tissues.