Copper fractionation and extractability in two contaminated variable charge soils
Introduction
Under intensifying pressure of industrialization in both developing and developed countries, more anthropogenic copper and other heavy metals enter agricultural fields and contaminate the food-chain of human beings. Dry and wet deposition around mining and smelting sites Karczewska, 1995, Grazebisz et al., 1997, Derome and Lindroos, 1998, wastewater irrigation Tam and Wong, 1996, Cao et al., 2000, Luo et al., 2003, compost application, including municipal waste, sewage sludge or their combination Pichtel and Anderson, 1997, Nyamangara, 1998, Planquart et al., 1999, Walter and Cuevas, 1999, Baldwin and Shelton, 1999, Veeken and Hamelers, 2002, Soumaré et al., 2003, Zheljazkov and Warman, 2003, and spraying of heavy metal-containing pesticides or herbicides Besnard et al., 2001, Balasoiu et al., 2001 have been reported to contribute to the input of anthropogenic copper and other heavy metals into agricultural soils.
Excess copper and heavy metals in soils are toxic to plants and soil organisms. Plant yield reduction and growth retardation (Moreno et al., 1997) and structural community changes of soil microorganisms and nematodes (Ellis et al., 2001) were reported to occur in heavy metal contaminated soils. More effect-based limits have been legislated by several nations or organizations, such as China, the Commission of European Communities, France, New Zealand, and the USA, to limit the use of heavy metal-contaminated wastes or composts in agricultural land. A series of permissible concentrations were established for each metal mainly based on total metal content. However, phytotoxicities or toxicities to soil organisms are often not determined by total metal contents in soils but by metal's ‘bioavailable’ concentrations (Qian et al., 1996). Moreover, the risk assessments of heavy metals in soils for surface and ground water contamination or other environmental issues are based on their chemical ‘lability’ in soils rather than on their total contents. ‘Bioavailability’ or chemical ‘lability’ of heavy metals strongly depends on their specific physicochemical features in soils, i.e. chemical fraction or speciation (Kabata-Pendias, 1993) and, consequently, on soil physicochemical characteristics (Planquart et al., 1999).
Fractionation studies of copper in soils using a sequential extraction procedure (SEP) can provide an understanding of its chemical fractions and potential bioavailability (Grazebisz et al., 1997), although the SEP is more operationally than theoretically defined because few of the extractants are specific enough to isolate a well-defined phase (Schramel et al., 2000). Routinely, the SEP fractionates soil copper into water soluble (WS), exchangeable (EX), weakly specifically adsorbed (SP), Fe/Mn oxides-bound (OX), organically bound (OR), and residual (RE) fractions using specific extractants by successive extractions Tessier et al., 1979, Planquart et al., 1999, Schramel et al., 2000.
Heavy metals associated with different fractions have different impacts on the environment (Tam and Wong, 1996). Copper in soils is mainly associated with clay minerals and organo-clay associations (<2 μm fractions) and particulate organic matter according to the analyses of the physical fractionation scheme (Besnard et al., 2001). Results from the SEP studies showed that in Cu-contaminated soils, fractionation of copper was determined by organic matter and Fe, Al and Mn oxides. Up to 57% of total soil fractionated Cu was bound to organic constituents in contaminated soils around a Poland copper smelter within a 4-km-diameter area (Grazebisz et al., 1997). However, in unpolluted soils, most Cu was found in the residual fraction Kuo et al., 1983, Levesque and Mathur, 1986, Grazebisz et al., 1997, Nyamangara, 1998. Agbenin and Olojo (2004) reported that the removal of amorphous hydrous oxides resulted in a greater decrease in Cu adsorption in an alfisol than the removal of organic matter. Therefore, for variable charge soils, especially those with low organic matter content, oxides of Fe, Mn, and Al are believed to play a key role in adsorption of anthropogenic Cu.
In southern China, there is a large range of variable charge soils, including oxisols, ultisols, inceptisols, and alfisols. These soils are usually acidic and poor in plant nutrients but rich in Fe and Al oxides. Since the 1980s, with a rapid economic development in this region, vast citrus groves and vineyards have been planted, and many small Cu smelters have also been established to traditionally recycle used copper products. Copper contaminations from application of Cu-containing herbicides and municipal wastes in fruit gardens, and from dust emission, and solid and liquid wastes produced by small Cu smelters have resulted in severe environmental problems, such as reduced crop growth and degraded water quality. Risk assessment of Cu contamination and remediation of Cu-contaminated soils are of great importance in this region. Although Cu adsorption–desorption has been studied for these variable charge soils (Yu et al., 2002), the relationships between Cu bioavailability/lability and soil Cu fractions remain to be evaluated. Information on the fates of anthropogenic Cu and its transformation dynamics in these acid variable charge soils is needed for improving soil and environmental quality.
This study was designed to investigate dynamic changes of Cu fraction and extractability as affected by loads of anthropogenic Cu under contaminated levels in two representative variable charge soils from southern China, and to understand the fate of anthropogenic Cu in these variable charge soils and the potential risks of Cu contamination to crop growth and environmental quality.
Section snippets
Soils
Two variable charge soils (an ultisol and an inceptisol) were sampled at the depth of 0–20 cm from two citrus groves in Longyou County, Zhejiang Province, China (E 119°02′–120°20′, N 28°44′–29°17′). Longyou County is located in a semitropical monsoon zone with an annual average temperature of 17 °C, an annual precipitation of 1400 mm, and an annual nonfrost period of up to 249 days. According to USDA soil taxonomy (USDA, 1988), the clayey loamy soil that developed on the Arenaceous rock (a
The soils
The ultisol was rich in kaolinite and Fe and Al oxides according to results of the X-ray diffraction of clay minerals (<0.002 mm) (Table 1). The inceptisol contains some chlorite and feldspar as well as kaolinite and Fe and Al oxides (Table 1). Both soils were limed for better citrus production resulting in a high content of exchangeable Ca for both the soils (Table 1). Compared with the inceptisol, the ultisol pH was approximately 0.5 units higher and the organic carbon content was 50% higher,
Conclusions
Variable charge soils were relatively vulnerable to contamination of anthropogenic Cu because of low organic matter contents and low pH. The sequential extraction procedure provided some useful information on the distribution of anthropogenic Cu in variable charge soils and on the “accurate” assessment of soil Cu availability estimated by the single extraction. The Fe/Mn oxides mainly controlled the distribution and fate of anthropogenic Cu in the two variable charge soils rather than soil
Acknowledgements
This study was, in part, supported by an Outstanding Young Scientist Fund grant (No 40025104), a research grant (No 20177020) from The Natural Science Foundation of China, and a 973 grant (2002CB410804) from the Science and Technology Ministry of China.
Florida Agricultural Experiment Station Journal Series Number R-09474.
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