Impact of Cultivation Change from Open Field to Greenhouse on Heavy Metal Contents and Fractions of Soil

We evaluated the impacts of changing from open field to greenhouse cultivation on the metal content, total content and fraction of heavy metals (Cr, Cd and Pb) in soil at a vegetable farm in Wuhan, China. Results showed there was no significant difference in the content of Cr and Pb between the open field and greenhouse soil, but there was a significant difference in Cd content. The greatest difference was in the acid extractable fraction, which increased from 0.62 mg/kg in the open field to 1.19 mg/kg in the greenhouse soil, followed by residual, organic bound, oxide, and carbonate-bound fractions. Total Cd content increased from 0.79 mg/kg in the open field to 1.58 mg/kg in the greenhouse soil, significantly higher than the soil environmental quality standards of Cd (0.3 mg/kg). The Cd content in greenhouse soil reached serious pollution levels. In addition, the percentages of carbonate bounded and oxidation form of Cd reduced, but acid extractable Cd significantly increased. These results suggest that a transition from open field to greenhouse cultivation could result in some carbonate bound and oxide Cd being transformed to acid exchangeable Cd, and the biological availability of soil Cd would be enhanced. Therefore, the application of greenhouse cultivation requires stronger Cd content control in soil to reduce heavy metal (Cd) toxicity to vegetable crops.