Hexavalent Chromium Dynamics and Uptake in Manure-Added Soil

Abstract

The soil dynamics of hexavalent Cr, a particularly mobile and toxic metal, is of a great environmental concern, and its availability to plants depends on various soil properties including soil organic matter. Thus, in a pot experiment, we added 50 mg Cr(VI) kg(-1) soil and studied Cr(VI) soil extractability and availability to spinach, where we applied both natural (zeolite), synthetic adsorptive materials (goethite and zeolite/goethite) and organic matter with farmyard manure. We found that, compared to the unamended control plants, dry matter weight in the Cr(VI)-added soil was greatly decreased to 17 % of the control, and height was decreased to 34% of the control, an indication of Cr toxicity. Also, exchangeable Cr(VI) levels in soil decreased back to the unamended control even in the first soil sampling time. This was much faster than the exchangeable Cr(VI) levels in the mineral-added soil, where Cr(VI) levels were decreased to the levels of the unamended control in the third sampling time. The positive effect of organic matter was also indicated in the Cr quantity soil-to-plant transfer coefficient (in grams of Cr in plant per kilogram of Cr added in soil), a phyto-extraction index, which was significantly higher in the manure-amended (1.111 gkg(-1)) than in the mineral-added treatments (0.568 gkg(-1)). Our findings show that organicmatter eliminates the toxicity of added Cr(VI) faster than the mineral phases do and enhances the ability of spinach to extract from soil greater quantities of Cr(VI) compared to mineral-added soils.