Abstract
An understanding of how redox conditions affect soil heavy metal fractions in rice paddies is important due to its implications for heavy metal mobility and plant uptake. Rice paddy soil samples routinely undergo oxidation prior to heavy metal analysis. Fraction distribution of Cu, Pb, Ni, and Cd from paddy soil with a wide pH range was investigated. Samples were both dried according to standard protocols and also preserved under anaerobic conditions through the sampling and analysis process and heavy metals were then sequentially extracted for the exchangeable and carbonate bound fraction (acid soluble fraction), iron and manganese oxide bound fraction (reducible fraction), organic bound fraction (oxidizable fraction), and residual fraction. Fractions were affected by redox conditions across all pH ranges. Drying decreased reducible fraction of all heavy metals. Curesidual fraction, Pboxidizable fraction, Cdresidual fraction, and Niresidual fraction increased by 25%, 33%, 35%, and >60%, respectively. Pbresidual fraction, Niacid soluble fraction, and Cdoxidizable fraction decreased 33%, 25%, and 15%, respectively. Drying paddy soil prior to heavy metal analysis overestimated Pb and underestimated Cu, Ni, and Cd. In future studies, samples should be stored after injecting N2 gas to maintain the redox potential of soil prior to heavy metal analysis, and investigate the correlation between heavy metal fraction distribution under field conditions and air-dried samples.
Highlights
Soil heavy metals Cu, Pb, Ni, and Cd, are regarded as ‘‘chemical time bombs’’ because of their propensity for accumulation in the soil and uptake by crops
Soil organic matter (OM) ranged from 19.40 g kg21 to 31.30 g kg21 and had a mean of 25.47 g kg21
The predominant fraction was residual fraction (60%), followed by reducible fraction (20%), oxidizable fraction and acid soluble fraction
Summary
Soil heavy metals Cu, Pb, Ni, and Cd, are regarded as ‘‘chemical time bombs’’ because of their propensity for accumulation in the soil and uptake by crops. This causes human toxicity in both the short- and long-term [1,2], making farmland ecosystems dangerous to health [3]. Morphological characteristics and processes of heavy metals have been studied to better understand heavy metal occurrence in various environments, transport pathways, and crop uptake. Redox changes the valence of ions, subsequently affecting the forms of various elements and compounds and their transport processes. Heavy metal behavior is strongly correlated with redox potential (Eh) [4]
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