Temporal evolution of copper distribution in soil fractions, influence of soil pH and organic carbon level on copper...

The present paper deals with the temporal evolution, over 6 years, of copper distribution in soil fractions using a sequential extraction procedure isolating five soil fractions which are operationally defined but commonly identified as : F1 = exchangeable metals, F2 = metals bound to organic matter, F3 = metals bound to manganese oxides, = metals bound to iron oxides and F5 = the residual fraction. The soil studied was a silty agricultural soil whose four plots were deliberately enriched with copper sulphate in October 1992 and one of them was also enriched with organic matter, another one with lime and the last one with both organic matter and lime. The great advantage of such an experimental procedure was that the five plots represent the same soil and thus had the same soil texture. It was then possible to fully describe anthropogenic copper behaviour, and to describe clearly the influence of pH and organic carbon level on copper distribution. Furthermore, the plots were subjected to natural climatic conditions allowing a temporal study of copper distribution under natural conditions. The total Cu level as well as Cu-F3 and Cu-F4 were clearly decreasing with time. Copper concentration in artificially defined soil fractions were described with multiple regression equations with the variable total copper content (TCu) and with the variables soil pH and soil organic carbon level when required. Cu-F1 and Cu-F2 depended on soil pH and soil organic carbon level, while Cu-F3 depended only on TCu. Copper level bound to iron oxides depended on soil pH.