Heavy metal retention by organo‐mineral particle‐size fractions from soils in long‐term agricultural experiments

Abstract

The retention of heavy metals by organo‐mineral particle‐size fractions from untreated and manured plots of fertilization experiments on sandy loess soils has been observed by analyses of the total Cd‐, Cu‐, Ni‐, Pb‐ and Zn‐concentrations, and by determinations of the adsorption isotherms for Cd, Ni and Pb. The heavy metal concentrations always decreased with increasing particle‐size from clay to sand. Longterm application of farmyard manure for 37 ("Field F") and 108 years ("Eternal Rye Cultivation") resulted in larger Pb‐ and Zn‐ concentrations with the largest differences in clay, fine silt and medium silt. This was also true for the elements Cu and Ni in the samples < 20 μm from “Field F”;. In the untreated plot of the “Eternal Rye Cultivation”;, clay and fine silt had larger Cd‐, Cu‐ and Ni‐concentrations than in the manured plot. The general order of heavy metal proportions was clay (29–55%) > silt (23–45%) > sand (15–32%). Within this order, relative metal enrichments in clay of the unfertilized and in sand of the manured soils were the most clear management‐induced redistributions. The adsorption isotherms for binding of Cd, Ni and Pb by size‐fractions were well described by the Freundlich equation and showed increases in the adsorption capacity due to farmyard manure application, despite larger total concentrations. Accordingly, the amounts of adsorbed metals were closely correlated to organic C‐concentrations and cation exchange capacities. Moreover, the importance of individual molecular constituents of soil organic matter for the retention of Cd was shown by significant correlations to the relative abundances of distinct signals in pyrolysis‐field ionization mass spectra.