Heavy metal distribution in soil aggregates: a comparison of recent and archived aggregates from Russia

Use of soil archives provides the opportunity to retrospectively analyze changes in soil properties. We used the distribution of heavy metals between the exterior and interior of aggregates in recent and archived samples to derive (1) temporal trends in metal concentrations and distribution across aggregates and (2) to assess aggregate turnover times. We collected 11 topsoils under grassland and forest along a transect from Moscow to a distance of 50 km from the city centre and at a background location. Furthermore, we analyzed six topsoils sampled between 1910 and 1954 at some of the locations. We fractionated aggregates >1 cm into interior and exterior portions. Except at one contaminated site, heavy metal concentrations were low (Cd: 0.09-0.33 mg kg(-1), Cu: 6.8-24 mg kg(-1), Pb: 6.7-31 mg kg(-1), Zn: 25-54 mg kg(-1)), comparable with background levels in central Europe. They were not related to the distance to Moscow indicating that Moscow was no point source for the studied soils. During the last century, contents of heavy metals in soils increased inside and decreased outside the city. In Moscow, the heavy metal accumulation in the aggregate exterior relative to the interior was more pronounced in recent than in archived samples reflecting a higher recent than historic deposition. However, there was also an increase in heavy metal concentrations in the interior of recent compared with archived aggregates. Thus, the studied aggregates were turned over at least once during the last 50-90 years at all sites. This limited the build-up of metal concentration gradients in aggregates. (C) 2004 Elsevier B.V. All rights reserved.