Anthropogenic heavy metals in organic forest soils

Åkerblom, Staffan (2006) Anthropogenic heavy metals in organic forest soils. Doctoral diss. Dept. of Environmental Assessment, SLU. Acta Universitatis agriculturae Sueciae vol. 2006:67.

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Abstract

The continuous heavy metal accumulation in organic topsoils in forests will pose problems for soil microbial health for many years to come. The primary objective of this thesis was to investigate heavy metal pollution in forest soils and its effects on microbial function. Heavy metal distribution and solubility as well as field- and laboratory-derived dose-response relationships were determined. These aspects were combined to evaluate the environmental status of mor layers affected by local, regional and experimental heavy metal pollution.

Heavy metal distribution in humus layers was examined in field studies at two sites characterised by considerable local industrial pollution, one adjacent to a major road and the other beside a major steel plant. Around the road, Pb and Cu distribution indicated that these heavy metals were major pollutants, while Mo, Cr and Ni were highly elevated around the steel plant. There was considerable covariation among the metals studied. To improve the accuracy of heavy metal exposure assessment, different aspects of solubility were investigated. At the steelworks, deposition of alkaline dust had a significant influence on heavy metal solubility.

Two further studies focused on forested areas and soil material subject to low and diffuse heavy metal pollution. One simulated heavy metal concentration gradients (Cr, Zn, Mo, Pb, Ni, Cd) in mor samples, while the other was an investigation of remote areas subject to long-range aerial Hg transport.

Possible microbial risk was indicated by negative correlations between metal concentrations and soil respiration in small-scale field grids (50x50 m2 with 10 m distance between sampling points to allow for spatial independence). Negative correlations were found in plots around the major emission sources. Laboratory-derived toxicity doses for Pb and Cu supported the indication of microbial risk around the road. At the steel plant there were negative correlations between soil respiration and Mo and Ni. Heavy metal covariation obscured the effects of individual metals in the field. Examination of microbial community structure by phospholipid fatty analysis (PLFA) revealed that metals induced synchronous uniform shifts in soil microbial community structure and soil respiration in experiments.

Measurement of Hg mobility with respect to DOC concentration in soil water leachates provided important data for modelling critical loads of Hg deposition on forest soils.

These results show the benefits of integrated evaluation of chemical and biological indicators in soil microbial risk assessment. Heavy metal pollutants still have negative effects on forest soils at major roads and steel plants, despite recent reductions of emissions.