To Örebro University

Soil contamination with heavy metals may disrupt soil microorganisms with important roles in carbon (C) and nitrogen (N) cycling. However, there is a lack of understanding on how microorganisms are affected in soil, which may lead to a mismatch when assessing risks of contaminants to field soils.

The overall aim of this thesis was to assess effects on C and N cycling in heavy-metal contaminated soils under realistic conditions. Two historically contaminated sites and two outdoor field trials were studied. A variety of microbial responses, such as in situ microbial soil respiration, biomass, and N cycling microbial guilds was applied, which were linked to slower responding plant and soil variables and stable isotopic content δ13C and δ15N.

Lower microbial activity, accumulation of soil C and a lower soil and plant δ15N showed that high lead (2000 mg kg-1) content was slowing down C and N cycles in a grassland area. In a former wood impregnation site, microbial biomass ceased below 5 cm depth while no effects in upper soil (2300 mg kg-1 copper) were observed. In a mesocosm study, responses of N cycling microbial guilds were mostly shaped by soil type. Neither total nor soluble copper, a proxy for bioavailability, could explain the effects on N cycling microbial communities. Finally, addition of biochar and peat to a moderately contaminated soil was shown to immobilize contaminants and N simultaneously, thereby being a promising remediation method to improve ecological soil quality in situ.

In summary, this thesis provides an increased understanding and a reality-check on effects on C and N cycling in heavy-metal contaminated soils. The different intensities of the ecosystem effects in the two field soils, and soil specificity of microbial responses in the N cycle, stress the need for site-specific approaches.