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Tobor-Kapl on, M. A., Bloem, J., Rö mkens, P. F. A. M. and de Ruiter, P. C. 2005. Functional stability of microbial communities in contaminated soils. Á/ Oikos 111: 119 Á/129.Functional stability, measured in terms of resistance and resilience of respiration and growth rate of bacteria and fungi, was studied in soils that have been exposed to copper and low pH for more than twenty years. We used treatments, consisting of soil with no or high copper load (0 or 750 kg ha (1 ) and low or neutral pH (4.0 or 6.1). Stability was examined by applying an additional stress in the form of lead or salt. After addition of lead, respiration (decomposition of freshly added lucerne meal) showed lower resistance at low than at neutral pH and at high copper than at low copper. The most acid and contaminated soil was the least resistant. Respiration showed no resilience after addition of lead. Bacterial growth rate (thymidine incorporation) also showed resistance at low pH but only in soils that were not contaminated with copper.After addition of salt, respiration showed no differences in resistance but the soils without copper contamination showed higher resilience. Bacterial growth rate showed lower resistance at low pH than at neutral pH, the latter in which the growth rate increased by on average 123%. This increase at high pH was faster in soil without copper than in soil with copper contamination in which the growth rate initially decreased and then increased. The effects of secondary stress depended on the nature of the stress (lead or salt) and on the parameter measured (respiration or bacterial growth rate). In general the highest resistance and/or resilience were found in the least contaminated soils with neutral pH and/or no copper contamination. Thus, the microbial communities in the cleaner soils showed the highest functional stability. The results seem to confirm the notion that environmental stress alters ecosystems such that supplementary stress will have stronger impacts than in an unstressed system. The results may also confirm the insurance-hypothesis that reduced biodiversity due to the first stress negatively affected community stability. As an alternative, we discuss the observed effects in terms of altered energy budget.

Flocculation of soil-derived dissolved organic carbon (DOC) was used as a tool to study the molecular size distribution of DOC and the native amount of copper bound to different size fractions. DOC was extracted from arable soils that had received varying amounts of animal manure, inorganic fertilizer, or CuSO 4 . Addition of calcium to the DOC extracts resulted in flocculation of up to 50% of the DOC originally present in the samples. High performance size exclusion chromatography (HPSEC) analysis revealed that the DOC removed mainly consisted of high molecular weight (HMW) organic acids. Low molecular weight (LMW) substances remained soluble even at high calcium concentrations. Copper solution concentrations decreased almost linearly with decreasing DOC levels, suggesting that copper remained bound to the flocculated material despite increasing calcium concentrations. The amounts of copper bound to both LMW and HMW components depended on the copper content of the soil and were described by two Langmuir sorption equations. Maximum binding capacities varied between 250 µmol of copper g -1 of C for HMW components and 450 µmol of copper g -1 of C for LMW components, but binding affinities were higher for HMW components. The Langmuir equation described the measured copper solution concentrations from the batch experiments very well (R 2 ) 0.994). The observation that up to 50% of the DOC in solution was not removed by calcium indicates that a significant part of copper is potentially highly mobile in soils and can be prone to DOC-facilitated transport.