Although the toxic effect of heavy metals on soil microorganism activity is well known, little is known about the effects on different organism groups. The influence of heavy metal addition on total, bacterial, and fungal activities was therefore studied for up to 60 days in a laboratory experiment using forest soil contaminated with different concentrations of Zn or Cu. The effects of the metals differed between the different activity measurements. During the first week after metal addition, the total activity (respiration rate) decreased by 30% at the highest level of contamination and then remained stable during the 60 days of incubation. The bacterial activity (thymidine incorporation rate) decreased during the first days with the level of metal contamination, resulting in a 90% decrease at the highest level of contamination. Bacterial activity then slowly recovered to values similar to those of the control soil. The recovery was faster when soil pH, which had decreased due to metal addition, was restored to control values by liming. Fungal activity (acetate-in-ergosterol incorporation rate) initially increased with the level of metal contamination, being up to 3 and 7 times higher than that in the control samples during the first week at the highest levels of Zn and Cu addition, respectively. The positive effect of metal addition on fungal activity then decreased, but fungal activity was still higher in contaminated than in control soil after 35 days. This is the first direct evidence that fungal and bacterial activities in soil are differently affected by heavy metals. The different responses of bacteria and fungi to heavy metals were reflected in an increase in the relative fungal/bacterial ratio (estimated using phospholipid fatty acid analysis) with increased metal load.

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.

Functional stability, measured in terms of resistance and resilience of soil respiration rate and bacterial growth rate, was studied in soils from field plots that have been exposed to copper contamination and low pH for more than two decades. We tested whether functional stability follows patterns predicted by either the "low stress-high stability" or the "high stress-high stability" theory. Treatments consisting of soils with no or high copper load (0 or 750 kg/ha) and with low or neutral pH (4.0 or 6.1) were used. Stability was examined by applying an additional disturbance by heat (50 degrees C for 18 h) or drying-rewetting cycles. After heating, the respiration rate indicated that the soils without copper were less stable (more affected) than the soils with 750 kg/ha. Bacterial growth rate was more stable (resistant) to heat in the pH 6.1 than in the pH 4.0 soils. Growth rate was stimulated rather than inhibited by heating and was highly resilient in all soils. The respiration rate was less affected by drying-rewetting cycles in the pH 4.0 soils than in the pH 6.1 soils. Bacterial growth rate after drying-rewetting disturbance showed no distinct pattern of stability. We found that the stability of a particular process could vary significantly, depending on the kind of disturbance; therefore, neither of the two theories could adequately predict the response of the microbial community to disturbance.

Abstract. Environmental pollution causes adverse effects on many levels of ecosystem organization; it might affect the use efficiency of available resources which will make the system more sensitive to subsequent stress. Alternatively the development of community tolerance may make the system more resistant to additional stresses.In this study we investigate the functional stability, measured in the terms of resistance and resilience, of microbial populations inhabiting contaminated soils near a zinc smelter. With functional stability we mean that we look at processes rather than at population dynamics. We measure changes in respiration and bacterial growth rate in response to addition of stress (lead, salt) or disturbance (heat). We used soils that differ in the level of pollution with zinc and cadmium originating from an adjacent smelter.Our results showed, with regard to respiration, that the most polluted soils have the lowest stability to salt (stress) and heat (disturbance). This confirms the hypothesis that more stressed systems have less energy to cope with additional stress or disturbance.However, bacterial growth rates were affected in a different way than respiration. There was no difference between the soils in resistance and resilience to addition of lead. In case of salt treatment, the least polluted soils showed highest stability. In contrast, the least polluted soils were the least stable to increased temperature, which supports the hypothesis that more stressed soils are more stable to additional stress/ disturbance due to properties they gained when exposed to the first stress (pollution by the smelter).Thus, the responses of microbial processes to stress, their nature and size, depend on the kinds of stress factors, especially whether a subsequent stress is similar to the first stress, in terms of the mechanism with which the organisms deal with the stress.