Functional stability of microbial communities from long-term stressed soils to additional disturbance.

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.

Functional stability of microbial communities in contaminated soils near a zinc smelter (Budel, the Netherlands).

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.