Soil life under stress

In this thesis I studied how long-term soil contamination affects microbial populations and processes, ecosystem properties and functional stability. I also investigated which parameters are suitable as indicators of soil quality in long-term contaminated soils. I found that contamination had a negative impact on many examined microbial parameters, e.g. biomasses, respiration and growth rate (Chapter 2). Some parameters like protozoan biomass and metabolic quotient did not show any effect of stress probably due to strong variation. No single parameter indicated effects of both stressors. Thus, a set of indicators is needed to assess the condition of contaminated soils. In chapter 3 I tested information indices as a tool to describe ecological succession in belowground ecosystems. I used data from a primary succession on the island of Schiermonnikoog. I found that the indices that describe both size and organization of ecosystem followed trends predicted by the theory, but at the same time they were strongly correlated with total system biomass. Therefore I could not say whether the observed trends reflect succession or simply the build up of biomass. However, analysis of relative indices that are independent of biomass and describe only the organization of the ecosystem, showed that succession occurred only in soils between 0 and 10 years old. Since other authors clearly have shown that there has been succession in these soils in, I conclude that the relative information indices are not sensitive indicators of succession. In contaminated soils, however, relative information indices were sensitive to stress caused by high concentrations of copper and by low pH (chapter 4). Stress affected the organization of belowground ecosystems as predicted by the theory. Stressed soils were more vulnerable to external perturbations and less efficient in processing energy than unstressed soils. As the relative information indices responded to stress in predictable manner and each of them revealed effects of both stresses I concluded that these indices are useful indicators of environmental stress. In contrary the absolute indices responded in unpredicted manner to stress and therefore are not suitable as indicators of stress. In chapters 5 and 6, I used ¿stress on stress¿ experiments to test the functional stability of soil respiration and bacterial growth rate to additional stress or disturbance in experimentally contaminated soils. The results described in Chapter 5 seemed to confirm the hypothesis that microbial processes in not-stressed soils are more stable to additional stress. The microbial processes showed different responses to disturbances (Chapter 6) than to stress (Chapter 5). In some cases stressed soils appeared to be more stable to additional disturbance than not-stressed soils, in other cases the opposite was found. In chapter 7, I tested the functional stability in a real field situation with zinc and cadmium pollution. In this experiment processes responded differently than in the former experiments. Probably the response of a process depends on whether a co-tolerance towards a given (subsequent) stress was developed during exposure to the first stress.