On the Ecology of Saprotrophic Fungi and Bacteria in Soil: Biotic and Abiotic Control of Growth Rates

Sammanfattning: Two groups of organisms dominate the decomposition in soil: fungi and bacteria. One of the most important parameters to optimise for any organism is its growth, and thus a direct way to study the effect of environmental factors on fungi and bacteria in soil is to measure their growth rate. However, methodological constraints have historically prevented this direct approach in soil. Using leucine/thymidine incorporation into bacterial cells and acetate incorporation into fungal ergosterol I have attempted to partly resolve this shortcoming, and have investigated how fungal and bacterial growth was affected by (i) the composition of plant litter, (ii) soil pH and (iii) temperature. Finally, (iv) I have started to assess how these microbial groups interact. To summarise, a list of the main results of this thesis follows: First, fungi and bacteria differed in their relative importance in the decomposition of different plant material in soil. Bacterial growth was more promoted by adding the plant material alfalfa (C:N = 15) compared with straw (C:N = 75). Second, the turnover of fungi is slower than that of bacteria in soil, corroborating the suggestion of a slow and a fast energy channel, respectively, through the soil food web. Third, the relative importance of fungi and bacteria is dramatically affected by soil pH – fungi more dominating at lower pH and bacteria at higher. Fourth, the relative importance of fungi and bacteria is strongly influenced by the competitive interaction between the decomposer groups. This interaction could partly explain the relatively low fungal importance in high pH soils, but the fungal influence on the low bacterial growth in low pH soils could not be evaluated. Fifth, this thesis demonstrates that the temperatures mediated by global climate change will affect the temperature response of microbial growth in a quantitative and predictable manner, making it possible to estimate the effect on microbial temperature sensitivity of future predicted temperature changes. Growth rate measurements could reveal fungal and bacterial responses with high sensitivity, and greatly improved the resolution of effects on and due to fungi and bacteria. The techniques employed in this thesis therefore have the potential to become standard methods used to monitor environmental effects on the soil ecosystem both in the short-term (hours, for example following substrate additions) and the long-term (years, for example community adaptation to environmental factors). They should also be applied to study the energy flux through the basic trophic level of the soil food web, in investigating the effects of, and effects on, secondary consumers.

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