Risch Anita Christina

Top-down effects of different sized herbivores on soil microbial biomass C and bacterial community structure in subalpine grasslands.

Project Number: CH-4656
Project Type: Master
Project Duration: 01/01/2011 - 12/31/2011 project completed
Funding Source: other ,
Project Leader: Prof. Anita Christina Risch
Head of Animal Ecology
├ľkologie der Lebensgemeinschaften
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8903 Birmensdorf
Phone: ; +41 (0) 44 739 21 11
FAX: +41 (0) 44 739 22 15
e-Mail: anita.risch(at)wsl.ch

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Grassland ecosystems support large populations of herbivores, which can be key determinants and regulators for grassland processes as grazing has impacts on several trophic levels. Herbivores are not only influencing aboveground, but also belowground properties. Trampling as well as deposition of organic matter in form of dung and urine are directly influencing belowground processes. Indirect herbivore impact on belowground processes via grazing can origin from changes in plant physiological properties such as nutrient allocation, root exudation and - in the long-term - plant community composition, which, in turn, alters the quantity and quality of the organic material incorporated into the soil. The effect of herbivores on grassland properties, however, highly depends on the grazing intensity, which is correlated to body size of the animal and the productivity of the system. Positive effects of grazing are generally found in heavily grazed productive ecosystems, negative ones in rarely grazed unproductive systems. All these impacts have consequences for microorganisms and as they are involved in a wide variety of soil processes, which ultimately have an impact on the functioning of all trophic levels, it is important to gain information on how herbivores affect the abundance and composition of these organisms. Consequently, a gradual exclosure experiment was established in this study to exclude four groups of herbivores of different body size in two different subalpine grassland types of different productivity. The main objective was to investigate how the exclusion of the different herbivores affects the abundance and composition of the soil bacterial community, examined using T-RFLP analyses. More specifically, I investigated how herbivore exclusion affected the soil bacterial community composition in the rhizosphere and mineral soil and the microbial biomass carbon (C) of the mineral soil after the first and second year of treatment establishment. After the first year I did not detect any impact of the treatments on microbial biomass. After the second year, herbivore exclusion resulted, however, in lower microbial biomass the more herbivores were excluded after the second year. This would indicate that grazing would stimulate microbial biomass C in the Swiss National Park, possibly due to enhanced root exudation or faeces deposition. Yet, no differences in the response to grazing exclusion on microbial biomass C were found between the two vegetation types. Neither exclusion of different herbivores nor the differences in vegetation type were able to explain the differences in the bacterial community structure detected. It rather seemed that differences in abiotic and biotic parameters such as soil temperature, soil moisture, root and microbial biomass were responsible for the differences in community structures found as I found a strong annual pattern for both rhizosphere and mineral soil layer, being more pronounced in the rhizosphere. Abiotic and biotic factors might be outweighing grazing effects as they could initiate variations in resource availability in the soil. In addition, bacterial diversity increased from the first to the second year, even though Evenness was reduced. Grazing induced changes in microbial biomass C did not result in changes in the bacterial community structure or diversity. Two years of exclusion might be insufficient to show the impact of grazing on soil microbes, since functioning ecosystems have some resilience. Nevertheless, I could show that the microbial compartment is not an inert system since strong temporal effects could be shown.

Hodel, Melanie. 2011: Top-down effects of different sized herbivores on soil microbial biomass C and bacterial community structure in subalpine grasslands. Master thesis University of Zurich.
pdf Masterthesis

Last update: 7/18/17
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