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Title: Plant Input Effects on the Diversity and Function of Grassland Bacterial Communities
Author: Thomson, Bruce Craig
ISNI:       0000 0001 3532 2104
Awarding Body: University of Newcastle Upon Tyne
Current Institution: University of Newcastle upon Tyne
Date of Award: 2008
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By decomposing organic matter, bacterial communities are essential for plant growth and ecosystem functioning. Conversely, aboveground plant communities are known to select for particular bacterial communities by depositing C resources of different palatability belowground. Aboveground communities are known drivers of bacterial community structure which in turn affects soil C cycling. However, the exact mechanisms and feedbacks of this intimate relationship remain unclear. This thesis examines the effects of vegetation on bacterial community structure and whether plant driven differences in bacterial community composition affect soil C cycling and substrate utilisation in a grassland soil at the NERC experimental field site, Sourhope, Scotland. Firstly, the effects of vegetation on bacterial community structure .and soil respiration were examined. Bacterial community structure and function were shown to differ depending on the presence or ab~nce of plants. Particularly, differences in the abundances of certain bacteria were observed and soil respiration rates were higher in vegetated soil 'compared to bare soil. These differences in soil CO2-C efflux were attributed to differences in the physiological traits of the dominant bacterial taxa. Next, an assessment of laboratory soil pre-treatments was carried out to establish a suitable microcosm design for analysing soil bacterial communities. The results from this study were then implemented in the design of subsequent experiments to investigate soil bacterial community structure and C functional responses to added substrates. Using soil microcosms it was then investigated whether plant-induced differences in bacterial community structure affected the decomposition of different substrate types and amounts. Also, whether the presence or absence of plants selected for different bacterial community members responsible for the decomposition of labile and recalcitrant substrates added at a high and low concentration. Functional responses were shown to differ depending on soil type, substrate complexity and loading rate. Respiration responses were thought to be due to differences in bacterial community structure between soil treatments. Taxonomic responses also differed depending on the presence or absence of plants, decomposability of substrate and concentration. However, there were no consistent patterns in community changes. Finally, 13C-Iabelled substrates were added to soil with or without vegetation to accurately measure substrate specific respiration and assess priming effects. Also rRNASIP was performed in an attempt to unambiguously identify bacteria responsible for the assimilation of labelled substrates. Following labelled-substrate additions, mineralisation of substrates differed between soil treatments and was dependent on substrate type. Priming effects were strongest in bare soil as in vegetated soil added substrate was preferentially mineralised over native SOM. Both substrate specific respiration and priming effects were thought to be linked to bacterial community composition. Whilst all substrates were rapidly mineralised and significant differences existed between vegetated and bare soils, 13C was not incorporated into RNA with similar efficiency. Using rRNASIP the entire bacterial community in vegetated and bare soils was shown to have utilised 13C-substrates as a general bacterial community shift into enriched gradient fractions was observed. However, similar to initial bacterial diversity, there were differences in the abundances of functionally active bacteria between vegetated and bare soils.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available