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Title: The effects of freeze-thaw cycles on the composition and concentration of low molecular weight dissolved organic carbon in Arctic tundra soils
Author: Foster, Anna
ISNI:       0000 0004 5371 5931
Awarding Body: Prifysgol Bangor University
Current Institution: Bangor University
Date of Award: 2015
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The Arctic is currently an important store of terrestrial carbon, but may become a net source of carbon to the atmosphere in the future in response to climate change. However, the intricacies of how the carbon cycle is likely to respond to predicted changes are unclear, as soil organic carbon (SOC) is a mixture of many different pools and compounds. Low molecular weight dissolved organic carbon (LMW-DOC) is a particularly important and dynamic C pool as it is highly available to microorganisms. This thesis explored the effects of freezing temperature and freeze-thaw cycles (FTC) on the composition, concentration and fate of this SOC pool, and on the carbon and nitrogen cycles in general, in arctic tundra soils. FTC were shown to have the potential to increase LMW-DOC in arctic tundra soils. This was only observed after 1 or 2 FTC and this also depended on tundra type. The composition of this LMW-DOC also changed with new LMW-DOC compounds, changes to aromaticity and the fluorescence spectrum being observed. Plants were a source of LMW-DOC during FTC. Salix polaris vegetation produced similar compounds and fluorescence spectra to FTC subjected soil. Initial experiments on sterilised Zea mays showed plants as well as microbes could be a source of glucose and amino acids released into soil after FTC. The fate of some rapidly utilised LMW-DOC compounds was relatively unaffected by FTC in tundra soils suggesting rapid recovery of C and N cycling and the microbial community within 12 hours of thaw. Freezing temperature decreased uptake and mineralisation of LMW-DOC in a manner that was substrate specific. Progress was made in measuring and modelling unfrozen water content. The mineralisation rate of glucose was modelled using temperature and unfrozen water content. Warmer winters seemed to increase nitrification and amino acid turnover early in the growing season and also to decrease microbial biomass carbon. Earlier thaws could promote soil CO2 emissions and increase susceptibility to FTC. In conclusion, LMW DOC concentration, composition and dynamics can be affected by FTC with the vegetation being a notable source. Microbial use of LMW DOC is affected by freezing temperature and, whilst this recovers quickly on thaw, over winter freezing temperature can have longer term impacts on C and N cycling in the arctic tundra.
Supervisor: Roberts, Paula ; Jones, David Sponsor: Natural Environment Research Council (NERC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available