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University of Sydney (2014)

Studies of soil respiration in eucalypt forests of south east Australia

De Remy De Courcelles, Vivien

Titre : Studies of soil respiration in eucalypt forests of south east Australia

Auteur : De Remy De Courcelles, Vivien

Université de soutenance : University of Sydney

Grade : Doctor of Philosophy (PhD) 2014

Résumé
This thesis addresses gaps in knowledge of soil respiration in forests of south-east Australia. Soil respiration plays a major part in the cycle of carbon between soils - the biggest pool of terrestrial carbon - and the atmosphere. Despite its global significance, we have only a limited understanding of the magnitude and responses of soil respiration, and especially of its components, to abiotic (temperature, moisture, soil fertility) and biotic (photosynthesis, seasonality of belowground C allocation patterns and root growth, quality and quantity of above and belowground litter) controls. Furthermore, vegetation type may modulate the influences of these abiotic and biotic controls and with soil respiration research having been based mostly in the northern hemisphere, it is crucial that regional studies be conducted further afield. This thesis also considers the context of the current increase in atmospheric [CO2] and resulting predicted climate change that will directly or indirectly impact on soil respiration through extreme weather events, changes in the frequency and intensity of fires or increase in growth. Using both field and laboratory based techniques I measured respiration from soils supporting a variety of Eucalypts. Elevated atmospheric [CO2] did not have an effect on rates of soil respiration in a Eucalyptus saligna plantation, contrary to usual findings. Drought on the other hand slowed rates of respiration, owing to a slowing of the transfer of photosynthates from leaves to roots. The impact of an increase in above-ground litter deposition, a possible consequence of extreme weather events, or continuous increase in primary production can be subdued by the nature and quality of the litter in Eucalyptus pauciflora woodlands. No effect was recorded in the field but ground litter added to soils in the laboratory triggered a response including a priming effect. Root priming effect was also found to increase basal heterotrophic respiration by 54% on average in Eucalyptus regnans. The study on the contribution of roots to total soil respiration showed that it is necessary to use hybrid techniques to separate and estimates the contribution of components of soil respiration ; in this thesis’ case the use of collars and chambers in the field and respirometer in the laboratory was determinant in identifying root priming effect. Great spatial variation in respiration rates was measured both in the simple ecosystem of a Eucalyptus saligna plantation and as a result of fire disturbance at the Messmate 1 site supporting Eucalyptus obliqua and Eucalyptus radiata. Finally, a synthesis of the results of the whole thesis considered the effect of soil temperature on soil respiration and showed that contrary to what is commonly agreed by the Q10 model, respiration rates reached a plateau for temperatures between 16°C to 23°C.

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