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Accueil du site → Doctorat → Australie → 2011 → Methane and carbon dioxide exchange in the tropical savannas of northern Australia : the role of termites

University of Melbourne (2011)

Methane and carbon dioxide exchange in the tropical savannas of northern Australia : the role of termites

JAMALI, HIZBULLAH

Titre : Methane and carbon dioxide exchange in the tropical savannas of northern Australia : the role of termites

Auteur : JAMALI, HIZBULLAH

Université de soutenance : University of Melbourne

Grade : Doctor of Philosophy (PhD) 2011

Présentation
Termites are one of the most uncertain components of global CH4 budget mainly because of the lack of long-term field based studies from different biogeographical regions. This thesis investigated the exchange of CH4 and CO2 between termites and atmosphere, and between soil and atmosphere in the tropical savannas of northern Australia. Diurnal variations in CH4 fluxes were measured from mounds of Microcerotermes nervosus, Microcerotermes serratus and Tumulitermes pastinator every four hours over a 24 hour period. There was large diurnal variation in mound CH4 fluxes caused by diurnal temperature patterns. Mound CH4 fluxes measured between 10:00 and 12:00 hours best represented the mean daily flux. Seasonal measurements of mound CH4 fluxes were up to 25-fold greater in the wet season than the dry season and always greater in the wet season for all investigated species. Detailed studies in M. nervosus revealed that these differences were not associated with changes in environmental pattern but seasonal changes in termite mound population size. The magnitude of diurnal and seasonal variations in mound CH4 fluxes measured in this study suggest that estimates of global CH4 emissions from termites that do not account for such variations will contain larger errors and uncertainty. The contribution of mound-building, hypogeal and wood-nesting termites to the CH4 balance was estimated for a savanna woodland at Howard Springs near Darwin. Methane fluxes were measured from termite mounds and from the soil - from which CH4 fluxes from hypogeal termites were estimated. Methane fluxes from wood-nesting termites were estimated based on known species abundance. Termites were an annual CH4 source of +0.24 kg CH4-C ha-1 y-1 and soils a CH4 sink of -1.14 kg CH4-C ha-1 y-1. Thus, termites offset 21% of CH4 consumed by soil methanotrophs, but overall the savanna ecosystem was a sink for CH4 of -0.90 kg CH4-C ha-1 y-1.
Two indirect methods were tested to predict CH4 and CO2 fluxes from termite mounds. The first predicted mound CH4 fluxes from ‘easier-to-measure’ mound CO2 fluxes. The second predicted CH4 and CO2 fluxes from termite mounds based on the relationship between internal mound concentrations and external mound flux. For both indirect methods the prediction errors were small when calculated separately for each species, whereas, a generic relationship or predictions between species resulted in large errors, probably associated with different mound structures for different species.
This study shows that CO2 emissions from termite mounds are up to two orders of magnitude greater than CH4 emissions, when expressed in CO2-equivalents. There was large variation in both CH4 and CO2 fluxes from termite mounds and soil among different sites which suggests caution when scaling up fluxes from the plot or site scale to a regional or greater scale.
This study filled important knowledge gaps in the ecosystem ecology of termites and Australian savannas. This study establishes North Australian savannas as one of the few biogeographical regions where the contribution of termites to ecosystem CH4 exchange has been investigated. The study highlights the difficulties associated with predicting CH4 flux from termites on a biome scale, which are caused by the high temporal and species-specific variability in flux. Future studies will have to consider these issues in order to reduce the uncertainty of the role of termites in the global CH4 budget.

Mots Clés : termites ; savannas ; methane ; carbon dioxide ; seasonal flux ; diurnal flux

Présentation

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Page publiée le 9 septembre 2016, mise à jour le 9 juin 2017