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Albert-Ludwigs-Universität Freiburg im Breisgau (2020)

Nitrous oxide emissions from excreta deposited onto tropical pasture in Kenya

Zhu, Yuhao

Titre : Nitrous oxide emissions from excreta deposited onto tropical pasture in Kenya

Auteur : Zhu, Yuhao

Université de soutenance : Albert-Ludwigs-Universität Freiburg im Breisgau

Grade : Doctor rer.nat 2020

Résumé partiel
Livestock production systems are dominant greenhouse gas sources in sub-Sahara African (SSA), including emissions of nitrous oxide (N2O) from excreta deposited onto pasture. However, a limited number of studies are available to date. Using the default emission factor (EF) from the Intergovernmental Panel on Climate Change (IPCC) guidelines, which represents a global mean value mainly based on observations in OECD countries, may cause strong uncertainties due to differences in the management practices, climate and soils between OECD countries and countries and livestock systems in SSA. To narrow uncertainties of national greenhouse gas inventories in SSA it is thus needed to develop region-specific N2O emission factors for excreta dropped on tropical pasture which do consider regional characteristics. In my thesis the calculation of N2O emission factors for excreta dropped on tropical pastures was based on a series of field trials in which I varied dung quantity, quality and urine amount. These field trials were carried out during the dry and the rainy season to also investigate how weather conditions and hydric seasons (dry versus rainy) are affecting emissions. In a final experiment I investigated the importance of soil properties on N2O emissions due to excreta deposition. Measurements of N2O fluxes were done using an automated chamber systems as well as manual chambers to measure N2O. The use of automated chambers allowed to obtain a high resolution dataset and to quantify short-term emission peaks. However, as the number of automated chambers was limited also manual chambers were used, specifically while investigating the effect of soil types on N2O emissions from excreta dropped on tropical pastures. My results show that differences in dung mass, dung quality and urine volume did not cause significant differences in terms of dung or urine N2O EF. Surprisingly, dung application only marginally stimulated the soil N2O fluxes regardless of the mass or quality of dung patches, with EFs being a magnitude lower as found for dung applications to temperate grasslands. This significant difference in N2O EFs was due to the low nitrogen (N) concentration, high carbon (C) to N ratio, as well as the rapid crust formation of dung under tropical weather and soil conditions. In contrast to my observations for dung, application of urine to tropical pastures resulted in rapid increases in N2O fluxes, though the stimulatory effect was higher in the rainy as compared to the dry seasons. Again, EFs for urine were significantly lower in my experiments as compared to experiments carried out in temperate regions.


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