Title: Quantifying Fine-Scale Temporal Variations in Greenhouse Gas Emissions in Tropical Highland Valley Bottom Wetlands, A Case Study in Taita Hills, Kenya
Author: Amose Ouko Obonyo
Supervising Institution: IHE Delft Institute for Water Education
Year: 2024
Abstract:
Despite the importance of wetlands for storing carbon (>30 % globally), their size has reduced by 50% since 1700, with 80% in the last 2 centuries. Conversion to agriculture is a major cause of wetlands decline (accounting for 35%), with even higher rates in Africa (>30%) where population pressure and food insecurity drive rapid wetland loss. Accurate assessment of climate feedback through GHG flux estimation is lacking because annual flux measurements are not frequent enough (often monthly) to capture hot moments related to rain or fertilization events. Furthermore, diurnal variation is poorly understood, creating uncertainty when upscaling to annual time scales.
Tropical valley-bottom wetlands in the highlands of Taita Hills, Kenya represent important but understudied systems where livelihoods strongly depend on agriculture in wetlands. This study aimed to understand the magnitude and causes of sub-daily and episodic variation in GHG emissions in recovering and converted wetlands in Taita Hills. Specifically, the study (1) assessed the diurnal variation in greenhouse gas emissions; (2) determined the effects of rewetting and rain events on the pulses of greenhouse gas emissions; (3) determined the effect of fertilizer addition on GHG emissions.
Ten converted and nine recovering wetlands were selected for this study where gas fluxes for CO2, CH4, and N2O were measured using the static chamber method and portable field analyzers between January and April 2024. Objective 1 was determined by conducting several diurnal measurement campaigns: 2 converted and 2 recovering wetlands were measured every two hours over 24 hours at 2-hour intervals. Objective 2 was achieved by conducting dry season rewetting experiments where dry soils were watered to simulate rain events in 10 converted and 5 recovering wetlands. In addition, sampling in the dry season and the onset of the wet season in March allowed comparison of dry season and early wet season measurements. Objective 3 was achieved by conducting a field trial for growing maize with no fertilizer added (control), manure only added, and manure and fertilizer added. The latter represented a typical farmer strategy.
Diurnally, the converted wetlands showed greater variation in flux rate than recovering wetlands, and the gasses behaved differently in response to soil moisture and temperature. In the converted type of wetland, CO2 and N2O had a positive relationship with soil temperature and a negative relationship with soil moisture while CH4 showed a positive relationship with soil moisture and a no relationship with temperature. For rewetting experiments, no significant difference was observed in GHG fluxes, but the early onset of the rainy season caused significantly increased CO2 and N2O fluxes in converted wetlands. No effect was observed in the recovering wetlands. For the fertilization experiment, the combined addition of manure and synthetic fertilizer significantly increased CO2 fluxes while manure alone had intermediate effects on N2O, with no very little effects on both CH4 and CO2. Fertilization significantly increased N2O emissions with no significant differences in both CO2 and CH4. In conclusion, GHG flux measurements should consider diurnal variation, response to rain and fertilization events when upscaling GHG measurements from weekly or monthly measurements to annual time scales.
Keywords: wetlands, ecosystem degradation, greenhouse gas, carbon storage, climate feedback, diurnal emission variation, rewetting, rain events, fertilization, CO2, CH4, N2O