Title: Influence of wetland conversion on greenhouse gas emission in valley bottom wetlands in Taita Hills, Kenya

Author: Damaris Guranya Kisha

Supervising Institution: IHE Delft  - Institute for Water Education

Year: 2023



Wetlands serve an important role of atmospheric GHG reduction and supporting communities whose livelihoods depend on them. However, the conversion of wetlands to agricultural land is increasing at a rapid rate at local, regional and global scale driven by the increasing human population and the need to tackle food insecurity problems. In East Africa the valley-bottom wetlands in the highland systems continue to face degradation through intensive small shareholder agricultural practises but little is known how this impacts GHG. To partly fill this knowledge gap, this study was conducted in the valley-bottom wetlands of Wundanyi, a small catchment located in Taita hills South East of Kenya. The study was done in November 2022, December 2022 and January 2023 during the short rain season. Four specific objectives were addressed during the study; (i) to assess wetland use and management by small shareholder farmers in valley-bottom wetlands in Wundanyi catchment; (ii) to compare GHG fluxes (CO2, CH4 and N2O) in the intact and converted wetland plots; (iii) to relate soil characteristics to GHG fluxes in the converted and intact wetlands inorder to assess potential controls and; (iv) to assess the potential impact of conversion on stream GHG fluxes.

Fifty-nine farmers owning land in the valley bottom wetlands were interviewed on the use and management of wetlands. Among the key information gathered was the time since the wetlands were converted, type of agricultural activities, soil, crop and water management approaches. Results, showed that most wetlands have been used for small shareholder mixed farming for more than 10 years with maize, beans and vegetables being the dominant crops. Crop rotation, intercropping and manure/fertilizer application were the main soil and crop management approaches. Flood and drought events were managed through channelization and irrigation in wet and dry seasons, respectively.

Soil GHG fluxes were determined using static chamber method in seven intact and converted wetlands. My results showed that, significantly higher CH4 flux in the intact than the converted plots (mean ± SE of intact versus converted: 6.51 ± 1.25 and 0.01 ± 0.02 mg C m-2 h-1). N2O flux was significantly higher in the converted plots than the intact plot (mean ± SE of intact versus converted: 3.80 ± 1.97 and 29.57 ± 5.99 μg m-2 h-1). CO2 did not vary significantly between intact and converted plots (mean ± SE of intact versus converted: 173.66 ± 12.72 and 183.47 ± 11.21 mg C m-2 h-1). CO2 flux showed a negative relationship with moisture, CH4 showed a positive relationship with soil moisture, temperature and organic carbon to nitrogen molar ratio, and N2O was positively correlated with NO3.

Stream GHG fluxes were determined at four streams; two in the wetland (intact) and two in the agricultural plots (converted). Intact streams had about 3-folds higher CO2, CH4 and N2O fluxes than agricultural streams. The mean fluxes (± SE) in intact versus converted were: CO2 (1505 ± 128.25 and 445.5 ± 63.16 mg C m-2 h-1), CH4 (41.10 ±7.25 and 17.21 ±4.28 mg C m-2 h-1) and N2O (0.3 ± 0.039 and 0.02 ± 0.004 mg N m-2 h-1) CO2 and CH4 showed a negative relationship with DO and N2O also showed negative relationship with DO and positive relationship with NO3.