Title: Ecosystem services, land use change, and greenhouse gas emission from Cheleleka Wetland, Rift Valley, Ethiopia
Author: Simo Raphael Ongaya
Supervising Institution: IHE Delft Institute for Water Education
Year: 2024
Abstract:
Wetlands serve an important function in ecosystem services provision such as climate regulation through soil carbon storage and livelihood support. However, the increasing population calls for more need for settlement space and food production, resulting in the conversion of the wetlands to land uses for provisioning services, which may cause a reduction in climate regulating services through greenhouse gas emissions and reduction of soil organic carbon (SOC) storage. In East Africa, wetlands face threats of degradation due to both intensive and extensive agricultural activities, however, little is known about how these have effects on ecosystem services, soil organic carbon (SOC) storage or greenhouse gas (GHG) emissions. For these gaps to be filled partly, a study was conducted in Cheleleka wetland, located in southern Ethiopia. Cheleleka is a rift valley wetland that forms part of Lake Hawassa catchment which has been experiencing conversion for the last 20 years and like other East African wetlands, experiencing increasing pressure, loss, and degradation. The study was conducted from January to March 2024 and addressed three specific objectives: (i) to describe the ecosystem services provided by the wetland and dominant management practices; (ii) to determine the effects of LULC on soil C, nitrogen (N) and GHG emissions and their relationships in management practices; and (iii) to use a chrono-sequence to examine the effects of wetland conversion on soil carbon storage in different LULC.
For objective (i), forty-two respondents including farmers, fishermen, experts and other wetland users in Cheleleka wetland were interviewed for their views on the ecosystem services, wetland use and management. The outcome information of the interviews were the types of fish in L. Cheleleka, age of the plots since conversion, soil and water management, and types of agricultural activities. The ages of the plots range from 0 to 15 years since conversion, and vegetable and maize production have been the dominant agricultural practices for the past few years. Organic manure and fertiliser (DAP, CAN and Urea) application, mixed farming and inter-cropping were major practices of soil management. Conversion of the wetland to eucalyptus plantations for timber was also common.
A synoptic sampling approach was used to measure soil C, N, and GHG fluxes for a total of 24 sites, which were arranged along 5 transects. These transects extended from the centre of the wetland, (used predominantly for dry season grazing and flooded during wet seasons) to the outer border of the wetland where conversion first occurred. LULCs along these transects were crop production, eucalyptus, and grazing. It was only possible to find one location with intact wetland vegetation (Typha), which was considered as a reference site. These 24 sites were sampled monthly from Jan. to March for GHG fluxes using a manual static chamber method, for a total of 72 number of fluxes. Soil sampling to characterize each plot up to 1 m depth at 10 cm intervals was also conducted. Bulk density, SOC, and extractable NH4 and NO3 were measured for 21 of the 24 sites.
Results showed a decrease of SOC with depth in all the sites. The results showed a significant decrease of SOC with age (p<0.05) and across different LULCs. Specifically, the intact (reference) site had higher SOC stocks of 5.13 ± 0.39 kg C/m2, while crops had about half that amount ((2.59 ± 0.21 kg C/m2) (mean ± SE). Grazing LULC and Eucalyptus. Soil NH4-N and NO3 -N were related to soil moisture and presumed soil oxygen conditions with NH4-N higher in wetter sites (in grazing and in the reference site) whereas NO3-N was highest in the crops and eucalyptus sites.
CH4 fluxes were mainly related to LULC, likely due to differences in SOC and soil N, as shown by the positive relationship between CH4 and SOC (R2 = 0.32, p<0.05) and CH4 and NH4-N (p=0.003, R2 = 0.14). This led to high CH4 emissions in grazing LULC (12.30 ± 3.23) and lower emissions in eucalyptus and crop LULC (0.539 ± 0.539 and 4.02 ± 1.08 respectively). In contrast, N2O was not significantly affected by LULC, but was significantly affected by sampling campaign with higher N2O-N emissions in March after a period of short rains. N2O was not significantly correlated with SOC, NO3 or NH4. These results suggest that N2O was controlled by both denitrification in grazing areas, which maintained higher N2O fluxes than other LULCs during the drier months of Jan. and Feb. and by nitrification or coupled nitrification-denitrification in crop areas which experienced a~5x increase in N2O emission in crop areas in March after short rains. This also corresponds to a sowing period where urea was applied as top-dressing.
In conclusion, these results show that LULC and age are important factors affecting GHG emission from Cheleleka wetland, with younger wetter areas related to higher CH4 emissions, but with higher SOC storage. In contrast, N2O emissions appeared to be more responsive to seasonality in precipitation and crop management (planting and fertilization rate), showing less clear relationships to LULC. Eucalyptus plantations overall had low rates of all gas fluxes, despite intermediate SOC and N content among all sites, which bears further research.
Keywords: wetlands, ecosystem services, soil carbon, greenhouse gas, land use change, degradation, Ethiopia, methane emissions, N2O emissions, eucalyptus, crop management