Greenhouse gas emissions from valley-bottom wetlands in an agricultural tropical highland system, Taita Hills, East Africa

Globally, agriculture is one of the main drivers of wetland loss, leading to reduced soil carbon (C) and changes in greenhouse gas (GHG) emissions. In recent decades, wetland loss in Africa appears to be faster than the global losses, at about 43% compared to 35% globally. Valley-bottom wetlands in African highland regions support the livelihoods of >65% of the people who live there, but the effect of agricultural conversion on soil C and GHG emissions is understudied. This study compares GHG emissions between 1 intact, 12 agricultural (converted), and 10 recovered valley-bottom wetlands in Taita Hills, Kenya. Using the static gas chamber method, CO2, CH4, and N2O emissions were measured monthly from April 2023 to date along with soil NO3-N, NH4-N, soil C, and soil moisture. The results indicate that CO2 emissions from the converted wetlands is similar to recovered wetlands (mean = 183 ± 11 SE mg CO2-C m-2 h-1 and mean = 174 ± 13 SE mg CO2-C m-2 h-1 respectively; p > 0.05). This is in contrast with both CH4 and N2O emissions, which showed strong differences (p<0.005). The average CH4 emission in agricultural versus intact wetlands was mean = 0.31 ± 9 SE mg CO2-C m-2 h-1 and mean = 10 ± 1 SE mg CO2-C m-2 h-1, respectively, and the N2O mean emission was mean = 41 ± 0.2 µg N m-2 h-1 vs. 9 ± 3 µg N m-2 h-1, respectively. Addition of organic and inorganic fertilizer to the agricultural wetlands showed an increase in NO3-N in the soil and a high correlation with N2O.  High soil moisture levels and organic matter in the intact wetlands was a major contributing factor for the high CH4 emissions while low soil moisture in the converted wetlands led to low CH4 emissions. The soil organic carbon in the recovered wetlands was higher (Mean = 11 ± 0.1 SE Kg C m-2) compared to the converted wetlands (Mean= 8 ± 0.2 SE Kg C m-2) indicating higher carbon storage in the recovered wetlands. Overall, recovered wetlands contribute more to Global Warming Potential GWP (0.84 CO2-equivalents), but these estimates do not take into account losses in soil C storage, which amount to 1043 Kg C m-2 year-1. On-going data analysis and field work will use seasonal variation and take into account historical losses in C storage to refine annual emission estimates.