Valley-bottom wetland agricultural conversion and recovery shape greenhouse gas dynamics and soil organic carbon stocks in an African tropical highland system

Tropical wetlands significantly impact global greenhouse gas (GHG) budgets and carbon storage; however, data from sub-Saharan Africa (SSA) remain limited. Highland valley-bottom wetland (HVBW) agriculture supports millions, yet its effects on GHG emissions and carbon storage remain undocumented. This study quantified soil emissions of nitrous oxide (N 2 O), carbon dioxide (CO 2 ), and methane (CH 4 ) in the Taita Hills, Kenya, from 12 converted, 10 recovering, and one reference (intact) HVBW. Agricultural conversion shifted wetland emissions from CH 4 to N 2 O dominance. Converted HVBWs were N 2 O sources (1.6 kgN2O-Nha-1yr-1), driven by elevated soil nitrate, whereas the intact HVBW was an N 2 O sink (−0.3 kgN2O-Nha-1yr-1), with high soil moisture and a high soil C/NO3--N ratio suggesting complete denitrification. Recovering HVBWs showed intermediate N 2 O emissions (0.5 kgN2O-Nha-1yr-1). CO 2 emissions were similar between converted and recovering HVBWs (9534 vs. 9610 kgCO2-Cha-1yr-1) but lower in the intact (2923 kgCO2-Cha-1yr-1). CH 4 emissions were highest in the intact HVBW (2757 kgCH4-Cha-1yr-1), intermediate in recovering sites (781 kgCH4-Cha-1yr-1), and lowest in converted sites (38 kgCH4-Cha-1yr-1). The intact HVBW had an SOC stock of 224 Mg C ha −1 , indicating carbon loss rates of 2.7 MgCha-1yr-1 over 45 years for converted HVBWs. Comparing recovering with converted HVBWs suggests an SOC recovery of about 1.1 MgCha-1yr-1 after abandonment, offsetting ∼0.0005 % of Kenya's annual agricultural GHG emissions, or 9.8 % when scaled nationally. However, recovering HVBWs also emitted considerably more CH 4 , so their net climate effect cannot be derived from SOC stocks alone. These findings highlight trade-offs between GHG emissions and carbon storage in HVBWs, with wetland recovery promoting partial functional restoration and SOC recovery in SSA, but at the cost of increased CH 4 emissions.