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

Abstract. Tropical wetlands significantly impact greenhouse gas (GHG) budgets and carbon storage: however data from sub-Saharan African (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 (N₂O), carbon dioxide (CO₂), and methane (CH₄) in the Taita Hills, Kenya, from 12 converted, 10 recovering, and one reference (intact) HVBWs. Agricultural conversion shifted wetland emissions from CH₄ to N₂O dominance. Converted HVBWs were N₂O sources (2.7 kg N₂O–N ha⁻¹yr⁻¹), driven by elevated soil nitrate, whereas the intact wetland was an N₂O sink (−0.3 kg N₂O–N ha⁻¹ yr⁻¹), with high soil moisture and high soil C/NO₃⁻–N ratio suggesting complete denitrification. Recovering HVBWs showed intermediate N₂O emissions (0.6 kg N₂O–N ha⁻¹ yr⁻¹). CO₂ emissions were similar between converted and recovering HVBWs (10,850 vs. 11,031 kg CO₂–C ha⁻¹ yr⁻¹) but lower in the intact (2,923 kg CO₂–C ha⁻¹ yr⁻¹). CH₄ emissions were highest in the intact HVBW (2,757 kg CH₄–C ha⁻¹ yr⁻¹), intermediate in recovering sites (879 kg CH₄–C ha⁻¹ yr⁻¹), and lowest in converted sites (37 kg CH₄–C ha⁻¹ yr⁻¹). The intact HVBW had 224 Mg C ha⁻¹, indicating carbon loss rates of 2.6 Mg C ha⁻¹ yr⁻¹ over 45 years for converted HVBWs. Restoring Taita Hills wetlands would sequester 1.1 Mg C ha⁻¹ yr⁻¹, offsetting ~0.0005 % of Kenya's annual agricultural GHG emissions, or 9.8 % when scaled nationally. These findings highlight trade-offs between GHG emissions and carbon storage in HVBWs, with wetland recovery promoting functional restoration and long-term carbon sequestration in SSA.