Trace Greenhouse Gas Fluxes from Soils and Tree Stems of Rainforests and Cacao Agroforests in the Congo Basin, Cameroon

Abstract:

Tree stems act as conduits of soil-borne greenhouse gases (GHG), but the magnitudes of tree contributions to total (soil + stem) N₂O and CH₄ fluxes from African ecosystems remain unknown. Moreover, soil GHG fluxes are largely understudied in African rainforests and converted land uses, which are identified as an important research gap in the global GHG budget. Here, we quantified the changes in stem and soil N₂O and CH₄ fluxes as well as soil CO₂ fluxes with forest conversion to cacao agroforestry, measured monthly for a year (2017–2018) in four replicate plots per land use at three sites across central and southern Cameroon.

Forest conversion to extensively managed, mature cacao agroforestry had no effect on stem and soil GHG fluxes, because of similarities in soil moisture and texture, absence of fertilizer application, and comparable abundance of leguminous trees and basal area in both land uses.

The annual total N₂O emissions were 1.55 ± 0.20 kg N ha⁻¹ yr⁻¹ from the forest and 1.15 ± 0.10 kg N ha⁻¹ yr⁻¹ from cacao agroforestry, with tree N₂O emissions contributing 11 to 38% for forests and 8 to 15% for cacao agroforestry. Tree stems were a net source of CH₄, despite the soil acting as sinks. The annual total soil CH₄ fluxes were −2.62 ± 0.37 for the forest and −3.23 ± 0.45 for the cacao agroforestry, with tree emissions offseting 5–18% of the soil sink in the forest, and 3–14% of the soil CH₄ sink in the cacao agroforestry. The balance between the soil and stem CH₄ fluxes indicated that there was a net CH₄ sink in both land uses. The substantial contributions of tree stems to total N₂O and CH₄ fluxes highlight the importance of including tree-mediated fluxes in ecosystem GHG budgets. Annual soil CO₂ emissions were 10.1 ± 0.27 Mg C ha⁻¹ yr⁻¹ for the forest, and 10.3 ± 0.42 Mg C ha⁻¹ yr⁻¹ for the cacao agroforestry.

¹⁵N-isotope tracing from soil mineral N to stem-emitted ¹⁵N₂O and correlations between stem and soil N₂O, vapour pressure deficit and other drivers suggest that stem-emitted N₂O originated predominantly from the soil. Our results also points a possible soil origin of tree stem CH₄ emissions, driven by transpiration.

Considering the biophysical conditions of our forest sites, which represented two-thirds of the Congo Basin, our extrapolated emissions from this rainforest area were 0.18 ± 0.05 Tg N₂O-N yr⁻¹ and −0.30 Tg CH₄-C yr⁻¹.