Exchange fluxes of CO₂, CH₄, N₂O and NO between soil and atmosphere along an ecoclimatic gradient in West African savannas: a missing piece for regional budgets

Abstract. This study investigated greenhouse gas (CO₂, CH₄, N₂O) and reactive nitrogen (NO) fluxes at three West African savanna sites: the international research reserve of Lamto (Taabo district, Côte d’Ivoire), the Observatoire de Recherche en Environnement de Nambekaha (OREN) (Korhogo, Côte d’Ivoire), and the Centre de Recherches Zootechniques (Dahra, Senegal). Measurements were carried out during intensive field campaigns conducted in 2024 and 2025, during the wet seasons at the three sites, across tree areas and grassy areas in savannas, and cropland ecosystems subjected to different treatments from March 2023 to September 2025.

Overall, soil moisture, vegetation type (grassy areas, trees areas, crops) and site location (Lamto, Dahra, Nambekaha) were the main factors controlling gas fluxes (CO₂, NO and CH₄), whereas treatments containing different ratio of nitrates and ammonium had no significant effect according to the statistical analysis (ANCOVA).

CO₂ fluxes ranged from 8.21 ± 2.5 to 91.35 ± 73.2 µg C m⁻² s⁻¹ and were controlled by soil moisture, with a decrease in soil respiration as water content increased (β = −1.105 ± 0.236 µg C m⁻² s⁻¹; p <0.001), due to a limitation of oxygen diffusion in the soil, highlighting the key role of soil moisture in regulating both heterotrophic microbial respiration and autotrophic plant respiration, in relation to soil aeration conditions. NO emissions, ranging from 0.01 ± 0.0 to 497.39 ± 146.3 ng N m⁻² s⁻¹, showed a significant correlation with vegetation type. The highest values were observed in the cropland plots of Nambekaha (β = +76.779 ± 15.82 ng N m⁻² s⁻¹; p < 0.001) compared with natural savannas, reflecting intensified nitrification processes linked to background fertilization inputs (150 kg NPK ha⁻¹ yr⁻¹).

CH₄ fluxes were primarily determined by vegetation type: grassy areas within savannas behaved as net sources (β = +3.836 ± 0.62; p < 0.0001), whereas croplands acted as sinks, suggesting methanotrophic activity capable of oxidizing atmospheric methane in the soil. In contrast, N₂O fluxes were mostly low or even negative across all ecosystems and treatments, with no significant relationship to soil moisture, vegetation type, or treatments. The results indicate that soils could occasionally function as net N₂O sinks: indeed, N₂O uptake may occur in nitrogen-poor soils under oxic conditionswhere the limited availability of mineral nitrogen restricts N₂O production and where atmospheric N₂O diffuses easily into the soil.

These findings highlight the microbial and environmental coupling of carbon and nitrogen dynamics in tropical savanna soils and provide critical insight for predicting greenhouse gas and reactive gas emissions under changing land-use conditions.