the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Jul 2026
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.
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Status: open (until 25 Sep 2026)
- RC1: 'Comment on egusphere-2026-3592', Anonymous Referee #1, 24 Aug 2026 reply
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I would like to congratulate the authors on a very nice piece of work that addresses an important gap in the understanding of biogeochemical processes in a significantly understudied region.
Scientific Significance
This paucity of research on the African continent has impacts on the global understanding of these processes and the global understanding of soil atmosphere exchange on a global basis.
The composition of the research team, comprising researchers from many countries and institutions in West Africa, should be lauded as an example of regional collaboration and scientific exchange.
I believe that this submission is work of excellent scientific significance. it highlights an in-situ study of the emissions of CO2, CH4, N2O and NO from three different sites with contrasting bioclimatic characteristics representing some of the major systems of West Africa and being indicative of tropical and subtropical regions in Africa and globally. Specifically, the sites studied include Lamto and Nambekaha in the humid savanna systems with approximately 1100mm MAP and the Dahra site in a Sahelian dry savanna biome MAP 380mm. These sites are subjected to agricultural activity and therefore provide an outstanding gradient of land uses and agro-ecological contexts.
Scientific Quality
The experimental design followed a multifactorial approach, and sampling occurred over multiple years, under differing land use and vegetation cover, and with differing levels of nitrogen addition. This design required significant efforts to deploy and to maintain, and the authors and their support teams should be commended on the dedication required to deploy and maintain this experiment in what are likely trying conditions.
The experimental design is appropriately replicated with 4 replicates per fertilisation treatment (4 treatments) per vegetation cover (3 cover types), resulting in 48 plots per site. Therefore, a total of 144 plots. I'd like to reiterate that this is a significant amount of work to set up and maintain this experimental design, and the results emanating from this are invaluable. At this point I'd like to question the use of the term "forage water", which I couldn't find defined in the text, I assume this may refer to something like irrigation water. I do, however, suggest the Authors check the translation to English.
The flux measurements seem to have been well conducted, utilising high-precision sensors and a well-described chamber system; this approach seems appropriate and is likely to yield defensible results. Analyses applied for the flux calculations seem appropriate. Measurements of ancillary data such as soil moisture and soil pH seem appropriate. I do recommend the authors rephrase the description of the soil texture at the Nambekaha site (line 393) and the depth to which the soil was sampled for pH analysis (Line 400) (rather "collected to a depth of 20 cm" ).
Presentation Quality
Presentation of the results is generally appropriate by separating the presentation of the results into the gas species of interest, i.e CO2, CH4, N2O, NO.
In the presentation of the fluxes of each of the gaseous species Id like to suggest that the authors consider reorganising the presentation of the results to account for the multifactorial design of the experiment, first by site, then by vegetation cover, then by the N addition treatments. This is done to some extend (eg fig, fig 6 and fig 9 ) but it would be better to have the treatment effects (Site, vegetation, moisture) represented consistently for each of the gas fluxes. As it stands I find the presentation of the results somewhat confusing and hard to follow and the statistical tables are not clear.
Please remove the lines of connection between the point and variance figures (4b, 6a, 6b, 9a, 9b); these should not be included.
The discussion section is generally well written and clear although I believe there would be value in following the logic of the multifactorial design in the presentation of the results (Site effects, vegetation effects, fertilisation effects, soil moisture effects), if the authors restructure the presentation of the results the presentation of the results the discussion will need to be updated to reflect those changes.
While the introduction and discussion appropriately reference existing research in West Africa, due to the paucity of studies on this (and related) topics in general, I would encourage the authors to expand their literature assessment to include work undertaken in Southern Africa.