Hydroelectric reservoirs, though fundamental to renewable energy generation, are increasingly recognized as important sources of greenhouse gases (GHGs) in tropical and subtropical regions. However, substantial uncertainty persists regarding their contributions to the GHG cycle and their overall climate impact. Here, we present a 14 year dataset (2009-2022) of CH4 and CO2 emissions from the Nam Theun 2 reservoir in Lao PDR. We report emissions through diffusion, ebullition, and downstream degassing below turbines and spillways. This work represents one of the longest continuous records of reservoir GHG emissions in a subtropical system. Complementary eddy covariance approach was also employed, the results showed that CO2 emissions were consistently higher than those estimated from discrete sampling, likely due its ability to capture real-time turbulence and hot moments, and to the location of the EC system in shallow, high-emission areas for the last two campaigns. In contrast, CH4 emissions upscaled from EC measurements were often lower than those derived from discrete sampling, particularly during later campaigns. This difference was attributed to spatial coverage limitations, meteorological influences, wind filtering, and the lower sensitivity of the EC system to episodic ebullition events. CH4 showed a clear diurnal pattern; while CO2 fluxes differed between day and night only during periods of strong stratification. In this study, discrete sampling provided broader spatial coverage and higher data availability; therefore, it was used for emission calculations. CH4 emissions peaked during the warm dry period due to lower water levels and intensified stratification that favored methanogenesis and ebullition, whereas CO2 emissions peaked during cold dry season overturn events that released accumulated hypolimnetic carbon. Across the study period, ebullition accounted for 77 % of total CH4 emissions and remained relatively stable, supported by substantial flooded organic matter reserves. In contrast, during that same period, diffusive CH4 fluxes declined by 97 %, and CO2 emissions - dominated by diffusive fluxes (96 %) - declined by 87 %, indicating reservoir aging and pro gressive depletion of labile organic matter. Over 14 years, cumulative gross emissions totaled 10 736 GgCO2eq., with CH4 (51 %) slightly exceeding CO2 (49 %). Annual emissions were greatest in 2010 (1276 GgCO2eq.), declining by similar to 70 % by 2021. These findings provide new insight into long-term GHG budgets in subtropical reservoirs, refine global carbon budget estimates, and inform climate-sensitive hydropower planning.

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.

Agroforestry systems - combining trees with crops and/or livestock - are increasingly promoted as sustainable and climate-resilient land-use strategies. Despite their widespread presence in the Sahel, experimental data on their potential as carbon sinks are scarce. This study presents a full-year, high-frequency dataset of CO2 fluxes in a Sahelian agro-silvo-pastoral parkland dominated by Faidherbia albida, located in Senegal's groundnut basin. CO2 fluxes were continuously measured using automated dynamic chambers, allowing the quantification of soil and crop respiration (Rch), gross primary production (GPPch), and net carbon exchange (FCO2ch) under both full sun and shaded (under tree canopies) environments. Seasonal patterns of CO2 fluxes were similar in both environments, with peaks during the rainy season. Rch and GPPch were significantly higher under tree canopies, indicating a "fertile island" effect. CO2 flux variability was primarily driven by soil moisture and leaf area index. Chamber-based GPP estimates closely matched those from Eddy Covariance measurements. On an annual scale, F. albida trees contributed approximately 23 % of total ecosystem GPP, with a carbon use efficiency of 0.48. Net annual vertical CO2 exchange was estimated at -1.4 +/- 0.46 and -1.8 +/- 0.17 Mg C-CO2 ha-1 using chamber and Eddy Covariance methods, respectively. These findings underscore the role of F. albida-based agroforestry systems as effective carbon sinks in Sahelian landscapes, supporting their potential contribution to climate change mitigation.

Abstract. Hydroelectric reservoirs, though fundamental to renewable energy generation, are increasingly recognized as important sources of greenhouse gases (GHGs) in tropical and subtropical regions. However, substantial uncertainty persists regarding their contributions to the GHG cycle and their overall climate impact. Here, we present a 14 year dataset (2009–2022) of CH4 and CO2 emissions from the Nam Theun 2 reservoir in Lao PDR. We report emissions through diffusion, ebullition, and downstream degassing below turbines and spillways. This work represents one of the longest continuous records of reservoir GHG emissions in a subtropical system. Complementary eddy covariance approach was also employed, the results showed that CO2 emissions were consistently higher than those estimated from discrete sampling, likely due its ability to capture real-time turbulence and hot moments, and to the location of the EC system in shallow, high-emission areas for the last two campaigns. In contrast, CH4 emissions upscaled from EC measurements were often lower than those derived from discrete sampling, particularly during later campaigns. This difference was attributed to spatial coverage limitations, meteorological influences, wind filtering, and the lower sensitivity of the EC system to episodic ebullition events. CH4 showed a clear diurnal pattern; while CO2 fluxes differed between day and night only during periods of strong stratification. In this study, discrete sampling provided broader spatial coverage and higher data availability; therefore, it was used for emission calculations. CH4 emissions peaked during the warm dry period due to lower water levels and intensified stratification that favored methanogenesis and ebullition, whereas CO2 emissions peaked during cold dry season overturn events that released accumulated hypolimnetic carbon. Across the study period, ebullition accounted for 77 % of total CH4 emissions and remained relatively stable, supported by substantial flooded organic matter reserves. In contrast, during that same period, diffusive CH4 fluxes declined by 97 %, and CO2 emissions – dominated by diffusive fluxes (96 %) – declined by 87 %, indicating reservoir aging and pro gressive depletion of labile organic matter. Over 14 years, cumulative gross emissions totaled 10 736 Gg CO2 eq., with CH4 (51 %) slightly exceeding CO2 (49 %). Annual emissions were greatest in 2010 (1276 Gg CO2 eq.), declining by ∼70 % by 2021. These findings provide new insight into long-term GHG budgets in subtropical reservoirs, refine global carbon budget estimates, and inform climate-sensitive hydropower planning.

Understanding greenhouse gas fluxes in semi-arid ecosystems is critical for improving our understanding of biogeochemical cycles, particularly in underrepresented regions like the African Sahel. In these landscapes, greenhouse gas exchange arises from ground, trees, and water ponds, and is further shaped by environmental conditions and grazing. The carbon dioxide, methane, and nitrous oxide fluxes were quantified from these components in a Sahelian savanna in Senegal, while also assessing grazing impacts and environmental drivers (soil water content, temperature, vapor pressure deficit, photosynthetically active radiation). The ground was a net carbon dioxide sink during the rainy season but shifted toward neutrality or weak emission in the dry season, consistently acted as a methane sink, and was a year-round nitrous oxide source. Seasonal ponds were strong methane and nitrous oxide emission hotspots, with methane emissions being high enough to offset the sink of the overall savanna landscape. Trees contributed to carbon dioxide and methane uptake via branches, whereas stems were net methane and carbon dioxide emitters. Both stems and branches emitted nitrous oxide, and the presence of trees enhanced carbon dioxide, methane and nitrous oxide fluxes from soils beneath their canopies. Grazing enhanced ground gross primary production, reduced methane uptake, while no effect was seen on nitrous oxide fluxes. Temporal variability of carbon dioxide and nitrous oxide fluxes was strongly linked to soil water content and temperature, whereas methane fluxes showed no correlations with any of the measured drivers. These results demonstrate that tree- and pond-mediated fluxes, together with grazing, substantially alter the greenhouse gas fluxes of savanna ecosystems and incorporating these effects is essential for accurately representing semi-arid savannas in global greenhouse gas budgets.

Human activities such as fertilization of agricultural lands and human-induced biomass burning strongly impact nitrogen (N) dynamics and losses, with many consequences on the environment. The quantification of N budgets (N inputs and outputs) between the surface and the atmosphere is a prerequisite to understand the N biogeochemical cycle, i.e. how N is transferred from the atmosphere to the biosphere, through the soil and back to the atmosphere from surface emissions. Sub Saharan Africa (SSA) is characterized by an increase in demography, with strong impacts on biodiversity, and on the sustainability of human activities including agriculture. In Africa, the increase in demography and the associated increased fertilizer inputs (to supply growing food and energy demands) will lead to increased emissions from amended soils, which will in turn increase atmospheric N deposition and induce feedbacks to the ecosystems and the atmosphere.In this context, the NitroAfrica project (2023-2026) is designed to study the impact of N wet deposition on the soil – plant – atmosphere continuum. We make the hypothesis that changes of wet N deposition in West African ecosystems over the 21th centuries will induce important changes in biogenic emissions from the ecosystems to the atmosphere with impacts on regional atmospheric chemistry and further N deposition. Indeed, increasing trends of N wet deposition has already been observed, especially in the NH4+ form. Three ecoclimatic zones in West Africa are studied, in Guinean (Lamto, Côte d’Ivoire), Sudanese (Korhogo, Côte d’Ivoire) and Sahelian (Dahra, Senegal) zones, where solutions with different NH4+/NO3- partition are used to mimic the increase in N wet deposition.Results on N (N2O, NO) and CO2 emissions from soils from plots amended with solutions as well as control plots will be presented. N wet deposition fluxes from recent years will also be presented within the context of existing long-term studies on N wet deposition. This comparison is particularly relevant for the Lamto station where the International Network to study Deposition and Atmospheric chemistry in Africa (INDAAF) is based and provides long-term data since 1995.This study contributes to fill in the lack of studies in SSA, and to understand the processes involved in N emissions and deposition in tropical regions.

Abstract. Agroforestry systems — combining trees with crops and/or livestock — are increasingly promoted as sustainable and climate-resilient land-use strategies. Despite their widespread presence in the Sahel, experimental data on their potential as carbon sinks are scarce. This study presents a full-year, high-frequency dataset of CO2 fluxes in a Sahelian agro-silvo-pastoral parkland dominated by F. albida, located in Senegal’s groundnut basin. CO2 fluxes were continuously measured using automated static chambers, allowing the quantification of soil and crop respiration (Rch), gross primary production (GPPch), and net carbon exchange (FCO2ch) under both full sun and shaded (under tree canopies) environments. Seasonal patterns of CO2 fluxes were similar in both environments, with peaks during the rainy season. Rch and GPPch were significantly higher under tree canopies, indicating a ‘fertile island’ effect. CO2 flux variability was primarily driven by soil moisture and leaf area index. Chamber-based GPP estimates closely matched those from Eddy Covariance measurements. On an annual scale, F. albida trees contributed approximately 50 % of total ecosystem GPP, with a carbon use efficiency of 0.48. Net annual CO2 exchange was estimated at −1.4 ± 0.02 and −1.8 ± 0.01 Mg C-CO2 ha⁻¹ using chamber and Eddy Covariance methods, respectively. These findings underscore the role of F. albida-based agroforestry systems as effective carbon sinks in Sahelian landscapes, supporting their potential contribution to climate change mitigation.

After this paper was published, one of the readers of our paper pointed out that CA-Est and FI-Van did not have any T water data.Therefore, for each panel in figure 4, the mutual information score for T water and CA-Est will now be replaced with a white color.The same will be carried out for T water and FI-Van.A brief sentence has also been added at the end of the figure 4 caption to indicate lack of T water data for CA-Est and FI-Van.This correction does not affect our results.We apologize for any inconvenience these errors may have caused.

Plants emit biogenic volatile organic compounds (BVOCs) in response to changes in environmental conditions (e.g. temperature, radiation, soil moisture). In the large family of BVOCs, isoprene is by far the strongest emitted compound and plays an important role in ozone chemistry, thus affecting both air quality and climate. In turn, climate change may alter isoprene emissions by increasing temperature as well as the occurrence and intensity of severe water stresses that alter plant functioning. The Model of Emissions of Gases and Aerosols from Nature (MEGAN) provides different parameterizations to account for the impact of water stress on isoprene emissions, which essentially reduces emissions in response to the effect of soil moisture deficit on plant productivity. By applying the regional climate–chemistry model RegCM4chem coupled to the Community Land Model CLM4.5 and MEGAN2.1, we thus performed sensitivity simulations to assess the effects of water stress on isoprene emissions and near-surface ozone levels over the Euro-Mediterranean region and across the drier and wetter summers over the 1992–2016 period using two different parameterizations of the impact of water stress implemented in the MEGAN model. Over the Euro-Mediterranean region and across the simulated summers, water stress reduces isoprene emissions on average by nearly 6 %. However, during the warmest and driest selected summers (e.g. 2003, 2010, 2015) and over large isoprene-source areas (e.g. the Balkans), decreases in isoprene emissions range from −20 % to −60 % and co-occur with negative anomalies in precipitation, soil moisture and plant productivity. Sustained decreases in isoprene emissions also occur after prolonged or repeated dry anomalies, as observed for the summers of 2010 and 2012. Although the decrease in isoprene emissions due to water stress may be important, it only reduces near-surface ozone levels by a few percent due to a dominant VOC-limited regime over southern Europe and the Mediterranean Basin. Overall, over the selected analysis region, compared to the old MEGAN parameterization, the new one leads to localized and 25 %–50 % smaller decreases in isoprene emissions and 3 %–8 % smaller reductions in near-surface ozone levels.

Tropical peatlands cycle and store globally significant amounts of carbon in their soil and biomass. Climate and land-use change alter greenhouse gas (GHG) fluxes of tropical peatlands, but the magnitude of these changes remains highly uncertain owing to limited measurements. We measured net ecosystem exchanges of carbon dioxide (CO2) and methane (CH4) as well as soil nitrous oxide (N2O) fluxes between mid-2016 and mid-2022 from Acacia crassicarpa plantation, degraded forest and intact forest within the same peat landscape to represent land-cover change trajectories in Sumatra, Indonesia. Here we report the first full plantation rotation GHG balance investigation undertaken in any fiber wood plantation on peatland globally. The Acacia plantation was found to have lower GHG emissions than the degraded peatland, which had a similar mean groundwater level, despite more intensive land-use. The GHG emissions from the Acacia plantation over a full plantation rotation (38.3 ± 4.8 tCO2-eq ha−1 yr−1, average ± standard deviation) were two times higher than those from the intact forest (20.1 ± 3.7 tCO2-eq ha−1 yr−1), but only around half of the current IPCC Tier 1 emission factor for this land-use. Our results should help to reduce the uncertainty in the estimation of GHG emissions from globally important ecosystems, provide a complete estimate of the impact of land-use change on tropical peat, and develop science-based peatland management practices that help to minimize GHG emissions.

Accounting for temporal changes in carbon dioxide (CO _2 ) effluxes from freshwaters remains a challenge for global and regional carbon budgets. Here, we synthesize 171 site-months of flux measurements of CO _2 based on the eddy covariance method from 13 lakes and reservoirs in the Northern Hemisphere, and quantify dynamics at multiple temporal scales. We found pronounced sub-annual variability in CO _2 flux at all sites. By accounting for diel variation, only 11% of site-months were net daily sinks of CO _2 . Annual CO _2 emissions had an average of 25% (range 3%–58%) interannual variation. Similar to studies on streams, nighttime emissions regularly exceeded daytime emissions. Biophysical regulations of CO _2 flux variability were delineated through mutual information analysis. Sample analysis of CO _2 fluxes indicate the importance of continuous measurements. Better characterization of short- and long-term variability is necessary to understand and improve detection of temporal changes of CO _2 fluxes in response to natural and anthropogenic drivers. Our results indicate that existing global lake carbon budgets relying primarily on daytime measurements yield underestimates of net emissions.

Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Geophysical Research Letters. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]Diel to interannual variation in carbon dioxide emissions from lakes and reservoirsAuthorsMalgorzataGolubiDNikaanKoupaei-AbyazaniiDTimoVesalaIvanMammarellaiDAnneOjalaGilBohreriDGesa AWeyhenmeyeriDPeter D.BlankeniDWernerEugsteriDFranziskaKoebschJiquanCheniDKevin P.CzajkowskiiDChandrashekharDeshmukhFrédéricGuérinJouniHeiskanenElynHumphreysiDAndersJonssoniDJanKarlssoniDGeorge W.KlingiDXuhuiLeeiDHepingLiuAnnaleaLohilaiDErik JohannesLundiniDTimothy HectorMorinEvaPodgrajsekMariaProvenzaleAnnaRutgersoniDTorstenSachsiDErikSahléeDominiqueSerçaiDChangliangShaoiDChristopherSpenceIan B.StrachaniDWeiXiaoiDAnkur RashmikantDesaiiDSee all authors Malgorzata GolubiDDundalk Institute of TechnologyiDhttps://orcid.org/0000-0001-9361-0331view email addressThe email was not providedcopy email addressNikaan Koupaei-AbyazaniiDUniversity of Wisconsin-MadisoniDhttps://orcid.org/0000-0001-6982-230Xview email addressThe email was not providedcopy email addressTimo VesalaUniversity of Helsinki, Institute for Atmospheric and Earth System Researchview email addressThe email was not providedcopy email addressIvan MammarellaiDUniversity of HelsinkiiDhttps://orcid.org/0000-0002-8516-3356view email addressThe email was not providedcopy email addressAnne OjalaNatural Resources Instituteview email addressThe email was not providedcopy email addressGil BohreriDOhio State UniversityiDhttps://orcid.org/0000-0002-9209-9540view email addressThe email was not providedcopy email addressGesa A WeyhenmeyeriDEcology and Genetics/LimnologyiDhttps://orcid.org/0000-0002-4013-2281view email addressThe email was not providedcopy email addressPeter D. BlankeniDUniversity of Colorado BoulderiDhttps://orcid.org/0000-0002-7405-2220view email addressThe email was not providedcopy email addressWerner EugsteriDETH ZurichiDhttps://orcid.org/0000-0001-6067-0741view email addressThe email was not providedcopy email addressFranziska KoebschGFZ German Research Centre for Geosciencesview email addressThe email was not providedcopy email addressJiquan CheniDMichigan State UniversityiDhttps://orcid.org/0000-0003-0761-9458view email addressThe email was not providedcopy email addressKevin P. CzajkowskiiDUniversity of ToledoiDhttps://orcid.org/0000-0002-0472-4204view email addressThe email was not providedcopy email addressChandrashekhar DeshmukhAPRIL Asiaview email addressThe email was not providedcopy email addressFrédéric GuérinIRD - Marseille, France.view email addressThe email was not providedcopy email addressJouni HeiskanenUniversity of Helsinkiview email addressThe email was not providedcopy email addressElyn HumphreysiDCarleton UniversityiDhttps://orcid.org/0000-0002-5397-2802view email addressThe email was not providedcopy email addressAnders JonssoniDDepartment of Ecology and Environmental ScienceiDhttps://orcid.org/0000-0002-0807-0201view email addressThe email was not providedcopy email addressJan KarlssoniDUmea UniversityiDhttps://orcid.org/0000-0001-5730-0694view email addressThe email was not providedcopy email addressGeorge W. KlingiDUniversity of Michigan-Ann ArboriDhttps://orcid.org/0000-0002-6349-8227view email addressThe email was not providedcopy email addressXuhui LeeiDYale University, School of Forestry and Environmental StudiesiDhttps://orcid.org/0000-0003-1350-4446view email addressThe email was not providedcopy email addressHeping LiuWashington State Universityview email addressThe email was not providedcopy email addressAnnalea LohilaiDFinnish Meteorological InstituteiDhttps://orcid.org/0000-0003-3541-672Xview email addressThe email was not providedcopy email addressErik Johannes LundiniDSwedish Polar Research SecretariatiDhttps://orcid.org/0000-0002-3785-8305view email addressThe email was not providedcopy email addressTimothy Hector MorinState University of New York College of Environmental Science and Forestryview email addressThe email was not providedcopy email addressEva PodgrajsekOX2view email addressThe email was not providedcopy email addressMaria ProvenzaleUniversity of Helsinkiview email addressThe email was not providedcopy email addressAnna RutgersoniDUppsala UniversityiDhttps://orcid.org/0000-0001-7656-1881view email addressThe email was not providedcopy email addressTorsten SachsiDHelmholtz Centre Potsdam - German Research Centre for Geosciences (GFZ)iDhttps://orcid.org/0000-0002-9959-4771view email addressThe email was not providedcopy email addressErik SahléeEarth Sciencesview email addressThe email was not providedcopy email addressDominique SerçaiDLaboratoire d'Aérologie, Université de Toulouse, CNRS, UPS, FranceiDhttps://orcid.org/0000-0001-8688-1440view email addressThe email was not providedcopy email addressChangliang ShaoiDInstitute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural SciencesiDhttps://orcid.org/0000-0002-4968-8577view email addressThe email was not providedcopy email addressChristopher SpenceEnvironment and Climate Change Canadaview email addressThe email was not providedcopy email addressIan B. StrachaniDMcGill UniversityiDhttps://orcid.org/0000-0001-6457-5530view email addressThe email was not providedcopy email addressWei XiaoiDNanjing University of Information Science and TechnologyiDhttps://orcid.org/0000-0002-9199-2177view email addressThe email was not providedcopy email addressAnkur Rashmikant DesaiiDCorresponding Author• Submitting AuthorUniversity of Wisconsin-MadisoniDhttps://orcid.org/0000-0002-5226-6041view email addressThe email was not providedcopy email address

Based on rare and original in situ measurements together with published data, we estimate a complete nitrogen (N) budget for a semi-arid Sahelian grazed grassland located in Dahra (Senegal) in 2014 and 2017. Nitrogen inputs include biological fixation, dry and wet atmospheric deposition, and input from livestock manure. Nitrogen outputs include nitric oxide (NO) and nitrous oxide (N 2 O) emissions from soils, NO and ammonia (NH 3 ) emissions from biomass burning, NH 3 volatilization from manure, ingestion from livestock grazing, uptake by trees and soil leaching. Nitrogen inputs ranged between 11.7 ± 0.5 and 34.4 ± 0.5 kg N ha −1 yr −1 for low and high estimates respectively, and N outputs ranged between 16.4 ± 1.5 and 45.7 ± 1.5 kg N ha −1 yr −1 for low and high estimates respectively, on average for both years. Nitrogen depletion was estimated between 4.7 ± 2.0 and 11.3 ± 2.0 kg N ha −1 yr −1 , which involves N mining from soils. The budget is dominated by the impact of livestock through grazing (63% of the outputs), NH 3 volatilization (15% of the outputs), manure (68% of the inputs) and atmospheric deposition (19% of the inputs). The N critical load (Steady State Mass Balance method) ranged from 16.7 ± 0.8 to 47.5 ± 1.7 kg N ha −1 yr −1 , showing that the grazed grassland of Dahra was not yet threatened by an excess of N. The assessment of the critical load in Sahelian landscapes depends heavily on livestock participation to the ecosystem equilibrium.

Estimates of greenhouse gas (GHG) emissions from soil are essential to understand possible climate change mitigation from ecosystems. There is currently very limited and reliable information on GHG emission factors for most land-use types of Sahelian Africa. GHG (CO2, H2O, CH4, N2O) and ammonia (NH3) emissions were measured in a Sahelian agro-silvo-pastoral parkland dominated by Faidherbia albida trees (Niakhar, Senegal) using 8 automatic chambers coupled to a Picarro G2508 gas analyser. The measurements were carried out in 2021 covering the late dry season (bare soil), the full rainy season (with groundnut plants in the chambers) and the beginning of the next dry period (senescent vegetation and bare soil). The chamber-based CO2 fluxes were compared to the Net Ecosystem Exchange of CO2 (NEE) as measured by a 4.5m-eddy covariance tower (below tree crowns) installed over the same agro-silvo-pastoral field. To avoid small scale heterogeneity, we compared here EC fluxes with chamber measured fluxes far from the Faidherbia albida area of influence. Indeed, for a given day, soil CO2 respiration is significantly higher under trees (shade) than far from trees (full sun) due to trees ‘island effect’ (p<0.0001). Soil CO2 respiration was very low at the end of the dry season, with an average of about 0.6 µmol CO2 m-2 s-1. During the wet season, the maximum soil respiration at night was about 5 µmol CO2 m-2 s-1 and the maximum net CO2 uptake during the day was around -6 µmol CO2 m-2 s-1. Only negligible fluxes of CH4, N2O and NH3 were recorded for all seasons. The low N2O fluxes could be related to low soil fertility and lack of nitrogen supply, and low soil moisture in these sandy soils does not favor soil gas production processes for both N2O and CH4. The CO2 fluxes from the automatic chambers showed similar typical semi-arid ecosystem patterns as that of the EC tower. We saw large emission peaks during the first rain events of the rainy season, positive and negative fluxes at night and day, respectively, high fluxes when the soil was wet, and decay during the next dry season. However, in average the soil CO2 respiration magnitude of the chambers with groundnut plant were much lower (1.26 µmol CO2 m-2 s-1) than the ecosystem respiration as seen from the EC tower (3.74 µmol CO2 m-2 s-1), and the difference was even worse for diurnal net CO2 uptake (by a factor of 7).

Accounting for temporal changes in carbon dioxide (CO2) emissions from freshwaters remains a challenge for global and regional carbon budgets. Here, we synthesize 171 site-months of e...

Based on rare and original in situ measurements together with published data, we estimate a complete N budget for a semi-arid Sahelian grazed grassland ecosystem (or open tree savanna) located in Dahra (Senegal) for the years 2014 and 2017. Nitrogen (N) budgets in Africa have been estimated primarily for croplands, and very little data are available for grassland ecosystems. N inputs include biological N fixation, dry and wet N atmospheric deposition, and N input from livestock manure. N outputs include NO and N 2 O emissions from soils, NO and NH 3 emissions from domestic fires and biomass burning, NH 3 volatilization from manure, N ingestion from livestock grazing, N uptake by trees and N soil leaching. Upper and lower bounds were estimated, depending on the method used to quantify the contribution of livestock to the N budget. N inputs ranged between 11.7 ± 0.5 and 34.4 ± 0.5 kg N ha -1 yr -1 and N outputs ranged between 16.8 ± 1.5 and 46.1 ± 1.5 kg N ha -1 yr -1 on average for both years. N depletion (inputs minus outputs) was consequently estimated to be between -5.1 ± 2.0 and -11.1 ± 2.0 kg N ha -1 yr -1 . The budget is dominated by the impact of livestock through grazing (62% of the outputs), NH 3 volatilization (15% of the outputs), manure (68% of the inputs) and atmospheric deposition (19% of the inputs). The nitrogen critical load (estimated by the Steady State Mass Balance approach) ranged from 16.7 ± 0.8 to 47.5 ± 1.7 kg N ha -1 yr -1 , showing that the grazed grassland of Dahra was not yet threatened by an excess of nitrogen. The main uncertainties are related to the assessment of the critical load and to the quantification of parameters linked to livestock.

This study is based on the analysis of field-measured nitrous oxide (N2O) emissions from a Sahelian semi-arid grassland site in Senegal (Dahra), tropical humid agricultural plots in Kenya (Mbita region) and simulations using a 1D model designed for semi arid ecosystems in Dahra. This study aims at improving present knowledge and inventories of N2O emissions from the African continent. N2O emissions were larger at the agricultural sites in the Mbita region (range: 0.0 ± 0.0 to 42.1 ± 10.7 ngN m−2 s−1) than at the Dahra site (range: 0.3 ± 0 to 7.4 ± 6.5 ngN m−2 s−1). Soil water and nitrate (NO3−) contents appeared to be the most important drivers of N2O emissions in Dahra at the seasonal scale in both regions. The seasonal pattern of modelled N2O emissions is well represented, though the model performed better during the rainy season than between the rainy and dry seasons. This study highlighted that the water-filled pore space threshold recognised as a trigger for N2O emissions should be reconsidered for semi-arid ecosystems. Based on both measurements and simulated results, an annual N2O budget was estimated for African savanna/grassland and agricultural land ranging between 0.17–0.26 and 1.15–1.20 TgN per year, respectively.

Quantification of biogenic volatile organic compound (BVOC) fluxes into the atmosphere is crucial to understand their role in atmospheric oxidation and biogeochemical cycles. BVOC flux measurements were carried out in nine forest ecosystems using a relaxed eddy accumulation (REA) based sampling system, which is easily transportable, simple to operate and designed to be low-cost and therefore can easily be deployed at multiple remote locations. The REA measurements were carried out during daytime between 06:00 and 18:30 (Local Time) with a flux averaging period of 30 min. A detailed description of the REA sampling setup, operational procedure and validation by comparison with full eddy covariance (EC) BVOC flux measurements is provided. BVOC flux measurements from established long-term carbon and water flux tower sites in nine forest ecosystems are compared including Manitou Forest Observatory in Colorado, USA (pine woodland forest), Niwot Ridge AmeriFlux site in Colorado, USA (subalpine forest), Deer Canyon Preserve in New Mexico, USA (pinyon-juniper forest), Lei bamboo forest site near Taihuyuan, China, Qianyanzhou ChinaFLUX site in China (pine forest), Baskett Wildfire Refuge MOFlux site in Missouri, USA (deciduous oak forest), University of Michigan Biological Station PROPHET site in Michigan, USA (mixed deciduous forest), Changbai Mountain Forest Research Station in China (mixed deciduous forest) and the Guyaflux site (GF-Guy) in French Guiana (tropical rainforest). BVOC flux measurements using our REA setup confirm dominance of 2,3,2- methylbutenol (2,3,2-MBO) at the Manitou Forest Observatory and Niwot Ridge sites in Colorado. Monoterpene fluxes measured by REA showed good agreement (within +/- 10%) with monoterpene fluxes measured by PTR-MS at the Manitou Forest Observatory. The MOFlux site in Missouri was dominated by isoprene emissions (average flux of similar to 9.5 mg m(-2) h(-1)) whereas the Deer Canyon site was dominated by alpha-pinene emissions (average flux similar to 0.73 mg m(-2) h(-1)). Mixed deciduous forest sites at the PROPHET Station in Michigan and Changbai Mountain Forest Research Station in China primarily emitted isoprene along with some alpha-pinene, beta-pinene and d-Limonene. Isoprene and alpha-pinene were the dominant BVOCs emitted from the subtropical Lei bamboo plantation at the Taihuyuan site in China while the pine forest site at Qianyanzhou in China were dominated by alpha-pinene emissions along with significant isoprene. BVOC measurements across different seasons (during 2009-2011) at a tropical forest site in French Guiana (Guyaflux site) revealed the dominance of isoprene emissions during all seasons. Irrespective of the type of the forest ecosystem, alpha-pinene was among the dominant monoterpenes emitted from all nine forests.

This paper presents the NitroCOSMES campaign, aimed at testing and evaluating the performance of three methods for monitoring N2O fluxes over an agricultural field. The experiment was conducted from May to August 2012 at a site located in the south-west of France. N2O fluxes from a 24 ha irrigated maize field were measured using eddy covariance (EC), automated chamber (AC) and static chamber (SC) methodologies. Uncertainties were calculated according to the specificities of each set-up. Measurements were performed over a large range of water-filled pore spaces (WFPS), soil temperatures, and mineral nitrogen availability, and offered the opportunity to compare methodologies over a wide range of N2O emission intensities. The average N2O fluxes were compared among the three methodologies during the same periods of measurement and for different intensities of emissions (low, moderate and high). Periods of comparison were determined according to the AC results. On average, the three methods gave comparable results for the low (SC: 14.7 +/- 2.2, EC: 15.7 +/- 10.1, AC: 17.5 +/- 1.6 ng N2O-N m(-2) s(-1)) and the high (SC: 131.7 +/- 22.1, EC: 125.3 +/- 8, AC: 125.1 +/- 8.9 ng N2O-N m(-2) s(-1)) N2O emission ranges. For the moderate N2O emission range, AC measurements gave higher emissions (57.2 +/- 3.9 ng N2O-N m(-2) s(-1)) on average than both the SC (41.6 +/- 6.6 ng N2O-N m(-2) s(-1)) and EC (33.8 +/- 3.9 ng N2O-N m(-2) s(-1)) methods, which agreed better with each other. The relative standard deviation coefficient (RSD) indicated that EC methodology gave highly variable values during periods of low N2O emissions, from -52.2 +/- 88.1 to 62.2 +/- 50.7 ng N2O-N m(-2) s(-1), with a mean RSD of 151%. Water vapour effects (dilution and spectroscopic cross-sensitivity) were discussed in an attempt to explain the high variability in low N2O emission measurements. Even after applying the Webb term correction, there could still be a spectroscopic cross-sensitivity effect of water vapour on the N2O trace gas signal because of the layout of the analysers, which was not determined during the experiment. This study underlined that EC methodology is a promising way to estimate and refine N2O budgets at the field scale and to analyse the effects of different agricultural practices more finely with continuous flux monitoring. It also highlighted the need to continue the effort to assess and develop chambers and EC methodologies, especially for the low N2O emission measurement range, for which values and systematic uncertainties remain high and highly variable.