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Tropical peatlands cycle and store large amounts of carbon in their soil and biomass1–5. Climate and land-use change alters greenhouse gas (GHG) fluxes of tropical peatlands, but the magnitude of these changes remains highly uncertain6–19. Here we measure net ecosystem exchanges of carbon dioxide, methane and soil nitrous oxide fluxes between October 2016 and May 2022 from Acacia crassicarpa plantation, degraded forest and intact forest within the same peat landscape, representing land-cover-change trajectories in Sumatra, Indonesia. This allows us to present a full plantation rotation GHG flux balance in a fibre wood plantation on peatland. We find that the Acacia plantation has lower GHG emissions than the degraded site with a similar average groundwater level (GWL), despite more intensive land use. The GHG emissions from the Acacia plantation over a full plantation rotation (35.2 ± 4.7 tCO2-eq ha−1 year−1, average ± standard deviation) were around two times higher than those from the intact forest (20.3 ± 3.7 tCO2-eq ha−1 year−1), but only half of the current Intergovernmental Panel on Climate Change (IPCC) Tier 1 emission factor (EF)20 for this land use. Our results can help to reduce the uncertainty in GHG emissions estimates, provide an estimate of the impact of land-use change on tropical peat and develop science-based peatland management practices as nature-based climate solutions.

Accounting for temporal changes in carbon dioxide (CO2) effluxes from freshwaters remains a challenge for global and regional carbon budgets. Here, we synthesize 171 site-months of flux measurements of CO2 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 CO2 flux at all sites. By accounting for diel variation, only 11% of site-months were net daily sinks of CO2. Annual CO2 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 CO2 flux variability were delineated through mutual information analysis. Sample analysis of CO2 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 CO2 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.

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 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).

Three different models (STEP–GENDEC–NOflux, Zhang2010, and Surfatm) are used to simulate NO, CO2, and NH3 fluxes at the daily scale for 2 years (2012–2013) in a semi-arid grazed ecosystem at Dahra (15∘24′10′′ N, 15∘25′56′′ W, Senegal, Sahel). Model results are evaluated against experimental results acquired during three field campaigns. At the end of the dry season, when the first rains re-wet the dry soils, the model STEP–GENDEC–NOflux simulates the sudden mineralization of buried litter, leading to pulses in soil respiration and NO fluxes. The contribution of wet season fluxes of NO and CO2 to the annual mean is respectively 51 % and 57 %. NH3 fluxes are simulated by two models: Surfatm and Zhang2010. During the wet season, air humidity and soil moisture increase, leading to a transition between low soil NH3 emissions (which dominate during the dry months) and large NH3 deposition on vegetation during wet months. Results show a great impact of the soil emission potential, a difference in the deposition processes on the soil and the vegetation between the two models with however a close agreement of the total fluxes. The order of magnitude of NO, NH3, and CO2 fluxes is correctly represented by the models, as well as the sharp transitions between seasons, specific to the Sahel region. The role of soil moisture in flux magnitude is highlighted, whereas the role of soil temperature is less obvious. The simultaneous increase in NO and CO2 emissions and NH3 deposition at the beginning of the wet season is attributed to the availability of mineral nitrogen in the soil and also to microbial processes, which distribute the roles between respiration (CO2 emissions), nitrification (NO emissions), volatilization, and deposition (NH3 emission/deposition). The objectives of this study are to understand the origin of carbon and nitrogen compounds exchanges between the soil and the atmosphere and to quantify these exchanges on a longer timescale when only a few measurements have been performed.

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...

International audience

International audience; 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 ~ 9.5 mg m−2 h−1) whereas the Deer Canyon site was dominated by α-pinene emissions (average flux ~ 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 α-pinene, β-pinene and d-Limonene. Isoprene and α-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 α-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, α-pinene was among the dominant monoterpenes emitted from all nine forests.

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 &plusmn, 0.0 to 42.1 &plusmn, 10.7 ngN m&minus, 2 s&minus, 1) than at the Dahra site (range: 0.3 &plusmn, 0 to 7.4 &plusmn, 6.5 ngN m&minus, 1). Soil water and nitrate (NO3&minus, ) 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&ndash, 0.26 and 1.15&ndash, 1.20 TgN per year, respectively.

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.

Freshwater reservoirs are a known source of greenhouse gas (GHG) to the atmosphere, but their quantitative significance is still only loosely constrained. Although part of this uncertainty can be attributed to the difficulties in measuring highly variable fluxes, it is also the result of a lack of a clear accounting methodology, particularly about what constitutes new emissions and potential new sinks. In this paper, we review the main processes involved in the generation of GHG in reservoir systems and propose a simple approach to quantify the reservoir GHG footprint in terms of the net changes in GHG fluxes to the atmosphere induced by damming, that is, ´€˜what the atmosphere sees’. The approach takes into account the pre-impoundment GHG balance of the landscape, the temporal evolution of reservoir GHG emission profile as well as the natural emissions that are displaced to or away from the reservoir site resulting from hydrological and other changes. It also clarifies the portion of the reservoir carbon burial that can potentially be considered an offset to GHG emissions.

Two greenhouse gases –methane (CH4) and nitrous oxide (N2O) - were monitored monthly during one year (2011) at the Eguzon Reservoir in France. The objective of the study was to quantify for the first time in a temperate area the total emissions of these gases through the main emission pathways (diffusion and bubbling from the reservoir, degassing and downstream diffusion). The reservoir was impounded in 1926 and had, in 2011, a eutrophic status promoting high organic matter degradation and nitrification-denitrification, all favouring CH4 and N2O production. CH4 and N2O emissions were dominated by diffusion from the reservoir surface (respectively 78.0% and 92.3%). Ebullition was only observed for CH4 and accounted for 14.0% of total CH4 emissions. Downstream degassing and diffusion represented 8.1% of the total CH4 emissions and 7.7% of the total N2O emissions.

Inland waters in general and freshwater reservoirs specifically are recognized as a source of CH4 into the atmosphere. Although the diffusion at the air–water interface is the most studied pathway, its spatial and temporal variations are poorly documented. We measured temperature and O2 and CH4 concentrations every 2 weeks for 3.5 years at nine stations in a subtropical monomictic reservoir which was flooded in 2008 (Nam Theun 2 Reservoir, Lao PDR). Based on these results, we quantified CH4 storage in the water column and diffusive fluxes from June 2009 to December 2012. We compared diffusive emissions with ebullition from Deshmukh et al. (2014) and aerobic methane oxidation and downstream emissions from Deshmukh et al. (2016). In this monomictic reservoir, the seasonal variations of CH4 concentration and storage were highly dependent on the thermal stratification. Hypolimnic CH4 concentration and CH4 storage reached their maximum in the warm dry season (WD) when the reservoir was stratified. Concentration and storage decreased during the warm wet (WW) season and reached its minimum after the reservoir overturned in the cool dry (CD) season. The sharp decreases in CH4 storage were concomitant with extreme diffusive fluxes (up to 200 mmol m−2 d−1). These sporadic emissions occurred mostly in the inflow region in the WW season and during overturn in the CD season in the area of the reservoir that has the highest CH4 storage. Although they corresponded to less than 10 % of the observations, these extreme CH4 emissions (> 5 mmol m−2 d−1) contributed up to 50 % of total annual emissions by diffusion. During the transition between the WD and WW seasons, a new emission hotspot was identified upstream of the water intake where diffusive fluxes peaked at 600 mmol m−2 d−1 in 2010 down to 200 mmol m−2 d−1 in 2012. The hotspot was attributed to the mixing induced by the water intakes (artificial mixing). Emissions from this area contributed 15–25 % to total annual emissions, although they occur in a surface area representative of less than 1 % of the total reservoir surface. We highly recommend measurements of diffusive fluxes around water intakes in order to evaluate whether such results can be generalized.

Methane (CH4) emissions from hydroelectric reservoirs could represent a significant fraction of global CH4 emissions from inland waters and wetlands. Although CH4 emissions downstream of hydroelectric reservoirs are known to be potentially significant, these emissions are poorly documented in recent studies. We report the first quantification of emissions downstream of a subtropical monomictic reservoir. The Nam Theun 2 Reservoir (NT2R), located in the Lao People's Democratic Republic, was flooded in 2008 and commissioned in April 2010. This reservoir is a trans-basin diversion reservoir which releases water into two downstream streams: the Nam Theun River below the dam and an artificial channel downstream of the powerhouse and a regulating pond that diverts the water from the Nam Theun watershed to the Xe Bangfai watershed. We quantified downstream emissions during the first 4 years after impoundment (2009–2012) on the basis of a high temporal (weekly to fortnightly) and spatial (23 stations) resolution of the monitoring of CH4 concentration. Before the commissioning of NT2R, downstream emissions were dominated by a very significant degassing at the dam site resulting from the occasional spillway discharge for controlling the water level in the reservoir. After the commissioning, downstream emissions were dominated by degassing which occurred mostly below the powerhouse. Overall, downstream emissions decreased from 10 GgCH4 yr−1 after the commissioning to 2 GgCH4 yr−1 4 years after impoundment. The downstream emissions contributed only 10 to 30 % of total CH4 emissions from the reservoir during the study. Most of the downstream emissions (80 %) occurred within 2–4 months during the transition between the warm dry season (WD) and the warm wet season (WW) when the CH4 concentration in hypolimnic water is maximum (up to 1000 µmol L−1) and downstream emissions are negligible for the rest of the year. Emissions downstream of NT2R are also lower than expected because of the design of the water intake. A significant fraction of the CH4 that should have been transferred and emitted downstream of the powerhouse is emitted at the reservoir surface because of the artificial turbulence generated around the water intake. The positive counterpart of this artificial mixing is that it allows O2 diffusion down to the bottom of the water column, enhancing aerobic methane oxidation, and it subsequently lowered downstream emissions by at least 40 %.

Soil deposition is an essential pathway for tropospheric ozone (O3) removal, but its controlling factors remain unclear. Here, we explored the variability of soil O3 resistance in response to soil texture. To this aim, data of O3 deposition over bare soil obtained from micrometeorological measurements under contrasted meteorological conditions for five sites were used. The results obtained are twofold: (i) soil resistance (Rsoil) increased with soil surface relative humidity (RHsurf), but (ii) this relationship exhibited large site-by-site variability. Further analysis showed that the minimum soil resistance (corresponding to completely dry soil surface or RHsurf = 0%) and the increase of Rsoil with RHsurf are both linked to soil clay content. These results can be explained by (i) the soil surface available for O3 deposition at a microscopic scale which is a function of the soil specific surface area, and (ii) the capacity of a soil to adsorb water according to its clay content and therefore to reduce the surface active for O3 deposition. From these results, a new parameterization has been established to estimate Rsoil as a function of RHsurf and soil clay fraction.

Three different models (STEP-GENDEC-NOflux, Zhang2010 and Surfatm) are used to simulate NO, CO2, and NH3 fluxes at the daily scale during two years (2012–2013) in a semi-arid grazed ecosystem at Dahra (15°24′10′′ N, 15°25′56′′ W, Senegal, Sahel). Model results are evaluated against experimental results acquired during three field campaigns. At the end of the dry season, when the first rains rewet the dry soils, the model STEP-GENDEC-NOflux simulate the sudden mineralization of buried litter, leading to pulses in soil respiration and NO fluxes. The contribution of wet season fluxes of NO and CO2 to the annual mean is respectively 51 % and 57 %. NH3 fluxes are simulated by two models: Surfatm and Zhang2010. During the wet season, air humidity and soil moisture increase, leading to a transition between low soil NH3 emissions (which dominate during the dry months) to large NH3 deposition on vegetation during wet months, Results show a great impact of the soil emission potential and a close agreement between the two models. The order of magnitude of NO, NH3 and CO2 fluxes are correctly represented by the models, as well as the sharp transitions between seasons, specific to the Sahel region. The role of soil moisture on flux magnitude is highlighted, whereas the role of soil temperature is less obvious. The simultaneous increase of NO and CO2 emissions and NH3 deposition at the beginning of the wet season is attributed to the availability of mineral nitrogen in the soil and also to microbial processes which distribute the roles between respiration (CO2 emissions), nitrification (NO emissions), volatilization and deposition (NH3 emission/deposition). This objective of this study is to understand the origin of carbon and nitrogen compounds exchanges between the soil and the atmosphere, and to quantify these exchanges on a longer time scale when only few measurements have been performed.

Hydro-reservoirs are recognized as potentially significant emitters of methane at the global scale. However, there are still large uncertainties in estimating such emissions at this scale due to the lack of comprehensive measurements of methane emissions in a wide range of climatic zones, for hydro-reservoirs with variable characteristics, and including all emissions pathways. This study presents an assessment of methane emissions for the 2013 year, that is four years after commissioning, for the Nam Theun 2 hydro-system, a 489 km² subtropical reservoir with a large drawdown area. All of the major pathways have been computed here, i.e., ebullition, diffusion, degassing, emissions from the drawdown area, and diffusion from downstream, and included in an emission budget. Emissions from the upstream area strongly dominate when compared to the downstream ones, a feature quite specific to the Nam Theun 2 reservoir. The drawdown area and the emissions by diffusion downstream represented a small contribution to the total emissions. About half of the methane is emitted during the four months of the warm dry season (from mid-February to Mid-June). Total methane emissions in NT2 have decreased from 35.6±2.6 Gg(CH4 ) year-1 in 2010 to 24.5±1.5 Gg(CH4 ) year-1 in 2013 though this difference was not found to be quite statistically significant.

The physical (temperature, conductivity and turbidity) and chemical water quality (O 2 , pH, conductivity, nutrients, major anions and cations) characteristics have been monitored in the Nam Theun 2 system (hydroelectric reservoir and rivers downstream) for the first 5 years after impoundment. The results emphasize that the impoundment of the reservoir induced a substantial modification of the water quality in the whole aquatic system which is partially controlled by the hydrodynamics in the reservoir. During the warm seasons, the reservoir water column is thermally stratified with a warm oxic epilimnion and a colder anoxic hypolimnion. During the cold dry season or during floods, the reservoir water column over-turns which enhances oxygen penetration down to the bottom waters and contributes to a global improvement of the water quality. Downstream of the reservoir, the oxygen level was always above the minimum requirement for aquatic life due to the specific design of the water intakes at the Nakai Dam and upstream of the Power House which both favour the withdrawal of a large proportion of oxygenated epilimnic waters. Over the five years, the water quality in the reservoir itself globally improved. However, the diversion of the Nam Theun River into the Xe Bang Fai watershed via the reservoir has impacted the Xe Bang Fai River in terms of temperature, conductivity and total suspended solids.

Release of hypolimnetic water from man-made reservoirs can be a problem for downstream rivers. These effects can be significant mainly during the first years after the reservoir impoundment, especially in thermally stratified reservoirs favouring the release of anoxic methane-rich water. In tropical areas, higher temperatures decrease the oxygen solubility and enhance chemical processes responsible for the rise of reduced compounds. The Nam Theun 2 Reservoir was first filled in 2008. It experienced hypolimnetic deoxygenation and significant methane concentrations during the first 2 years. Dedicated structures to oxygenate water and degas methane were designed during the study phase. The overall aerating and degassing effects of these hydraulics structures varied from very good to moderate. Results depend on the continuous water quality improvement with time as well as on the limited range of upstream oxygen and methane concentrations tested. The hollow jet valve and the concrete tooth shaped structure were very efficient together with downstream natural turbulence in aerating/degassing compared to the staggered baffle blocks. Contrary to other structures, the efficiency of the baffle blocks structure is reduced with high discharges. The aeration weir showed a moderate efficiency in supplying oxygen to the water due to the high upstream oxygen saturations (close to 100%). However, it was very efficient for methane degassing even at low concentrations. Hydraulics structures of the Nam Theun 2 project are an efficient, reliable and low maintenance way to improve oxygen content and to degas methane.

International audience; With around 490 km2 at full level of operation, the Nam Theun 2 Reservoir (NT2R) is one of the largest hydroreservoir in Southeast Asia. This study presents a first estimation of the atmospheric inorganic nitrogen deposition into the NT2R based on a 2-year monitoring (June 2010 to July 2012) including gas concentrations and precipitation. Dry deposition fluxes are estimated by the inferential method using, on the one hand, surface measurements of gas concentrations (NO2, HNO3, NH3) from passive samplers and, on the other hand, modeled exchange rates. Wet deposition fluxes are calculated from NH4+ and NO3- concentrations determined in samples of rain from an automatic precipitation collector. The average nitrogen deposition flux is estimated at 1.26 ± 0.14 kgN·ha-1·year-1 from dry processes and 5.01 ± 0.92 kgN·ha-1·year-1 from wet ones, that is, an average annual total nitrogen flux of 6.27 ± 1.06 kgN·ha-1·year-1 deposited at the NT2R with 80% from wet deposition. Before impoundment, the mean N flux has been estimated at 3.42 ± 1.88 kgN·ha-1·year-1 for dry deposition and 5.01 ± 2.12 kgN·ha-1·year-1 for wet deposition, or a total N deposition flux of 8.43 ± 4.01 kgN·ha-1·year-1 over the studied area dominated by forests with little agriculture soil and water surfaces. Thus, the total N deposition over the studied area has been reduced of 26% (or 63% for dry deposition) following the reservoir impoundment based on our working hypothesis.

This study deals with the problem of identifying atmospheric data that are influenced by local emissions which cause spikes in time series of greenhouse gases and long-lived tracer measurements. We considered three spike detection methods known as coefficient of variation (COV), robust extraction of baseline signal (REBS), and standard deviation of the background (SD), to detect and filter positive spikes in continuous greenhouse gas time series from four monitoring stations representative of the ICOS (Integrated Carbon Observation System) European Infrastructure network. The results of the different methods are compared to each other and against a manual detection performed by station managers. Four stations were selected as test cases to apply the spike detection methods: a continental rural tower of 100 m height in Eastern France (OPE); a high mountain observatory in the south-west of France (PDM); a regional marine background site in Crete (FKL); and a marine clean-air background site in the southern hemisphere in Amsterdam island (AMS). This panel allows addressing the spike detection problems in time series with different variability. Two years of continuous measurements of CO2, CH4 and CO were analyzed. All the methods were found to be able to detect short-term spikes (lasting from a few seconds to few minutes) in the time series. Analysis of the results of each method leads us to exclude the use of the COV method because of its requirement to arbitrarily specify an a priori percentage of rejected data in the time series, which may over- or under-estimate the actual number of spikes. The two other methods freely determine the number of spikes for a given set of parameters, and the values of these parameters were calibrated to provide the best match with spikes known to reflect local emissions episodes well documented by the station managers. More than 96 % of the spikes manually identified by station managers were successfully detected both in the SD and the REBS methods after the best adjustment of parameter values. At PDM, measurements made by two analyzers 200 m from each other allow to confirm that the CH4 spikes identified in one of the time-series but not in the other correspond to a local source from a sewage treatment facility in one of the observatory buildings. From this experiment, we found that the REBS method underestimates the number of positive anomalies in the CH4 data caused by local sewage emissions. As a conclusion, we recommend the use of the SD method, which also appears as the easiest one to implement as automatic data processing, for the operational filtering of spikes in greenhouses gases time series at global and regional monitoring stations of networks like ICOS.

A 3D water quality model has been applied to predict medium term evolution of the water quality in the Nam Theun 2 Reservoir and also to quantify the effect of various scenarios. 15-year simulations show that the oxygen concentration will continue to increase in the water column although the hypolimnion will remain anoxic in some areas of the Reservoir. In parallel, the concentration of reduced compounds will decrease with time. The significance of the hydrodynamics in water quality evolution is pointed out with two scenarios in which natural or human forcings have been modified. The comparison of simulations made for years with contrasted hydrometeorological conditions shows that and duration of major hydrometeorological related events (rainfall, flood and air temperature drop) have a major influence on the seasonal evolution of water quality in the whole Reservoir. Simulations also show that the physico-chemical quality of the water released downstream of the power house would have been different if the commissioning had been carried out immediately after the impoundment. Finally, the model has been used to quantify the impact of an increase of the NO3 - and PO4 3- incoming flux consecutive to potential changes in the watershed land use. The fluxes have been multiplied by a factor two separately (2 scenarios) and together. While the additional load of NO3 - has almost no impact on physico-chemistry and phytoplankton activity, the increase of PO4 3- leads to a larger increase of chlorophyll-a close to the Intake than close to the dam.

We describe the implementation of a biochemical model of isoprene emission that depends on the electron requirement for isoprene synthesis into the Farquhar–Ball–Berry leaf model of photosynthesis and stomatal conductance that is embedded within a global chemistry-climate simulation framework. The isoprene production is calculated as a function of electron transport-limited photosynthesis, intercellular and atmospheric carbon dioxide concentration, and canopy temperature. The vegetation biophysics module computes the photosynthetic uptake of carbon dioxide coupled with the transpiration of water vapor and the isoprene emission rate at the 30 min physical integration time step of the global chemistry-climate model. In the model, the rate of carbon assimilation provides the dominant control on isoprene emission variability over canopy temperature. A control simulation representative of the present-day climatic state that uses 8 plant functional types (PFTs), prescribed phenology and generic PFT-specific isoprene emission potentials (fraction of electrons available for isoprene synthesis) reproduces 50% of the variability across different ecosystems and seasons in a global database of 28 measured campaign-average fluxes. Compared to time-varying isoprene flux measurements at 9 select sites, the model authentically captures the observed variability in the 30 min average diurnal cycle (R2 = 64–96%) and simulates the flux magnitude to within a factor of 2. The control run yields a global isoprene source strength of 451 TgC yr−1 that increases by 30% in the artificial absence of plant water stress and by 55% for potential natural vegetation.

Greenhouse gas (GHG) emissions from the surface soils and surface water receiving animal excreta may be important components of the GHG balance of terrestrial ecosystems, but the associated processes are poorly documented in tropical environments, especially in tropical arid and semi-arid areas. A typical sylvo-pastoral landscape in the semi-arid zone of Senegal, West Africa, was investigated in this study. The study area (706 kmA(2) of managed pastoral land) was a circular zone with a radius of 15 km centered on a borehole used to water livestock. The landscape supports a stocking rate ranging from 0.11 to 0.39 tropical livestock units per hectare depending on the seasonal movements of the livestock. Six landscape units were investigated (land in the vicinity of the borehole, natural ponds, natural rangelands, forest plantations, settlements, and enclosed plots). Carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) fluxes were measured with static chambers set up at 13 sites covering the six landscape units, and the 13 sites are assumed to be representative of the spatial heterogeneity of the emissions. A total of 216 fluxes were measured during the one-year study period (May 2014 to April 2015). At the landscape level, soils and surface water emitted an average 19.8 t C-CO2 eq/(hm(2).a) (CO2: 82%, N2O: 15%, and CH4: 3%), but detailed results revealed notable spatial heterogeneity of GHG emissions. CO2 fluxes ranged from 1148.2 (+/- 91.6) mg/(m(2).d) in rangelands to 97,980.2 (+/- 14,861.7) mg/(m(2).d) in surface water in the vicinity of the borehole. N2O fluxes ranged from 0.6 (+/- 0.1) mg/(m(2).d) in forest plantations to 22.6 (+/- 10.8) mg/(m(2).d) in the vicinity of the borehole. CH4 fluxes ranged from-3.2 (+/- 0.3) mg/(m(2).d) in forest plantations to 8788.5 (+/- 2295.9) mg/(m(2).d) from surface water in the vicinity of the borehole. This study identified GHG emission "hot spots" in the landscape. Emissions from the surface soils were significantly higher in the landscape units most frequently used by the animals, i.e., in the vicinity of the borehole and settlements; and emissions measured from surface water in the vicinity of the borehole and from natural ponds were on average about 10 times higher than soil emissions.

The alternating between dry and wet seasons and the consecutive microbial responses to soil water content in semiarid ecosystems has significant consequences on nitrogen exchanges with the atmosphere. Three field campaigns were carried out in a semi arid sahelian rangeland in Dahra (Ferlo, Senegal), two at the beginning of the wet season in July 2012 and July 2013, and the third one in November 2013 at the end of the wet season. The ammonia emission potentials of the soil ranged from 271 to 6628, indicating the soil capacity to emit NH3. The ammonia compensation point in the soil ranged between 7 and 150 ppb, with soil temperatures between 32 and 37 °C. Ammonia exchange fluctuated between emission and deposition (from −0.1–1.3 ng N.m−2 s−1), depending on meteorology, ambient NH3 concentration (5–11 ppb) and compensation point mixing ratios. N2O fluxes are supposed to be lower than NO fluxes in semi arid ecosystems, but in Dahra N2O fluxes (5.5 ± 1.3 ng N m−2 s−1 in July 2013, and 3.2 ± 1.7 ng N m−2 s−1 in November 2013) were similar to NO fluxes (5.7 ± 3.1 ng N m−2 s−1 in July 2012, 5.1 ± 2.1 ng N m−2 s−1 in July 2013, and 4.0 ± 2.2 ngN m−2 s−1 in November 2013). Possible reasons are the influence of soil moisture below the surface (where N2O is produced) after the beginning of the wet season, the potential aerobic denitrification in microsites, the nitrifier denitrification, and nitrification processes. The presence of litter and standing straw, and their decomposition dominated N compounds emissions in November 2013, whereas emissions in July 2012 and 2013, when the herbaceous strata was sparse, were dominated by microbial processes in the soil. CO2 respiration fluxes were high in the beginning (107 ± 26 mg m−2 h−1 in July 2013) and low in the end of the wet season (32 ± 5 mg m−2 h−1 in November 2013), when autotrophic and heterotrophic activity is reduced due to low soil moisture conditions These results confirm that contrasted ecosystem conditions due to drastic changes in water availability in semi arid regions have important non linear impacts on the biogeochemical nitrogen cycle.

International audience

The disjunct eddy covariance (DEC) method is an interesting alternative to the conventional eddy covariance (EC) method because it allows the estimation of turbulent fluxes of species for which fast sensors are not available. We have developed and validated a new disjunct sampling system (called MEDEE). This system is built with chemically inert materials. Air samples are taken quickly and alternately in two cylindrical reservoirs, the internal pressures of which are regulated by a moving piston. The MEDEE system was designed to be operated either on the ground or aboard an aircraft. It is also compatible with most analysers since it transfers the air samples at a regulated pressure. To validate the system, DEC and EC measurements of CO2 and latent heat fluxes were performed concurrently during a field campaign. EC fluxes were first compared to simulated DEC (SDEC) fluxes and then to actual DEC fluxes. Both the simulated and actual DEC fluxes showed a good agreement with EC fluxes in terms of correlation. The determination coefficients (R2) were 0.93 and 0.91 for DEC and SDEC latent heat fluxes, respectively. For DEC and SDEC CO2 fluxes R2 was 0.69 in both cases. The conditions of low fluxes experienced during the campaign impaired the comparison of the different techniques especially for CO2 flux measurements. Linear regression analysis showed an 14% underestimation of DEC fluxes for both CO2 and latent heat compared to EC fluxes. A first field campaign, focusing on biogenic volatile organic compound (BVOC) emissions, was carried out to measure isoprene fluxes above a downy oak (Quercus Pubescens) forest in the south-east of France. The measured standard emission rate was in the lower range of reported values in earlier studies. Further analysis will be conducted through ground-based and airborne campaigns in the coming years.

We have incorporated a semi-mechanistic isoprene emission module into the JULES land-surface scheme, as a first step towards a modelling tool that can be applied for studies of vegetation – atmospheric chemistry interactions, including chemistry-climate feedbacks. Here, we evaluate the coupled model against local above-canopy isoprene emission flux measurements from six flux tower sites as well as satellite-derived estimates of isoprene emission over tropical South America and east and south Asia. The model simulates diurnal variability well: correlation coefficients are significant (at the 95 % level) for all flux tower sites. The model reproduces day-to-day variability with significant correlations (at the 95 % confidence level) at four of the six flux tower sites. At the UMBS site, a complete set of seasonal observations is available for two years (2000 and 2002). The model reproduces the seasonal pattern of emission during 2002, but does less well in the year 2000. The model overestimates observed emissions at all sites, which is partially because it does not include isoprene loss through the canopy. Comparison with the satellite-derived isoprene-emission estimates suggests that the model simulates the main spatial patterns, seasonal and inter-annual variability over tropical regions. The model yields a global annual isoprene emission of 535 ± 9 TgC yr−1 during the 1990s, 78 % of which from forested areas.

The atmospheric nitrogen budget depends on emission and deposition fluxes both as reduced and oxidized nitrogen compounds. In this study, a first attempt at estimating the Sahel nitrogen budget for the year 2006 is made, through measurements and simulations at three stations from the IDAF network situated in dry savanna ecosystems. Dry deposition fluxes are estimated from measurements of NO2, HNO3 and NH3 gaseous concentrations and from simulated dry deposition velocities, and wet deposition fluxes are calculated from NH4+ and NO3− concentrations in samples of rain. Emission fluxes are estimated including biogenic emission of NO from soils (an Artificial Neural Network module has been inserted into the ISBA-SURFEX surface model), emission of NOx and NH3 from domestic fires and biomass burning, and volatilization of NH3 from animal excreta. Uncertainties are calculated for each contribution of the budget. This study uses original and unique data from remote and hardly-ever-explored regions.The monthly evolution of oxidized N compounds shows that emission and deposition increase at the beginning of the rainy season because of large emissions of biogenic NO (pulse events). Emission of oxidized compounds is dominated by biogenic emission from soils (domestic fires and biomass burning of oxidized compounds account for 0 to 13% at the most at the annual scale, depending on the station), whereas emission of NH3 is dominated by the process of volatilization from soils. At the annual scale, the average gaseous dry deposition accounts for 47% of the total estimated deposition flux, for both oxidized and reduced compounds. The average estimated wet plus dry deposition flux in dry savanna ecosystems is 7.5±1.8 kgN ha−1 yr−1, with approximately 30% attributed to oxidized compounds, and the rest attributed to NHx. The average estimated emission flux ranges from 8.4(±3.8) to 12.4(±5.9) kgN ha−1 yr−1, dominated by NH3 volatilization (72–82%) and biogenic emission from soils (11–17%), depending on the applied volatilization rate of NH3. While larger, emission fluxes are on the same order of magnitude as deposition fluxes. The main uncertainties are linked to the NH3 emission from volatilization. When scaled up from the 3 measurement sites to the Sahelian region (12° N:18° N, 15° W:10° E), the estimated total emission ranges from 2(±0.9) to 3(±1.4) TgN yr−1, depending on the applied volatilization rate of NH3 and estimated total deposition is 1.8(±0.4) TgN yr−1. The dry savanna ecosystems of the Sahel contribute around 2% to the global (biogenic + anthropogenic) nitrogen budget.

Nitrogen oxide biogenic emissions from soils are driven by soil and environmental parameters. The relationship between these parameters and NO fluxes is highly non linear. A new algorithm, based on a neural network calculation, is used to reproduce the NO biogenic emissions linked to precipitations in the Sahel on the 6 August 2006 during the AMMA campaign. This algorithm has been coupled in the surface scheme of a coupled chemistry dynamics model (MesoNH Chemistry) to estimate the impact of the NO emissions on NOx and O3 formation in the lower troposphere for this particular episode. Four different simulations on the same domain and at the same period are compared: one with anthropogenic emissions only, one with soil NO emissions from a static inventory, at low time and space resolution, one with NO emissions from neural network, and one with NO from neural network plus lightning NOx. The influence of NOx from lightning is limited to the upper troposphere. The NO emission from soils calculated with neural network responds to changes in soil moisture giving enhanced emissions over the wetted soil, as observed by aircraft measurements after the passing of a convective system. The subsequent enhancement of NOx and ozone is limited to the lowest layers of the atmosphere in modelling, whereas measurements show higher concentrations above 1000 m. The neural network algorithm, applied in the Sahel region for one particular day of the wet season, allows an immediate response of fluxes to environmental parameters, unlike static emission inventories. Stewart et al (2008) is a companion paper to this one which looks at NOx and ozone concentrations in the boundary layer as measured on a research aircraft, examines how they vary with respect to the soil moisture, as indicated by surface temperature anomalies, and deduces NOx fluxes. In this current paper the model-derived results are compared to the observations and calculated fluxes presented by Stewart et al (2008).

The biogenic volatile organic compound (BVOC) composition of ambient air at a rural field site near Djougou, Benin has been studied as part of the AMMA (African Monsoon Multidisciplinary Analysis) project. Ambient air was sampled during day and night during the period 2 June 2006 to 13 June 2006. Gas samples from within the forest canopy and from branch and cuvette enclosure systems for four vegetation species were also obtained and emissions flux estimates made. All samples were analysed for the presence of isoprene, monoterpenes and sesquiterpenes by either gas chromatography-time of flight mass spectrometry (GC-TOF/MS) or comprehensive gas chromatography-time of flight mass spectrometry (GCxGC-TOF/MS). Concentrations of isoprene ranged from a few tens of pptV to in excess of 3000 pptV. Similar concentration ranges for certain monoterpenes were also observed. Limonene was seen at a maximum concentration in ambient air of 5000 pptV. The combination of leaf-level observations and direct analysis of dried vegetation samples suggests that emissions of terpene species from indigenous species are unlikely to account for the unexpectedly high ambient concentrations of monoterpenes. Leaf scale emission measurements and biological sample analysis indicated that \\textit{Anacardium occidentale}, a non-native crop species found throughout the tropics, was the dominant source of monoterpenes at this location. These preliminary findings suggest that activities involving species replacement have potential implications for the chemistry of the African troposphere that have not been widely considered previously.

In recent years evidence has emerged that the amount of isoprene emitted from a leaf is affected by the CO2 growth environment. Many – though not all – laboratory experiments indicate that emissions increase significantly at below-ambient CO2 concentrations and decrease when concentrations are raised to above-ambient. A small number of process-based leaf isoprene emission models can reproduce this CO2 stimulation and inhibition. These models are briefly reviewed, and their performance in standard conditions compared with each other and to an empirical algorithm. One of the models was judged particularly useful for incorporation into a dynamic vegetation model framework, LPJ-GUESS, yielding a tool that allows the interactive effects of climate and increasing CO2 concentration on vegetation distribution, productivity, and leaf and ecosystem isoprene emissions to be explored. The coupled vegetation dynamics-isoprene model is described and used here in a mode particularly suited for the ecosystem scale, but it can be employed at the global level as well. Annual and/or daily isoprene emissions simulated by the model were evaluated against flux measurements (or model estimates that had previously been evaluated with flux data) from a wide range of environments, and agreement between modelled and simulated values was generally good. By using a dynamic vegetation model, effects of canopy composition, disturbance history, or trends in CO2 concentration can be assessed. We show here for five model test sites that the suggested CO2-inhibition of leaf-isoprene metabolism can be large enough to offset increases in emissions due to CO2-stimulation of vegetation productivity and leaf area growth. When effects of climate change are considered atop the effects of atmospheric composition the interactions between the relevant processes will become even more complex. The CO2-isoprene inhibition may have the potential to significantly dampen the expected steep increase of ecosystem isoprene emission in a future, warmer atmosphere with higher CO2 levels; this effect raises important questions for projections of future atmospheric chemistry, and its connection to the terrestrial vegetation and carbon cycle.

Inland waters in general and specifically freshwater reservoirs are recognized as source of CH4 to the atmosphere. Although the diffusion at the air–water interface is the most studied pathway, its spatial and temporal variations are poorly documented. We measured fortnightly CH4 concentrations and physico-chemical parameters at nine stations in a subtropical monomictic reservoir which was flooded in 2008 (Nam Theun 2 Reservoir, Lao PDR). Based on these results, we quantified CH4 storage in the water column and diffusive fluxes from June 2009 to December 2012. We also compared emissions with aerobic methane oxidation calculated from Deshmukh et al. (2015). In this monomictic reservoir, the seasonal variations of CH4 concentration and storage were highly dependant of the thermal stratification. Hypolimnic CH4 concentration and CH4 storage reached their maximum in the warm dry season (WD) when the reservoir was stratified. They decreased during the warm wet (WW) season and reached its minimum after the reservoir overturned in the cool dry season (CD). The sharp decreases of the CH4 storage were concomitant with sporadic extreme diffusive fluxes (up to 200 mmol m−2 d−1). These hot moments of emissions occurred mostly in the inflow region in the WW season and during the overturn in the CD season in the area of the reservoir that has the highest CH4 storage. Although they corresponded to less than 10 % of the observations, these CH4 extreme emissions (> 5 mmol m−2 d−1) contributed up to 50 % of total annual emissions by diffusion. Based on our fortnightly monitoring, we determined that accurate estimation of the emissions can be determined from measurements made at least at a monthly frequency. During the transition between the WD and WW seasons, a new hotspot of emissions was identified upstream of the water intake where diffusive fluxes peaked at 600 mmol m−2 d−1 in 2010 down to 200 mmol m−2 d−1 in 2012. In the CD season, diffusive fluxes from this area were the lowest observed at the reservoir surface. Emissions from this area contributed 15–25 % to total annual emissions although they occur on a surface area representative of less than 1 % of the total reservoir surface. We highly recommend measurements of diffusive fluxes around water intakes in order to evaluate if such results can be generalized.

Research during the past decades has outlined the importance of biogenic isoprene emission in tropospheric chemistry and regional ozone photo-oxidant pollution. The first part of this article focuses on the development and validation of a simple biogenic emission scheme designed for regional studies. Experimental data sets relative to Boreal, Tropical, Temperate and Mediterranean ecosystems are used to estimate the robustness of the scheme at the canopy scale, and over contrasted climatic and ecological conditions. A good agreement is generally found when comparing field measurements and simulated emission fluxes, encouraging us to consider the model suitable for regional application. Limitations of the scheme are nevertheless outlined as well as further on-going improvements. In the second part of the article, the emission scheme is used on line in the broader context of a meso-scale atmospheric chemistry model. Dynamically idealized simulations are carried out to study the chemical interactions of pollutant plumes with realistic isoprene emissions coming from a Mediterranean oak forest. Two types of anthropogenic sources, respectively representative of the Marseille (urban) and Martigues (industrial) French Mediterranean sites, and both characterized by different VOC/NOx are considered. For the Marseille scenario, the impact of biogenic emission on ozone production is larger when the forest is situated in a sub-urban configuration (i.e. downwind distance TOWN-FOREST

Surface and remote-sensing instruments deployed during ESCOMPTE experiment over the Marseille area, along the Mediterranean coast, were used to investigate the fine structure of the atmospheric boundary layer (ABL) during sea-breeze circulation in relation to pollutant transport and diffusion. Six sea-breeze events are analyzed with a particular focus on 25 June 2001. Advection of cool and humid marine air over land has a profound influence on the daytime ABL characteristics. This impact decreases rapidly with the inland distance from the sea. Nearby the coast (3 km inland), the mixing height Zi rises up to 750 m and falls down after 15:00 (UT) when the breeze flow reaches its maximum intensity. A more classical evolution of the ABL is observed at only 11-km inland where Zi culminates in the morning and stabilizes in the afternoon at about 1000 m height. Fine inspection of the data revealed an oscillation of the sea-breeze with a period about 2 h 47 min. This feature, clearly discernable for 3 days at least, is present in several atmospheric variables such as wind, temperature, not only at the ground but also aloft in the ABL as observed by sodar/RASS and UHF wind profilers. In particular, the mixing height Zi deduced from UHF profilers observations is affected also by the same periodicity. This pulsated sea-breeze is observed principally above Marseille and, at the northern and eastern shores of the Berre pond. In summary, the periodic intrusion over land of cool marine air modifies the structure of the ABL in the vicinity of the coast from the point of view of stability, turbulent motions and pollutants concentration. An explanation of the source of this pulsated sea-breeze is suggested.

Measurements of the dry deposition velocity of ozone have been made by the eddy correlation method during ESCOMPTE (Etude sur Site pour COntraindre les Modèles de Pollution atmosphérique et de Transport d'Emissions). The strong local variability of natural ecosystems was sampled over several weeks in May, June and July 2001 for four sites with varying surface characteristics. The sites included a maize field, a Mediterranean forest, a Mediterranean shrub-land, and an almost bare soil. Measurements of nitrogen oxide deposition fluxes by the relaxed eddy correlation method have also been carried out at the same bare soil site. An evaluation of the deposition velocities computed by the surface module of the multi-scale Chemistry and Transport Model MOCAGE is presented. This module relies on a resistance approach, with a detailed treatment of the stomatal contribution to the surface resistance. Simulations at the finest model horizontal resolution (around 10 km) are compared to observations. If the seasonal variations are in agreement with the literature, comparisons between raw model outputs and observations, at the different measurement sites and for the specific observing periods, are contrasted. As the simulated meteorology at the scale of 10 km nicely captures the observed situations, the default set of surface characteristics (averaged at the resolution of a grid cell) appears to be one of the main reasons for the discrepancies found with observations. For each case, sensitivity studies have been performed in order to see the impact of adjusting the surface characteristics to the observed ones, when available. Generally, a correct agreement with the observations of deposition velocities is obtained. This advocates for a sub-grid scale representation of surface characteristics for the simulation of dry deposition velocities over such a complex area. Two other aspects appear in the discussion. Firstly, the strong influence of the soil water content to the plant response, specifically in conditions of stress, is confirmed. Second, we point out the difficulty in interpreting measurements of nitrogen oxide deposition velocities: a synergetic approach combining measurements and modeling is practical.

This work is an attempt to provide seasonal variation of biogenic NO emission fluxes in a Sahelian rangeland in Mali (Agoufou, 15.34° N, 1.48° W) for years 2004, 2005, 2006, 2007 and 2008. Indeed, NO is one of the most important precursors for tropospheric ozone, and previous studies have shown that arid areas potentially display significant NO emissions (due to both biotic and abiotic processes). Previous campaigns in the Sahel suggest that the contribution of this region in emitting NO is no longer considered as negligible. However, very few data are available in this region, therefore this study focuses on model development. The link between NO production in the soil and NO release to the atmosphere is investigated in this modelling study, by taking into account vegetation litter production and degradation, microbial processes in the soil, emission fluxes, and environmental variables influencing these processes, using a coupled vegetation–litter decomposition–emission model. This model includes the Sahelian Transpiration Evaporation and Productivity (STEP) model for the simulation of herbaceous, tree leaf and faecal masses, the GENDEC model (GENeral DEComposition) for the simulation of the buried litter decomposition and microbial dynamics, and the NO emission model (NOFlux) for the simulation of the NO release to the atmosphere. Physical parameters (soil moisture and temperature, wind speed, sand percentage) which affect substrate diffusion and oxygen supply in the soil and influence the microbial activity, and biogeochemical parameters (pH and fertilization rate related to N content) are necessary to simulate the NO flux. The reliability of the simulated parameters is checked, in order to assess the robustness of the simulated NO flux. Simulated yearly average of NO flux ranges from 2.09 to 3.04 ng(N) m−2 s−1 (0.66 to 0.96 kg(N) ha−1 yr−1), and wet season average ranges from 3.36 to 5.48 ng(N) m−2 s−1 (1.06 to 1.73 kg(N) ha−1 yr−1). These results are of the same order as previous measurements made in several sites where the vegetation and the soil are comparable to the ones in Agoufou. This coupled vegetation–litter decomposition–emission model could be generalized at the scale of the Sahel region, and provide information where few data are available.

As part of the EXPRESSO program (EXPeriment for the REgional Sources and Sinks of Oxidants), biosphere-atmosphere exchanges of trace gases were investigated in a ground-based forest site of the Republic of Congo. Experiments were carried out in March and NovemberDecember 1996. A 60-meter walkup tower was erected in an undisturbed mixed tropical forest typical of upland vegetation in the Nouabale´-Ndoki National Park. Eight belt transects radiating from the tower were used to characterize the species composition and structure of the upland mixed forest. As a comparison, and to investigate horizontal heterogeneity of the trace gases exchanges, additional measurements were made in a nearby monospecific forest stand characteristic of lowland Gilbertiodendron dewevrei (Gilbert. dew.) forest. Micrometeorological data, trace gas concentrations and flux measurements were made from the tower. We report daily above-canopy variation in temperature and radiation, energy partitioning into latent and sensible heat flux, volatile organic compound (VOC) mixing ratios, isoprene and CO 2 fluxes. Fluxes of isoprene and CO 2 were measured above the canopy using relaxed eddy accumulation and eddy covariance methods, respectively. These fluxes show a seasonal variation between the two experiments, as does energy partitioning. However, diVerence in isoprene emission between the two seasons are diYcult to reconcile with meteorological (T, PAR) data only, and more data such as plant water potential are needed to modeled the seasonal isoprene emission cycle. Isoprene emission at the leaf level was also determined for plant species at both upland and lowland sites using environmentally controlled leaf enclosures. Together with the ecological survey, the leaf level work suggests that lowland Gilbert. dew. forests act as hot spots in terms of isoprene emissions. Future climate and land use changes could greatly aVect the isoprene regional emission estimate through changes in the respective proportion of the upland and lowland forests, and the extent of dry versus wet season.

A global three-dimensional (3-D) chemistry and transport model was used to demonstrate that a factor of 2 decrease in isoprene and monoterpene emissions results in significant (10-30%) changes in predicted concentration distributions of compounds such as OH, MPAN, NOx, H2O2, O3, and CO. Isoprene and monoterpenes were predicted to have a particularly strong impact on tropical regions, including central Africa. The 1996 Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) study included a number of experiments that improved our ability to predict isoprene and monoterpene emissions from central Africa. The results of these experiments have been incorporated into an isoprene emission model that predicts hourly emissions on a spatial scale of about 1 km2. The model uses procedures that are suitable for estimating global emissions but use’s regional measurements to accurately parameterize the model. Comparisons with above-canopy aircraft and tower flux measurements demonstrate that the model can estimate emissions within a factor of 2 for regions where ground measurements of model inputs are available. The annual central African isoprene emission predicted by our revised model (35 Tg C) is only 14% less than that predicted by our earlier model, but there are considerable differences in estimates of individual model variables. The models differ by more than a factor of 5 for specific times and locations, which indicates that .there are large uncertainties in emission estimates for at least some locations and seasons. The good agreement obtained for the EXPRESSO study field sites, however, suggests that the model can predict reasonable estimates if representative field measurements are used to parameterize the model. Copyright 1999 by the American Geophysical Union.

Surface emission and deposition fluxes of reactive nitrogen compounds have been studied in three sites of West Africa during the year 2006, representative of dry savannas ecosystem, and part of the IDAF network: Agoufou (Mali, 15.3°N, 1.4°W), Banizoumbou (Niger, 13.3°N, 2.4°E) and Katibougou (Mali, 12.5°N, 7.3°W). Dry deposition fluxes are calculated from surface measurements of NO2, HNO3 and NH3 concentrations (from passive samplers) and simulated deposition velocities, and wet deposition fluxes are calculated from NH4 + and NO3 − concentration in samples of rain. Emission fluxes are evaluated including simulated NO biogenic emission from soils, emissions of NOx and NH3 from biomass burning and domestic fires, and volatilization of NH3 from animal excreta. From these 3 sites, the average deposition flux, attributed to dry savanna ecosystems in Sahel, is 7.5 (± 1.8) kg N ha−1 year−1, and the average emission flux is from 8.5 ± 3.8 kg N ha−1 year−1. This budget is dominated by the NH3 contribution (due to volatilization of animal excreta). Biogenic emissions from soils are the second most important contribution in emission fluxes. Total estimated deposition is 1.84 ± 0.44 Tg N year−1 and the estimated emission is 2.0 ± 0.9 Tg N year−1 for the Sahel region. Limited interannual variability (between 2002 and 2007) in precipitation is responsible for small variability in local emission sources and hence deposition fluxes.

Emissions of nitrogen compounds from heavily fertilized and irrigated maize fields have been studied in the Southwest of France, over an annual cultivation cycle. Strong nitrous oxide emissions from denitrification were observed after application of nitrogen fertilizer. Flux intensity appears to be stimulated by rain or irrigation. Emission algorithms, taking into account both nitrogen input and soil water content were established on the basis of the experimental data set. They allowed us to estimate annual nitrogen loss in the form of nitrous oxide modulated by rainfall. Production of methane is observed at the level of the water table under anoxic conditions. Nevertheless, the net flux between soil and atmosphere is negative for most of the time. When methane is produced, fluxes were very low due to methane oxidation in the soil surface layer.

During October to December 1994, we measured diel and seasonal variability in nitric oxide (NO) exchange by a southern African savanna ecosystem (Grasslands Research Station, Marondera, Zimbabwe). A set of automated dynamic chambers was installed in three sub-sites: Miombo forest, natural grassland, and cropped soils (maize, sorghum, groundnut). The latter received a single application of commercialy available in-organic or cowdung fertilizer. The dynamic chamber system is described in detail, including correction of the resulting data for wall losses and gas-phase reactions of the NO-NO2-O3 triad. During the dry season, net vertical NO fluxes were close to the detection limit (0.44 ngN m-2s-1) regardless of vegetation type. During the transition from dry to wet season strong "pulsing" effects of the NO flux were observed: NO emissions increased by a factor of up to 60 within hours. During the wet season, NO emissions varied between the different sub-sites of the ecosystem; corresponding means are in ranking order of fertilized agricultural plots (27.2 ngN m-2 s-1), unfertilized agricultural plots (8.5 ngN m-2 s-1 ), grassland (4.4 ngN m-2 s-1 ), and Miombo forest (1.5 ngN m-2 s-1). Soil moisture was found to be the dominant factor controlling the NO fluxes. Whenever it was not limiting, the diel behaviour of NO emission followed the daily variation of soil temperature measured 1 cm below the surface.

Emissions of nitrogen compounds (NO, NH3, N2O and N2) from heavily fertilized (280 kg(N) ha-1) and irrigated maize fields were studied over an annual cultivation cycle in southwestern France. NO and N2O emissions were measured by chamber techniques throughout the year. During fertilization and maize growth periods, chamber measurements were intensified and complemented by flux-gradient micrometeorological measurements of NOx and NH3. The two methods used, Bowen ratio and a simplified aerodynamical techniques, agree quite well and quantify NOx and NH3 flux variations during the period of intense emission which followed fertilizer application. Over a yearly cycle, nitrogen loss in the form of NH3, NO and N2O were calculated using micrometeorological flux measurements and emission algorithms calibrated with field data (chambers). The soil denitrification potential represented by the ratio N2O/(N2O+N2) was measured in the laboratory to calculate potential total gaseous nitrogen loss. Taking into account all uncertainties, the total N loss into the atmosphere represents 30 to 110 kg(N) ha-1 with about less than 1% as NH3, 40% as NO, 14% as N2O and 46% as N2. This is in agreement with the agronomic nitrogen budget based on the N fertilizer input and soil furniture and, on the N-output by crops and crop residues, which displays a net imbalance of 50 to 100 kg(N) ha-1.

Nitrogen oxides are key compounds for the oxidation capacity of the troposphere. NOx concentrations depend on the proximity of sources because of their short atmospheric lifetime. An accurate knowledge of the distribution of their sources and sinks is therefore crucial. At global scale, the dominant sources of nitrogen oxides - combustions of fossil fuel (∼50%) and biomass burning (∼20%) - are basically anthropogenic. Natural sources, including lightning and microbial activity in soils, represent therefore less than 30% of total emissions. Fertilizer use in agriculture constitutes an anthropogenic perturbation to the microbial source. The methods to estimate the magnitude and distribution of these dominant sources of nitrogen oxides are discussed. Some minor sources which may play a specific role in tropospheric chemistry such as NOx emission from aircraft in the upper troposphere or input from production in the stratosphere from N2O photodissociation are also considered.

International audience; The static chamber method (non-flow-through-non-steady-state chambers) is the most common method to measure fluxes of methane (CH4) from soils. Laboratory comparisons to quantify errors resulting from chamber design, operation and flux calculation methods are rare. We tested fifteen chambers against four flux levels (FL) ranging from 200 to 2300 μg CH4 m−2 h−1. The measurements were conducted on a calibration tank using three quartz sand types with soil porosities of 53% (dry fine sand, S1), 47% (dry coarse sand, S2), and 33% (wetted fine sand, S3). The chambers tested ranged from 0.06 to 1.8 m in height, and 0.02 to 0.195 m3 in volume, 7 of them were equipped with a fan, and 1 with a vent-tube. We applied linear and exponential flux calculation methods to the chamber data and compared these chamber fluxes to the reference fluxes from the calibration tank. The chambers underestimated the reference fluxes by on average 33% by the linear flux calculation method (Rlin), whereas the chamber fluxes calculated by the exponential flux calculation method (Rexp) did not significantly differ from the reference fluxes (p < 0.05). The flux under- or overestimations were chamber specific and independent of flux level. Increasing chamber height, area and volume significantly reduced the flux underestimation (p < 0.05). Also, the use of non-linear flux calculation method significantly improved the flux estimation; however, simultaneously the uncertainty in the fluxes was increased. We provide correction factors, which can be used to correct the under- or overestimation of the fluxes by the chambers in the experiment.

Emissions of nitric oxide from soils of equatorial rain forest were measured in the Dimonika Natural Park (4°30′S, 12°30′E) in the Mayombe Forest in Congo. Three research campaigns were carried out in June and July 1991 and in February 1992. Fluxes were measured by dynamic chamber techniques using a chemiluminescence instrument Scintrex LMA3. NO fluxes measured on natural soils are in between 5 and 17 × 10 9 molecules cm -2 s -1 ; they are of the same order of magnitude as those observed in similar tropical forest media. Soil treatment experiments show that the auto-decomposition of HNO 2 in these acid soils (pH# 4) (chemodenitrification) is a potentially important cause of nitric oxide production in this type of ecosystem. Nitrous acid comes from autotrophic nitrification all the year round, and also from biological denitrification, shown by N 2 0 emissions, during the rainy season. The regulation of NO release from soils is linked to ammonia production from litter mineralisation and to direct NH 4 input by throughfall. DOI: 10.1034/j.1600-0889.1994.t01-3-00001.x

The atmospheric composition of West Africa reflects the interaction of various dynamical and chemical systems (i.e. biogenic, urban, convective and long-range transport) with signatures from local to continental scales. Recent measurements performed during the African Monsoon Multidisciplinary Analyses (AMMA) observational periods in 2005 and 2006 provide new data which has allowed new insight into the processes within these systems that control the distribution of ozone and its precursors. Using these new data and recently published results, we provide an overview of these systems with a particular emphasis on ozone distributions over West Africa during the wet season. Copyright (C) 2010 Royal Meteorological Society

Surface emission and deposition fluxes of reactive nitrogen compounds have been studied in five sites of West Africa during the period 2002 to 2007. Measurements of N deposition fluxes have been performed in IDAF sites representative of main west and central African ecosystems, i.e., 3 stations in dry savanna ecosystems (from 15° N to 12° N), and 2 stations in wet savanna ecosystems (from 9° N to 6° N). Dry deposition fluxes are calculated from surface measurements of NO2, HNO3 and NH3 concentrations and simulated deposition velocities, and wet deposition fluxes are calculated from NH4+ and NO3− concentration in samples of rain. Emission fluxes are evaluated including simulated NO biogenic emission from soils, emissions of NOx and NH3 from biomass burning and domestic fires, and volatilization of NH3 from animal excreta. This paper is a tentative to understand the eventual impact of the monsoon variability from year to year, with the natural variability of local sources, on the emission and deposition N fluxes, and to compare these evolutions between dry and wet savanna ecosystems. In dry savanna ecosystems where the rain season lasts mainly from June to September, the occurence of rain correlates with the beginning of emission and deposition fluxes. This link is less obvious in wet savanna ecosystems (wet season mainly from May to October), where the surface is less submitted to drastic changes in terms of water content. Whatever the location, the natural variability of rain from year to year does not exceed 15 %, and the variability of emission and deposition magnitude ranges between 15 % and 28 %. While quasi providing the same total N budget, and due to the presence of different types of soils and vegetation, wet and dry savanna do not present the same distribution in emission and deposition fluxes contributions: in dry savanna, the emission is dominated by ammonia volatilization, and the deposition is dominated by the dry contribution. In wet savanna, emission is equally distributed between ammonia volatilization, emissions from biomass burning and natural NO emissions from soils, and wet and dry deposition are equivalent. Due to the scarcity of available data on the African continent, and despite the numerous uncertainties resulting from the different calculations and assumptions, this work is a combination of data from different origins (surface measurements, satellite and modelling) to document the atmospheric Nitrogen cycle in tropical regions.