Net greenhouse gas balance of fibre wood plantation on peat in Indonesia.

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.

Greenhouse Gas Emissions from Freshwater Reservoirs: What Does the Atmosphere See?

Freshwater reservoirs are a known source of greenhouse gas (GHG) to the atmosphere, but their quantitative significance is still only loosely con- strained. 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.

Carbon dioxide fluxes from a degraded woodland in West Africa and their responses to main environmental factors.

BACKGROUND: In West Africa, natural ecosystems such as woodlands are the main source for energy, building poles and livestock fodder. They probably behave like net carbon sinks, but there are only few studies focusing on their carbon exchange with the atmosphere. Here, we have analyzed CO2 fluxes measured for 17 months by an eddy-covariance system over a degraded woodland in northern Benin. Specially, temporal evolution of the fluxes and their relationships with the main environmental factors were investigated between the seasons. RESULTS: This study shows a clear response of CO2 absorption to photosynthetic photon flux density (Qp), but it varies according to the seasons. After a significant and long dry period, the ecosystem respiration (R) has increased immediately to the first significant rains. No clear dependency of ecosystem respiration on temperature has been observed. The degraded woodlands are probably the "carbon neutral" at the annual scale. The net ecosystem exchange (NEE) was negative during wet season and positive during dry season, and its annual accumulation was equal to +29 ± 16 g C m-2. The ecosystem appears to be more efficient in the morning and during the wet season than in the afternoon and during the dry season. CONCLUSIONS: This study shows diurnal and seasonal contrasted variations in the CO2 fluxes in relation to the alternation between dry and wet seasons. The Nangatchori site is close to the equilibrium state according to its carbon exchanges with the atmosphere. The length of the observation period was too short to justify the hypothesis about the "carbon neutrality" of the degraded woodlands at the annual scale in West Africa. Besides, the annual net ecosystem exchange depends on the intensity of disturbances due to the site management system. Further research works are needed to define a woodland management policy that might keep these ecosystems as carbon sinks.

PROBLÉMATIQUE: En Afrique de l'Ouest, les écosystèmes naturels comme les forêts claires constituent la principale source d'énergie, de bois d'oeuvre et de fourrage pour le bétail. Ces forêts claires se comportent probablement comme de puits nets de carbone, mais très peu d'études ont porté sur les échanges de carbone de celles-ci avec l'atmosphère. Cette étude a analysé les flux de CO2 mesurés pendant 17 mois à l'aide d'un système d'eddy-covariance placé au dessus d'une forêt claire dégradée au nord du Bénin. De façon spécifique, l'évolution temporelle des flux de CO2 et leurs relations avec les principaux facteurs environnementaux ont été étudiées suivant les saisons. RÉSULTATS: Cette etude montre une réponse claire de l'absorption du CO2 à la densité de flux de photons photosynthétiques, mais elle est différente selon les saisons. Après une longue et significative période sèche, la respiration de l'écosystème (R) augmente immédiatement en réaction aux premières pluies significatives. Aucune dépendance claire de la respiration de l'écosystème à la température n'a été observée. Les forêts claires dégradées ouest-africaines sont probablement neutres en considérant leurs échanges de carbone à l'échelle annuelle avec l'atmosphère. L'échange net de l'écosystème (NEE) est négatif pendant la saison humide et positif durant la saison sèche, et son cumul annuel est égal + 29 ± 16 g C m−2. L'écosystème apparaît être plus efficient dans la matinée et en saison humide que pendant l'après-midi et en saison sèche. CONCLUSION: Cette étude a montré d es variations journalières et saisonnières contrastées des flux de CO2 en relation avec l'alternance entre les saisons sèche et humide. Le site investigué est à l'état d'équilibre en considérant ses échanges de carbone avec l'atmosphère. La durée de la période d'observation était trop courte pour justifier l'hypothèse de la neutralité des forêts claires dégradées ouest-africaines par rapport aux échanges de carbone avec l'atmosphère à l'échelle annuelle. En outre, l'échange net de l'écosystème dépend de l'intensité des perturbations dues au système de gestion du site. D'autres recherches sont nécessaires pour définir une politique de gestion des forêts claires qui contribueraient à maintenir ceux-ci comme de puits nets de carbone.

Gross CO2 and CH4 emissions from the Nam Ngum and Nam Leuk sub-tropical reservoirs in Lao PDR.

Gross CO2 and CH4 emissions (degassing and diffusion from the reservoir) and the carbon balance were assessed in 2009-2010 in two Southeast Asian sub-tropical reservoirs: the Nam Ngum and Nam Leuk Reservoirs (Lao PDR). These two reservoirs are within the same climatic area but differ mainly in age, size, residence time and initial biomass stock. The Nam Leuk Reservoir was impounded in 1999 after partial vegetation clearance and burning. However, GHG emissions are still significant 10 years after impoundment. CH4 diffusive flux ranged from 0.8 (January 2010) to 11.9 mmol m(-2) d(-1) (April 2009) and CO2 diffusive flux ranged from -10.6 (October 2009) to 38.2 mmol m(-2) d(-1) (April 2009). These values are comparable to other tropical reservoirs. Moreover, degassing fluxes at the outlet of the powerhouse downstream of the turbines were very low. The tentative annual carbon balance calculation indicates that this reservoir was a carbon source with an annual carbon export (atmosphere+downstream river) of about 2.2±1.0 GgC yr(-1). The Nam Ngum Reservoir was impounded in 1971 without any significant biomass removal. Diffusive and degassing CO2 and CH4 fluxes were lower than for other tropical reservoirs. Particularly, CO2 diffusive fluxes were always negative with values ranging from -21.2 (April 2009) to -2.7 mmol m(-2) d(-1) (January 2010). CH4 diffusive flux ranged from 0.1 (October 2009) to 0.6 mmol m(-2) d(-1) (April 2009) and no degassing downstream of the turbines was measured. As a consequence of these low values, the reservoir was a carbon sink with an estimated annual uptake of - 53±35 GgC yr(-1).