Le cycle du carbone en domaine tropical humide : exemple du bassin versant forestier du Nyong au sud Cameroun

Les suivis spatio-temporels des teneurs en carbone et de sa signature isotopique d13C dans le bassin versant du Nyong permettent d'une part, à une échelle locale (petit bassin versant du Mengong), d'identifier les sources de carbone et d'autre part, à une échelle régionale (bassin du Nyong), de comprendre les processus biogéochimiques qui contrôlent les transformations et les échanges de carbone entre les différents réservoirs au cours du transfert de l'amont vers l'aval. Dans ces écosystèmes tropicaux humides, les sols marécageux de bas fonds présentent en surface un épais horizon humifère riche en carbone organique (12 à 16%), avec des signatures isotopiques d13C de -29‰ en équilibre avec celle des plantes en C3 (-32‰ à -28‰). La minéralisation de la MO et le lessivage des zones marécageuses sont responsables en saison des pluies des fortes teneurs en COD (jusqu'à 38 mg/l), des fortes pCO2 et des signatures isotopiques d13CCID appauvries (-20‰ en moyenne) des cours d'eau. En saison sèche, la réduction des zones marécageuses favorise l'alimentation des cours d'eau par les eaux de nappe pauvre en COD (< 1mg/l) et enrichies en 13C (d13CCID jusqu'à -7‰). Les flux de carbone vers l'océan pour le bassin du Nyong, estimés à 1,2.106 t C/an (soit 6,24 t C/km2/an) sont constitués à 80% de COD, 8% de COP et 12% de CID. Au cours du transfert d'amont en aval, une diminution de COD de 33% est observée en relation avec des processus d'oxydation. Les fortes pCO2 qui en résultent, favorisent un dégazage du CO2 vers l'atmosphère, estimé pour le réseau hydrographique du Nyong à 2,3. 105 t C/an ; ce dégazage s'accompagne d'un enrichissement isotopique d'amont en aval de l'ordre de 4‰. L'altération des minéraux silicatés par les acides organiques entraîne un excédent de Ca+Mg (0,5< Ca+Mg/HCO3- <2,2) par rapport à une altération par l'acide carbonique (Ca+Mg/HCO3- = 0,5). Cet excédent est corrélé positivement aux teneurs en COD et négativement au d13CCID dans les eaux de surface. Par contre, dans les eaux de nappe pauvres en COD, Ca+Mg/HCO3- est < 0,5 et il n'y a pas de corrélation avec COD et d13CCID, montrant que l'altération est ici principalement contrôlée par l'acide carbonique. La charge spécifique des acides organiques, estimée en moyenne à 6,7 µeq/mgC dans le bassin versant du Nyong, est responsable du déficit anionique observé dans les eaux de surface. Sa décroissance d'amont (7,5 µeq/mgC en moyenne à Awout) en aval (5,4 µeq/mgC en moyenne à Olama) entraîne une diminution de la capacité complexante du CODSpatial and temporal follow ups of carbon contents and its isotopic signature d13C in the Nyong river basin allow on one hand, at a local scale (small experimental catchment of Mengong), to identify the carbon sources, and on the other hand, at a regional scale (Nyong river basin), to understand the biogeochemical processes which control the carbon transformations and exchanges between the different reservoirs during the transfer from upstream to downstream. In these wet tropical ecosystems, the swampy soils of bottom valley low marshes present in surface a thick organic carbon rich horizon (12 to16 %), with isotopic signatures d13C of -29 ‰ in equilibrium with C3 plants (-32 ‰ to -28 ‰). During the rainy season, the mineralization of organic matter and the leaching of swampy areas are responsible of hight DOC contents (until 38 mg / L) in streamwaters, strong pCO2 and impoverished d13CDIC isotopic signatures (-20 ‰ on average). During the dry season, the shrinkage of the swampy zones facilitates the supply of streams by low DOC content (< 1mg / L) and enriched d13CDIC (d13CDIC until 7 ‰%) grounwaters. The carbon flux into the ocean is estimated to 1.2*106 t C / year, i.e. 6.24 t C/km2.year, of which 80 % of DOC, 8 % of POC and 12 % of DIC. During the transfer from upstream to downstream, a DOC decrease of 33 % is observed in relation with oxidation processes. The resulting strong pCO2 facilitate the CO2 degassing towards the atmosphere, estimated for the Nyong river system at 2.3* 105 t C / year. This CO2 degassing is accompanied by an isotopic enrichment of about 4 ‰. The weathering of silicate minerals by organic acids leads to a surplus of Ca+Mg (0.5 < Ca+Mg / HCO3- < 2.2) with regard to a natural weathering pathway by the carbonic acid (Ca+Mg / HCO3- = 0.5). This Ca+Mg surplus is correlated positively with DOC content and negatively with d13CDIC in surface waters. On the contrary, in poor DOC content grounwaters, Ca+Mg / HCO3- is < 0.5 and there is no correlation with DOC and d13CDIC, showing that the weathering is mainly controlled by carbonic acid. The specific charge of organic acids, estimated to 6.7 µeq / mgC on average in the Nyong river basin, is responsible of the observed anionic deficit in surface waters. Its decreasing from upstream (7.5 µeq / mgC on average to Awout) to downstream (5.4 µeq / mgC on average to Olama) leads to a reduction of the DOC complexing capacit

Terrestrial and fluvial carbon fluxes in a tropical watershed: Nyong basin, Cameroon

The Nyong watershed, with an area of 27 800 km2 and a mean annual discharge of 390 m3 s−1, is the second largest river in Cameroon. The Nyong watershed serves as an outstanding study area for the examination of carbon fluxes in humid tropical environments because of its limited anthropogenic impact and homogeneous silicate bedrock. Between April 2005 and April 2007, we sampled water at seven stations, from the small watershed of the Mengong (0.6 km2) to the Nyong at Edea (24 500 km2), and monitored temperature, pH, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) contents, as well as the isotopic composition of DIC (δ13CDIC)andDOC(δ13CDOC).We estimated terrestrial net ecosystemproductivity in theNyong River watershed and measured fluvial fluxes of carbon to the ocean and the atmosphere. The Nyong River basin sequesters significant amounts of carbon on an annual basis: ~7 920 000t C year−1 (300 g C m−2 year−1). The combined dissolved organic, dissolved inorganic and atmospheric fluxes of carbon from the Nyong River only export 3% of this flux fromthe basin on an annual basis. This includes a minimumCO2 outgassing of 1487 g Cm−2 year−1, comparable to 115% of the annual flux of DOC and four times greater than the flux of DIC

Stable Carbon Isotopes δ13C as a Proxy for Characterizing Carbon Sources and Processes in a Small Tropical Headwater Catchment: Nsimi, Cameroon

Stream carbon fluxes are one of the major components in the global C cycle, yet the discrimination of the various sources of stream carbon remains to a large extent unclear and less is known about the biogeochemical transformations that accompany the transfer of C from soils to streams. Here, we used patterns in stream water and groundwater δ13C values in a small forested tropical headwater catchment to investigate the source and contribution from the soil carbon pools to stream organic and inorganic carbon behavior over seasonal scales. Stream organic carbon (DOC and POC) comes mainly from the upper rich soil organic carbon horizons and derived from total organic carbon (TOC) of biogenic source. The isotopic compositions δ13CTOC, δ13CDOC and δ13CPOC of these carbon species were very close (− 30‰ to − 26‰) and typical of the forested C3 vegetation. The relationship observed between DOC and log pCO2 and δ13CDIC indicated that besides the considerable CO2 evasion that occurs as DIC is transported from soils to streams, there were also other processes affecting the stream DIC pool. In-stream mineralization of DOC and mixing of atmospheric carbon had a significant influence on the δ13CDIC values. These processes which varied seasonally with hydrological changes represent the main control on DOC and DIC cycling in the wet tropical milieu. The rapid turnover of carbon on hillside soils, the transformation of TOC to DOC in wetland soils and further mineralization of stream DOC to DIC favor the evasion of C, making the zone a source of carbon to the atmosphere

Le cycle du carbone en domaine tropical humide (exemple du bassin versant forestier du Nyong au sud Cameroun)

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF