The carbon cycle in wet tropical domain: example of the forested Nyong river basin in the south Cameroon

Spatio-temporal follow ups of carbon contents and its isotopic signature δ¹³C 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 to 16 %), with isotopic signatures δ¹³C of -29 o/oo in equilibrium with C3 plants (-32 o/oo to -28 o/oo). During the rainy season, the mineralization of organic matter and the leaching of swampy areas are responsible of high DOC contents (until 38 mg / L) in stream waters, strong pCO₂ and impoverished δ¹³C_DIC isotopic signatures (-20 o/oo on average). During the dry season, the shrinkage of the swampy zones facilitates the supply of streams by low DOC content (< 1 mg / L) and enriched δ¹³C_DIC (δ¹³C_DIC until 7 o/oo) groundwaters. 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 o/oo. 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 δ¹³C_DIC in surface waters. On the contrary, in poor DOC content groundwaters, Ca+Mg / HCO₃^- is < 0.5 and there is no correlation with DOC and δ¹³C_DIC, 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 capacity. (author)