Soil Carbon Stock and River Carbon Fluxes in Humid Tropical Environments: The Nyong River Basin (South Cameroon)- Chapter 18

This study investiged the concentrations of carbon (C) in different water reservoirs in different forms in relation to the weathering rate and the C content in the soils of five nested catchments belonging to the Nyong basin in South Cameroon. This forested granitic watershed is representative of the humid tropical domain. The five nested catchments (from upstream to downstream) are : the experimental Mengong catchment near Nsimi village, the Awout watershed (tributary of the So'o) at Messam, the So'o watershed (tributary of the Nyong) at Pont So'o, the upper Nyong basin at Mbalmayo, and at Olama station after the Nyong and So'o confluence. Soil samples were collected and analysed only in the Mengong catchment for different horizons. The C contents exhibited a very heterogeneous distribution in the different soil profiles. The organic C stock was estimated at 32 000 t in soils of this catchment. For the hydrolic year 1998/99, the mean annual DOC concentration was 0.31 mg/litre in the spring and 14.3 mg/litre in the stream at the outlet of the Mengong catchment. the mean dissolved organic C (DOC) river fluxes were between 4700 and 9300 kg/km2 per year. In coloured river and swamp waters, DOC fluxes were between 4700 and 9300 kg/km2 per year, whereas POC fluxes were between 600 and 1400 kg/km2 per year. In contrast, clear spring waters or groundwaters contain low DOC contents. The mean annual bicarbonate concentrations were 3.6 mg/litre in the spring water, and between 3.4 and 7.7 mg/litre in different rivers. In a non-carbonated environment, the dissolved inorganic carbon fluxes are normally low. Whereas for the Nyong River basin, these fluxes are lower than for other tropical silicatedd catchments due to low chemical weathering rate of silicate rocks for the upper Nyong Basin at Olama. Particulate organic C (POC) was measured (17 samples) during the flood event of 28 and 29 April 2002 on the Mengong River. These measurements indicated that POC content represented on average 25.3% if the TSS content, with relatively low variations compared to water discharge fluctuations