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

Partitioning carbon sources in a tropical watershed (Nyong River, Cameroon) between wetlands and terrestrial ecosystems – Do CO<sub>2</sub> emissions from tropical rivers offset the terrestrial carbon sink?

“…Based on TA concentrations (53±26 µmol L -1 ) in forest groundwater and seepage of the groundwater of ~0.005 m 3 s -1 , we estimated a weathering rate in forest groundwater and overlaying lateritic soil of 0.2±0.1 t C km -2 yr -1 . The low weathering rates are mainly attributed to the thickness of the lateritic cover on hills and slopes, which considerably slows the advance of percolating water in the bedrock (Boeglin et al, 2005). This was confirmed by the low mineral dissolved load in the aquifer of the Nyong basin (Braun et al, 2002) and by the dissolved silica fluxes in rivers that were significantly lower compared to the annual rainfall (e.g., White and Blum, 1995).…”

“…In the Mengong source, groundwater pCO2 and O2 were not correlated (p>0.05, data not shown) and groundwater was free of DOC (Table 2; Braun et al, 2012;Brunet et al, 2009). In the lateritic soil overlaying the Mengong source, iron oxides represent up to 50% of the average volumetric weight of the soil (Braun et al, 2005). Thus, the low concentrations of dissolved iron (Fe 2+ ) observed in the Mengong source indicated that despite the oxidizing environment above the groundwater, the transfer of Fe 2+ from the soil to the aquifer is not allowed (Braun et al, 2005).…”

“…Then, the Mengong flows donwstream trough a swamp located in the lowland part of the Mengong basin to eventually reach the Mengong outlet 850 m downstream of the seepage point (Maréchal et al, 2011). In the lowland part of the Mengong catchment, the Mengong stream is indeed adjacent to the swamp whose extent is on average through the year ~20% of the total surface area of the catchment (Braun et al, 2005). Accordingly, water at the Mengong outlet is fed by two different sources: the forest groundwater that seeps from the hillside of the Mengong catchment and by the swamp located in the lowland part of the basin (Maréchal et al, 2011).…”

“…Considering the granitic lithology (i.e., absence of carbonate minerals) of the Nyong basin (Maurizot et al, 1986), TA in forest groundwater originates from the weathering of silicate minerals as dissolved CO2 can react with silicate minerals to produce bicarbonates (Meybeck, 1987). In forest groundwater, the low TA concentrations and the absence of significant seasonal variations of TA were likely related to low weathering rates in the lateritic soil cover, which exhibits a relatively inert mineralogy (Braun et al, 2005(Braun et al, , 2012. Based on TA concentrations (53±26 µmol L -1 ) in forest groundwater and seepage of the groundwater of ~0.005 m 3 s -1 , we estimated a weathering rate in forest groundwater and overlaying lateritic soil of 0.2±0.1 t C km -2 yr -1 .…”

“…In the lateritic soil overlaying the Mengong source, iron oxides represent up to 50% of the average volumetric weight of the soil (Braun et al, 2005). Thus, the low concentrations of dissolved iron (Fe 2+ ) observed in the Mengong source indicated that despite the oxidizing environment above the groundwater, the transfer of Fe 2+ from the soil to the aquifer is not allowed (Braun et al, 2005). OM in the upper organic-rich horizons of the soil is probably well complexed with iron oxides and prevents DOC https://doi.org/10.5194/bg-2021-69 Preprint.…”