Soil Carbon Stock and River Carbon Fluxes

in Humid Tropical Environments: The Nyong River Basin (South Cameroon)

INTRODUCTION

The role of organic and inorganic carbon (C) in the environmental equilibrium is now generally accepted. Therefore, on a global scale, the interactions between the C cycle and the weathering of rocks and soils or the continental water circulation have been frequently demonstrated [START_REF] Berner | The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years[END_REF][START_REF] Meybeck | Global geochemical weathering of surficial rocks estimated from river dissolved loads[END_REF][START_REF] Amiotte-Suchet | Modelling of atmospheric CO 2 consumption by chemical weathering of rocks: Application to the Garonne, Congo and Amazon basins[END_REF][START_REF] Ludwig | Atmospheric CO 2 consumption by continental erosion: Present-day controls and implication for the last glacial maximum[END_REF][START_REF] Amiotte-Suchet | Worldwide distribution of continental rock lithology: Implication for the atmospheric/soil CO 2 uptake by continental weathering and alkalinity river transport to the ocean[END_REF]. Hydrochemical studies of C transfers, realized on different-size catchments located in all climatic areas, are complex because this element is found under several forms (mineral or organic, dissolved or particulate). The C origin in the continental waters (atmospheric or soil CO 2 , dissolution and erosion of rocks, soil organic matter, aquatic microorganisms), so that possible exchange or transformation reactions can be determined using isotopic tracing of C [START_REF] Amiotte-Suchet | 13 C pattern of dissolved inorganic carbon in a small granitic catchment: The Strengbach case study (Vosges Mountains, France)[END_REF][START_REF] Aucour | Use of 13 C to trace origin and cycling of inorganic carbon in the Rhône River system[END_REF][START_REF] Barth | Carbon cycle in the St Lawrence aquatic ecosystem at Cornwall (Ontario), Canada: Seasonal and spatial variations[END_REF]Brunet et al., in press) and of other elements like strontium (Negrel et al. 1993;[START_REF] Gaillardet | Global silicate weathering and CO 2 consumption rates deduced from the chemistry of large rivers[END_REF], or geochemical modeling [START_REF] Probst | Carbon river fluxes and global weathering CO 2 consumption in the Congo and Amazon river basins[END_REF][START_REF] Amiotte-Suchet | A global model for present-day atmospheric/soil CO 2 consumption by chemical erosion of continental rocks (GEM-CO 2 )[END_REF]Gaillardet et al. 1997, Mortatti andProbst 2003). While data of C concentrations and fluxes are available for some rivers, including the Niger [START_REF] Martins | Biogeochemistry of major African rivers: Carbon and mineral transport[END_REF][START_REF] Boeglin | Transports fluviaux de matières dissoutes et particulaires sur un bassin versant en région tropicale: Le bassin amont du Niger au cours de la période 1990-1993[END_REF], the Congo [START_REF] Probst | Carbon river fluxes and global weathering CO 2 consumption in the Congo and Amazon river basins[END_REF][START_REF] Seyler | Concentrations, fluctuations saisonnières et flux de carbone dans le bassin du Congo[END_REF], and the Amazon [START_REF] Richey | Biogeochemistry of carbon in the Amazon river[END_REF], in the case of tropical basins, the C contents and stocks in the soil cover of corresponding watersheds have generally not been determined.

The objective of this chapter is to analyze the C concentrations 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. Such a study has been rarely performed on this type of environment (in Puerto Rico, McDowell and Asbury 1994;[START_REF] White | Chemical weathering in a tropical watershed, Luquillo Mountains, Puerto Rico. I. Longterm versus short-term weathering fluxes[END_REF]in the Ivory Coast, Stoorvogel et al. 1997a;Stoorvogel et al. 1997b), contrary to the temperate North American or European environments (see for example the compilations of [START_REF] Velbel | Interaction of ecosystem processes and weathering processes[END_REF][START_REF] White | Effects of climate on chemical weathering in watersheds[END_REF][START_REF] Drever | Chemical weathering rates of silicate minerals[END_REF].

CHARACTERIZATION OF THE NYONG RIVER BASIN

The Nyong basin, covering an area of 27,800 km 2 between 2°58' and 4°32' N latitude, is located in Cameroon. Five nested catchments have been sampled in the present study, i.e., from upstream to downstream: the experimental Mengong catchment near to 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 (Figure 18.1).

The study took place on the southern Cameroon plateau, a smooth undulating area, with altitudes between 650 and 850 m. The morphology, presenting large depressed swampy zones (about 20% of the Nsimi catchment area) between eroded hills, is derived from an original half-orange landscape [START_REF] Bilong | Séquence d'évolution des paysages cuirasses et des sols ferrallitiques en zones forestières tropicales d'Afrique Centrale. Place des sols à horizons d'argiles tachetées[END_REF]).

The regional substratum is granite-type. The southern part of the study area is mainly constituted of Liberian granitoids of the Ntem group (2600-2900 M yr), and corresponds to the edge of the Congo craton, whereas the northern part, with gneisses and migmatites of the Yaoundé series metamorphized during the Panafrican orogenesis (~600 M yr) is the western continuation of the Oubanguide chain [START_REF] Vicat | Esquisse géologique du Cameroun. Collect. Géocam[END_REF].

A spatial distribution of pedologic horizons in the Nsimi catchment was done from pits and holes [START_REF] Nyeck | Séquence d'évolution des sols sur granite dans le Sud du Cameroun[END_REF]) and using resistivity measurements along several cross sections [START_REF] Ritz | Improvement to resistivity pseudosection modelling by removal of near surface inhomogeneity effects: Application to a soil system of Southern Cameroon[END_REF]. The lateritic profile developed on slopes and hills (up to 50 m thick) is characterized by a deferruginisation and a nodulation within the ferricrete, which is capped by a sandy-clayey homogeneous cover yellowish to reddish in color. In the depressed zone, hydromorphic soils (1-3 m thick), essentially constituted by grey colluvial sandy clays containing high organic matter content (> 10%) at their upper part, overlay the truncated mottled clay horizon.

Southern Cameroon is characterized by a four-season Guinean climate. The two rainy seasons (March-May, September-November) are separated by a short dry season, whereas the well-marked long dry season is from December to February. The annual rainfall is between 1500 and 2000 mm with a mean temperature of about 25°C presenting a 2 to 3°C annual amplitude, and a potential evapotranspiration of 1250 mm.yr -1 [START_REF] Olivry | Fleuves et rivières du Cameroun[END_REF]. For the hydrologic year 1998/99, rainfall (P) and runoff (R) averaged 1822 and 387 mm, respectively, for the Mengong catchment, and 1667 and 283 mm for on Nyong basin at Olama. These values correspond to a runoff ratio (R/P) of 21 and 17%, respectively.

The Yaoundé-Sangmélima-Abong Mbang triangular area (Figure 18.1) is mainly the domain of the semi-deciduous forest, characterized by Stertuliaceae and Ulmaceae. This type of vegetation is found in limited sectors surrounded with more degraded vegetation zones (Villiers in [START_REF] Santoir | Atlas regional Sud-Cameroun[END_REF]. The swampy depressions are generally colonized by raffias (Raphia montbuttorum). The anthrogenic effects on the environment are rather weak on the southern Cameroon plateau, because of the low population density (5-10 in km -2 , except in the urban zone of Yaoundé). Human activities include deforestation, shifting cultivation without fertilizer input, and growing food crops (tubers, banana, corn) or peanuts, tomatoes, cocoa bean, etc.

METHODOLOGY

Soils were sampled on Nsimi catchment, using profiles on summit and side slopes, and by auger in the valley. The organic C was analyzed in two ways: (1) wet oxidation method using potassium dichromate and titration with ammonium-iron sulfate (Anne's method, Rouiller et al., in Bonneau and Souchier 1994), performed at the Hydrological Research Center in Yaoundé (Cameroon), and (2) measurement of the CO 2 volume evolved consecutively to the combustion of a soil aliquot, performed in the laboratories of the Centro de Energia Nuclear na Agricultura in Piracicaba (Brazil). The first method, appropriate for C contents > 1%, is not precise in the case of soils with low C content (C < 0.2%).

Waters samples were collected monthly at six stations (one on a spring, two on streams, three on rivers) of the Nyong network. After measurement of the physicochemical parameters (temperature, pH, electrical conductivity) and filtration in the field, samples were sent to the Centre de Géochimie de la Surface in Strasbourg (France) for determination of cations and anions, dissolved silica, alkalinity, and dissolved organic C (DOC). The alkalinity (= "acid neutralizing capacity") was determined by titration with H 2 SO 4 using Gran method on a Mettler memotitrator DL 40RC, with an analytical reproducibility better than 2%. The Shimadzu TOC 5000A analyzer was used to measure DOC content by catalytic combustion; the reproductibility is about 5%, the detection limit is 0.1 mg L -1 . The determination of particulate organic C (POC) was done on the total suspended solids (TSS) collected on glass microfiber filters for the April 22 and 23, 2002 flood event. The concentrations were determined by catalytic combustion on a LECO CS125 carbon analyzer at the Centre de Recherche sur les Environnements Sédimentaires et Océaniques of Bordeaux I University (France).

RESULTS

Soil Carbon

Soils of the southern Cameroon plateau are laterites developed on hills and slopes, or hydromorphic soils in depressions and valleys, all formed on a granitoid-type bedrock. Carbonates have not been observed in the geological substratum and in the soil. Thus, total C present in the soil is contained in the organic matter. Soil samples were collected and analyzed only in the Mengong catchment for different horizons. The C contents exhibited a very heterogeneous distribution in the different soil profiles (Table 18.1). High concentrations were found in the first meter of the soil on hillslopes and hydromorphic soils (up to 6% in the sandy-clayey horizon) of the swampy depression, and the C concentration was very low in all subsoil horizons. These results are in accord with those reported by [START_REF] Humbel | Quantité de matières organiques associées aux sols du domaine ferrallitique au Cameroun[END_REF] for different ferrallitic soils of Cameroon.

Total soil C stock was estimated for the Mengong catchment, using the data of organic C analyses in the present study, pedological studies (description of toposequences, determination of apparent density) conducted by [START_REF] Nyeck | Séquence d'évolution des sols sur granite dans le Sud du Cameroun[END_REF], and geophysical measurements (surficial resistivity allowing an assessment of the soil profile depth along cross sections) conducted by [START_REF] Ritz | Improvement to resistivity pseudosection modelling by removal of near surface inhomogeneity effects: Application to a soil system of Southern Cameroon[END_REF]. Different parameters used in the calculation for each horizon are summarized in Table 18.1. The organic C stock is estimated at 35,000 Mg in soils of the Mengong catchment. The surface horizons of hills, slopes and especially of depression, which contain high organic matter content, represent only 13% of the total C stock, of which 51% is contributed by saprolite in which low C content is compensated by the thickness of this formation, which covers the entire catchment. Note: A % is the percentage of the catchment area (0.60 km 2 ), T is the mean thickness (in m), V is the volume (in 10 6 *m 3 ), and AD the apparent density of an horizon; OC% is the percentage of organic carbon content; S OC is the stock of organic carbon, S OC % in the percentage of organic carbon stock in a given horizon compared the whole catchment; data in the last line correspond to the whole catchment.

Carbon in Surface Waters

Dissolved Organic Carbon (DOC)

In the Mengong catchment, an obvious distinction can be made between the clear spring waters and the colored outlet river waters. This color difference has previously been reported by [START_REF] Viers | Chemical weathering in the drainage basin of a tropical watershed (Nsimi-Zoetele site, Cameroon): Comparison between organicpoor and organic-rich waters[END_REF] and [START_REF] Oliva | The effect of organic matter on chemical weathering: Study of a small tropical watershed, Nsimi Zoetele, Cameroon[END_REF], and is attributed to differences in DOC content (Table 18.2). For the hydrologic year 1998/1999, the mean annual DOC concentration was 0.31 mg.L -1 in the spring and 14.3 mg.L -1 in the stream at the outlet.

The mean DOC river fluxes were between 4700 and 9300 kg km -2 yr -1 (Table 18.2). The values corresponding to the southern part of the Nyong watershed are higher than those observed for the eastern part of the basin (Mbalmayo station). But this difference may be due to a higher drainage intensity in the southern part, rather than to differences in the vegetation and soil covers.

The organic-rich waters exhibit a very high ionic imbalance between the cationic sum (S c ) and the anionic sum (S a ), due to an important anionic deficit, as was reported by [START_REF] Probst | Dissolved major element exported by the Congo and the Ubangui rivers during the period 1987-1989[END_REF] for the Congo River. The mean annual deficit averaged 44 to 84% depending on the stations. This imbalance is mainly due to organic anions that are not taken into account in the measurement of alkalinity [START_REF] Fillion | Natural organic matter contribution to throughfall acidity in French forests[END_REF]. Consequently, there is a good relationship between ionic deficit (in µeq L -1 ) and DOC content (in mg L -1 ) for the Mengong River (Figure 18.2). The slope of the regression lines obtained for different stations varies between 4.3 and 9.1 µeq.mg -1 C; the correlation coefficients R calculated for all catchments are > 0.91.

Dissolved Inorganic Carbon (DIC)

Dissolved inorganic C (DIC) is present mainly as bicarbonates at the pH of most river waters. This ion is released consecutively to hydrolysis in which soil and rock minerals are weathered under the effect of the carbonic acid (Equation 18.3). This acid, originating from the atmospheric CO 2 (Equation 18.2), is abundant in soil solutions and is produced by the oxidation of soil organic matter (Equation 18.1): Note: S is the catchment area in km 2 ; Qm is the annual mean runoff expressed in mm yr -1 ; C is the mean annual concentration in mg L -1 ; F, the annual specific flux in kg km -2 yr -1 ; DIC is the dissolved inorganic carbon contained in bicarbonate; TDS is total inorganic dissolved solids; DOC is dissolved organic carbon; TSS is total suspended solids; FCO 2 is the atmospheric/soil CO 2 specific flux consumed by weathering (in 103 mol km -2 yr -1 ).

CH 2 O + O 2 CO 2 + H 2 O (18.1) CO 2 + H 2 O H 2 CO 3 (18.2)
For example, the weathering reaction of orthose (or microcline) into kaolinite can be written as follows [START_REF] Garrels | Evolution of Sedimentary Rocks[END_REF]:

2KAlSi 3 O 8 + 3H 2 O + 2CO 2 Si 2 Al 2 O 5 (OH) 4 + 2K + + 4SiO 2 + 2HCO 3 - (18.3)
As shown in Equation 18.3, all bicarbonate ions released into solution originate from atmospheric/soil CO 2 .

The mean annual bicarbonate concentrations are 5.3 mg L -1 in the spring water, and between 4.8 and 7.8 mg L -1 in different rivers. The corresponding concentrations of DIC for the Nyong River stations are summarized in Table 18.2.

Since the substratum of the Nyong drainage basin is exclusively composed of silicate rocks, the total bicarbonate fluxes measured in the river waters sampled at different stations can be directly related to the atmospheric/soil CO 2 (Equation 18.3). Consequently the calculation of the CO 2 fluxes used in chemical weathering is directly derived from the river's alkalinity fluxes. The values calculated for different stations range between 22 and 27 10 3 mol km -2 yr -1 (Table 18.2).

Total Suspended Solids (TSS) and Particulate Organic Carbon (POC)

Studies on the drainage waters of the Nyong basin showed that the suspended matter is enriched in organic matter; the mineral fraction is mainly composed of kaolinite, quartz and goethite, and amorphous silica is present in diatom frustules or in phytoliths [START_REF] Allard | Crystal chemistry of suspended matter in a tropical hydrosystem, Nyong basin (Cameroon, Africa)[END_REF]. No measurement of POC concentration was made for the TSS during the hydrologic year 1998/1999. However, POC was measured (17 samples) during the flood event of April 28 and 29, 2002 on the Mengong River. These measurements indicated that POC content represented on average 25.3% of the TSS content, with relatively low variations compared to water discharge fluctuations. This percentage is close to that (25.4%) obtained from the mean annual POC and TSS fluxes calculated by [START_REF] Ndam-Ngoupayou | Bilans hydrogéochimiques sous forêt tropicale humide en Afrique: Du bassin expérimental de Nsimi-Zoétélé aux réseaux hydrographiques du Nyong et de la Sanaga (Sud-Cameroun)[END_REF] for the hydrologic year 1995/1996 on the Mengong River. Whereas this percentage is higher than those obtained for the Nyong at Mbalmayo (17.3%) and Olama (13.8%), it shows that the suspended material decreases in organic C from upstream to downstream during the same time the TSS concentration increases due to the physical erosion of more inorganic materials. This pattern is comparable to those observed for most world rivers [START_REF] Martins | Biogeochemistry of major African rivers: Carbon and mineral transport[END_REF][START_REF] Ludwig | Predicting the oceanic input of organic carbon by continental erosion[END_REF]. Using these data, the POC specific fluxes were estimated between 550 and 750 kg km -2 yr -1 for different stations of the Nyong River basin.

DISCUSSION

The Major Role of Colored Waters

There are two main types of water in the Nyong basin: clear waters and colored waters. The colored water, which remains after filtration and cannot be related to suspended matters, is attributed to a high content of dissolved organic matter (Ndam-Ngoupayou 1997). This corroborates with our results, with mean DOC concentrations > 15 mg L -1 in all river waters, whereas these are about 0.3 mg L -1 in the Mengong spring waters. Moreover, [START_REF] Ndam-Ngoupayou | Bilans hydrogéochimiques sous forêt tropicale humide en Afrique: Du bassin expérimental de Nsimi-Zoétélé aux réseaux hydrographiques du Nyong et de la Sanaga (Sud-Cameroun)[END_REF] shows that in the Mengong catchment clear waters come not only from springs, but also from hill or slope water tables and from the depressional deep water table, whereas colored waters, constituting the river waters, are also found in the surficial depression water table. The major role of dissolved organic matter in the chemical weathering was demonstrated by [START_REF] Viers | Chemical weathering in the drainage basin of a tropical watershed (Nsimi-Zoetele site, Cameroon): Comparison between organicpoor and organic-rich waters[END_REF] and [START_REF] Oliva | The effect of organic matter on chemical weathering: Study of a small tropical watershed, Nsimi Zoetele, Cameroon[END_REF] in this catchment, but also by [START_REF] Idir | Contribution des surfaces saturées et des versants aux flux d'eau et d'éléments exportés en période de crue: Traçage à l'aide du carbone organique dissous et de la silice. Cas du petit bassin versant du Strengbach (Vosges, France)[END_REF] in the Strengbach catchment in the Vosges Mountains (France). Organic acids, abundant in the swampy depression waters, are responsible for an enhanced dissolution of soil minerals and for the transport in a complexed form of insoluble metallic elements.

The Important Contribution of DOC Fluxes

In terms of specific fluxes in river waters, Table 18.2 shows that the DOC values are comparable to those of inorganic TDS (in which dissolved silica flux represents 39 to 52% according to the stations), whereas these DOC fluxes are one to two times higher than TSS fluxes. Considering that POC averages 20% of TSS, the mean annual flux of total organic carbon (DOC + POC) represents 38 to 48% of the total inorganic and organic materials (TDS + DOC + TSS) exported by the river at different stations; these percentages correspond to (DOC + POC) specific fluxes between 5300 and 10,100 kg km -2 yr -1 . Present results are comparable to those of [START_REF] Sigha-Nkamdjou | Bilans hydrologique et géochimique d'un écosystème forestier équatorial de l'Afrique Centrale: La Ngoko à Mouloundou[END_REF] for other forested southern Cameroon catchments (Ntem, Kadéi, Boumba, Dja, Ngoko), where the calculated fluxes were between 3300 and 5300 kg km -2 yr -1 for DOC, and between 600 and 1200 kg km -2 yr -1 for POC. For savannah Cameroonian basins (Sanaga, Mbam), [START_REF] Ndam-Ngoupayou | Bilans hydrogéochimiques sous forêt tropicale humide en Afrique: Du bassin expérimental de Nsimi-Zoétélé aux réseaux hydrographiques du Nyong et de la Sanaga (Sud-Cameroun)[END_REF] reported lower values for DOC (~1500 kg km -2 yr -1 ) due to lower vegetation biomass and to lower drainage intensity. In contrast, the POC fluxes are rather high (1000 kg km -2 yr -1 for the Sanaga, 4200 kg km -2 yr -1 for the Mbam), probably due to a notable TSS transfer (18 and 98 Mg km -2 yr -1 , respectively). A comparison between the DOC fluxes exported on the Niger basin: 593 kg km -2 yr -1 at Lokodja [START_REF] Martins | Biogeochemistry of major African rivers: Carbon and mineral transport[END_REF], 455 kg km -2 yr -1 at Bamako [START_REF] Boeglin | Transports fluviaux de matières dissoutes et particulaires sur un bassin versant en région tropicale: Le bassin amont du Niger au cours de la période 1990-1993[END_REF], and on the Congo basin: 2.9 Mg km -2 yr -1 (Nkounkou and Probst 1986) or 3.1 Mg km -2 yr -1 [START_REF] Seyler | Concentrations, fluctuations saisonnières et flux de carbone dans le bassin du Congo[END_REF], is a significant illustration of the influence of the vegetation type on the organic C solubilization. [START_REF] Ludwig | Predicting the oceanic input of organic carbon by continental erosion[END_REF] proposed a mean DOC flux of 1043 kg km -2 yr -1 in the dry tropical zone, and of 3818 kg km -2 yr -1 in the humid tropical zone.

Low CO 2 Uptake by Silicate Weathering

Concerning the dissolved inorganic C, the specific fluxes of CO 2 used in chemical weathering on the silicated Nyong watersheds are between 22 and 27*10 3 mol km -2 yr -1 , which are equivalent to 1000 to 1200 kgCO 2 km -2 yr -1 . These values are low compared with those obtained on other lateritic basins, 30 to 120*10 3 mol km -2 yr -1 [START_REF] Mortatti | Hydrological and geochemical characteristics of the Jamari and Jiparana river basins (Rondonia, Brazil)[END_REF][START_REF] Probst | Carbon river fluxes and global weathering CO 2 consumption in the Congo and Amazon river basins[END_REF][START_REF] Boeglin | Physical and chemical weathering rates and CO 2 consumption in a tropical lateritic environment: the upper Niger basin[END_REF], except the one determined for the Senegal River whose watershed has a very low runoff Q (F CO2 = 17*10 3 mol km -2 yr -1 for Q = 45 mm yr -1 ). Considering only the part of F CO2 used in the silicate weathering, [START_REF] Boeglin | Physical and chemical weathering rates and CO 2 consumption in a tropical lateritic environment: the upper Niger basin[END_REF] demonstrated that, contrary to common belief, the F CO2 corresponding to lateritic basins -where the chemical weathering is supposedly very intense -are about two times lower than for the nonlateritic watersheds. In the case of the southern Cameroon catchments, the F CO2 values are three times lower than those previously calculated for the lateritic basins (Figure 18.3). In the case of the Nyong catchments, especially low F CO2 are attributed to the thickness of the lateritic cover on hills and slopes, which considerably slows the advance of percolating waters to the bedrock. This hypothesis is confirmed by the low mineral dissolved load in the clear waters and in the colored waters [START_REF] Braun | Biogeohydrodynamic in the forested humid tropical environment: the case study of the Nsimi experimental watershed (South Cameroon)[END_REF]. The low chemical weathering intensity indicated by these weak F CO2 values is confirmed by the calculation done using the dissolved silica fluxes in the different upper Nyong catchments, which are low compared to the annual rainfall (see [START_REF] White | Effects of climate on chemical weathering in watersheds[END_REF]. Supposing that 35% of the quartz of the bedrock remains in the saprolite, the weathering rate determined from the method proposed by [START_REF] Boeglin | Physical and chemical weathering rates and CO 2 consumption in a tropical lateritic environment: the upper Niger basin[END_REF] at Olama station averages 3.2 mm/1000yr under present conditions, which is low compared to values generally obtained for tropical basins (6-15 mm/1000 yr; Nkoukou and Probst 1987;[START_REF] Tardy | Petrologie des latérites et des sols tropicaux[END_REF][START_REF] Mortatti | Silicate rock weathering and atmospheric/soil CO 2 uptake in the Amazon basin estimated from the river water geochemistry: Seasonal and spatial variations[END_REF]. However, the weathering rate of 2.8 mm/1000 yr was reported by [START_REF] Seyler | Hydrochemistry of the Ngoko River, Cameroon: Chemical balances in a rain forest equatorial basin[END_REF] for the equatorial Ngoko basin in southeastern Cameroon.

Chemical Properties of the DOC and Transport Capacity of Trace Elements

The high correlation obtained for all stations of the Nyong basin between the anionic deficit and the DOC content indicates that the presence of organic anions compensates for the lack of negative mineral charges as was previously reported by [START_REF] Sullivan | Influence of aqueous aluminium and organic acids on measurements of Acid Neutralizing Capacity in surface waters[END_REF] and Munson and Guerini (1993). The specific charge of the organic matter (in µeq mg -1 DOC), given by the slope of the regression line, differs among stations: 7.2 for the Mengong, 9.1 for the Awout at Messam, 5.3 for the So'o at Pont So'o, 4.6 for the upper Nyong at Mbalmayo, and 4.3 for Nyong at Olama. The decrease of this specific charge from upstream to downstream (with a particularly high value for the Awout stream) may be attributed to a progressive structural modification of the organic compounds in the river waters. For example, the length of the water course from Nsimi to Olama (~100 km) is about 3 to 6 days. That means that the capacity of the dissolved organic matter to complex the cations, particularly the heavy metals, decreases from upstream to downstream, thereby increasing the transport of heavy metals by the suspended matters.

Global Carbon Budget at the Mengong Catchment Scale

The amount of C transfer for the Mengong catchment is shown in Figure 18.4. Atmospheric inputs, essentially constituted by rainfalls, is a variable C content: mean DOC is 0.61 mg L -1 in open field precipitations and 3.6 mg L -1 in throughfalls [START_REF] Ndam-Ngoupayou | Bilans hydrogéochimiques sous forêt tropicale humide en Afrique: Du bassin expérimental de Nsimi-Zoétélé aux réseaux hydrographiques du Nyong et de la Sanaga (Sud-Cameroun)[END_REF]. In comparison, Freydier et al. (2002) determined a mean total organic C concentration (including fine suspended particles resulting from the combustion of biomass) of 2.5 mg L -1 in open field precipitations. However, these values were obtained from a low number of samples that did not cover the entire hydrologic year. Considering an annual rainfall of 1800 mm with a mean carbon content ranging from 1 to 3 mg L -1 , the corresponding input is 1.8 to 5.4 MgC km -2 yr -1 .

The organic C stock in the soil of the Mengong catchment was estimated in this study at 59,000 Mg km -2 ; however, only 26% of this amount of C is concentrated in the two upper meters of the profiles and can be drained by the surface and subsurface runoff.

Litter fall measurements made by Odigui Ahana (2000) in an experimental site at 100 km north of the Mengong catchment showed an annual value of 10.75 Mg of total wet organic matter with a mean C content of 38%, which is equivalent to 4100 MgC km -2 yr -1 . For biomass [START_REF] Likens | Biochemistry of a Forested Ecosystem[END_REF] indicated a mean density of 450,000 Mg km -2 with an annual production of 22,000 t km -2 for the forested tropical area.

In such a lithological environment without any carbonate rocks, the riverine inorganic C is only found under dissolved inorganic C, there is no particulate inorganic C. In the Mengong catchment, the DIC riverine specific flux has been estimated from the alkalinity to 320 kgC km -2 yr -1 , corresponding to the same amount of atmospheric/soil CO 2 consumed by the chemical weathering of the silicate minerals.

Concerning the organic C, the riverine fluxes have been estimated for the Mengong catchment to 5700 and 575 kgC km -2 yr -1 respectively for the DOC and the POC. With the exception of Messam on the Awout brook, the DOC and DIC specific fluxes are comparable from one station to another over the Nyong River basin, indicating that the transfer dynamic of dissolved C does not change from upstream to downstream of the Nyong River basin, which presents the same type of lithology and vegetation covers, irrespective of the subcatchment size.

The residence time (R t ) of carbon in the Mengong catchment soils can be estimated from the total carbon stock (S OC ) in the soil cover (35,000 t), and from the annual fluxes of riverine carbon {F DOC (3400 kg yr -1 ) + F POC (345 kg yr -1 ) + F DIC (190 kg yr -1 )}: R t = S OC /(F DOC + F POC + F DIC ) (18.4)

The residence time of 9000 yr indicates a rather slow turn over of the C in this catchment under present conditions. That means also that the riverine C fluxes represent only 0.1% of the soil C stock. This value is of the same order of magnitude as the global estimate (0.2%) from the data of [START_REF] Meybeck | Carbon, nitrogen and phosphorus transport by world rivers[END_REF]. However, if one considers that the reactive and mobilizable C is essentially contained in the upper part of the profiles (stock = 9200 Mg), or only in the hydromorphic depression soils (stock = 3700 Mg), the residence time notably decreases to 2350 yr and 950 yr, thereby increasing also the percentage of C exportation respectively to 0.4% and 1%, respectively.

CONCLUSION

Analyses of the different forms of inorganic and organic C in the soils and in the waters for silicated catchments of the southern Cameroon forested area provide information on the transfer dynamic and on the mass balance for a tropical humid environment.

• The upper horizons of the soil and the corresponding draining waters exhibit rather high concentrations of organic C. Considering the soil cover containing a specific C stock of 59,000 Mg km -2 , about 26% of this amount is concentrated in the upper 2 m of the soil profiles (mean thickness = 30 m). In colored river and swamp waters, DOC fluxes are between 4700 and 9300 kg km -2 yr -1 (which is comparable to inorganic TDS fluxes), whereas POC fluxes (representing about 20% of TSS) are between 600 and 1400 kg km -2 yr -1 . In contrast, clear spring waters or ground waters contain low DOC contents. • In such a noncarbonated environment, the DIC fluxes are normally low. Whereas for the Nyong River basin, these fluxes (about 300 kgC km -2 yr -1 ) are lower than for other tropical silicated catchments. This can be related mainly to the low chemical weathering rate of silicate rocks estimated to be 3.2 mm/1000yr for the upper Nyong basin at Olama. Such a weak weathering rate is explained by the thickness of the soil cover (until 50 m), insulating the bedrock from the interactions with the surficial draining waters. • The turn over of the C (organic + inorganic) in the Mengong catchment is estimated to 9000 yr if one takes into account the whole C stock of the soil cover. However, considering that only the C contained in the hydromorphic soil horizons of the swampy zone is involved in the exchange and recycling processes, the residence time decreases to 950 yr. • The high anionic deficit (44-84%) observed in all drainage waters is strongly correlated with the DOC content. The specific charge of the dissolved organic matter is estimated to 4.3 to 9.1 µeq mg -1 C. This specific charge decreases from upstream to downstream in the Nyong River basin, which indicates a progressive structural modification of the dissolved organic species during the transfer, reflecting a decreasing capacity of trace element complexation and transport.

• Values of C specific fluxes (DIC and DOC) obtained at different stations of the Nyong network are rather close. On the contrary, the POC specific fluxes are variable -depending on the TSS flux -from one station to another. However, the POC/TSS ratio is slightly variable and is about 20%. These results are attributed to a comparable C dynamic at the scale of the Nyong River basin, independently of the surface area of the different subcatchments.

Figure 18 . 1

 181 Figure 18.1 Location map of the Nyong basin (southern Cameroon); the studied stations are indicated by black circles.

Figure 18 . 2

 182 Figure 18.2 Relationship between the anionic deficit and the DOC content in the Mengong River during the year 1998-99. DOC content (mg/L)

Figure 18 . 3

 183 Figure 18.3 Relationship between annual CO 2 fluxes consumed by silicate weathering and mean annual runoff for rivers of different areas in the world (see Boeglin and Probst 1998, for the list of river basins).

Figure 18 . 4

 184 Figure 18.4 Carbon cycle and annual budget on the experimental Mengong catchment.

Table 18 .1 Organic Carbon Content and Stock in Different Soil Horizons of the Mengong Catchment

 18

			T	V			OC	S OC	S OC
	Horizon	A %	(m)	(10 3 .m 3 )	V %	AD	%	(Mg)	%
	Lateritic Profile (

with overlay on hills and slopes) from Top to Depth

	Upper overlay	80	0.2	96	0.6	1.30	1.5	1,870	5.3
	Overlay	80	2	960	5.6	1.28	0.30	3,690	10.5
	Ferricrete/nodular	80	2	960	5.6	1.66	0.15	2,390	6.8
	Mottled clays	99	5	2,970	17.4	1.29	0.15	5,750	16.3
	Saprolite	99	20	11,900	69.7	1.50	0.10	17,850	50.7
	Hydromorphic Soils of the Depression (covering truncated mottled clays) from Top to Depth
	Organo-mineral	15	0.5	45	0.3	1.44	4.0	2,590	7.3
	Colluvial	15	1.5	135	0.8	1.62	0.50	1,090	3.1
	Total catchment	-	-	17,066	100	-	-	35,230	100

Table 18 .2 Hydrological Characteristics of the Nyong Catchment Network and Average Values for Some Physicochemical Parameters Measured during the Year 1998/1999 in the Spring and River Waters

 18

				DIC		TDS	DOC	TSS	FC
	Station	A	Qm	C	F	C	F	C	F	C	F	O2
				Mengong Experimental Catchment
	Spring	-	-	1.05	-	13.0	-	0.31	-	-	-	-
	Stream	0.60	387	0.94	322	16.8	6200	14.3	5677	8.1	2873	26.8
		Nyong River Stations from Upstream to Downstream
	Messam	206	431	0.84	292	21.7	7570	22.9	9304	13.9	4112	24.4
	Pont So'o	3,070	370	1.24	321	23.4	7244	15.1	6315	20.5	6966	26.7
	Mbalmayo	13,555	257	1.53	269	24.7	5027	15.9	4670	12.8	3237	22.4
	Olama	18,510	283	1.29	295	22.3	5664	15.6	4849	14.3	4488	24.6

ACKNOWLEDGMENTS

The authors would like to thank D. Million and G. Krempp from the Centre de Géochimie de la Surface in Strasbourg for chemical analyses of river waters, J. P. Bedimo-Bedimo and J. Nlozoa from the Hydrological Research Center of Yaoundé for hydrological measurements and field assistance. This study has been realized with the financial support of a CAMPUS/CORUS cooperation project no. 99.0026 (04) between the University Paul Sabatier in Toulouse and the University of Yaoundé I. This work is also a contribution to the IGCP project no. 459 (UNESCO-IUGS) on "Carbon Cycle and Hydrology" and to DYLAT project of IRD.