The study was conducted in Mbe subdivision mainly at Wack village and Mbe during dry season (Figure 1). The soudanoguinean climate with dry season from November to March and wet season from April to October prevails in the study area. The mean annual rainfall is 1500 - 2000 mm in this transition zone between the subhumid and dry climates. The mean annual temperature is 22˚C. However, the minimum temperature is between 10˚C - 19˚C and it occurs during December and January. Then a maximum temperature is between 27˚C - 34˚C in March.

Wack and Mbé repose respectively on granites of Precambrian and gneiss from upper Cretaceous and Tertiary formations. The soils are classified as oxisols and ultisols [10]. These soils were mainly sandy and display a low exchangeable cations and exchangeable cations capacity (CEC). Wack and Mbé are located in the North of the Adamawa plateau with 1200 m altitude [11]. Most of the lands are between 500 m and 1100 m above the sea level (Figure 1). A positive correlation between relief and soil erosion by water in the zone is widely observed [12].

Soils samples were collected from topsoil (0 - 25 cm depth). Five soil samples were collected in Mbé on a gentle slope and six samples were collected in two toposequences with three samples each one in Wack (Figure 1). In each series, samples were collected according to land uses and position on the slope (Table 1).

The sampled soils which were air-dried and sieved to pass through a 2 mm were analysed at the Institute of Science of the University of Leibniz, Hanover in Germany.

Particle size distribution was determined by the pipette method after dispersion with Na-hexametaphosphate and organic matter destruction by hydrogen peroxide and deferritisation with hydrochloric acid followed by 16 h of mechanical agitation using an end-over-end shaker. Soil pH water was measured

with a pH meter equipped with a glass electrode in 2/5 soil-water suspensions. The pHCaCl₂ was determined in 2/5 soil-water suspensions + CaCl₂. Organic carbon (OC), nitrogen (N) and sulphur (S) were determined by CHNS-O method (Vario EL III). The percentage of organic matter (OM) was calculated by multiplying the OC value by 1.724 and to determine the rate of OM degradation, OC/N was calculated by dividing OC by N contents. Exchangeable cations (Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺) were determined by extraction with silver thiourea and cation exchange capacity (CEC) was measured at soil pH by performing the Na-thiourea method. The percentage of base saturation was calculated as (TEB/CEC)*100. Amorphous and crystalline Fe and Al in the soil were extracted using ammonium oxalate and citrate-bicarbonate-dithionate (CBD). Soluble cations and anions were extracted with deionised water (1:10) and detected by Inductively Coupled Plasma (ICP-AES) and anions chromatography (DIONEX ICS-90).

The water dispersible particles were determined followed the same method as particles size distribution described above except that no chemical dispersant was used and without organic matter destruction. The erodibility indices are calculated as follow [13] [14]:

Clay aggregation: C A = T C ( g ⋅ kg − 1 ) − W D C ( g ⋅ kg − 1 ) (1)

Clay floculation index: C F I = T C ( g ⋅ kg − 1 ) − W D C ( g ⋅ kg − 1 ) T C ( g ⋅ kg − 1 ) (2)

Clay dispersion ratio: C D R = W D C ( g ⋅ kg − 1 ) T C ( g ⋅ kg − 1 ) (3) [15]

Dispersion ratio: D R = W D C ( g ⋅ kg − 1 ) + W D S ( g ⋅ kg − 1 ) T C ( g ⋅ kg − 1 ) + T S ( g ⋅ kg − 1 ) (4) [16]

Exchangeable sodium percentage: E S P = exchangeable Na + ∗ 100 C E C (5) [17]

Exchangeable sodium ratio E S R = exchangeable Na + exchangeable Ca + + Mg + + K + (6)

Total clay (TC) and total silt (TS) are clay fraction and silt fraction obtained by chemical dispersion. WDC is water dispersible clay and WDS is water dispersible silt.

The obtained data were been subjected statistically to simple correlation and regression analysis to determine the extent of relationships between soils parameters and their contribution to clay dispersion.

Specific properties of the soils and their classification are shown in Table 2 and Table 3.

The total clay (TC) varies from 6.54% (MCY) to 15.03% (MNV). The soils from Mbé and Wack are sandy loam (SL) (Table 2). The soil under native vegetation (MNV) was most clayey; displayed the highest pH (pH water = 6.68, pH CaCl₂ = 5.70), organic components and amorphous and crystalline elements (Table 3). As in Mbé, the soil on natural vegetation (WNV) from Wack, located at upslope presents the highest Fe content while the soil from the footslope (WCS) is the most clayey (14.74% of clay) with higher organic matter content (1.88%) and the highest Al content. However, the studied soils under yams (MCY, WCY) and cassava (MCC, WCC) both at Mbé and Wack are the sandiest, more acidic, poorest in organic components (Table 2), Fe and Al (Table 3).

The studied soils have low content in exchangeable cations Ca²⁺ (9.214 - 53.473 mmolc∙kg⁻¹), Mg²⁺ (2.949 - 20.422 mmolc∙kg⁻¹), K⁺ (0.813 - 5.677 mmolc∙kg⁻¹), Na⁺ (0.065 - 0.457 mmolc∙kg⁻¹), Al³⁺ (0 - 1.201 mmolc∙kg⁻¹), CEC (22.3 - 118.57 mmolc∙kg⁻¹) and TEB/CEC (0.592 - 0.7753) which vary in conformity with texture (SL) and pH (Table 4). The most clayey soils (MNV, WCM, and WCS), less acidic, richer in N and OM, have the highest exchangeable cations concentration and poorest in Al³⁺ (responsible for exchange acidity) than the sandiest soils (MCY, MCC, WCY, and WCC) which are the more acidic, more rich in Al³⁺ and poorest in the exchangeable cations (Table 4).

The soils soluble ions concentration are less than 1 mmolc∙kg⁻¹ (0 - 0.6 mmolc∙kg⁻¹) except K⁺ (0.27 - 2.54 mmolc∙kg⁻¹) and Ca²⁺ (0.47 - 1.58 mmolc∙kg⁻¹) (Table 5). They also vary in accordance with textural class and land use (cereals, tubers). Their variation and those of electric conductivity (6.9 - 24.7 µS∙cm⁻¹) and exchangeable cations were similar. MNV recorded the highest values of

SL = sandy-loamy.

ox = oxalate, dith = dithionite, d = ox + dith.

TEB = total exchangeable bases.

divalent cations (Ca²⁺, Mg²⁺) and EC. MCC and WCC are the poorer in sulphate, phosphate, nitrate, ammoniac, chloride and sodium (Table 5).

The studied soils are acidic (pH < 7). C/N values which is ranged between 12 and 25 reveals that OM is less matured. Despite of their position, the most clayey samples (MNV, WCS) content approximately the double of clay fraction than sandiest samples under tubers (MCC, MCY, WCC, WCY) mainly located at footslope.

The sandy-loam soils of Mbé and Wack show that real and potential acidities were proportional to clay or sand contents. More a soil is sandy, more it is acidic. Then, from the more acidic or sandy to the less acidic (more clayey) in Mbé, we noted MCY, MCF, MCC, MCM (footslope), MNV (topslope); in Wack, there is, WCY, WNV, WCC, WCM, WCS, WCF (Table 2).

Disappearance of organic matter due to erosion and oxidation degrade soils and their agricultural potential aptitudes. Reduction of organic matter weakens organo-minerals stability which can cause soil impermeability by crusting increasing streaming and erosion. In addition, cultivations weaken soils through yielding by nutrients exportation and non-restitution of organic matter to soils. Organic matter plays an important role in soil aggregation and structuring and it informs on soil potential fertility. MCM has lowest organic matter content because cereal crops are generally used as substitute crops to yams. However, MNV recorded higher organic matter elements in spite of his topslope position because that it never ploughed and plant cover favours soil structural stability. But soil under tubers crops are impoverished by plants needs and more by ploughing and ridge.

Globally, cultivated soils are more acidic and sandy. However, those which are under tubers crops (yam, Cassava) are more acidic than those under cereals (maize, millet). The soils are low in crystalline Fe_ox and Al_ox, amorphous Fe_di and Al_di which also vary with regard to texture and land use (Table 3). MNV is naturally richer in Fe which probably derived from rock (gneiss) weathering but WCS recorded more Al because of alteration, agricultural input and position on footslope. The CEC vary similarly to the exchangeable cations contents in the soils and in conformity with the clay ratio. Soils under yams culture are the less saturated and cultivated soils are more saturated than soils never cultivated (Table 4).

The total clay (TC) of the studied soils varies from 65.4 g∙kg⁻¹ (MCY) to 150.27 g∙kg⁻¹ (MNV) while the WDC ranges between 34.92 g∙kg⁻¹ (MCY) and 121.75 g∙kg⁻¹ (MNV). The WDC was globally high and varied with regard to land use. It is noted that the not cropped soil mainly located at the upslope (MNV) recorded the highest WDC (Table 6). The CDR ranged between 0.45 and 0.84 g∙kg⁻¹.

In Mbé, the soil under natural vegetation (MNV) at topslope recorded the highest TC (150.27 g∙kg⁻¹), WDC (121.75 g∙kg⁻¹), CDR (0.81) while MCY (located at midslope) presented the lowest TC (65.4 g∙kg⁻¹), WDC (47.46 g∙kg⁻¹) and CDR (0.73) (Table 6). In Wack, it is WCS (on footslope) which presents the highest value of TC (147.35 g∙kg⁻¹) and WDC (112.49 g∙kg⁻¹). WCY at midslope under yam culture display slow TC (77.19 g∙kg⁻¹), WDC (34.92 g∙kg⁻¹) and CDR (0.45). However, in the first sequence, WCM (on topslope) has the highest TC (90.4 g∙kg⁻¹), WDC (50.90 g∙kg⁻¹), CDR (0.56) and WCY (on midslope) has the lowest ones. In the second sequence at Wack, WCS (on footslope) recorded the highest TC (147.35 g∙kg⁻¹) and WDC (112.49 g∙kg⁻¹) but the lowest CDR (0.76). WCC (on topslope) presents the lowest TC (81.39 g∙kg⁻¹), WDC (68.23 g∙kg⁻¹) and the highest CDR (0.84) (Table 6).

The WDC and CDR are good estimators of soil susceptibility to water erosion. They express an ability of clays particles from soils to be eroded by water. Higher WDC and CDR mean high soil susceptibility to erosion [7] [18] [19] [20]. Hence, according WDC values, the not cropped soils both on topslope at Mbe is the more susceptible to erosion while those from Wack at footslope is the less erodible. This means that the slope has impact on soil susceptibility to erosion. It is also noted that WDC and TC were good estimators of soils erodibility [3] [6] [7] [8]. In fact, as observed, soils which are more clayey and those on topslope are more erodible than soils with less clays content. But those on midslope are generally more stable and show low CDR and WDC values.

In Mbé, the DR varied from 0.85 to 0.89; ESP ranged from 0.22 to 0.83 and ESR varied from 0.001 to 0.017. MCY (on midleslope) has a lowest DR (0.85) and a highest ESR (0.01). Then, MCC (on midleslope) has the highest DR (0.89), and lowest ESP (0.24), ESR (0.003). In Wack, the DR varied from 0.75 to 0.87. In Wack, in the first sequence, WCM (on topslope) has highest DR (0.88) and

lowest ESP (0.22). Then, WCY (on midleslope) has highest ESP (0.83), ESR (0.01). In the second sequence, WCS (on footslope) has highest ESP (0.44), ESR (0.017) and lowest DR (0.85). While, WCC (on topslope) presents lowest ESP (0.23), ESR (0.004) and highest DR (0.89) values (Table 6). The DR is the ability of clay and silt to be dispersed by water. So, high DR means high susceptibility of the soil to erosion because of disorganization of their structure which facilitates the mobilization of fine particles [19]. However, ESP and ESR vary opposite to DR. This means that, Na⁺ do not plays it dispersive role [7], because of very low contents in the soil.

The clay aggregation (CA) ranged between 13.16 and 42.27 g∙kg⁻¹ while the CFI varied between 0.16 and 0.55. In Mbé, the more clayey soil (MNV) at topslope presents the highest CA (28.52 g∙kg⁻¹) and the low CFI (0.19) than MCY (on midslope) which presents the highest CFI (0.27). However, in Wack, it is WCS (on footslope) that presents the highest CA (34.86 g∙kg⁻¹), CFI (0.24) and the highest TC (147.35 g∙kg⁻¹) and WDC (112.49 g∙kg⁻¹) while, in the second toposequence, WCY (on midleslope) presents the highest CA (42.27 g∙kg⁻¹) and CFI (0.55) in accordance with its lowest TC (77.19 g∙kg⁻¹) and WDC (34.92 g∙kg⁻¹) (Table 6).

In Mbé, soil with higher clays content and not cropped (MNV on topslope) is more erodible than degraded soils with high values of sand. In Wack, on each toposequence, there are cultivated soils (WCM on topslope and WCS on footslope) more clayey, which are more erodible. However, in spite of their position, soils under culture of tubers (MCY and WCY on middle slope, WCC on topslope) are more erodible. Thus, contrary to land use, the gentle slope not really influences soils susceptibility to erosion. The clay aggregation is very low at upslope (30 g∙kg⁻¹) while in the middle and lower slopes these values are relatively high and similar. As CA, CFI are lowest in upslope soils. The CA and CFI is indicative of the ability of soils particles to be aggregated and flocculated or more stable. Higher CA or CFI means higher soil stability and thus lower erodibility. Thus, as indicated by WDC, CDR and DR, in this study, CA and CFI shows that: soils located at the up part of the toposequence are more erodible and the cropped soils located at the up part of the toposequence are more erodible and less stable than not cropped soils suggesting that agricultural practices and slope gradient increase their erodibility [19]. [19] Obtained similar results in the irrigated and flooded vertisols from the sudano-sahelian part of Cameroon.

Correlations between erodibility indexes and soil properties are presented in Table 7. The TC shows significant positive correlation with WDC (r = 0.94), pH water (r = 0.7), organic components (r = 0.6 - 0.75), amorphous Fe_dith (r = 0.9), amorphous Al_dith (r = 0.85), exchangeable Ca (r = 0.8), Mg (r = 0.8) and CEC (r = 0.8) (Table 7). The CA is positively correlated with CFI (r = 0.79), crystalline Fe_ox (0.7) and crystalline Al_ox (r = 0.76). Also, CDR is positively correlated with DR (0.78), WDC (0.7), ESP, Na and Al (Table 7). However, there is significant negative correlation between WDC and CFI (r = –0.7); CA and CDR (r = –0.79); CFI and CDR (r = –1.0) and DR (r = –0.78); CDR and Fe_ox (r = –0.7) and Al (r = –0.75); DR and Fe_ox (r = –0.77), Al (r = –0.87). ESP and DR are also negatively correlated (r = –0.8) (Table 7).

Generally, OM is cementing particles agent. It has a capacity to bind mineral particles together developing soil structure. Intense tillage degrade soil structure and contribute to decrease OM content which holds particles together, enabling the surface soil to resist to the detachment forces of raindrop and flood [19]. In the current study, OM and sulphur show positive but not significant correlation with CA (Table 7). Despite its low content, it contribution on reducing clay dispersion can be important [3] [19].

Fe and Al can be flocculants by establishing linkage between clays and organic polymers or by acting as cement like gel on clayey particles surfaces. They increase cohesion [21] [22] [23] which contribute to reduce soils erodibility. Fe and Al (responsible for soils acidity) correlated positively with TC, WDC, CA and CFI. The CFI which express the stability of the soils shows negative correlation with WDC, CDR and DR (Table 7). This confirms that flocculation of clays is opposed to their dispersion. The significant positive correlation between CFI and CA means that clay flocculation leads to clay aggregation. So, this last results from the rearrangement of particles through flocculation and cementation [24]. The positive correlation between amorphous Al and Fe with WDC, CA, CFI and negative correlation with CDR and DR show that Al_ox and Fe_ox contributes to soil aggregation (Table 7). Exchangeables Ca, Mg, Na and Al are positively

correlated to CA (Table 7). They have stabilizing and aggregating effect. Na is not dispersive agent as usual. Then, K negatively correlated to CFI and positively to CDR and DR is dispersive agent (Table 7) [7] [13] [19].

Despite Na⁺ is positively correlated to ESP, globally, it is no significant between erodibility indexes and soluble ions and EC because their low contents in these soils. These elements are washed out or lixiviate due to agricultural practices (Table 7). Clay dispersion normally increases with exchangeable Na⁺ (ESP) content [25]. However, [26] postulate that clay dispersion was affected by increasing of exchangeable Na⁺. In this study, K⁺ and NH 4 + are positively correlated to CDR and DR, but Na⁺ and NO 3 − are positively correlated to CA and CFI. So, K⁺ and NH 4 + are dispersive then Na⁺ and NO 3 − are aggregative. But Na and EC not significantly correlated to erodibility indexes (Table 7). Hence, they are not more influencing clay dispersion.

In cultivated soils, the matrix and structure of the soils is regularly disorganized by labour. Conventional tillage which inverts soil is generally accompanied by a loss of organic matter which binds soils particles together. Then soils cultivated by tubers (yams, cassava) are less erodible than those under cereals (maize, sorghum). Finally, cultural practices influence stability of aggregates and carbon ratio of the soils [9] [27] [28].

Considering obtained data and correlations coefficient, clay dispersion of sandy soils of Mbé and Wack is mostly caused by soil utilization. Soils under tubers culture (Yam, Cassava) are cultivated during six months (august to February); they are the most acidic, less fertile, less saturated and most erodible. Thus, sandier soil is less erodible due to its heavy coarse materials which are low soluble and which cannot constitute suspension that can be easily mobilized. Consequently, most sandy (cultivated) soils that are already eroded are less erodible than the more clayey (never cultivated) soils. Soils under tubers culture are poorer in clay than soils under cereals (maize, millet) sometimes richer than never cultivated soils.