Variation of Soils Erodibility in Mbe Agropastoral Area in Relation with Land Utilization, Central Cameroon

The study of the soils from Mbé and Wack is carried out in the framework of the knowledge of soils from the Adamawa Region of Cameroon and their erodibility was investigated using erodibility indices obtained through physi-co-chemical data. Eleven topsoils (0 - 20 cm) samples were collected on dif-ferent land use and their susceptibility to erosion was assessed. The water dispersible clay (34.92 - 121.75 g∙kg −1 ), the clay dispersion ratio (0.45 - 0.84) and the dispersion ratio (0.75 - 0.89) were high in the studied soils while the clay aggregation (13.16 - 42.27g∙kg −1 ) and the clay flocculation index (0.16 -0.55) were low to moderate indicating their high erodibility. The soils under natural vegetation, more clayey, displayed the highest amount of water dispersible clay while cropped soils recorded the smaller ones. Globally, in cropped soils, those under cereals displayed the highest clay dispersion indices than those under tubers. This suggests that tubers cropping practices in studied soils enhance their erodibility. Statistical analyses revealed that amorphous Al and Fe are elements which limit soils erodibility while K + and 4 NH + promote soils particles dispersion. Sustainable management of these soils will consist on limiting runoff through agricultural practices such as di-rect seedling and orienting tillage perpendicularly to slope gradient.

. Location map of Mbé and Wack.
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).

Laboratory Methods
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 Open Journal of Soil Science

Data Analyses
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: Clay floculation index: Clay dispersion ratio: 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.

Variation of Soil Properties According Land Use
Specific properties of the soils and their classification are shown in Table 2 and       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 Open Journal of Soil Science 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).

Total Clay (TC), Water Dispersible Clay (WDC), Clay Dispersion Ratio (CDR) and Soil Erodibility
The total clay (TC) of the studied soils varies from 65. 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.   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), Clay Flocculation Index (CFI) and Soil Erodibility
The clay aggregation (CA) ranged between 13. 16 (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 Open Journal of Soil Science 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.

Relationships between Erodibility Indexes and Soils Properties
Correlations between erodibility indexes and soil properties are presented in Table 7.  (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].  (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  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]

Conclusion
The study shows that soils of Mbé and Wack are sandy-loam and gravelly. They