The watershed of Kpacomey is located in the district of Aplahoué. With a surface area of 82,585 km², the Kpacomey watershed is between 6˚48'00"N-7˚0'0''N and 1˚39'00"E-1˚42'0''E (Figure 1). The watershed is characterized by a four-season sub-equatorial climate (two dry and two wet seasons) and annual rainfall varies from 900 to 1100 mm/year. Being part of the plateau of Adja, thewatershed of Kpacomey has an average altitude of 80 m and slopes slightly southward where it connects sensitively to the median “Tchi” depression. Ferrallitic soils with low desaturation in (B) are the dominant soils of the Kpacomey watershed which are subject to strong agro-fertilization pressure due to their degradation and nutrient depletion. Moreover, this degradation is due to the topography and the different cropping systems that are adopted and practiced on this type of soil. The vegetation cover of the Kpacomey watershed is being reduced more and more to artificial vegetation and is composed of a few forest islands in the form of fetish or sacred forests and Saxicolian forests, but of very small dimensions. The watershed of Kpacomey is drained by a permanent stream called Kpaco.

The first factor of this study is “cropping systems” with four modalities: Maize-Cassava system, pure Palm grove system, Palmgrove-Cassava system and Teak Plantation system. The “slope” is the second factor of this study with three modalities: Low Slope, Medium Slope and High Slope. The slope steepness in degrees was measured with a SUUNTO type clinometer at each sampling point on each transect. From these measurements, three classes of slope in degrees were identified: <5˚; 5˚ to 10˚ and >10˚ and were assigned to the Low Slope, Medium Slope and High Slope modalities.

All cropping systems are rainfed. In the Maize-cassava cropping system, cassava is generally planted in association with sown maize (direct sowing after no-tillage soil preparation) about two weeks earlier. After the maize harvest, cassava occupies the field for the rest of its growth cycle. Cassava root harvesting can start from the sixth to the twelfth month after planting depending on the variety. For the Pure Palm Grove cropping system, the palm trees are planted 10 years ago. The teak plantations are around five years-old and are installed for timber production. The effect of cropping systems and slope on the soil degradation in the Kpacomey watershed was assessed using the approach based on the physical and chemical characteristics of the soil. To achieve this goal, soil samples were collected along three parallel transects in the slope direction (Figure 2).

Transects are 30 meters from each other. On each transect, soil samples were taken every 20 meters using a soil sampler in the depth of 30 cm. The number of soil samples that were collected per transect is a function of the length of the transect and the occupancy of the cropping systems. A total of 51 soil samples were collected: 20 soil samples on transect 1, 18 soil samples on transect 2 and 13 soil samples on transect 3. The repartition on the number of sampling taken by cropping is as follow: 16 in the Maize-Cassava system, 14 samples in the pure Palm grove system, 11 in the Palm grove-Cassava system and 10 in the Teak Plantation system. As for the slope modalities, 15; 16 and 20 soil samples were taken respectively for Low Slope, Medium Slope and High Slope. The geographic coordinates of all soil sampling points were taken using GPS. A clinometer and a wooden stake were used to take slope measurements. In addition, all soil samples were taken from ferrallitic soils with low desaturation in (B) at the watershed of Kpacomey.

After sampling, the soil samples were air dried and sieved at 2 mm mesh size. The physical and chemical soil analyses were made at the Laboratory of Soil Microbiology and Microbial Ecology (LMSEM) of the Faculty of Agronomic Sciences at the University of Abomey-Calavi. The targeted parameters were soil texture, bulk density, threshing index, root biomass, current and potential soil acidity (pH_H2O, pH_KCl), soil organic matter and available phosphorus. The appropriate analytical method for each parameter was used. Soil texture, actual acidity (pH H₂O) and potential acidity (pH KCl), soil organic matter content (Organic Carbon (%) × 1.724) and available phosphorus were determined respectively by the Robinson pipette method, the potentiometric method in a ratio of 1/2.5 sol/distilled water and sol/KCl, the method of Walkley and Black [14] and available phosphorus by extraction method of the Bray I.

The soil bulk density Da was calculated as the ratio of the dry weight to the volume of the cylindrical probe.

D a = P V [15] with P: dry weight (in g) of soil sampled; V (volume in cm³) =

3.14 ∗ H ∗ r 2 ; H = depth of sampling and r = radius of the base of the core barrel.

Soil crusting Index was calculated according to the formula developed by [16].

S C I = [ ( 1.5 ∗ F S ) + ( 0.75 ∗ C S ) ( C + ( 10 ∗ S O M ) ) ] − C t

with SCI: Soil crusting Index; FS: fine silt content (%); CS: coarse silt content (%); C: clay content (%); SOM: Soil organic matter content (%); Ct = Constant; If pH < 7, Ct = 0; if pH > 7, Ct = 0.2 (pH - 7).

For the interpretation of the data resulting from the calculation of the soil flapping index, the following scale will be used: SCI > 2: much crusted soil; 1.8 < SCI < 2: crusted soil; 1.6 < SCI < 1.8: fairly crusted soil; 1.4 < SCI < 1.6: slightly crusted soil; SCI < 1.4: uncrusted soil.

Plant roots contained in the soil samples were recovered and weighed. The percentage of biomass root (Br) in the soil was calculated as follows:

R B ( % ) = w r w s ∗ 100 [17] with RB = RootBiomass; w_r = root weight and w_s =

soil weight.

The main effects tested are cropping systems, slope and their interaction. Statistical analyses were performed with SAS version 9.2 software. After verification of the conditions of normality and equality of variances, the data collected were subjected to an analysis of variance (ANOVA) following the GLM procedure. Separation of the means was performed using the Least Significant Difference (LSD) test at the 5% significance level. The variograms of the distribution of organic matter and available phosphorus were carried out with the Gs+ software. The extrapolation model used is an exponential model. The validation parameters used are the RMS (Root Mean Square), the coefficient of determination (R²), the naguet, the nugget. The distribution maps of organic matter and available phosphorus according to the toposequence of the Kpacomey watershed have been made by ordinary kriging with the software Sufer (version 11) and ArcGIS (version 10.1).

Compaction and crusting are factors that characterize the physical degradation of soils. Apparent density is considered an indicator of low soil porosity and an indicator of soil compaction [17] [18]. The results of the present study show that the modalities of cropping systems are significantly different in terms of soil bulk density. Our results are consistent with those of [19] who showed that agricultural activities significantly influence the bulk density in the A horizon of cultivated soils. Our study showed that soil bulk density increases with the pure Palm grove cropping system as opposed to the Palm grove-Cassava maize cropping system. These results confirm the results of [20] which showed that soil bulk density values are high in the row spacing of adult palm groves whereas they are low under swaths. Other researchers observed mean bulk density values ranging from 1.4 to 1.6 g·cm⁻³ for soils under palm groves at a depth of 0 - 50 cm [21]. Thus, a high bulk density value means that voids are reduced and particles are highly compacted. The increase in bulk density values with depth indicates the progressive compacting of the soil towards depth [22].

From the point of view of root biomass, the modalities of cropping systems and slope are significantly different. The abundance of root biomass in the soil varies from one cropping system to another and it remains higher with the pure palm grove cropping system. This abundance is due to the density and root system of oil palm cultivation. Soils can have a shallow compacted horizon, which limits the rooting of plants and thus restricts their supply of water and mineral elements [23].

Structural disaggregation is at the origin of the formation of crustal crust and erodibility must therefore be a direct function of structural stability, the measurement of which gives a precise indication of the dimensional distribution of the fragments [24]. These authors have shown that there is a relationship between this sensitivity of soils to disintegration and flapping, but this relationship is complex, multifaceted and probably difficult to put into equation. The results of our study showed that the soils of the watershed of Kpacomey are not flapping under the effect of cropping systems and slope according to the interpretation grid proposed by [16] Rémy Marin-Laflèche]. This confirms the results of [24] who stated that the measurement of structural stability is not then a good indicator of soil degradation, because in some cases of heavy rainfall, runoff can occur even without the formation of flapping crusts and large aggregates or fragments can be transported.

Our results show that the interactive effect of cropping systems and slope significantly affects soil water pH and soil KCl pH, and an increase in these two types of acidity is induced by the Palm grove-Cassava maize cropping system on a high slope. According to the interpretation grid proposed by [25], the soil of the Kpacomey watershed is weakly acidic under the pure Palm grove-Cassava, Palm grove-Maize-Cassava and Teak plantation cropping systems. Soil acidity is an indication of the equivalent chemical elements at soil level in general and is directly related to exchangeable cations and anions [19]. The rate of acidification varies greatly depending on soil type, land use, productivity and management of the farming system [26].

Soil organic matter, because of its close relationship with fertility, is one of the important factors in controlling sustainable plant productivity [27]. Indeed, through its degradation and mineralization, soil organic matter improves the physical and chemical quality of soils and provides the nutrients necessary for crop development [28]. Available phosphorus mainly associated with soil practice is found in runoff to the river and soil levels of available phosphorus are strongly accumulated in the first five centimeters of soil and decrease with depth [29]. Analyses from our results show that cropping systems significantly influence soil organic matter content. Indeed, for the cropping system modalities, a higher level of organic matter was found in the pure Palm Grove cropping system than in the Palm Grove-Cassava maize cropping system. These results are consistent with those of [22] who proved that this variability in organic matter content is due to the ridge management of food crops associated with the young planting age and the row planting method and the swathing of plant matter resulting from palm pruning. Our results can be explained by the fact that nutrient loss by runoff is strongly related to cropping systems in agricultural watersheds. Moreover, organic matter is either transported by surface runoff or as individual particles or adsorbed to mineral particles in the soil [17]. The results of our studies thus explain the deficit of the watershed soils in available phosphorus and organic matter.