This study was conducted at the Soubré dam (Figure 1) stripping site in southwestern Côte d’Ivoire in the Nawa region, located between 6˚12' and 7˚08' west longitude and 5˚19' and 6˚34' north latitude. The climate is humid sub-equatorial, characterized by two rainy seasons (April-June and September-November) and two dry seasons (July-August and December-March). Under the direct influence of the Atlantic Ocean and relatively dense vegetation cover due to the proximity of Taï National Park, average temperatures range between 24˚C and 27˚C.

Figure 1. Location of the study site.

The plant material used in the study consists of the cover crop Arachis repens(Figure 2). It was chosen primarily as a cover crop because of the thick, prostrate growth of its stolons and strong taproots. It is a leguminous plant with characteristics that enable it to adapt to different soil conditions, fix atmospheric nitrogen, and add organic matter to the soil. It is also a very effective plant for combating erosion [7].

Figure 2.Arachis repenscollected and used as plant material.

2.3.1. Selection and Preparation of the Experimental Plot

The experimental plot for the trial was selected from the site of the stripping work carried out during the construction of the Soubré hydroelectric dam, which left large areas of bare earth. Two types of plots were selected: steeply upland surfaces (>5%) and relatively flat surfaces (<5%). The pH of the plot measured using the glass electrode method [11] was acid around 5 whatever the soil surface type. This stripped site reveals a ferralitic subsoil with a sandy-clay texture and a B horizon rich in iron (lateritic). Each plot had a basic area of 100 m², where staking and hole digging were carried out after watering, at intervals of 30 cm × 30 cm across the entire surface of the plot. The holes, approximately 5 cm deep, were made using machetes, dabas, and stakes (Figure 3).

Figure 3. Preparation of the experimental plot.

2.3.2. Collection of the Cover Plant and Preparation of Cuttings

The cover plant was collected by carefully removing it from its substrate so that the roots were obtained in fairly good condition. It was then moistened. The plants were transported to the study site, where the stems were cut into cuttings using scissors so that each cutting had at least five nodes. The cuttings were moistened and immediately transplanted.

2.3.3. Transplanting Cuttings and Maintenance

The cuttings were transplanted individually into holes that had been dug beforehand. The cuttings were planted with two nodes in the soil, positioned close together to ensure faster ground cover. Maintenance consisted of watering the transplanted cuttings with a water jet, using a motor pump to deliver water to the plot, then watering cans to water the transplanted cuttings. Watering was done twice a day, in the morning before sunrise and in the evening after sunset for two weeks.

2.3.4. Experimental Design

The experimental design consisted of three simple blocks, each comprising an upland and a flat surface. Each surface had an area of 100 m² (10 m × 10 m). Five subplots were set up within each surface, diagonally and in the center. Each subplot with an area of 1.5 m² contained 12 plants among which 10 plants were selected for growth measurements and observations.

2.3.5. Data Collection

Data collection focused on the recovery rate of cover plants, the number of branches, internodes, and the length and coverage of cover plants. Surveys were conducted over thirteen months, starting one month after the cuttings were transplanted and continuing at three-month intervals. The recovery rate of cover plants was calculated based on the ratio of the number of cuttings that have recovered growth to the number of cuttings transplanted. The branches emitted by each individual were determined by counting. The number of internodes was also determined by counting. The length of the aerial axis was measured at the apical end of the main stems using a tape measure. During plant growth, the vegetation cover rate (area occupied by a species or vegetation if projected onto the soil surface) was estimated at regular intervals (every 3 months) starting from the first month after the vegetation cover was established [10].

2.3.6. Analysis of Data

The data obtained were entered using EXCEL version 2016 and analyzed using XLSTAT 2016 software. To study the effects of surface type on the recovery and growth of cover crops, a one-way analysis of variance was performed. In cases of significant differences, Tukey’s test was used at a 5% probability threshold to identify homogeneous groups.

The analysis of variance shows a significant difference between the recovery rates of the cover plant Arachis repens depending on the surface area of the stripped site (p < 0.05). The highest recovery rate was obtained on the flat plot, with 90.67%, while the lowest was recorded on upland surface, with 75.67% (Table 1). Figure 4 illustrates that the recovery of cuttings transplanted on flat surface is higher than that obtained on upland area.

Table 1. Recovery rate of the cover crop A. repens according to soil surface.

Values assigned the different letter are statistically different at the α = 0.05 threshold according to Tukey’s test. F: Tukey’s value; p: probability.

Figure 4. Recovery of cuttings on a tray (A) and on a slope (B) one month after cutting.

The number of branches and internodes of Arachis repensgradually increases over the twelve months following transplanting, regardless of the type of soil surface stripped from the Soubré hydroelectric dam (Figure 5and Figure 6). During the first few months (M1 to M3), vegetative growth remains moderate, with values ranging from 2 to 6 branches and 5 to 11 internodes. At M1, a significant difference is observed between the two conditions, with the flat surface showing the highest numbers of branches and internodes. From M3 onwards, the differences between the surfaces were no longer significant. Both types of surface supported similar and regular growth of the cover plant A. repens. At M12, both types of soil surface showed maximum development, with values of around 17 to 18 branches and 37 to 38 internodes.

For a same period, values assigned the different letter are statistically different at the α = 0.05 threshold according to Tukey’s test. The orange color indicates the flat plots, and the blue color indicates the upland plots.

Figure 5. Number of branches of the cover crop A. repens according to soil surface.

For a same period, values assigned the different letter are statistically different at the α = 0.05 threshold according to Tukey’s test. The orange color indicates the flat plots, and the blue color indicates the upland plots.

Figure 6. Number of internodes of the cover crop A. repens according to soil surface.

The length of the stems and the rate of ground cover by the cover crop Arachis repens gradually increased over the twelve months following transplanting, with significant differences between periods but not between soil surface types within the same period (Table 2). At M1, the flat surface has a slightly greater stem length (13.91 cm) than those located in the upland area (10.06 cm), although the coverage rates remain statistically similar (11.00% and 8.00%, respectively). From M3 onwards, stem lengths remained higher on flat surface (24.72 cm compared to 21.81 cm in upland areas), while coverage rates evolved in parallel, reaching 19.67% and 17.33%. This trend is confirmed at M6, where vegetative growth reaches 40.34 cm versus 34.48 cm, accompanied by coverage rates of 32.13% and 27.47%. In the ninth month, the differences persist, with 52.75 cm on flat surface compared to 49.16 cm on upland. However, the coverage rates remain statistically indifferent. At M12, A. repens reaches its maximum development, with stems measuring 89.22 cm on flat surface and 86.80 cm on upland, accompanied by high coverage rates (71% and 69%). At this stage, no significant differences are observed in either the coverage rate or the length of the plants.

Table 2. Plant length and coverage rate of the cover crop A. repens according to soil surface.

For a same period, values assigned the different letter are statistically different at the α = 0.05 threshold according to Tukey’s test.