The degradation of natural resources, particularly soil, is now one of the main challenges facing global agriculture. In sub-Saharan Africa, this degradation is a particular cause for concern due to the low intrinsic fertility of the soil, climatic constraints and intensive farming practices, which accelerate the depletion of soil resources [1]-[3]. This situation is leading to a gradual decline in agricultural productivity, jeopardising food security and the sustainability of production systems [4]. In this context, soil quality assessment appears to be an essential tool for evaluating the sustainability of agroecosystems and guiding sustainable management strategies. Soil quality refers to its ability to simultaneously fulfil the biological, physical and chemical functions necessary to maintain crop productivity, to the preservation of the environment and the functioning of ecosystems. It depends not only on its physico-chemical properties, in particular the availability of essential nutrients such as carbon, nitrogen, phosphorus, potassium, calcium and magnesium, but also on its biological component. This biological fertility relies primarily on the activity of soil organisms, which break down organic matter and recycle nutrients, the formation of aggregates and the improvement of soil structure. Among these organisms, soil macrofauna plays a vital role. Consisting mainly of earthworms, termites, ants, millipedes, molluscs and various insect larvae, it comprises the invertebrates visible to the naked eye that spend a significant part of their life cycle in the soil ([5] Bachelier, 1978; [6] Brown et al., 2002). These organisms, often referred to as “ecosystem engineers” [7], alter the physical, chemical and biological properties of the soil by promoting the incorporation of organic matter and aeration, water infiltration and the availability of the minerals essential for crop growth. The diversity and abundance of macrofauna are therefore relevant indicators of biological fertility and soil quality. In Côte d’Ivoire, where agriculture is a strategic sector of the national economy, cocoa farming plays a key role. As the world’s leading cocoa producer, the country derives a significant proportion of its export earnings from this sector, on which more than a million producers are directly dependent [8][9]. However, the expansion of cocoa plantations—often in the form of monoculture—has led to a sharp decline in forest cover, a reduction in biodiversity and progressive soil degradation, thereby jeopardising the sustainability of the plantations and cocoa productivity [10]. In light of these challenges, agroforestry appears to be a promising approach that reconciles agricultural production with the conservation of natural resources. By deliberately combining cocoa trees with forest or fruit trees, agroforestry systems help to improve the microclimate, enrich the soil with organic matter, limit erosion, promote the recycling of nutrients and maintain functional biodiversity, which is essential for the proper functioning of agroecosystems [11]-[13]. These systems have the potential to simultaneously improve the chemical properties of soils and their biological fertility through the development of an abundant and diverse macrofauna. In the Lôh-Djiboua region, particularly in the Lakota department where cocoa farming is the predominant agricultural activity, agroforestry systems are characterised by considerable diversity. However, our understanding of their effects on soil quality remains limited, particularly with regard to the interactions between soil chemical properties, biological fertility and the structure of macrofauna communities. A better understanding of these relationships appears essential for promoting agroforestry practices capable of ensuring sustainable cocoa production whilst preserving the ecological functions of the soil. It is against this backdrop that this study has been conducted, with the aim of assessing the influence of cocoa agroforestry systems on soil quality through a combined analysis of soil chemical properties and soil macrofauna in the plantations of the Lakota department.

Agroforestry systems are recognised as models of sustainable production that combine crops with woody species. The adoption of these systems on cocoa farms could prove significant for producers, as they improve soil quality and productivity. We will explain our findings, give our views on them and relate them to some related research.

The results of the survey of trees associated with cocoa trees revealed that agricultural fruit species were predominant, accounting for 62.40 per cent in the traditional agrosystems of the Lakota area. These species, including Citrus sinensis (orange tree), Musa paradisiaca (banana tree) and Mangifera indica (mango tree), are mostly grown for food, whilst others, such as Cola nitida (cola tree), are planted to diversify farmers’ incomes [22]. Following the floristic survey of our agroforestry systems, a complete system was identified, comprising the three associated tree strata. Trees in stratum 3 stood out (39 per cent) from those in strata 2 and 1, which accounted for 36 per cent and 25 per cent respectively. Indeed, the prevalence of associated trees that are considerably larger than the cocoa trees (strata 3) would appear to indicate that the agroforestry orchards found in the area are ageing. This was revealed by [21] who showed that no new cocoa plantations are being established in Côte d’Ivoire’s main cocoa-producing areas and that all of them are existing plantations that have been in production for more than five years. However, some producers retain certain tree species in their plantations in order to use them as timber, which is sold to logging companies [23]. As for the species comprising strata 2 and 3 consisting of Citrus sinensis (orange tree), Musa paradisiaca (banana tree) and Mangifera indica (mango tree), their presence is thought to be due to dietary requirements [22].

Furthermore, analysis of the physical properties of the soil has shown that the bulk density of the soil decreases as the density of trees growing alongside the cocoa trees increases. In fact, the value was 1.20 g/cm³ for treatment T1 (0 - 25 associated trees), 1.26 g/cm³ for treatment T2 with a medium density (25 - 50 associated trees) and 1.27 g/cm³ for more than 50 associated trees (T3). These results confirm that these plots are indeed cultivated systems, as described by [24]. They maintain that the bulk density of cultivated systems generally ranges between 1.2 and 1.5 g/cm³. Furthermore, the decrease in bulk density associated with tree density is thought to be due to good soil structure and improved aeration, resulting from the seamless interaction between the soil components. This idea is supported by [25], who demonstrates that agroforestry, through the continuous addition of plant litter and the development of the root system, reduces the bulk density of the soil whilst improving its porosity. As for the usable water reserve, the results indicated that this decreases as tree density increases. Indeed, the T1 (0 - 25 associated trees) and T2 (25 - 50 associated trees) treatments recorded higher values than the high-density treatment (˃50 associated trees), with values of 39 mm, 44 mm and 34 mm respectively. This finding could be explained by competition between trees and cocoa plants for access to water in the soil. [25] explain that if the trees growing alongside cocoa trees do not complement this crop, competition may arise for the available water in the soil. These results highlight the importance of selecting an appropriate density of trees to be planted alongside cocoa trees within modern agroforestry systems, which are now recommended in Côte d’Ivoire. The proportion of coarse particles does not vary from one treatment to another. Furthermore, regardless of the associated tree density, the proportion of coarse material remains within the range of values for cocoa cultivation reported by [26]. Indeed, soils with a low content of coarse particles have a good usable water-holding capacity. However, an abundance of coarse particles reduces the soil’s ability to retain water.

From a chemical perspective, the research focused on organic matter, the rate of mineralisation, cation exchange capacity (CEC), total exchangeable bases, base saturation and nutrients. The results relating to soil organic matter content showed that the values obtained were higher in agroforestry systems than in full-sun systems, but remained within the normal range for cocoa cultivation [27]. However, the optimal values were observed for T2 (30.53 g.kg⁻¹) and T3 (32.16 g.kg⁻¹). These values represent the optimum MO levels for cocoa cultivation. They are thought to be linked to a high availability and diversity of litter debris. Furthermore, the presence of trees in a plantation is thought to increase the soil’s organic carbon content through the permanent presence of plant litter and the microclimatic conditions essential for the development of organisms that break down plant matter, which is then returned to the soil. These findings are consistent with those of [28][29], who argue that agroforestry increases carbon and organic matter stocks compared with agricultural systems without associated trees. Furthermore, the C/N ratio, which indicates the rate of organic matter mineralisation, showed normal values for treatments T2 (25 - 50 trees/ha) and T3 (˃50 trees/ha), at 12.40 and 11.33 respectively. These values indicate good mineralisation of the organic matter. The optimal range (9 < C/N < 15) ensures a balance between mineralisation and immobilisation, beneficial for the nutrition of cocoa trees [30]. Indeed, [31] established threshold values for nitrogen uptake by plants. If the C/N ratio is less than 9, mineralisation is too rapid (nitrogen loss), and if the ratio is greater than 15, mineralisation is too slow (nitrogen deficiency). The nitrogen input observed in treatments T2 and T3 may be due to the presence of associated trees capable of fixing atmospheric nitrogen in the soil, as mentioned in the survey of trees associated with cocoa trees. Indeed, the results showed that in these two densities (T2 and T3), there is a good supply of nitrogen, at 1.40 and 1.65 mg/kg respectively.

As for the adsorbent complex, the results of this study revealed identical CEC values in soils of all tree densities associated with cocoa trees. These rates ranged from 17.21 at the control level (T0) to 29.93 for the high-density treatment (cmol.kg⁻¹). These values fall within the standards for cocoa cultivation. Indeed, soils that receive a daily supply of organic matter from associated trees will develop a well-formed clay-humus complex [32]. This is the “reservoir” for basic cations in the soil, which explains why systems with increasingly high tree densities exhibit higher CEC values as density increases. Furthermore, the soil pH was statistically identical across all cropping systems (6.4 to 6.7), with values close to neutrality (7). These results show that agroforestry systems have helped to maintain, or even improve, soil pH. Indeed, the ideal pH for growing cocoa is 7 [27][33][34]. The same authors had confirmed that this plant can still grow in slightly acidic (pH 4.5 - 6) or slightly alkaline (pH 6.5 - 7.5) soils. The near-neutral pH levels found in the various soils are thought to promote the effective decomposition of organic matter and the optimal structuring of soil components. In agroforestry systems, soil chemical properties are strongly influenced by daily litter inputs and nutrient recycling [35]. Furthermore, the base saturation (V %) showed statistically identical values in the soils of the different cropping systems. However, the tree-based system yielded acceptable values of 57.86 per cent, 71.69 per cent and 63.38 per cent respectively for treatments T1, T2 and T3, compared with the 60 per cent benchmark set by [36]. Furthermore, these slightly elevated saturation values confirm that the soils are slightly neutral, with a pH of around 6.7; indeed, the base saturation is closely linked to soil pH [37]. Thus, when the pH is low, the saturation level is low (˂60 per cent), and when the pH is around neutral, the base saturation level is high (˃60 per cent). Furthermore, the presence of trees in plantations helps to maintain the base saturation level through nutrient recycling and the transfer of exchangeable bases from deeper soil layers to surface layers [38]. With regard to the sum of exchangeable bases, the results showed the highest values in the T2 (16.62 cmol/kg) and T3 (18.97 cmol/kg) treatments compared with the other treatments. These values are consistent with those reported by [39] which defines an optimal value for cocoa cultivation of more than 15 cmol/kg. These normal values are thought to be due to the well-structured clay-humus complex of the soils studied.

Following the assessment of the soils’ physico-chemical properties, the biological characterisation of the soil is then addressed. Indeed, the results of the biological characterisation revealed an abundance of biological organisms at medium (25 to 50 associated trees) and high (more than 50 associated trees) densities, compared with the lower associated tree densities (0 and 25 associated trees) and the “full sun” system. These results suggest that an increase in the density of associated trees is correlated with a higher abundance of soil macrofauna, thus reflecting an increase in biological activity within the ecosystem. Indeed, the presence of trees in cropping systems appears to promote the establishment, survival and activity of beneficial soil organisms, in particular by improving microclimatic conditions, the continuous supply of organic matter. These findings are consistent with those of [9] who reported that agroforestry systems promote high levels of biological activity thanks to the microclimatic conditions they create. Principal component analysis (PCA) yielded the same results as those from the analysis of variance, which shows that systems with increasing levels of cover are rich in biological organisms in terms of both abundance and diversity.

The findings of this study indicate that traditional cocoa-based agroforestry systems have a positive impact on cocoa plantations. However, densities of between 25 and 50 trees per hectare and those with more than 50 trees per hectare could be beneficial for the sustainability of cocoa plantations, given that the physical, chemical and biological indicators of soil fertility have reached optimal levels at these densities. But further studies are needed on the effect of these densities on the growth and yield of cocoa trees in order to draw definitive conclusions regarding the recommended density.