Mineral Fertilizers Influence the Macrofauna of Soils Under Cocoa Trees in the South-Western Côte d’Ivoire

A study on mineral fertilization of soils was carried out for two years in three cocoa farms in the Nawa region of south-western Côte d’Ivoire. The aim of this study was to evaluate the effects of mineral fertilizers on soil macrofauna. The experimental design was Fisher block with four treatment with three repetitions. The treatments were control without fertilizer (T0), NPK 0-23-19 fertilizer (T1), NPK 0-15-15 + 17 CaO + 5 MgO + 1 B2O3 + 0.5 Zn (T2) and NPK 4-10-10 + 19 CaO + 4 MgO + 0.8 B2O3 + 0.3 Zn (T3). Twelve soil monoliths were made per treatment and per site, to count the macrofauna of the soil. Each soil monolith was subdivided into three strata 0-10 cm, 10-20 cm and 20-30 cm. The organisms were counted and classified into functional groups. The abundance, species richness and diversity indices of Shannon-weaver and Piélou were determined. The analysis of variance of the numbers of individuals in the macrofauna did not show a significant difference between the treatments on different strata of the monoliths during the second year of the trial. However, during the first year, all the treatments with fertilizer in the 0-10 cm stratum had a more abundant macrofauna than that of the control without fertilizer at Soubré and Mayo. Multivariate analyses (AFC) showed links between functional groups of organisms and treatments. Myriapods and earthworms were linked to the T2 and T3 treatments, microarthropods to T1 and other organisms to T0. As for macrofauna diversity, the highest value of species richness (7 species), Shannon-weaver (2) and Piélou (1) indices were found in the fertilized treatments. Mineral fertilizers thus presented better conditions for the expansion of macrofauna. A reasoned application of mineral fertilizers creates a favourable living environment for certain soil organisms.


Introduction
Falling cocoa yields due to declining crop soil fertility (Koko et al., 2009) are prompting producers to explore remedial measures. One such avenue is soil fertilization to improve yields. Mineral fertilization is the most widely adopted to correct the state of soil fertility in order to obtain better yields. The application of these inputs is not without consequences on the environment (Stadelmann et al., 2002). Thus, the soil fauna that has a very important role in soil fertility (Lavelle et al., 2006) is in some cases impacted by external inputs. A first cause of decline in species diversity at the landscape/territory/regional scale is thus land use change which may substitute spaces with species-poor communities for spaces with richer communities (either through intensification or abandonment). Moreover, in the case of agricultural intensification (significant fertilization with synthetic fertilizers, deep drainage, intensive mowing, mowing early in the year), spatial heterogeneity tends to diminish at all scales, from the plot to the regions to the landscape (Benton et al., 2003), leading to a decrease in diversity at the landscape scale. Thus, the source and/or composition of the fertilizer may have an influence on the distribution of soil fauna. Indeed, chemical fertilizers in particular are sometimes responsible for soil acidification (Boli et al., 2000;Bado, 2002) and a decrease in microbiological activities in fertilized soils  (Anderson & Ingram, 1993). Each year, three soil monoliths per treatment were collected; one per elementary plot. Each soil monolith had a size of 25 cm × 25 cm × 30 cm. When a soil monolith is collected, it is divided into 0-10 cm, 10-20 cm and 20-30 cm strata. The soil monoliths were positioned at a distance of 80 cm from the collar of the fertilized cocoa trees. In each soil monolith stratum, manual harvesting of the macrofauna was carried out. The extracted organisms were preserved in 70 p.c. formaldehyde. The recorded individuals were then classified by morphotype or functional group and the abundance and species diversity (species richness, Shannon-Weaver index and Piélou equitability index) (L. Legendre & P. Legendre, 1984) were determined.
Specific diversity is defined according to the stationary (wildlife) data processing method proposed by Grall and Hily (2003).
Specific Richness was the total number of species in a stand.
The Shannon-Weaver H' index defined the diversity of species in relation to all the individuals in the stand. It is based on: where, H': number of individuals of a given species, i ranging from 1 to S (S = total number of species); P i : total number of individuals.
H' is minimal if all the individuals in the stand belong to the same species. H' is also minimal if each species is represented by a single individual, except one species that is represented by all other individuals in the stand. The index is maximal when all individuals are equally distributed over all species (Issouf et al., 2008;Pedel & Fabri, 2012).
The equitability index (J) of Piélou, represented the ratio of H' to the theoretical maximum index in the stand: where, H'max = log 2 (S).
This index varies from 0 to 1, it is maximal when species have identical abundances in the stand and it is minimal when a single species dominates the entire stand. Insensitive to species richness, it is very useful for comparing potential dominance between stations or between sampling dates.

Soil and Litter Data Collection Produced
After fertilizer application, a sampling of the 0-20 cm soil horizon was carried out in June 2019 using an auger. Three composite samples were made by treatment and sent to the laboratory after air drying and sieving with a 2 mm mesh sieve. In addition to the sampling of the 0-20 cm horizon, the 0-10, 10-20 and 20-30 cm layers were sampled using a cylinder for bulk density with a volume of 554.2 cm 3 . These samples were used to determine the soil bulk density. As for the litter, using litter traps installed under the canopy of the cocoa trees, it was quantified. Three metal cages per treatment constituted these traps. Each cage measuring 1m x 1m x 1m with a removable grid installed at 0.5 m from the ground in this cage made it possible to collect the fall of cocoa tree leaves from June 2018 to November 2019.

Data Processing
A normality test was performed on all the data. Those found abnormal were transformed by the square root function (√x + 0.5) before being all subjected to an analysis of variance using the GLM procedure in SAS 9.4 software. The mean values were separated by the Student-Newman-Keul (SNK) method at a ≤ 0.05. To see the impact of fertilizers on the macrofauna over time, a comparison of the macrofauna means was carried out between the two study periods. In order to establish links between treatments and groups of organisms, the data were subjected to multivariate analyses (PCA, AFC) by first forming taxonomic groups (earthworms, myriapods, microarthropods and other organisms encountered). This multivariate analysis was performed using the XLSTAT 2016 software. The Excel interface was used to enter the data and graph the shannon-weaver and Piélou diversity indices.

Morpho-pedological and Physico-chemical Characteristics of Study Site Soils and Quantity of Litter
On the Soubré site, the soils were silty-sandy in the T0 treatment and silty-clay-sandy in the other treatments. As a result, a higher proportion of sand was determined in the soil of the T0 treatment. On the other hand, the other treatments had the highest levels of fine elements. In terms of bulk density, no significant difference was observed between treatments. Stratum averages were 0-10 cm, 10-20 cm and 20-30 cm, 1.7, 1.8 and 1.8 respectively. In terms of chemical parameters, the levels of the variables pH(water), V, organic matter (OM), soil organic carbon (SOC), total nitrogen, assimilable phosphorus, exchangeable potassium, exchangeable calcium and exchangeable magnesium in the T0 treatment were lower than those recorded in soil samples taken in the fertilizer treatments. As for litter production, no significant differences were found between treatments (Table 1).
On the Méagui site, all treatments were installed on sandy-silt soils. Therefore, no difference was observed between soils at the level of each mineralogical fraction. However, in terms of bulk density, differences were observed in the 0-10 cm stratum, and the highest value (1.6) was obtained in the soil of the T1 treatment. In terms of chemical parameters, the levels of the variables pH(water), V, organic matter (OM), soil organic carbon (SOC), total nitrogen, assimilable phosphorus, exchangeable potassium, exchangeable calcium and exchangeable magnesium in the T0 treatment were lower than those recorded in soil samples taken in the fertilizer treatments.
As for litter production, no significant differences were found between treatments (Table 2).
At the Mayo site, the T0 treatment was installed on a clay loamy-sandy textured soil, the T1 treatment on a silty-sandy textured soil, the T2 treatment on a silty-clay loamy-sandy textured soil and the T3 treatment on a silty-sandy textured soil. Therefore, a higher clay content was determined in the soil on which the T0 treatment was installed and a higher proportion of sand in the soils where the T1 and T3 treatments were installed. On the other hand, in terms of bulk density, no difference was observed between the soils of the treatments in all strata.
In terms of chemical parameters, the levels of pH(water), V, total nitrogen, exchangeable potassium and exchangeable calcium in the T0 treatment were lower than those recorded in the soil samples taken in the fertilizer treatments. On the other hand, no significant differences were observed between the treatments in organic matter, soil organic carbon. As for litter production, during the year 2018 significant differences were observed between treatments. Treatments T1 and T2 produced more litter 271.6 g and 257.2 g respectively. In contrast, during the year 2019, litter production was statistically similar between all treatments (Table 3).

Species Identified at the Three Sites During the Trial
Twenty-two (22) species were recorded on the three study sites. These included 6 species of earthworms, 4 species of diplopods and chilopods, 4 species of microarthropods, 2 species of pterygotic insects and 2 species of mites. On the Soubré site, sixteen (16) species were counted. On the Méagui site, seventeen (17) species were encountered. Finally, on the site of Mayo, eighteen (18) species were identified. All the species of earthworms and diplopods mentioned in Table 1 were identified at the three sites. At the level, chilopods Scolopendra cingulata, Cryptopidae sp. and Geophilus ssl. were identified at the three sites. Among the microarthropods, only one species was identified at the three sites (Table 4). Note. Presence = +, absence = -.

Effect of Treatments on the Abundance of Soil Macrofauna
The analysis of variance did not show significant differences between treatments in the localities of Soubré, Méagui and Mayo for the abundance of soil fauna during the years 2018 and 2019. In the year 2018, the means were respectively 4.67, 6.42 and 2.58 organisms in Soubré, Méagui and Mayo. In 2019, the averages were 5.31 in Soubré, 4.47 in Méagui, and 1.50 in Mayo (Table 5). . On the other hand, in the 0-10 cm stratum in Méagui and in the underlying strata (10-20 cm and 20-30 cm), the analysis of variance did not show a significant difference between treatments in all localities (Table 6).
However, in the year 2019, there were no significant differences between treatments in different localities across all strata (Table 7). However, from 0-10 cm the averages were 3.6 in Soubré, 6.0 in Méagui and 3.1 in Mayo. From 10-20 cm, the means were 10.0 in Soubré, 5.7 in Méagui, and 2.1 in Mayo. Finally, for the 20-30 cm stratum, the means were 2.1 in Soubré, 1.5 in Méagui and 0.1 in Mayo. Note. The averages followed by the same letters in each column are not significantly different at a ≤ 0.05.

Effect of Fertilizers on Soil Macrofauna Abundance as a Function of Test Duration
In Soubré, the analysis showed significant differences between the two study periods (2018, 2019) in all treatments. For the T0 treatment, the difference was found in the 10-20 cm stratum, with more organisms (2.6) in 2019. For the T1 treatment, the differences were observed in the 0-10 and 10-20 cm strata. In the 0-10 cm stratum, macrofauna was more abundant in 2018 with 14 organisms. For the T2 treatment, differences were observed in the 10-20 cm and 20-30 cm strata. In both strata, abundance was highest in 2019 with 4.6 organisms in the 10-20 cm and 1.6 organisms in the 20-30 cm stratum. Finally, for the T3 treatment, in the 0-10 cm stratum, the highest value was obtained in 2018 with 21 organisms; in the other two strata, the highest values were obtained in 2019 (Table 8).
In Méagui, the analysis showed significant differences between the two study periods (2018,2019) in the T0, T1 and T2 treatments (Table 9). For the T0 treatment, the differences were observed in the 10-20 and 20-30 cm strata. In these two strata, the highest values were found in 2019 with 3 and 1.6 organisms respectively. For the T1 treatment, the difference was observed in the 10-20 cm stratum, with. Thus, a higher abundance (34.3 organisms) in 2018. For T2, the difference was observed in the 20-30 cm stratum, with a high abundance (3,000 organisms) in 2019. On the other hand, in the T3 treatment, at the level of all strata, no significant difference was observed between the two periods.
Finally, in Mayo, significant differences between the two periods (2018, 2019) were observed in the T1 and T0 treatments (Table 10). For the T1 treatment, the difference was observed in the 0-10 cm stratum, with a high presence of organisms (12.6) in 2018. For the T0 treatment, the difference was observed in the 10-20 cm stratum, with a high presence of organisms (1.6) in 2019.   Note. The averages followed by the same letters in each column are not significantly different at a ≤ 0.05.  Note. The averages followed by the same letters in each column are not significantly different at a ≤ 0.05.

Relationship Between Soil Macrofauna Taxonomic Groups and Treatments
All the information is carried by the F1 and F2 axes with an inertia of 89.66%. Regarding the graphical representation of the variables on the F1 and F2 axis, the taxonomic group of earthworms and the other unidentified organisms grouped under the term "others", contribute to the formation of the F1 axis respectively to the extent of 35.26% and 37.6% with square cosines of 0.77 and 0.82. The F2 axis was formed by the taxonomic groups of myriapods and microarthropods respectively to the extent of 57.63% and 41.88% with square cosines of 0.82 and 0.59. Concerning the coordinates, on the F1 axis, an opposition was observed between the group formed by earthworms, myriapods and that formed by microarthropods and other organisms recorded. And along axis F2, an opposition was observed between the group formed by myriapods and earthworms and that formed by microarthropods and others.
As for the observations, the control treatment was very well correlated to the F1 axis with a cosine square equal to 1. Treatments T1 and T2 were correlated to the F2 axis with square cosines of 0.813 and 0.626 respectively.
With respect to the F1 axis, the T1, T2 and T3 treatments were opposed to the T0 treatment. The T2 and T3 treatments were opposed to the T1 treatment on the F2 axis ( Figure 2).
With regard to correspondence factor analysis (CFA) (Figure 3), all the information was carried by the F1 (79.96%) and F2 (18.18%) axes with an inertia of 98.14%. It made it possible to constitute three entities, that of the treatment T1 and the microarthropods, that of the others and the control and finally the last group consisting of the couples (myriapods, earthworms) and (T2, T3).

Specific Richness
In Soubré, the results showed that during the years 2018 and 2019, the specific richness was greater in the T1, T2 and T3 treatments compared to the control (T0). From 0 to 20 cm depth, treatments T1 and T3 respectively with 6 and 7 species had the highest species richness (Figure 4).
In Méagui, the results showed that in 2018, the specific richness was approximately equal between treatments in all strata. However, in 2019, the highest species richness (6 species) was obtained in T1 ( Figure 5).
Finally, in Mayo, the results revealed that in 2018, the T0 and T3 treatments obtained the highest values in species richness (4 species). In 2019, the second year of testing, the highest values of species richness were recorded by T1, T2 and T3 with 4 species (Figure 6).  T1 T3 T2 T0 T1 T3 T2 T0 T1 T3 T2 T0 T1 T3 T2 T0 T1 T3 T2 T0 T1 T3 T2  In Soubré during 2018, treatments T1 (2.41) and T3 (2.02) obtained the highest Shannon-Weaver diversity indices in the 0-10 cm and 10-20 cm strata respectively, while T0 had the lowest value (0). In contrast, in 2019, the T0 treatment with 1.72 recorded the highest value. From 2018 to 2019, there was a decrease in the diversity index in all treatments except the T0 treatment at the 0-10 cm stratum. In contrast, at the 20-30cm stratum, all treatments had a zero diversity index during the trial (Figure 7).
Concerning the equitability index of Piélou, in year 1, treatments T1 (0.933) and T2 (0.844) had the highest values in the 0-10 cm stratum and T0 recorded a zero value. In the 10-20 cm stratum, treatment T0 (0.811) had the highest index, while T1 had the lowest.
In 2019, Q3 (1), Q0 (0.86) and Q1 (0.81) treatments had the highest indices across all strata. However, from 2018 to 2019, treatments T1, T2 and T3 had a decrease in the equitability index in the 0-10 cm stratum, while in the 10-20 cm horizon, T1 and T3 had an increase in this index. For the 20-30 cm stratum, the equitability index was zero for all treatments (Figure 8).

Discussion
Comparison of the average abundance between treatments on the 0-30 cm layer showed no significant difference in all localities overall. However, differences appeared between treatments at the 0-10 cm layer. This difference shows a higher abundance of organism populations in the mineral fertilizer treatments. This result shows that these fertilizers improve organism populations. These phosphate fertilizers were applied to supplement the phosphate status of the soil, but their dissolution also provides other nutrients including potassium, calcium and magnesium. These exchangeable cations saturate the leaves with colloids and increase the saturation rate, which in turn raises the pH. Thus, a favourable living environment for the development of soil fauna is created. Soils that received fertilizer had higher saturation rates than the control without fertilizer. In the same order, the pH levels in the fertilized soils were close to neutral (6.5 to 8 in Soubré, 6.5 to 7.9 in Méagui and 6.8 to 6.9 in Mayo).
Knowing that most soil organisms proliferate in less acidic environments that tend towards neutrality (pH(water) = 7) (Ponge, 2004), a higher abundance of soil macrofauna in fertilized environments is justified. Because, according to this author, environments with a strong biological activity present pHs close to neutrality. In addition to acidity, the choice of litter leaves, size, hardness, mineral content (calcium in particular) and carbohydrate and protein richness for feeding soil organisms play a major role (Benton et al., 2003). The application of moderately or highly reactive natural phosphates in strongly leached acidic tropical soils has a potential trigger effect on plant growth and the resulting crop yield. The products and residues from these crops have a better food quality (higher content of phosphorus, calcium, magnesium than unfertilized plants). The incorporation of such organic residues increases the biological activity in the soil, resulting in an improvement of the physical and chemical properties of the soil (FAO, 2004). Two others, and not the least important factors that determine soil life are soil organic matter (SOM) and soil organic carbon. They enable the maintenance of soil biological activity and the productivity of ecosystems (Chenu, 2009). Indeed, they are the energy source of most soil organisms with the exception of certain protozoa with chlorophyll chromatophores that are autotrophic. Fertilized soils have higher MOS (32,7 to 49.8 g kg -1 in Soubré, 13.7 to 27.5 g kg -1 in Méagui and 27.0 to 36.0 g kg -1 in mayo) and COS contents than the soils of the control without fertilizer. Thus, a larger population of macrofauna resides there. In addition to the nutrient conditions that positively impact the abundance of soil fauna (Guéï and Tondoh 2012; Moço et al., 2010), habitat is another determining factor. Even if litter production does not show a significant difference, it is sufficient to create an environment conducive to the life of soil macrofauna. Indeed, the production of abundant litter and fine dead wood covering the soil creates moisture conditions favourable to the development of soil fauna (Bertrand et al., 2015). The existence of favourable nutrition and housing conditions will positively impact the abundance of soil organisms in fertilized environments. This would explain the greater diversity of soil organisms observed in fertilized cocoa farms, given the higher species richness, Shannon-Weaver index and Piélou equitability index observed in these environments. This litter being of quality would attract soil organisms such as phytophages, saprophages, geophages, scavengers. However, there is greater diversity in the surface horizons than in the underlying horizons. Indeed, the deep horizons are less rich in organic matter and in particular in organic carbon. Also, it should be noted that the surface horizons are less dense than the deep horizons, making them easier to move. Thus, organisms migrate easily from the depths to the surface in search of food. The low values of the Shannon-Weaver index noted in the treatments without fertilizers indicate that there are preponderant species.  T1  T3  T2  T0  T1  T3  T2  T0  T1  T3  T2  T0 10 20 30 Equitability index of piélou Treatment and stratum J1 (2018) J2 (2019) jas.ccsenet.org Journal of Agricultural Science Vol. 13, No. 2;2021 This is best appreciated with the Equitability Index, which highlights ecosystem imbalance and poor species distribution, given its values below 0.6 in most sampling levels for non-fertilizer treatments. Moço et al. (2010) found that density and richness in total soil invertebrate and litterfall numbers varied significantly between sites, and in particular faunal richness was higher in cocoa-based agroforestry systems where leaf litter was more abundant, suggesting that the vegetation structure responsible for the development of a litter layer is a key factor in explaining the observed variations in soil invertebrate populations.
These fertilizers used do not modify the abiotic and biotic parameters of the soil that could significantly influence the abundance of organisms. This is because it is recognized that these animals are extremely sensitive to variations in the microclimate and can disappear when the changes in state, which are likely to occur in a man-managed ecosystem, are bad. Better still, here, these fertilizers have created environments that tend towards a climax for soil organisms. This translates into greater diversity in the environments fertilized by mineral fertilizers. Moreover, the results of Principal Component Analysis (PCA) and Correspondence Factor Analysis (CFA) reveal affinities between taxonomic groups and different treatments. Myriapods and earthworms are more encountered in T2 (NPK 0-15-15 + 17 CaO + 5 MgO + 1 B 2 O 3 + 0.5 Zn) and T3 (4-10-10 + 19 CaO + 4 MgO + 0.8 B 2 O 3 + 0.3 Zn) treatments while microarthropods have a preference for 0-23-19 (T1) fertilizers and other organisms have a filiation with the treatment without fertilizer. These environments favour the settlement of certain taxonomic groups to the detriment of others. This would be due to the nutritive potential of the soils (quality and quantity). Indeed, Arpin et al. (1980) had evoked this aspect to show the presence or absence and the location of such species or groups in a complex game that limits a global representation. Moreover, different species participating in the same functions react differently to the limiting factors (André, 2006). They are therefore not all affected by a disturbance in the environment (presence of toxic substances, change in water regime, change in physico-chemical conditions).

Conclusion
The application of phosphate and potassium fertilizers on soils under cocoa trees in order to determine their impact on the soil macrofauna reveals a positive effect on the latter. The abundance of macrofauna was greater in environments with fertilizers than in environments without fertilizers. This was the case for the 0-10 cm strata at the Soubré and Mayo sites in the first year. Also, it should be noted that the specific richness in the fertilized areas is much higher than in the unfertilized areas. The highest values were observed in the fertilized plots. The Shannon-Weaver diversity index study showed values above 2 in plots fertilized with phosphate and potassium mineral fertilizers. As for the equitability index of Piélou, it showed a greater imbalance of organisms in cocoa farms without fertilizer. Finally, treatments with mineral fertilizers showed a greater affinity for myriapods, earthworms and microarthropods, unlike the treatment without fertilizer, which was richer in organisms grouped under the term "others".