Relationships between soil morpho-chemical parameters and earthworm community attributes in tropical agro-ecosystems in the Centre-West region of Côte d’Ivoire, Africa

This study was carried out to investigate the relationship between earthworm trophic groups and soil morphology and chemical attributes, and moreover, to determine which of these attributes would be most significant in explaining the distribution of earthworm communities in agro-ecosystems in the Centre-West region of Côte d’Ivoire. Earthworms' soil morphology and soil samples were studied in three agro-ecosystems: 20-year-old cocoa plantations, 5-year-old mixed cocoa plantations and mixed crop-fields. The semi-deciduous forests near the agro-ecosystems were also sampled and considered as control plots. Earthworm global densities varied on average between 53.9 ± 7.9 and 86.0 ± 19.0 individuals m−2 and biomass between 16.5 ± 3.1 and 20.6 ± 4.1 g m−2 under these ecosystems. Path analysis produced a significant model: soil morphology and chemical attributes under different agro-ecosystems affected the density and biomass of earthworm trophic groups, and these attributes are potential regulators of the fauna communities. The morphological components related to dead leaves (r2 = 0.73, P < 0.05) and fine woods quantities (r2 = 0.71, P < 0.05) are most decisive for detritivore abundances, whereas geophageous mesohumic abundances were positively affected by soil organic carbon (r2 = 0.79, P < 0.05) and N (r2 = 0.84, P < 0.05) and geophageous polyhumic abundances were positively affected only by soil N (r2 = 0.63, P < 0.05). In agro-ecosystems the relationship between soil conditions and earthworm communities varied between earthworm trophic groups, so detritivores were more affected by litter quantity, whereas shallow geophageous populations were guided by soil organic matter.


Introduction
Earthworms represent an important soil faunal group commonly named soil ecosystem engineers (sensu Jones et al. 1994) that affect soil fertility and conservation through burrowing, ingesting and egesting soil as cast (Lavelle et al. 2006;Blouin et al. 2013;Kanianska et al. 2016). Moreover, earthworms are assumed to play the key ecological roles in litter comminution and soil organic matter decomposition processes which are conditioned by the functional traits of different species (Bouché 1977;Dewi and Senge 2015). They were mainly classified into trophic categories of detritivores and geophageous based on their feeding and living preferences (Lee 1985). Detritivores live and feed at the soil surface on plant litters; their burrowing activity is low when an adequate food source is available. Geophageous feed deeper in the soil organic matter and dead fine roots mixed with soil (Lavelle 1981). According to the feeding strategies in relation to soil organic matter amounts ingested, geophageous may be subdivided into three groups such as polyhumics, mesohumics and oligohumics. Polyhumics consume considerable amount of organic matter, while mesohumics and oligohumics feed on soil, respectively, fairly and poor in organic matter (Lavelle 1981).
Earthworm's abundance is affected by resource availability as well as disturbances (Jouquet et al. 2018). Agricultural practices induced disturbances that affect the size and composition of the earthworm communities by impacting International Society for Tropical Ecology their ecological groups (Smith et al. 2008;Spurgeon et al. 2016). However, the knowledge about effects of soil environmental variability in shaping earthworm assemblages is poorly understood (Ettema and Wardle 2002). In tropical soils, studies have documented the effects of different land use practices on earthworm communities (Koné et al. 2012;Tondoh et al. 2015). Tondoh et al. (2015) reported a reduction in detritivore species populations, namely Millsonia lamtoiana (Omodeo and Vaillaud 1967), Dichogaster baeri (Sciacchitano 1952) and Dichogaster erhrhardti (Michaelsen 1898) with forest conversion into cocoa plantations, while Koné et al. (2012) showed increases in both detritivore Dichogaster baeri (Sciacchitano 1952) and D. saliens (Beddard 1893) abundance with the adoption of legume-based fallows. Remarkably little is known about earthworm feeding ecology and their relationship to soil morphology and chemical quality in agro-ecosystems. Moço et al. (2010) reported that organic matter, soil acidity and litter quality were regulators of the soil fauna functional groups under cacao agroforestry systems, but litter quality was more decisive than soil quality. Also, Koné et al. (2012) reported a positive influence of soil organic matter on the populations of mesohumic worm M. omodeoi (Sims 1986) under Chromoleana odorata (L.) King and Robinson fallow, whereas litter feeders and polyhumic populations decreased.
In the same study area, investigations carried out by Guéi and Tondoh (2012) revealed that earthworm assemblage is guided by soil organic matter content, which is subject to the type of land use. Continuous conversions of forests into agricultural lands are likely to be a major source of threat to population conservation. However, the better understanding of the soil property impacts on earthworm trophic groups in agro-ecosystems may be required to better manage these organism communities and lead to more sustainable soil management strategies. This paper dealt with the current state of knowledge and aimed to determine the relationship between earthworm trophic groups and soil physical and chemical characteristics in contrasted agro-ecosystems as opposed to the semi-deciduous forests. We hypothesized that earthworm feeding assemblages are controlled by edaphic conditions, mainly soil organic matter.

Study sites
We conducted the study around the village Goulikao in the Centre-West region of Côte d'Ivoire (6°30′N, 5°31′W). In the 1970s, this area was part of the main cocoa production area characterized by a high rate of deforestation. As a result, the landscape is composed of a mosaic of landuses including forests, cocoa plantations, fallows and food crops spread around three settlements, namely Petit Bouaké (6°31.4′ N, 5°31.6′ W), Djè Koffikro (6°28.8′ N, 5°30.4′ W) and N'Kroiêdjô (6°30.9′ N, 5°30.2′ W) located each at least 2 km from the main village Goulikao. These settlements are exclusively occupied by farmers from the savanna areas of the country seeking arable lands for cocoa farming. The land-use system consisted in a mixture of perennial and food crops with fallow as intermediary step for soil regeneration in slash and burn agricultural systems. Food crops such as plantains (Musa spp.) are mixed with cocoa trees until 5-year-old with the aim of having ultimately a cocoa plantation.
The climate is a subequatorial type divided into four seasons. The long dry season starts from November to February, the long wet season from March to June, the short dry season from July to August and the short wet season from September to October. The annual mean rainfall is about 1626.7 mm with an average relative humidity of 79%. The average monthly temperature was about 26 °C with low monthly variability of 1.6 °C. Soils are ferralsols (World Soil Reference 2006), slightly acid in the top 20 cm (pH < 6.5) with a sandy-loam texture (Assié et al. 2008). Nutrient contents are low and decrease rapidly from the upper soil layer to 20 cm depth.

Sampling and extraction of earthworms' soil morphology
The sampling campaign took place from June to July 2008 during the major rainy season along a gradient of landuse types starting from forest (baseline plot) to food crops referred to as the most disturbed ecosystem. Specifically, four land-use types including semi-deciduous forests (SF), 20-year-old cocoa plantations (OCP), 5-year-old mixed cocoa plantations (MCP) and mixed crop fields (MCF) were selected in each locality (Petit Bouaké, Djè Koffikro and N'Kroiêdjô) of the landscape in order to obtain 15 replicates for each at the scale of the study area. The mixed-crop fields consisted of a mixture of annual and perennial food crops, such as: cassava, yam, plantains (Musa spp.), maize and vegetables.
We sampled earthworms using a modified method recommended for tropical soils (Anderson and Ingram 1993). It consists at each sampling point in excavating three soil monoliths (50 × 50 × 30 cm) spaced by 5 m interval along a transect (Guéi and Tondoh 2012). Earthworms, hand-sorted and preserved in 4% formaldehyde solution, were identified to species level (Tondoh and Lavelle 2005;Csuzdi and Tondoh 2007), counted and weighted. In this study, we assigned species into four trophic groups (Lavelle 1981) including detritivores and geophageous polyhumics, mesohumics and oligohumics (Table 1).

Soil and litter sampling for study of morphology and chemical properties
Soil morphology is an assessment of the contribution of soil aggregates of different sizes and origins (physical or biogenic), plants, gravels and stones and other components to the spatial architecture of the upper centimetres of soil and derived from visual separation of these items (Topoliantz et al. 2000). Near each soil monolith, a cube of soil, 10 × 10 cm down to 10 cm depth was taken. Soil physical or biogenic aggregates were gently separated as well as other remaining materials such as dead leaves and shoot debris, seeds, gravels and woody debris (Velasquez et al. 2007). The biogenic aggregates (casts, galleries, nests) mainly produced by earthworms, termites, ants and coleoptera were regrouped into three size classes: small biogenic aggregates (BA(1) ≤ 1 cm), medium biogenic aggregates (1 cm ≤ BA(2) ≤ 3 cm) and large biogenic aggregates (BA(3) ≥ 3 cm). Soil physical aggregates produced by chemical-physical processes were also distributed among small, medium and large classes as the biogenic aggregates. Separation was done by gently breaking the soil apart among its natural constituents. Depending on the soil and training of the operator, it took 1-3 h to process one sample. Separated items were counted and the total quantity was given in item numbers by square metre.
At each sampling point, nine soil cores (Ø = 5 cm) were randomly collected at 0-10 cm, air-dried, sieved and mixed thoroughly to form a composite sample. The soil samples were analysed for soil organic carbon (SOC), total N, available P and pH determination. SOC and total N were assessed by dry combustion in a CHN (Thermo-Electron NA-1500). Available P was extracted according the Olsen-Dabin method and was determined by colorimetry at 660 nm (Murphy and Riley 1962). Soil pH was measured in a soil:water suspension at a 1:1 ratio.

Statistical analysis
A total of 15 replicates were considered as the distance separating the three soil monoliths at each sampling point was not enough to consider them as true replicates, meaning that variables were averaged to form a single replicate. The impact of land-use change on earthworm density and biomass, and soil variables was examined using a one-way ANOVA with the Fisher's LSD test for multiple mean comparisons. The statistical tests were conducted using STATIS-TICA 7.0 (Statsoft, Tulsa, USA).

Co-inertia analysis of fauna communities and soil morphology and chemical attributes
The results of co-inertia analysis of earthworm feeding groups and soil morphology and chemical attributes showed significant relationships (RV coefficient: 0.85, P < 0.008 and RV coefficient: 0.90, P < 0.001 for soil morphology and chemical attributes, respectively) (Figs. 1, 2). The first factor accounted for twice more of the total variability in each co-inertia analysis indicating that it revealed all information. The first factor (61.7%) of co-inertia analysis between earthworms and morphology associated geophageous polyhumics biomass with seed, geophageous mesohumic density and biomass with roots and large biogenic aggregates, and detritivore populations with leaves and wood debris. Cocoa plantations and semideciduous forests, respectively, exhibited this pattern most strongly (Fig. 1). As far as the second co-inertia analysis (earthworms-chemical variables) is concerned, factor 1 (76.3%) dealt with associations on the one hand between geophageous polyhumics, SOC and C/N ratios, and on the other hand detritivore biomass and soil pH (Fig. 2).

Discussion
The co-inertia analyses showed significant relationships between earthworm trophic groups and soil morphology and chemical attributes, indicating that forests' conversion into agro-ecosystems associated with soil attributes changes affected earthworm communities. These observations are consistent with previous results yielded by Singh et al. (2016) in the agro-ecosystems of the northwestern part of the Punjab, India. Some feeding groups are more sensitive than others to the effect of forests' conversion into agro-ecosystems. Agricultural practises particularly affected BA (1) BA (2) BA (3) PA (1) PA (2) PA ( (1) small ≤ 1 cm, (2) 1 cm ≤ medium ≤ 3 cm, (3) large ≥ 3 cm detritivore, geophageous polyhumic and mesohumic densities while oligohumic populations were not influenced. These results highlight the idea that soil heterogeneity induced by land use practices contributed to the formation of population patches for some earthworm species (Jiménez et al. 2011). For instance, in soils with a direct seeded system with living mulch, epigeic earthworm populations were more abundant than those in conventional farming systems with ploughing (Pelosi et al. 2009). As detritivore earthworms feed on plant litters at the soil surface (Lee 1985), they are negatively impacted by decreasing in litter cover and ploughing as they cannot gain access to their trophic resource (Jiménez et al. 2011;Bertrand et al. 2015). This assertion is corroborated by the path analysis that showed  (1) BA (2) BA (3) PA (1) PA (2) PA ( (1) BA (2) BA (3) PA (1) PA (2) PA ( (1) BA (2) BA (3) PA (1) PA (2) PA ( C B D a positive control of dead leaves and fine woods over the abundance of detritivore worms. Moreover, detritivore earthworms are positively influenced by the availability of food and moist soil conditions, and such conditions are provided by soil cover by litter and dead fine woods (Bertrand et al. 2015). Earthworms like moist soils and are most active in such conditions because the water protection mechanisms in their bodies are not well developed. Their body functioning such as respiration rate strongly depends on the gas diffusion through the moist body wall (Lapied et al. 2009). Endogeic earthworms, i.e., geophageous polyhumics and mesohumics were affected by soil organic matter content mostly by soil organic carbon and total N that affected directly and positively their density. These results are consistent with previous observations in tropical (Guéi and Tondoh 2012;Huerta and Van der Wal 2012;Moço et al. 2010) and temperate agro-ecosystems (Schirrmann et al. 2016), which suggested that soil organic matter are strong drivers of the abundance of endogeic earthworm communities in agricultural ecosystems. Endogeic earthworm populations were guided by soil organic matter content. They rapidly responded to changes in C availability induced by soil management practices (Lapied et al. 2009), and this is one of the main reasons why they are considered as good bio-indicators of changes in soil quality induced by land use change (Guéi and Tondoh 2012).
This work also indicates that geophageous oligohumics were only the endogeic earthworm groups that were not affected by land use type and soil attributes. This is corroborated by the co-inertia result that showed no association between polyhumic earthworm populations and soil chemical and morphological properties. These observations tend to confirm that species of these trophic groups withstood adverse effects of forest conversion to agro-ecosystems as they lived deeper in soil and consumed soil less rich in organic matter (Bouché 1977;Lavelle 1981). The soil depth conditions offered protection against agricultural detrimental practices such as tillage, mineral fertilizers and herbicide applications. For instance, Fraser et al. (1996) observed in temperate soils that the endogeic earthworm Apporrectodea caliginosa dominates and is more tolerant to agricultural soils because it lived in relatively protected habitat within the soil.

Conclusions
Our studies demonstrated that the relationship between soil conditions and earthworm communities varied between earthworm functional groups in agro-ecosystems. The detritivore populations were more affected by litter quantity than shallow geophageous groups that were mainly guided by soil organic matter. Soil morphology components related to dead leaves and fine woods are the most decisive attributes for detritivore earthworm density. Chemical attributes which affected geophageous polyhumics and mesohumics in cocoa systems and mixed crop-fields included mainly soil organic carbon and nitrogen. Because earthworms are important as soil ecosystem engineers (sensu Jones et al. 1994), maintaining litter cover at soil surface could be a good practice to promote their healthy activities to improve ecosystem functioning in tropical agro-ecosystems.