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Spatial distribution of bowal and differences in physicochemical characteristics between bowal and woodland soils in Benin, West Africa
Catena 124 (2015) 45–52
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Spatial distribution of bowal and differences in physicochemical characteristics between bowal and woodland soils in Benin, West Africa Elie A. Padonou a,⁎, Yvonne Bachmann b, Romain Glèlè Kakaï a, Anne Mette Lykke c, Brice Sinsin a a b c
Faculty of Agronomic Sciences, University of Abomey-Calavi, 01 BP 526, Cotonou, Benin Institute of Ecology, Evolution and Diversity, J. W. Goethe University, Max-von-Laue-Str.13, 60438 Frankfurt am Main, Germany Aarhus University, Department of Bioscience, Vejlsøvej 25, DK-8600 Silkeborg, Denmark
a r t i c l e
i n f o
Article history: Received 11 October 2013 Received in revised form 17 August 2014 Accepted 18 August 2014 Available online xxxx Keywords: Bowal Ferricrete Ferruginous soil Physicochemical soil characteristics Phytogeographical district Benin
a b s t r a c t Bowal, is degraded land surface characterized by ferricrete exposure due to soil surface erosion. This study aims at assessing the spatial distribution of bowé in Benin and the physicochemical differences between bowé and soil of adjacent woodlands. Bowé surfaces were identified during field investigations and mapped. Soil samples were taken on the bowé surfaces and nearby woodland. Mann–Whitney test was applied to analyze the different physicochemical characteristics of bowé and woodland. The results show that bowé were directly related to ferruginous soils and rainfall regime. Bowé soils are characterized by significantly lower values of electrical conductivity, organic matter, extractable phosphorus, silt and total nitrogen than woodland soils, while potassium exchangeability of bowé soils is higher. Bowé can be expected wherever ferruginous soils and/or ferricretes are observed under unimodal rainfall regimes condition. The disaggregation of bowé ferricretes may improve the soil physicochemical characteristics and sustain the regrowth of forest under more humid conditions. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In the intertropical region, the drier conditions that occurred during paleoclimatic changes (Tardy and Roquin, 1998) led to ferricrete exposures due to degradation of the land surfaces and erosion of the upper leached horizon (Büdel, 1982; Butt, 1987; Millot, 1983). The exposition of ferricrete resulted in bowal expansion (plural bowé) (Aubréville, 1947; Goldman et al., 2011). Bowalization might be human induced (Aubréville, 1947; Goldman et al., 2011; Padonou et al., 2014) or a natural process (André et al., 2003; Thomas et al., 2003). The soil characteristics of bowé impede plant root growth because of absorption of water and burns in dry season. Trees on bowé are dwarfed, gnarled, and widely scattered (Thompson, 1911). Bowal occupies the top of plateaus and does not change appreciably over decades by either growing or disintegrating (Lacroix, 1913; Shantz and Marbut, 1923). However, under humid climatic conditions, the exposed ferricretes are disaggregating and degrading geochemically to form soil (Beauvais, 1999, 2009; Beauvais and Tardy, 1993). Thus one could expect change
⁎ Corresponding author. Tel.: +229 97212586. E-mail addresses: [email protected] (E.A. Padonou), [email protected] (Y. Bachmann), [email protected] (R.G. Kakaï), [email protected] (A.M. Lykke), [email protected] (B. Sinsin).
http://dx.doi.org/10.1016/j.catena.2014.08.022 0341-8162/© 2014 Elsevier B.V. All rights reserved.
in the spatial distribution of bowé since the climate has becoming more humid particularly in West Africa (Diallo et al., 2012; Diawara et al., 2014). In West Africa, bowé are found in the semiarid and the sub humid climate zone (Padonou et al., 2012, 2014; Sieglstetter et al., 2012; Zwarg et al., 2012). Hardened ferruginous soils (i.e., ferricretes) could be the only soil types on which bowé occur. Disaggregation of bowé ferricrete can sustain forest regrowth and lead to a soil formation (Beauvais, 2009). Since it is common to observe bowé adjacent to woodland in the protected area in West Africa (Fig. 1), we can assume that the woodland is formed after the biophysical disaggregation of the bowé. Thus, assessing the physical chemical differences between ferricretes exposed on bowé and soil of contiguous woodlands documents the change of the physical chemical characteristics linked to the disaggregation of bowé. The aim of this study is to assess the spatial distribution of bowé in Benin and to analyze the physicochemical characteristics of topsoil on bowé surfaces compared to the topsoil of adjacent woodland. If the spatial distribution of bowé is related to the occurring climate and soil types, it may be possible to determine what climate and soil types are more vulnerable for bowalization. In addition, if the physicochemical characteristics of topsoil on bowé are different to those of adjacent woodland, it may be possible to predict the physical chemical characteristics of soil, as the soils are derived from the disaggregation of bowé ferricretes.
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Fig. 1. Typical bowal surface in dry season (February, 2011).
2. Materials and methods 2.1. Study area The Republic of Benin is located in West Africa between the latitudes of 6°10′N and 12°25′N and longitudes of 0°45′E and 3°55′E. The size of the national territory is of 114,673 km2. Three rainfall regimes (Fig. 2b) are found: (1) bimodal rainfall between the coast and 7°30′N, with the first rainy season being more intense, (2) a broad peak with a tendency to unimodal rainfall in central Benin, and (3) unimodal rainfall characterized by a slow increase and a sudden decrease. The geomorphology of Benin is closely linked to its geologic
structure. The main geologic units of Benin are the sedimentary rocks and the metamorphic and crystalline rocks of the Dahomeyan basement (Faure and Volkoff, 1998). Three major geomorphologic units are observed on the sedimentary rocks and seven on the crystalline rocks besides the Atacora, the main relief and the quartzitic crest in the KandiBimbéréké (Faure and Volkoff, 1998). The geomorphologic units of the sedimentary rocks are the sedimentary basins located in north (Kandi basin and Volta basin) and in south Benin (Coastal basin) (Fig. 2a). Plateaus landscapes are important in the sedimentary basins compared to slopes and lowlands. The Coastal and Kandi Basins are characterized by landscapes with low plateaus. Erosion and flood plains of the Pendjari are other characteristics of the Volta Basin. The crystalline rocks are characterized by peneplain dominated by hills and small relict plateaus. The main geomorphological units are the peneplain of Kouandé-Péhonco westward, the plain of the Alibori in the north, the plateau of Djougou and the peneplain of Pira in the south, the plateau of Parakou and the peneplain of Nikki in the east and the erosionplain of the lower Ouémé in the south (Fig. 2a). Four major soil types with characteristic vegetation are distinguished in the territory of Benin (Volkoff and Willaine, 1976): (1) ferralitic soils covered by semi-deciduous forest, (2) ferruginous soils covered by dry forest, woodland, and savanna, (3) vertisol in the depression of Lama covered by a dry semi-deciduous forest, and (4) hydromorphic soils covered by swamp and riparian forests. The country is subdivided in ten phytogeographical districts (Adomou et al., 2006) (Table 1; Fig. 3). 2.2. Data collection and analysis 2.2.1. Spatial distribution of bowé The spatial distribution of bowal in Benin is assessed based on field work and phytogeographical districts maps (Adomou et al., 2006). Two main transects oriented according to rainfall gradients and soil types are investigated (Fig. 3). The first transect of 540 km long run
Fig. 2. (a) Landscape units in Benin (Giertz and Schönbrodt, 2008) and (b) mean annual rainfall in mm (Fink et al., 2008) for the period from 1961 to 1990 in Benin.
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Table 1 Ecological, chorological, and floristic characterisation of the phytogeographical districts (Adomou et al., 2006). Phytogeographic district
Rainfall Rainfall regime (mm)
Major soil type
Phytochorological Major plant zones formation
Coast Pobè Plateau Ouemé-Valley Bassila Zou North Borgou South Borgou Atacora Chaine Mékrou-Pendjari
Bi Bi Bi Bi TUn TUn TUn Un Un Un
Sandy + Hydromorphic & halomorphic soils Ferrallitic soils without concretions Ferrallitic soils without concretions Hydromorphic soils Ferrallitic soils with concretions and breastplates Ferruginous soils on crystalline rocks Ferruginous soils on crystalline rocks Ferruginous soils on crystalline rocks Poorly evolved & mineral soils Ferruginous soils with concretions on sedimentary rocks
GC GC GC GC GS GS GS S S S
900–1300 1200–1300 900–1100 1100–1300 1100–1300 1100–1200 1100–1200 1000–1200 1000–1200 950–1000
Coastal forest and derived thickets, Mangrove Semi-deciduous forest Semi-deciduous forest Swamp and semideciduous forest Semi-deciduous forest, woodland, and riparian forest Dry forest, woodland, and riparian forest Dry forest, woodland, and riparian forest Dry forest, woodland, and riparian forest Riparian forest, dry forest, and woodland Tree and shrub savanna, dry forest and riparian forest
R: Rainfall regime: Bi: Bimodal (2 rainy seasons), TUn: Tendency to unimodal, Un: Unimodal (1 rainy season). PChor: Phytochorological zones based on the composition in distribution range types: GC: Guineo-Congolian zone, GS: Guineo/Sudanian transition zone, S: Sudanian zone.
from the Coastal to the Mekrou-Pendjari phytogeographical district (in the north western part of Benin), across nine phytogeographical districts namely: Coast, Plateau, Pobè, Ouemé-Valley, Zou, Bassila, South-Borgou, Atacora-Chaine and Mekrou-Pendjari. The second transect (250 km) is located between the Mekrou-Pendjari phytogeographical districts (from north western to north eastern of Benin) and in addition includes Atacora-Chaine and North-Borgou districts. Each phytogeographical district was explored to detect bowal (ferricrete exposure) occurrence. A minimum unit of 30 m × 30 m of ferricrete landscape was considered for assigning a bowal surface. When bowal was detected, the center coordinate of the bowal was marked with a Global Positioning System (GPS) in order to relate it with ArcGIS 10 (ESRI Inc., Redlands, USA) to the soil map of Benin (Volkoff and Willaine, 1976) and to the map of the phytogeographical districts (Adomou et al., 2006). The minimum distance between consecutive points for GPS measures was 5 km along the transects. In total, 36 sampling sites were obtained on the first transect and 18 on the second. 2.2.2. Physicochemical characteristics of soil samples on bowé and adjacent woodlands Within each of the six phytogeographical districts where bowé were identified, nine homogeneous topsoil sample pairs were taken on bowé and adjacent woodlands. Woodland soil was sampled as adjacent material in protected areas (Fig. 3). 17 woodland samples were considered for the entire study as some of the bowé sites were adjacent to the same woodland sample. We used a soil auger to dig a hole to a depth of 10 cm because bowal has a hardened soil (ferricrete) with only thin shallow topsoil (Fig. 1). Out of the nine sample pairs we selected randomly one composite topsoil sample composed of the nine homogenous ones from bowal and woodland of each phytogeographical district for further analysis. The composite soil samples were dried in the laboratory at 105 °C, then crushed and sieved through a screen of 2 mm. The particle size was measured by the pipette method (Loveland and Whalley, 1991). The electrical conductivity (EC) and the pH-value were measured according to the methods of Rhoades (1982). Soil organic matter (OM) was determined by the K2Cr2O7– H2SO4 oxidation method of Walkey and Black (Nelson and Sommers, 1982). The total nitrogen was extracted by using the Kjeldahl System 1026 distilling unit. Potassium was determined after extraction of ammonium acetate-extractable cations using atomic absorption spectrometry (Thomas, 1982). The extractable phosphorus was analyzed using a Bray-2 extract (Bray and Kurtz, 1945). Principal component analysis (PCA) was applied to the composite soil data of the bowé plots in order to determine the physicochemical characteristic of the bowé soils of each phytogeographical district. With the non-parametric Mann–Whitney test the differences of the soil characteristics between the bowé and woodland sites were investigated per phytogeographical district. All analyses were performed with the SAS software (SAS Inc., 1999) and PC-ORD Version 5 (McCune et al., 2002).
3. Results 3.1. Spatial distribution of bowé Bowé of high and low plateaus and hillslopes expose three types of ferruginous soils: depleted tropical ferruginous soils, slightly leached tropical ferruginous soils and leached tropical ferruginous soils (Fig. 4). Bowé are frequent on leached tropical ferruginous soils (30 of 54) and slightly leached tropical ferruginous soils (22 of 54) than on depleted ferruginous soils (2 of 54) (Fig. 4). Bowé are encountered in six of the ten phytodistricts of Benin: in North-Borgou, South-Borgou, Mekrou-Pendjari, Bassila, AtacoraChaine and Zou. Bowé are not found in the phytodistricts of Plateau, Ouemé-Valley, Pobè and Coast (Fig. 3). Bowé are frequent in NorthBorgou (13 of 54), South-Borgou (11 of 54), Mekrou-Pendjari (10 of 54), Bassila (10 of 54) and Atacora-Chaine (8 of 54) than in Zou (2 of 54) (Fig. 3). In the phytogeographical districts where bowé are frequent (North-Borgou, South-Borgou, Mekrou-Pendjari and Atacora-Chaine) the rainfall regime is unimodal with one rainy season from April to October. Annual precipitation ranges here from 950–1,200 mm (Table 1). In the phytogeographical disctrict of Zou, with low occurrences of bowé, the rainfall regime is also unimodal with amounts of 1100–1200 mm per year. In the phytogeographical districts where bowé do not occur, the rainfall regime is bimodal with two rainy seasons (a long rainy season from March to July and a short from September to October). The annual precipitation varies between 900 and 1300 mm (Table 1). 3.2. Physicochemical characteristics of soil samples on bowé and woodlands A PCA performed on the physicochemical characteristics of soil samples of the bowé plots in six phytogeographical districts resulted in two axes explaining 77.3% of the overall variance. Organic matter, phosphorus, total nitrogen and silt are positively correlated with the first axis (Axis 1) while potassium is negatively correlated with the first axis (Table 2). The first axis (Axis 1) represents bowé plots with the highest value of organic matter, phosphorus, total nitrogen and silt and the lowest value of potassium. Electrical conductivity, pH, and sand are positively correlated with the second axis (Axis 2) while clay is negatively correlated with the second axis (Table 2). The second axis (Axis 2) represents bowé plots with the highest values of electrical conductivity, pH and sand and the lowest content of clay. The phytogeographical districts of North-Borgou, Atacora-Chaine and Mekrou-Pendjari are positively correlated with the first axis, while South-Borgou, Bassila and Zou are negatively correlated with this axis (Fig. 5). Bowé with the highest values of organic matter, phosphorus, total nitrogen and silt and the lowest value of potassium are located in the phytogeographical districts of North-Borgou, AtacoraChaine and Mekrou-Pendjari (Table 3). Bowé with lowest values of organic matter, phosphorus, total nitrogen and silt and the highest value
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Fig. 3. A map of the phytogegraphical district showing the location of the 2 transects with protected areas and bowé soil sampling sites considered.
of potassium are located in the phytogeographical districts of SouthBorgou, Bassila and Zou (Table 3). The phytogeographical districts of Mekrou-Pendjari, AtacoraChaine, South-Borgou and Zou are positively correlated with the second axis, while North-Borgou and Bassila are negatively correlated with it. We conclude that bowé from Mekrou-Pendjari, Atacora-Chaine, SouthBorgou and Zou have the highest value of electrical conductivity, pH
and sand and the lowest content of clay while bowé from NorthBorgou and Bassila have the lowest value of EC, pH and sand and the highest content of clay (Fig. 5; Table 3). The comparison between the soils from bowé and woodlands (Table 4) show that bowé plots have significantly lower (Mann–Whitney test, p b 0.05, n = 6) electrical conductivity (median of 25.96 ms/cm vs. 68.36 ms/cm), lower organic matter content (8.60 gkg− 1 vs.
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Fig. 4. Spatial distribution of bowal on the phytogegraphical districts (Adomou et al., 2006) and soil map of Benin (Volkoff and Willaine, 1976) in 2011.
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Table 2 Correlation coefficient of soil variables on the principal component axes. Soil variables
Organic matter Extractable phosphorus Electrical conductivity Total nitrogen pH Potassium Silt Sand Clay
Correlation coefficient Axis 1
Axis 2
0.65 0.89 −0.53 0.96 −0.48 −0.86 0.78 −0.34 −0.55
0.63 0.07 0.67 −0.21 0.83 0.12 −0.02 0.76 −0.70
23.8 gkg−1), lower extractable phosphorus (0.01 gkg−1 vs. 0.02 gkg−1), lower silt content (9.25% vs. 30.17%) and lower total nitrogen (0.30 gkg− 1 vs. 0.80 gkg− 1) than the woodland soils. Exchangeable potassium is higher (0.65 cmol(+)kg−1 vs. 0.27 cmol(+)kg−1) in bowé than woodland soils. 4. Discussion 4.1. Spatial distribution of bowé Bowé cover plateaus (high and low) and hillslopes in six of the ten phytogeographical districts of Benin, which correspond to the distribution described by earlier studies (Beauvais, 1999, 2009; Beauvais and Roquin, 1996; Faure and Volkoff, 1998). Bowé are confined to different ferruginous soil types namely depleted tropical ferruginous soils, slightly leached tropical ferruginous soils and leached tropical ferruginous soils. This corresponds to previous observations made by Maignien (1958), CPCS (Commission de Pédologie et de Cartographie des Sols) (1967), Grandin and Thiry (1983); and Faure and Volkoff (1998) also linked to the occurrence of hardened ferruginous soils (ferricrete) in West Africa. The low occurrence of bowé in the phytogeographical district of Zou may be explained by the lack of ferruginous soils. Indeed, the other five phytogeographical districts where bowé occur are mainly
covered with leached and slightly leached tropical ferruginous soils. The phytogeographical district of Zou is also covered by hydromorphic soils on which ferricrete is not found (Faure and Volkoff, 1998). The absence of bowé in the phytogeographical districts of Plateau, Ouemé-Valley, Pobè and Coast can be explained by the rainfall regime. Here two rainy seasons are observed with higher precipitation, which lead to an accelerated biophysical disaggregation of the ferricretes and a transformation to ferralitic soils. Ferralitic soils are the main soil types in these districts (Faure and Volkoff, 1998). The more humid condition leads to biodegradation and oxidation of organic matter that modifies the redox processes triggering the degradation of ferricrete (Beauvais, 2009). Since rainfall is an important factor of the biophysical disaggregation of ferricrete (Beauvais, 1999, 2009; Fernández-Caliani and Cantano, 2010; Huang et al., 2013; May et al., 2011; Porder et al., 2007; Soumya et al., 2011), we may assume here that the observed rainfall regime plays an important role in the spatial distribution of bowé in Benin. 4.2. Physicochemical characteristics of bowé and adjacent woodland soils The physicochemical characteristics of bowé vary according to the phytogeographical districts where they occur. In the northernmost districts (North-Borgou, Atacora-Chaine and Mekrou-Pendjari), bowé ferricretes have the highest value of organic matter, total nitrogen and silt and the lowest value of potassium. Mekrou-Pendjari, AtacoraChaine, South-Borgou and Zou have the highest value of electrical conductivity, pH and sand, and the lowest value of clay. These results suggest that even if bowal is referred to as a particular form of land degradation its physicochemical characteristics are not the same in all phytogeographical districts of Benin. The variations of the physicochemical characteristics of the bowé soils of the different phytogeographical districts may be a result of previous land used practices, which are known to affect the physicochemical characteristics of soils (Jha et al., 2010). The comparison of the physicochemical characteristics of bowal and woodland soils revealed that bowal plots had significantly lower values of electrical conductivity, organic matter, total nitrogen, silt and extractable phosphorus but higher values of exchangeable potassium than
Fig. 5. Projection of the phytogeographical districts with bowal on Principal Component Analysis axes defined by physicochemical characteristics. NBorgou (north-Borgou); SBorgou (south-Borgou); MPendj (Mekrou-Pendjari); Bassila (Bassila); Atchaine (Atacorachaine); Zou (Zou); OM (Organic matter); P (Extractable phosphorus); EC (Electrical conductivity); TotalN (Total nitrogen); pH (pH); K (Potassium); Silt (Silt); Sand (Sand); Clay (Clay).
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Table 3 Composite soil characteristics on bowé in the phytogeographical districts. Soil variables
Organic matter (g kg−1)
Extractable phosphorus (g kg−1)
Electrical conductivity (mS cm−1)
Total nitrogen (g kg−1)
pH
Potassium (cmol(+) kg−1)
Silt (%)
Sand (%)
Clay (%)
North-Borgou South-Borgou Mekrou-Pendjari Bassila Atacora Chaine Zou
11.8 10 16 4.9 18.7 10.9
0.01 0.01 0.01 0.01 0.01 0
14.2 24.8 30.1 24.8 25 28.3
0.7 0.31 0.56 0.31 0.5 0.2
4.25 4.68 4.75 4.54 4.8 4.92
0.15 0.76 0.22 0.52 0.18 0.67
17.5 11.5 16 6.25 8.75 10
56.25 68.5 76.75 42.5 55 78
13.75 20 17.5 51.25 11.75 12
woodland. Similar results were found in Burkina Faso where nutrient availability in soil is low on bowé compared to savanna (Zwarg et al., 2012). This may be explained by the level of degradation of these soils. Indeed on degraded land, these parameters are always low (Bewket and Stroosnijder, 2003; Jha et al., 2010; Mills and Fey, 2004; Zhao et al., 2005). The high values of exchangeable potassium observed on bowé may be attributed to the weathering of ferricrete or previous land use on bowé. During the process of weathering of ferricrete a strong chemical alteration is observed and potassium is highly leached (Fernández-Caliani and Cantano, 2010; Yang et al., 2009). High amounts of potassium increase soil erodibility (Auerswald et al., 1996; Miller and Scrifres, 1988) and cause land degradation. Indeed large quantity of exchangeable potassium in soil increases the potential for flocculation or dispersion processes within the soil (Quirk, 1978; Shainberg et al., 1987a; Keren, 1991). These processes influence the hydraulic properties of the soil, crust formation, runoff generation and soil erosion (Ahuja, 1990; Gerits et al., 1987; Lavee et al., 1991). 5. Conclusion In Benin, bowé are distributed exclusively in six of ten phytogeographical districts associated with hardened ferruginous soils (ferricrete) and unimodal rainfall distributions. Bowé soils have significantly lower values of electrical conductivity, organic matter, total nitrogen, silt and extractable phosphorus than woodland soils. However the amount of exchangeable potassium is higher in bowé soils than in woodland soils. The disaggregation of bowé ferricrete may improve the resulting soil physicochemical characteristics and sustain the regrowth of forest. Acknowledgment We wish to thank the Robert S. McNamara Fellowships Program and the UNDESERT (EU FP7 243906) project, “Understanding and combating desertification to mitigate its impact on ecosystem services” funded by the European Commission, Directorate General for Research and Innovation, Environment Programme for financial support (Grant Agreement number 243906). We are also grateful to the editor and Table 4 Mann–Whitney test results for bowé and woodland plots comparisons in Benin. Bowé
Woodland Mann–Whitney test
Organic matter (g kg−1) Extractable phosphorus (g kg−1) Electrical conductivity (mS cm−1) Total nitrogen (g kg−1) pH Potassium (cmol(+) kg−1) Silt (%) Sand (%) Clay (%)
m
m
8.60 0.01 25.96 0.30 4.71 0.65 9.25 63.00 27.75
23.80 0.02 68.36 0.80 5.41 0.27 30.17 49.58 20.58
−1.52* −13.11* −42.40* −0.05* −0.70 ns 0.38* −20.92* 13.42 ns 7.17 ns
The sample size is 6; m: median; ns: non significant at 0.05; *: significant at 0.05.
Anicet Beauvais for useful comments and suggestions which contributed to improve the quality of this paper.
Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.catena.2014.08.022. These data include Google map of the most important areas described in this article.
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Spatial distribution of bowal and differences in physicochemical characteristics between bowal and woodland soils in Benin, West Africa Elie A. Padonou a,⁎, Yvonne Bachmann b, Romain Glèlè Kakaï a, Anne Mette Lykke c, Brice Sinsin a a b c
Faculty of Agronomic Sciences, University of Abomey-Calavi, 01 BP 526, Cotonou, Benin Institute of Ecology, Evolution and Diversity, J. W. Goethe University, Max-von-Laue-Str.13, 60438 Frankfurt am Main, Germany Aarhus University, Department of Bioscience, Vejlsøvej 25, DK-8600 Silkeborg, Denmark
a r t i c l e
i n f o
Article history: Received 11 October 2013 Received in revised form 17 August 2014 Accepted 18 August 2014 Available online xxxx Keywords: Bowal Ferricrete Ferruginous soil Physicochemical soil characteristics Phytogeographical district Benin
a b s t r a c t Bowal, is degraded land surface characterized by ferricrete exposure due to soil surface erosion. This study aims at assessing the spatial distribution of bowé in Benin and the physicochemical differences between bowé and soil of adjacent woodlands. Bowé surfaces were identified during field investigations and mapped. Soil samples were taken on the bowé surfaces and nearby woodland. Mann–Whitney test was applied to analyze the different physicochemical characteristics of bowé and woodland. The results show that bowé were directly related to ferruginous soils and rainfall regime. Bowé soils are characterized by significantly lower values of electrical conductivity, organic matter, extractable phosphorus, silt and total nitrogen than woodland soils, while potassium exchangeability of bowé soils is higher. Bowé can be expected wherever ferruginous soils and/or ferricretes are observed under unimodal rainfall regimes condition. The disaggregation of bowé ferricretes may improve the soil physicochemical characteristics and sustain the regrowth of forest under more humid conditions. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In the intertropical region, the drier conditions that occurred during paleoclimatic changes (Tardy and Roquin, 1998) led to ferricrete exposures due to degradation of the land surfaces and erosion of the upper leached horizon (Büdel, 1982; Butt, 1987; Millot, 1983). The exposition of ferricrete resulted in bowal expansion (plural bowé) (Aubréville, 1947; Goldman et al., 2011). Bowalization might be human induced (Aubréville, 1947; Goldman et al., 2011; Padonou et al., 2014) or a natural process (André et al., 2003; Thomas et al., 2003). The soil characteristics of bowé impede plant root growth because of absorption of water and burns in dry season. Trees on bowé are dwarfed, gnarled, and widely scattered (Thompson, 1911). Bowal occupies the top of plateaus and does not change appreciably over decades by either growing or disintegrating (Lacroix, 1913; Shantz and Marbut, 1923). However, under humid climatic conditions, the exposed ferricretes are disaggregating and degrading geochemically to form soil (Beauvais, 1999, 2009; Beauvais and Tardy, 1993). Thus one could expect change
⁎ Corresponding author. Tel.: +229 97212586. E-mail addresses: [email protected] (E.A. Padonou), [email protected] (Y. Bachmann), [email protected] (R.G. Kakaï), [email protected] (A.M. Lykke), [email protected] (B. Sinsin).
http://dx.doi.org/10.1016/j.catena.2014.08.022 0341-8162/© 2014 Elsevier B.V. All rights reserved.
in the spatial distribution of bowé since the climate has becoming more humid particularly in West Africa (Diallo et al., 2012; Diawara et al., 2014). In West Africa, bowé are found in the semiarid and the sub humid climate zone (Padonou et al., 2012, 2014; Sieglstetter et al., 2012; Zwarg et al., 2012). Hardened ferruginous soils (i.e., ferricretes) could be the only soil types on which bowé occur. Disaggregation of bowé ferricrete can sustain forest regrowth and lead to a soil formation (Beauvais, 2009). Since it is common to observe bowé adjacent to woodland in the protected area in West Africa (Fig. 1), we can assume that the woodland is formed after the biophysical disaggregation of the bowé. Thus, assessing the physical chemical differences between ferricretes exposed on bowé and soil of contiguous woodlands documents the change of the physical chemical characteristics linked to the disaggregation of bowé. The aim of this study is to assess the spatial distribution of bowé in Benin and to analyze the physicochemical characteristics of topsoil on bowé surfaces compared to the topsoil of adjacent woodland. If the spatial distribution of bowé is related to the occurring climate and soil types, it may be possible to determine what climate and soil types are more vulnerable for bowalization. In addition, if the physicochemical characteristics of topsoil on bowé are different to those of adjacent woodland, it may be possible to predict the physical chemical characteristics of soil, as the soils are derived from the disaggregation of bowé ferricretes.
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Fig. 1. Typical bowal surface in dry season (February, 2011).
2. Materials and methods 2.1. Study area The Republic of Benin is located in West Africa between the latitudes of 6°10′N and 12°25′N and longitudes of 0°45′E and 3°55′E. The size of the national territory is of 114,673 km2. Three rainfall regimes (Fig. 2b) are found: (1) bimodal rainfall between the coast and 7°30′N, with the first rainy season being more intense, (2) a broad peak with a tendency to unimodal rainfall in central Benin, and (3) unimodal rainfall characterized by a slow increase and a sudden decrease. The geomorphology of Benin is closely linked to its geologic
structure. The main geologic units of Benin are the sedimentary rocks and the metamorphic and crystalline rocks of the Dahomeyan basement (Faure and Volkoff, 1998). Three major geomorphologic units are observed on the sedimentary rocks and seven on the crystalline rocks besides the Atacora, the main relief and the quartzitic crest in the KandiBimbéréké (Faure and Volkoff, 1998). The geomorphologic units of the sedimentary rocks are the sedimentary basins located in north (Kandi basin and Volta basin) and in south Benin (Coastal basin) (Fig. 2a). Plateaus landscapes are important in the sedimentary basins compared to slopes and lowlands. The Coastal and Kandi Basins are characterized by landscapes with low plateaus. Erosion and flood plains of the Pendjari are other characteristics of the Volta Basin. The crystalline rocks are characterized by peneplain dominated by hills and small relict plateaus. The main geomorphological units are the peneplain of Kouandé-Péhonco westward, the plain of the Alibori in the north, the plateau of Djougou and the peneplain of Pira in the south, the plateau of Parakou and the peneplain of Nikki in the east and the erosionplain of the lower Ouémé in the south (Fig. 2a). Four major soil types with characteristic vegetation are distinguished in the territory of Benin (Volkoff and Willaine, 1976): (1) ferralitic soils covered by semi-deciduous forest, (2) ferruginous soils covered by dry forest, woodland, and savanna, (3) vertisol in the depression of Lama covered by a dry semi-deciduous forest, and (4) hydromorphic soils covered by swamp and riparian forests. The country is subdivided in ten phytogeographical districts (Adomou et al., 2006) (Table 1; Fig. 3). 2.2. Data collection and analysis 2.2.1. Spatial distribution of bowé The spatial distribution of bowal in Benin is assessed based on field work and phytogeographical districts maps (Adomou et al., 2006). Two main transects oriented according to rainfall gradients and soil types are investigated (Fig. 3). The first transect of 540 km long run
Fig. 2. (a) Landscape units in Benin (Giertz and Schönbrodt, 2008) and (b) mean annual rainfall in mm (Fink et al., 2008) for the period from 1961 to 1990 in Benin.
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Table 1 Ecological, chorological, and floristic characterisation of the phytogeographical districts (Adomou et al., 2006). Phytogeographic district
Rainfall Rainfall regime (mm)
Major soil type
Phytochorological Major plant zones formation
Coast Pobè Plateau Ouemé-Valley Bassila Zou North Borgou South Borgou Atacora Chaine Mékrou-Pendjari
Bi Bi Bi Bi TUn TUn TUn Un Un Un
Sandy + Hydromorphic & halomorphic soils Ferrallitic soils without concretions Ferrallitic soils without concretions Hydromorphic soils Ferrallitic soils with concretions and breastplates Ferruginous soils on crystalline rocks Ferruginous soils on crystalline rocks Ferruginous soils on crystalline rocks Poorly evolved & mineral soils Ferruginous soils with concretions on sedimentary rocks
GC GC GC GC GS GS GS S S S
900–1300 1200–1300 900–1100 1100–1300 1100–1300 1100–1200 1100–1200 1000–1200 1000–1200 950–1000
Coastal forest and derived thickets, Mangrove Semi-deciduous forest Semi-deciduous forest Swamp and semideciduous forest Semi-deciduous forest, woodland, and riparian forest Dry forest, woodland, and riparian forest Dry forest, woodland, and riparian forest Dry forest, woodland, and riparian forest Riparian forest, dry forest, and woodland Tree and shrub savanna, dry forest and riparian forest
R: Rainfall regime: Bi: Bimodal (2 rainy seasons), TUn: Tendency to unimodal, Un: Unimodal (1 rainy season). PChor: Phytochorological zones based on the composition in distribution range types: GC: Guineo-Congolian zone, GS: Guineo/Sudanian transition zone, S: Sudanian zone.
from the Coastal to the Mekrou-Pendjari phytogeographical district (in the north western part of Benin), across nine phytogeographical districts namely: Coast, Plateau, Pobè, Ouemé-Valley, Zou, Bassila, South-Borgou, Atacora-Chaine and Mekrou-Pendjari. The second transect (250 km) is located between the Mekrou-Pendjari phytogeographical districts (from north western to north eastern of Benin) and in addition includes Atacora-Chaine and North-Borgou districts. Each phytogeographical district was explored to detect bowal (ferricrete exposure) occurrence. A minimum unit of 30 m × 30 m of ferricrete landscape was considered for assigning a bowal surface. When bowal was detected, the center coordinate of the bowal was marked with a Global Positioning System (GPS) in order to relate it with ArcGIS 10 (ESRI Inc., Redlands, USA) to the soil map of Benin (Volkoff and Willaine, 1976) and to the map of the phytogeographical districts (Adomou et al., 2006). The minimum distance between consecutive points for GPS measures was 5 km along the transects. In total, 36 sampling sites were obtained on the first transect and 18 on the second. 2.2.2. Physicochemical characteristics of soil samples on bowé and adjacent woodlands Within each of the six phytogeographical districts where bowé were identified, nine homogeneous topsoil sample pairs were taken on bowé and adjacent woodlands. Woodland soil was sampled as adjacent material in protected areas (Fig. 3). 17 woodland samples were considered for the entire study as some of the bowé sites were adjacent to the same woodland sample. We used a soil auger to dig a hole to a depth of 10 cm because bowal has a hardened soil (ferricrete) with only thin shallow topsoil (Fig. 1). Out of the nine sample pairs we selected randomly one composite topsoil sample composed of the nine homogenous ones from bowal and woodland of each phytogeographical district for further analysis. The composite soil samples were dried in the laboratory at 105 °C, then crushed and sieved through a screen of 2 mm. The particle size was measured by the pipette method (Loveland and Whalley, 1991). The electrical conductivity (EC) and the pH-value were measured according to the methods of Rhoades (1982). Soil organic matter (OM) was determined by the K2Cr2O7– H2SO4 oxidation method of Walkey and Black (Nelson and Sommers, 1982). The total nitrogen was extracted by using the Kjeldahl System 1026 distilling unit. Potassium was determined after extraction of ammonium acetate-extractable cations using atomic absorption spectrometry (Thomas, 1982). The extractable phosphorus was analyzed using a Bray-2 extract (Bray and Kurtz, 1945). Principal component analysis (PCA) was applied to the composite soil data of the bowé plots in order to determine the physicochemical characteristic of the bowé soils of each phytogeographical district. With the non-parametric Mann–Whitney test the differences of the soil characteristics between the bowé and woodland sites were investigated per phytogeographical district. All analyses were performed with the SAS software (SAS Inc., 1999) and PC-ORD Version 5 (McCune et al., 2002).
3. Results 3.1. Spatial distribution of bowé Bowé of high and low plateaus and hillslopes expose three types of ferruginous soils: depleted tropical ferruginous soils, slightly leached tropical ferruginous soils and leached tropical ferruginous soils (Fig. 4). Bowé are frequent on leached tropical ferruginous soils (30 of 54) and slightly leached tropical ferruginous soils (22 of 54) than on depleted ferruginous soils (2 of 54) (Fig. 4). Bowé are encountered in six of the ten phytodistricts of Benin: in North-Borgou, South-Borgou, Mekrou-Pendjari, Bassila, AtacoraChaine and Zou. Bowé are not found in the phytodistricts of Plateau, Ouemé-Valley, Pobè and Coast (Fig. 3). Bowé are frequent in NorthBorgou (13 of 54), South-Borgou (11 of 54), Mekrou-Pendjari (10 of 54), Bassila (10 of 54) and Atacora-Chaine (8 of 54) than in Zou (2 of 54) (Fig. 3). In the phytogeographical districts where bowé are frequent (North-Borgou, South-Borgou, Mekrou-Pendjari and Atacora-Chaine) the rainfall regime is unimodal with one rainy season from April to October. Annual precipitation ranges here from 950–1,200 mm (Table 1). In the phytogeographical disctrict of Zou, with low occurrences of bowé, the rainfall regime is also unimodal with amounts of 1100–1200 mm per year. In the phytogeographical districts where bowé do not occur, the rainfall regime is bimodal with two rainy seasons (a long rainy season from March to July and a short from September to October). The annual precipitation varies between 900 and 1300 mm (Table 1). 3.2. Physicochemical characteristics of soil samples on bowé and woodlands A PCA performed on the physicochemical characteristics of soil samples of the bowé plots in six phytogeographical districts resulted in two axes explaining 77.3% of the overall variance. Organic matter, phosphorus, total nitrogen and silt are positively correlated with the first axis (Axis 1) while potassium is negatively correlated with the first axis (Table 2). The first axis (Axis 1) represents bowé plots with the highest value of organic matter, phosphorus, total nitrogen and silt and the lowest value of potassium. Electrical conductivity, pH, and sand are positively correlated with the second axis (Axis 2) while clay is negatively correlated with the second axis (Table 2). The second axis (Axis 2) represents bowé plots with the highest values of electrical conductivity, pH and sand and the lowest content of clay. The phytogeographical districts of North-Borgou, Atacora-Chaine and Mekrou-Pendjari are positively correlated with the first axis, while South-Borgou, Bassila and Zou are negatively correlated with this axis (Fig. 5). Bowé with the highest values of organic matter, phosphorus, total nitrogen and silt and the lowest value of potassium are located in the phytogeographical districts of North-Borgou, AtacoraChaine and Mekrou-Pendjari (Table 3). Bowé with lowest values of organic matter, phosphorus, total nitrogen and silt and the highest value
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Fig. 3. A map of the phytogegraphical district showing the location of the 2 transects with protected areas and bowé soil sampling sites considered.
of potassium are located in the phytogeographical districts of SouthBorgou, Bassila and Zou (Table 3). The phytogeographical districts of Mekrou-Pendjari, AtacoraChaine, South-Borgou and Zou are positively correlated with the second axis, while North-Borgou and Bassila are negatively correlated with it. We conclude that bowé from Mekrou-Pendjari, Atacora-Chaine, SouthBorgou and Zou have the highest value of electrical conductivity, pH
and sand and the lowest content of clay while bowé from NorthBorgou and Bassila have the lowest value of EC, pH and sand and the highest content of clay (Fig. 5; Table 3). The comparison between the soils from bowé and woodlands (Table 4) show that bowé plots have significantly lower (Mann–Whitney test, p b 0.05, n = 6) electrical conductivity (median of 25.96 ms/cm vs. 68.36 ms/cm), lower organic matter content (8.60 gkg− 1 vs.
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Fig. 4. Spatial distribution of bowal on the phytogegraphical districts (Adomou et al., 2006) and soil map of Benin (Volkoff and Willaine, 1976) in 2011.
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Table 2 Correlation coefficient of soil variables on the principal component axes. Soil variables
Organic matter Extractable phosphorus Electrical conductivity Total nitrogen pH Potassium Silt Sand Clay
Correlation coefficient Axis 1
Axis 2
0.65 0.89 −0.53 0.96 −0.48 −0.86 0.78 −0.34 −0.55
0.63 0.07 0.67 −0.21 0.83 0.12 −0.02 0.76 −0.70
23.8 gkg−1), lower extractable phosphorus (0.01 gkg−1 vs. 0.02 gkg−1), lower silt content (9.25% vs. 30.17%) and lower total nitrogen (0.30 gkg− 1 vs. 0.80 gkg− 1) than the woodland soils. Exchangeable potassium is higher (0.65 cmol(+)kg−1 vs. 0.27 cmol(+)kg−1) in bowé than woodland soils. 4. Discussion 4.1. Spatial distribution of bowé Bowé cover plateaus (high and low) and hillslopes in six of the ten phytogeographical districts of Benin, which correspond to the distribution described by earlier studies (Beauvais, 1999, 2009; Beauvais and Roquin, 1996; Faure and Volkoff, 1998). Bowé are confined to different ferruginous soil types namely depleted tropical ferruginous soils, slightly leached tropical ferruginous soils and leached tropical ferruginous soils. This corresponds to previous observations made by Maignien (1958), CPCS (Commission de Pédologie et de Cartographie des Sols) (1967), Grandin and Thiry (1983); and Faure and Volkoff (1998) also linked to the occurrence of hardened ferruginous soils (ferricrete) in West Africa. The low occurrence of bowé in the phytogeographical district of Zou may be explained by the lack of ferruginous soils. Indeed, the other five phytogeographical districts where bowé occur are mainly
covered with leached and slightly leached tropical ferruginous soils. The phytogeographical district of Zou is also covered by hydromorphic soils on which ferricrete is not found (Faure and Volkoff, 1998). The absence of bowé in the phytogeographical districts of Plateau, Ouemé-Valley, Pobè and Coast can be explained by the rainfall regime. Here two rainy seasons are observed with higher precipitation, which lead to an accelerated biophysical disaggregation of the ferricretes and a transformation to ferralitic soils. Ferralitic soils are the main soil types in these districts (Faure and Volkoff, 1998). The more humid condition leads to biodegradation and oxidation of organic matter that modifies the redox processes triggering the degradation of ferricrete (Beauvais, 2009). Since rainfall is an important factor of the biophysical disaggregation of ferricrete (Beauvais, 1999, 2009; Fernández-Caliani and Cantano, 2010; Huang et al., 2013; May et al., 2011; Porder et al., 2007; Soumya et al., 2011), we may assume here that the observed rainfall regime plays an important role in the spatial distribution of bowé in Benin. 4.2. Physicochemical characteristics of bowé and adjacent woodland soils The physicochemical characteristics of bowé vary according to the phytogeographical districts where they occur. In the northernmost districts (North-Borgou, Atacora-Chaine and Mekrou-Pendjari), bowé ferricretes have the highest value of organic matter, total nitrogen and silt and the lowest value of potassium. Mekrou-Pendjari, AtacoraChaine, South-Borgou and Zou have the highest value of electrical conductivity, pH and sand, and the lowest value of clay. These results suggest that even if bowal is referred to as a particular form of land degradation its physicochemical characteristics are not the same in all phytogeographical districts of Benin. The variations of the physicochemical characteristics of the bowé soils of the different phytogeographical districts may be a result of previous land used practices, which are known to affect the physicochemical characteristics of soils (Jha et al., 2010). The comparison of the physicochemical characteristics of bowal and woodland soils revealed that bowal plots had significantly lower values of electrical conductivity, organic matter, total nitrogen, silt and extractable phosphorus but higher values of exchangeable potassium than
Fig. 5. Projection of the phytogeographical districts with bowal on Principal Component Analysis axes defined by physicochemical characteristics. NBorgou (north-Borgou); SBorgou (south-Borgou); MPendj (Mekrou-Pendjari); Bassila (Bassila); Atchaine (Atacorachaine); Zou (Zou); OM (Organic matter); P (Extractable phosphorus); EC (Electrical conductivity); TotalN (Total nitrogen); pH (pH); K (Potassium); Silt (Silt); Sand (Sand); Clay (Clay).
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Table 3 Composite soil characteristics on bowé in the phytogeographical districts. Soil variables
Organic matter (g kg−1)
Extractable phosphorus (g kg−1)
Electrical conductivity (mS cm−1)
Total nitrogen (g kg−1)
pH
Potassium (cmol(+) kg−1)
Silt (%)
Sand (%)
Clay (%)
North-Borgou South-Borgou Mekrou-Pendjari Bassila Atacora Chaine Zou
11.8 10 16 4.9 18.7 10.9
0.01 0.01 0.01 0.01 0.01 0
14.2 24.8 30.1 24.8 25 28.3
0.7 0.31 0.56 0.31 0.5 0.2
4.25 4.68 4.75 4.54 4.8 4.92
0.15 0.76 0.22 0.52 0.18 0.67
17.5 11.5 16 6.25 8.75 10
56.25 68.5 76.75 42.5 55 78
13.75 20 17.5 51.25 11.75 12
woodland. Similar results were found in Burkina Faso where nutrient availability in soil is low on bowé compared to savanna (Zwarg et al., 2012). This may be explained by the level of degradation of these soils. Indeed on degraded land, these parameters are always low (Bewket and Stroosnijder, 2003; Jha et al., 2010; Mills and Fey, 2004; Zhao et al., 2005). The high values of exchangeable potassium observed on bowé may be attributed to the weathering of ferricrete or previous land use on bowé. During the process of weathering of ferricrete a strong chemical alteration is observed and potassium is highly leached (Fernández-Caliani and Cantano, 2010; Yang et al., 2009). High amounts of potassium increase soil erodibility (Auerswald et al., 1996; Miller and Scrifres, 1988) and cause land degradation. Indeed large quantity of exchangeable potassium in soil increases the potential for flocculation or dispersion processes within the soil (Quirk, 1978; Shainberg et al., 1987a; Keren, 1991). These processes influence the hydraulic properties of the soil, crust formation, runoff generation and soil erosion (Ahuja, 1990; Gerits et al., 1987; Lavee et al., 1991). 5. Conclusion In Benin, bowé are distributed exclusively in six of ten phytogeographical districts associated with hardened ferruginous soils (ferricrete) and unimodal rainfall distributions. Bowé soils have significantly lower values of electrical conductivity, organic matter, total nitrogen, silt and extractable phosphorus than woodland soils. However the amount of exchangeable potassium is higher in bowé soils than in woodland soils. The disaggregation of bowé ferricrete may improve the resulting soil physicochemical characteristics and sustain the regrowth of forest. Acknowledgment We wish to thank the Robert S. McNamara Fellowships Program and the UNDESERT (EU FP7 243906) project, “Understanding and combating desertification to mitigate its impact on ecosystem services” funded by the European Commission, Directorate General for Research and Innovation, Environment Programme for financial support (Grant Agreement number 243906). We are also grateful to the editor and Table 4 Mann–Whitney test results for bowé and woodland plots comparisons in Benin. Bowé
Woodland Mann–Whitney test
Organic matter (g kg−1) Extractable phosphorus (g kg−1) Electrical conductivity (mS cm−1) Total nitrogen (g kg−1) pH Potassium (cmol(+) kg−1) Silt (%) Sand (%) Clay (%)
m
m
8.60 0.01 25.96 0.30 4.71 0.65 9.25 63.00 27.75
23.80 0.02 68.36 0.80 5.41 0.27 30.17 49.58 20.58
−1.52* −13.11* −42.40* −0.05* −0.70 ns 0.38* −20.92* 13.42 ns 7.17 ns
The sample size is 6; m: median; ns: non significant at 0.05; *: significant at 0.05.
Anicet Beauvais for useful comments and suggestions which contributed to improve the quality of this paper.
Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.catena.2014.08.022. These data include Google map of the most important areas described in this article.
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