Land use management effects on soil hydrophobicity and hydraulic properties in Ekiti State, forest vegetative zone of Nigeria

Catena 155 (2017) 170–182

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Land use management effects on soil hydrophobicity and hydraulic properties in Ekiti State, forest vegetative zone of Nigeria Idowu Ezekiel Olorunfemi a,⁎, Johnson Toyin Fasinmirin a,b a b

Department of Agricultural and Environmental Engineering, Federal University of Technology, Akure, Nigeria Department of Agricultural and Biosystems Engineering, Landmark University, Omu Aran, Nigeria

a r t i c l e

i n f o

Article history: Received 11 March 2016 Received in revised form 8 March 2017 Accepted 13 March 2017 Available online 22 March 2017 Keywords: Hydraulic conductivity Sorptivity Soil organic matter Croplands Plantations Natural forests

a b s t r a c t This study was conducted to characterize soil hydrophobicity, unsaturated hydraulic conductivity and sorptivity under different land uses (i.e croplands, plantation agriculture and natural forests) and soil types in southwestern Nigeria. In this study, a total of 105 different points in 35 different locations comprising of the 3 land uses were sampled in the study areas. Random sampling pattern of 3 sampling points per sample location were carried out and undisturbed soil samples were collected at depths up to 15 cm from the different locations. Handheld mini disk infiltrometer at a steady-state flow of −2 cm water suction rate was used to determine the unsaturated hydraulic conductivity, water and ethanol sorptivity at each land use site. In addition, the effects of antecedent soil moisture contents (MC), soil bulk density (BD), total porosity (PT), soil water holding capacity (WHC), organic matter content (SOM), and cation exchange capacity (CEC) on soil hydrophobicity, unsaturated hydraulic conductivity and sorptivity were determined. The mean hydrophobicity index, R, showed a decreasing trend in the order: natural forest ˃ plantation agriculture ˃ croplands, whereas, mean hydraulic conductivity values showed an increasing trend in the order: natural forest b plantation agriculture b croplands. Hydraulic conductivity resulted to a negative correlation with hydrophobicity among all sampled soils. In all the sampled soils, index of soil hydrophobicity (R) correlated significantly (p ≤ 0.01) with organic matter content, organic carbon and cation exchange capacity (CEC). Soil sorptivity to water correlated negatively with moisture content among all samples at p ≤ 0.05. Soil ethanol sorptivity showed significantly positive correlation with organic carbon, organic matter content and cation exchange capacity among all the soil samples at a p ≤ 0.05. Soil properties such as organic matter content, bulk density, and aggregate sizes influence the infiltration characteristics of soils of the study areas. Findings from this research has provided a better understanding of soil characteristics and water management under different land uses, which will be of utmost usefulness to land managers, growers, hydrologist and soil scientists. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Soil hydrophobicty (water repellency) has become a subject of global concern, with substantial effects on plant production, land use and management (Müller and Deurer, 2011; Vogelmann et al., 2013). Soil hydrophobicity is caused through the production of complex organic acids during the decomposition of organic matter and these complex organic acids are wax-like substances derived from plant material during organic matter decomposition or burning during a hot fire that form a coating over particles of soil (Franco et al., 2000). It is a phenomenon documented in several countries around the world, and can be responsible for enhanced surface runoff, erosion and preferential flow (Vogelmann et al., 2013). Soil hydrophobicity has been documented in ⁎ Corresponding author at: Federal University of Technology, Akure, Nigeria. E-mail address: [email protected] (I.E. Olorunfemi).

http://dx.doi.org/10.1016/j.catena.2017.03.012 0341-8162/© 2017 Elsevier B.V. All rights reserved.

cultivated lands, pastures, forests (Doerr et al., 2006) and wildlands (Crockford et al., 1991; Doerr et al., 1996; Lin et al., 1996; Scott, 2000; DeBano, 2000). Several soil factors affect the origin and severity of soil hydrophobicity one way or another (Cesarano et al., 2016). Some of these factors include organic matter content, soil texture, aggregation state, soil moisture content, fire intensity, and soil pH etc. (Vogelmann et al., 2010; Olorunfemi et al., 2014; Cesarano et al., 2016). The occurrence of soil water repellency or hydrophobicity is not limited to any particular soil type as numerous researches have reported that soil texture i.e. the proportion of different particle sizes (sand, silt and clay) in a soil influences the degree of hydrophobicity (Wallis et al., 1991; Dekker et al., 2005, Lellamanei et al., 2010). Though coarse-textured, sandy soils are most likely to become hydrophobic because of their relatively small surface area per unit of volume (Karnok and Tucker, 2002). The ease of coating of sand by hydrophobic substances (Wallis and Horne, 1992),

I.E. Olorunfemi, J.T. Fasinmirin / Catena 155 (2017) 170–182

and their susceptibility to acidification favour soil hydrophobicity (Deurer et al., 2011; Schwen et al., 2015). Similar results were found and documented in loamy, peaty clay and clayey peat soils (McGhie and Posner, 1980; Dekker and Ritsema, 1996a, 1996b), as well as in heavy clay soils with grass covers (Dekker and Ritsema, 1996c). Organic matter content has also been shown to have positive correlation with soil water repellency in range of studies (e.g. Berglund and Persson, 1996; Taumer et al., 2005), while others reported little or no relationship (Jungerius and de Jong, 1989; Doerr et al., 2005; Doerr et al., 2006). Soil pH is another static site-dependent controlling factor for the degree of soil hydrophobicity. Steenhuis et al. (2001); Woche et al. (2005); and Schwen et al. (2015) reported an inverse relationship between soil hydrophobicity and soil pH. The soil moisture is also an important component that can prevent or lead to the formation and persistence of a hydrophobic layer (Olorunfemi et al., 2014). It is the main risk factor responsible for the high variability of this phenomenon in the soil (Hallett, 2008). Subedi et al. (2013) reported that the coating of mineral soil particles or aggregates with partly hydrophobic soil organic matter caused the dependence between soil hydrophobicity and the soil water content. Prolong exposure to critically low water contents causes the arrangement of organic matter in the soil to change (i.e shape of the polar compound changes) so that the hydrophobic surface is exposed to the air/water in soil pores. Whereas under moist conditions, the hydrophilic surface of amphiphilic soil organic matter molecules is exposed to the air/water in soil pores (Olorunfemi et al., 2014; Schwen et al., 2015). Dekker and Ritsema (1994) established a transition zone or a critical soil moisture zone, defined by two water content thresholds. When soil moisture is above this critical value (which varies for every soil), the water repellency effect is temporarily eliminated but when it falls below this critical value, the soil returns to a hydrophobic condition. In general, the degree of soil hydrophobicity depends mainly on soil texture, quantity and quality of soil organic matter and the soil water content (Keck et al., 2016). These findings revealed that hydrophobicity is not an isolated curiosity as it has been found in soils all over the world (Dekker et al., 1999; Franco et al., 2000; Scott, 2000; Doerr et al., 2003). Wallis and Horne (1992) equally reported soil hydrophobicity under a range of crops and cropping systems (Wallis and Horne, 1992). Thus, it should be a point of concern in the rain forest region of Nigeria. Hydrophobic soils repel water, thus, reducing water infiltration into soil. Decreased infiltration into the soil results in damaging flows in stream channels. Erosion increases with greater amounts of runoff, and much of the fertile topsoil layer is lost. Increased runoff carries large amounts of sediment that can spread over lower lying areas, clog stream channels, and lower water quality (Olorunfemi et al., 2014). Few studies have investigated the impacts of soil hydrophobicity on soil hydraulic properties. Bauters et al. (2000) and Lamparter et al. (2010) found a linear relationship between contact angle (quantitative measure of the wetting of a solid by a Liquid) and the air-entry value (inverse of the van Genuchten α parameter) in controlled laboratory investigations on sand and glass beads with different degrees of soil water repellency. Laboratory studies by Arye et al. (2007) and Subedi et al. (2013) also confirmed this relationship. A contact angle of zero represents complete wetting (hydrophilic) while a contact angle N 90° is said to be non-wetting (hydrophobic/water repellent) (van Genuchten and Leij, 1992; Olorunfemi et al., 2014). However, most soils have a certain level of water resistance, where water will infiltrate but at a slower rate than expected as demonstrated by Tillman et al. (1989). These soils have contact angles between 0° and 90°. Schwen et al. (2015) studied the impacts of soil water repellency on effective soil hydraulic characteristics with the perspective to include water repellency effects into advanced soil hydrological models in a beech forest under simulated rainfall. Their findings confirm that the postulated linear relationship between contact angle and the air-entry value is applicable to natural field soils. They also

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affirmed that increased soil water repellency (SWR) levels strongly reduced near-saturated hydraulic conductivity. Studies by Hallett et al. (2001, 2004); Lamparter et al. (2006) and Orfánus et al. (2008) suggested that the impact of subcritical water repellency (WR) on water sorptivity, conductivity and infiltration rates is underestimated and Lamparter et al. (2006) found subcritically water repellent soils to reduce infiltration rates by a factor of 3 to 170. All these findings explain why the study of hydrophobicity is very important considering its impacts on soil hydraulic properties i.e. soil water sorptivity and unsaturated hydraulic conductivity. Assessment of the hydraulic properties of soil, such as infiltration and sorptivity, is very important component for the interpretation of the physical characteristics of soil and the management of agricultural practices (Green et al., 2003; Vogelmann et al., 2010). It is an important step in understanding the water dynamic and solution transport in the soil matrix. Soil hydraulic properties reflect the ability of a soil to retain or transmit water and its dissolved constituents (van Genuchten and Leij, 1992). Soil hydraulic properties are also important for modelling hydrological processes and related contamination transport (Xu et al., 2009). Soil hydraulic properties are active and changing, this is due to factors such as rainfall, irrigation, wetting and drying cycles and most especially cropping systems (Mapa et al., 1986). Lekamalage (2003) further reported that soil, soil surface and agricultural management are the three categories of factors affecting hydraulic properties. As human activities, such as agricultural practices (ploughing and sowing) change, land use related to deforestation or reforestation of abandoned agricultural land can significantly affect topsoil and first layers soil properties and consequently hydraulic (Gonzalez-Sosa et al., 2010) and soil water repellency properties. Therefore, research dealing with soil hydrophobicity and hydraulic properties under different land uses is of great interest as the evaluation of the soil properties affecting them is essential for understanding the influences of human activities on soil water movement and possible implications for livelihoods. In the literature of tropical soils, large areas in the humid tropics have been subjected to dramatic land use (LU) and land cover (LC) changes over the last few decades (Chang and Lau, 1993; Bonell et al., 2010). Giertz and Diekkruger (2003) and Giertz et al. (2005) assessed the effects of land use change on soil physical properties and hydrological processes in the sub-humid tropical environment of West Africa. Also, the effect of land use on saturated hydraulic conductivity and hydrological flow paths has been the focus of many studies in the last decades (Hanson et al., 2004; Zimmermann et al., 2006; Chaves et al., 2008; Germer et al., 2010; Hassler et al., 2011). Previous studies have also been carried out and documented on soil hydrophobicity in the humid tropical climates (Vogelmann et al., 2010; Cambronero et al., 2011). Vogelmann et al., 2013 reviewed the correlation between origin of soil hydrophobicity and hydro-physical processes and soil properties. Despite these research efforts, there is scarcity of data, typical for tropical soils especially on the soil hydrophobicity and hydraulic characteristics under cropland, plantations and natural forest in the forest vegetative zone of Nigeria. Beyond the above studies, little is known about the effects of soil water repellency on soil hydraulic characteristics and there is generally poor understanding of the soil parameters that affect water repellency, the prediction of its occurrence and severity in the humid tropical region of Nigeria and Africa. The increasing dry climate and reductions in the availability of irrigation water has led to a situation where soil water repellency has emerged as an issue facing gardeners, farmers, land managers, hydrologists, and soil scientists. Given the extensive researches on the prevalence of soil water repellency in the humid temperate regions of the world, and its effects on crop performance or soil erosion, one might expect a higher degree of soil hydrophobicity in terms of prevalence and severity in the tropical regions of Nigeria owing to increasing dry climate. We also expect the degrees of soil hydrophobicity to be very different among the different land uses depending on the various soil factors affecting the occurrence

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and severity of soil hydrophobicity. Therefore, the aims of this research were to: i. quantify and compare the effects of different land use management (natural forest, plantation, and cropland) on hydrophobicity and hydraulic characteristics of soil; ii. examine the relationship between soil hydrophobicity, hydraulic characteristics and soil physico – chemical properties for a better understanding of which soil parameters affect soil hydrophobicity and hydraulic characteristics; iii. identify the impacts of soil hydrophobicity on the hydraulic characteristics of the soils (i.e. unsaturated hydraulic conductivity and soil water sorptivity). 2. Materials and methods 2.1. Experimental site and procedure 2.1.1. Experimental site The study was conducted in Ekiti State in the forest vegetative zone of Nigeria. Ekiti State is located between Latitudes 7° 15′ to 8° 5′ N and Longitude 4° 45′ to 5° 45′ E and occupies a land area of about 6, 353 km2 (EKSG, 2009). The State is mainly an upland zone with elevation ranging from 250 to 540 m above mean sea level (a.m.s.l.) (Simon-Oke et al., 2012). The State lies on an area underlain by metamorphic rock and is potentially rich in mineral deposits, which include kaolin, columbite, channockete, iron ore, barite, aquamine, gemstone, phosphate, limestone, gold among others, largely deposited in different towns and

villages within the State. The climate of the State is tropical with two distinct seasons (rainy season (April – October) and dry season (November–March)). The mean air temperature ranges between 21° and 28° with high humidity. The dominant soils in Ekiti state are the Egbeda series and Iwo series (Smyth and Montgomery, 1962), which under the FAO/UNESCO classification can be classified as Orthic and Plinthic Luvisols, respectively (FAO, 1998). Ekiti State consists of guinea forest vegetation with its attendant climate, flora and fauna (EKSG, 2009). 2.1.2. Experimental procedure Field experiments were carried out between January and March 2014 during the dry season. Soil sampling was carried out across 35 different locations under different land uses in Ekiti State (Fig. 1) to determine their hydrophobicity/water repellency index. The different land uses (treatments) in the study area include intensive (row crops and minor grazing), agricultural tree crop and forest plantations (Tectona grandis, Gmelina arborea, Elaeis guineensis, Musa acuminata), dominantly trees/woodlands/shrubs, disturbed and undisturbed forests. The croplands have been put under manual tillage (using Cutlass and Hoe) with cassava and yam cultivation sometimes intercropped with maize for N15 consecutive years according to the farmers. Tectona grandis plantations range from 25 to 29 years ago, while Musa acuminata has been under cultivation for over 10 years. Elaeis guineensis plantations are about 23 years old. The forest soils are uncultivated and comprise of shrubs, woodlands and deciduous trees under the protection of the state forest reserve agency. The land uses studied were categorized under three (3) main treatments which include croplands (CP), plantation agriculture (PA) and natural forests (NF).

Fig. 1. Land use and land cover map of Ekiti State. (Source: EKSG, 2009).

I.E. Olorunfemi, J.T. Fasinmirin / Catena 155 (2017) 170–182

In all the 35 locations, 17 croplands, 11 agricultural plantation sites and 7 natural forests were examined. Three sampling points were randomly selected per location for each land use treatment and detailed infiltration measurements. In total, 105 sampling points were examined on the soil surface layer for the determination of soil hydrophobicity, hydraulic conductivity, and water and ethanol sorptivity. For the determination of soil moisture content, bulk density, total porosity, macro and micro porosity, three (3) sampling points were randomly selected per location and three (3) undisturbed samples were collected at each sampling point. Soil samples were collected using stainless steel cylindrical cores of height 5 cm and diameter 4.2 cm. Soil samples collected were packed in plastic bags and transferred to the laboratory for further analysis. 2.2. Measurements 2.2.1. Physico-chemical characterization of soils Chemical characterization of the collected soil samples included the analysis of organic matter (SOM), organic carbon (SOC), cation exchange capacity (CEC) at pH 7.0, base saturation, Al3+ saturation and soil pH; whereas the physical characterization consisted of particle size analysis, bulk density (BD) and total porosity (PT) determination. The samples were allowed to dry in the open air until reaching friability. The organic carbon was determined using the Walkley-Black wet oxidation procedure and the soil organic matter content was determined from the organic carbon (Nelson and Sommers, 1996). The cation exchange capacity (CEC) at pH 7.0 was determined following the procedures described by Reeuwijk (2002). The phosphorus (P), exchangeable potassium (K+) and sodium (Na+) were extracted with HCl solution and their levels determined by flame photometry (Vogelmann et al., 2010) and exchangeable magnesium (Mg2 +) and calcium (Ca2+) by atomic absorption spectrophotometer (Senjobi and Ogunkunle, 2010). Soil pH was determined in distilled water using the pH meter with water ratio of 1:2. The bulk density (BD) was obtained by the gravimetric soil core method described by Blake and Hartge (1986) and the particle density (PD) was assumed to be 2.65 g cm−3 (Li and Shao, 2006; Zhang et al., 2006; Price et al., 2010). The total porosity (PT) was obtained from BD and PD using the equation and relationship developed by Danielson and Sutherland (1986). Soil particle sizes were determined using the hydrometer method as described by Agbede and Ojeniyi (2009). Micro porosity (Mic) and macro porosity (Mac) were also obtained using Eqs. (1) and (2) as described by Adesigbin and Fasinmirin (2011); Micro Porosity ðMicÞ ¼

Ww − Wd Vc

Macro Porosity ðMacÞ ¼ PT−Mic

ð1Þ ð2Þ

where Ww and Wd are wet weight and constant oven dried weight of soil samples (g) respectively and Vc is the volume of the soil core (cm−3). 2.2.2. Hydraulic conductivity (K) and sorptivity (S) The minidisk infiltrometer (Decagon Devices, Inc., Pullman, WA), a hand-held field instrument was used to rapidly assess soil infiltration capacity. Unsaturated hydraulic conductivity, water sorptivity and ethanol sorptivity were calculated from the infiltration data obtained during field measurement. Suction rate of 2 cm per seconds was chosen at different points on the field for the infiltration measurement. The 2 cm setting is adequate for most soils (Decagon devices, 2011) and is optimal suction rate for infiltration measurement from our past field test and measurements. The data collected were then used to calculate the water infiltration rates of the soil. The unsaturated hydraulic conductivity of soil was calculated using the method of Zhang (1997). The method requires measuring

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cumulative infiltration vs. time and fitting the results with the infiltration function (3): I ¼ C1t þ C2√ t

ð3Þ

where C1 (m s−1) and C2 (m s−1/2) are parameters. C1 is related to hydraulic conductivity, and C2 is the soil sorptivity. The unsaturated hydraulic conductivity of the soil (K) was then computed using the relationship in Eq. (4): K¼

C1 A

ð4Þ

where C1 is the slope of the curve of the cumulative infiltration vs. the square root of time, and A is a value relating the van Genuchten parameters for a given soil type to the suction rate and radius of the infiltrometer disk. A is computed from: A¼

A¼


11:65 n0:1 −1 exp½2:92ðn−1:9Þαh0  ðαr 0 Þ0:91
11:65 n0:1 −1 exp½7:5ðn−1:9Þαh0  ðαr 0 Þ0:91

n ≥1:9

n≤1:9

ð5Þ

ð6Þ

where n and α are the van Genuchten parameters for the soil, r0 is the disk radius and h0 is the suction at the disk surface. 2.2.3. Hydrophobicity/water repellency index An Index of soil water repellency, R, was determined from the sorptivities of 95% ethanol and water. The water reservoir of the minidisk infiltrometer was filled with ethanol to conduct the ethanol sorptivity measurement, and later with fresh water to measure the water sorptivity of soil. The bubble chamber was filled with fresh water in both cases and the suction rate of 2 cm was selected for sorptivity measurement. Soil sorptivity to ethanol (Se), which is the slope of the cumulative infiltration vs. square root of time (√t) relationship was calculated as follows: I ¼ Se √ t

ð7Þ

where soil sorptivity to ethanol (Se) is in (cm s−1/2), I is the early cumulative infiltration (cm) and t is time (s). Likewise, soil sorptivity to water (Sw) which was determined from the slope of the cumulative infiltration vs. square root of time relationship was calculated as follows: I ¼ Sw √ t

ð8Þ

where the sorptivity of water (Sw) is also given in (cm s−1/2). Soil repellency index R by Tillman et al. (1989) is the ratio of the soilethanol sorptivity (Se; cm s− 1/2) to the soil-water sorptivity (Sw; cm s−1/2) and was estimated from the relationship in Eq. (7): R ¼ 1:95

Se Sw

ð9Þ

The constant (1.95) accounts for the difference in surface tension and the viscosity of ethanol and water (Vogelmann et al., 2010). The angle of soil–water contact (θ) in degree was obtained from the formula of Gryze et al. (2006) in Eq. (8) as follows: θ ¼ arccos

1 R

ð10Þ

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2.3. Statistical analysis Hydraulic conductivity and soil sorptivity were subjected to statistical analysis to determine the mean, standard deviation, and coefficient of variation, linear and non linear regressions using Statistical Analysis System (SAS) (SAS Institute Inc., Cary, NC) and MINITAB 17 under different land uses. The existence of inter-relationships between data set was tested by linear correlation and the correlation coefficients determined at 1 and 5% levels of significance. Pearson correlation coefficient at 1 and 5% significant levels was used to evaluate the relationships between soil hydrophobicity, unsaturated hydraulic conductivity, water and ethanol sorptivity and other soil properties. One way ANOVA was used to test for statistical significance in soil properties under different land uses and post hoc comparison was used to compare the hydrophobicity index. 3. Results and discussion 3.1. Hydrophobicity/water repellency index (R) and land use types The hydrophobicity of the different land uses (cropland, plantation agriculture and natural forest) is presented in Table 1. The mean hydrophobicity index decreased in order: NF N PA N CP with values 5.41 ± 4.20 N 1.77 ± 0.50 N 1.5 ± 1.48, respectively. The average repellency index (R) of the natural forest soils was approximately 3 times greater than plantations and 4 times greater than the croplands. Difference in hydrophobicity among croplands, plantations and natural forest soils (Fig. 3) were statistically significant (p ≤ 0.001). Difference in mean hydrophobicity index between natural forests and the other two land uses,

that is, plantations (t = − 3.391; p = 0.004) and croplands (t = − 3.962; p = 0.001) (Table 7) were statistically significant. Mean hydrophobicity index of croplands and plantations did not differ significantly (Table 7). The surface soil of the natural forests had a much greater repellency and lower hydraulic conductivity (Fig. 2). This is due to the activity of microorganisms capable of producing hydrophobic compounds. Microbial activity breaks down dead plant material in such a way that it contributes to the development of water repellence in susceptible soils (Moore, 2004). Natural forest soils generally feature higher litter material, organic inputs, plant roots and exudates in the topsoil (Price et al., 2010) which when decomposed produced hydrophobic substances (Franco et al., 2000) resulting in greater degree of hydrophobicity than soils under plantations and croplands. Croplands has the least mean hydrophobicity index which may be due to frequent soil tillage/cultivation, which causes the abrasion of hydrophobic soil particles thereby removing hydrophobic coatings from soil surfaces (Buczko et al., 2002, Buczko et al., 2006, Olorunfemi et al., 2014). Only two croplands sites (CP 2 and CP 5) exhibited strong water repellency (Table 1). CP 2 which exhibited strong hydrophobicity was noted for its high altitude and characteristically steep slope, while trace of bush burning was noticed in CP 5. Topography and type of land use are among the main environmental features and soil management factors that greatly influence surface soil hydraulic properties (Rawls et al., 1993). The unique topography with steep slope and high elevation of about 500 m above sea level of CP 2 might have resulted in accelerated erosion and a distinct severe harmattan with high temperatures in the dry season of the experimental period. Increase in soil temperature causes dehydration (drying process), leading to changes in the arrangement of organic matter in the soil (i.e. shape of the polar

Table 1 Means of soil sorptivity to water (Sw), sorptivity to ethanol (Se), water repellency index (R), contact angle and hydraulic conductivity in the sampled soils. Land use

Site code

Sw (cm h−1/2)

Se (cm h−1/2)

R

Contact Angle θ (degree)

K (cm h−1)

Repellency rating

Croplands

CP 1 CP 2 CP 3 CP 4 CP 5 CP 6 CP 7 CP 8 CP 9 CP 10 CP 11 CP 12 CP 13 CP 14 CP 15 CP 16 CP 17 PA 1 PA 2 PA 3 PA 4 PA 5 PA 6 PA 7 PA 8 PA 9 PA 10 PA 11 NF 1 NF 2 NF 3 NF 4 NF 5 NF 6 NF 7

62.04 ± 3.75 32.58 ± 7.74 64.80 ± 0.72 54.00 ± 1.44 33.00 ± 2.58 36.84 ± 9.66 109.56 ± 6.43 64.44 ± 3.91 79.80 ± 5.38 114.12 ± 2.29 113.88 ± 2.43 59.88 ± 3.26 117.60 ± 8.35 128.28 ± 11.23 50.22 ± 3.87 109.80 ± 11.85 84.72 ± 11.3 114.48 ± 6.89 135.48 ± 5.71 59.64 ± 3.06 71.88 ± 9.12 63.96 ± 6.48 102.6 ± 3.90 73.56 ± 5.95 108.72 ± 9.38 62.88 ± 3.41 82.32 ± 3.32 83.76 ± 3.95 31.68 ± 1.66 42.48 ± 2.99 26.28 ± 2.79 96.72 ± 8.71 92.52 ± 2.95 28.87 ± 3.99 18.36 ± 2.55

47.72 ± 2.28 110.77 ± 7.23 35.56 ± 2.91 48.46 ± 2.66 54.66 ± 4.41 35.14 ± 4.77 28.65 ± 1.36 28.75 ± 2.44 56.06 ± 3.29 43.31 ± 2.85 54.31 ± 3.82 41.46 ± 1.25 51.86 ± 5.04 34.87 ± 0.75 22.41 ± 0.25 41.67 ± 2.51 29.54 ± 2.42 94.52 ± 8.77 82.68 ± 3.95 62.70 ± 3.38 56.03 ± 3.58 62.65 ± 2.59 71.56 ± 2.31 37.72 ± 1.93 138.27 ± 8.59 80.94 ± 6.62 87.39 ± 1.83 78.61 ± 2.42 54.26 ± 2.52 64.26 ± 2.08 63.88 ± 2.47 132.93 ± 5.28 65.00 ± 2.79 169.22 ± 2.47 106.68 ± 1.53

1.50 ± 0.20 6.63 ± 2.94 1.07 ± 0.30 1.75 ± 0.86 3.23 ± 0.66 1.86 ± 0.82 0.51 ± 0.18 0.87 ± 0.18 1.37 ± 0.37 0.74 ± 0.05 0.93 ± 0.15 1.35 ± 0.29 0.86 ± 0.75 0.53 ± 0.04 0.87 ± 0.54 0.74 ± 0.02 0.68 ± 0.23 1.61 ± 0.10 1.19 ± 0.12 2.05 ± 0.87 1.52 ± 0.75 1.91 ± 0.30 1.36 ± 0.05 1.00 ± 0.33 2.48 ± 0.61 2.51 ± 0.14 2.07 ± 0.75 1.83 ± 0.29 3.34 ± 0.49 2.95 ± 0.52 4.74 ± 1.04 2.68 ± 0.11 1.37 ± 0.08 11.43 ± 3.91 11.33 ± 0.08

48.19 81.33 20.84 55.15 71.97 57.48 – – 43.12 – – 42.21 – – – – – 51.60 32.82 60.80 48.86 58.43 42.67 0.00 66.22 66.52 61.11 56.88 72.58 70.19 77.82 68.09 43.12 84.98 84.94

2.66 ± 0.47 1.43 ± 0.10 3.14 ± 0.82 2.72 ± 0.88 3.17 ± 1.50 1.42 ± 0.03 2.00 ± 0.16 0.71 ± 0.23 2.06 ± 0.40 3.24 ± 0.32 3.13 ± 1.36 1.50 ± 0.01 2.11 ± 0.34 1.00 ± 0.64 1.73 ± 0.53 4.82 ± 0.14 2.65 ± 0.19 1.87 ± 0.04 1.54 ± 0.06 1.87 ± 0.13 1.20 ± 0.77 1.49 ± 0.56 2.05 ± 0.57 2.74 ± 0.61 4.09 ± 0.69 1.35 ± 0.31 1.63 ± 0.15 0.76 ± 0.01 1.03 ± 0.28 1.10 ± 0.10 1.73 ± 0.15 1.80 ± 0.66 1.66 ± 0.05 0.54 ± 0.01 0.62 ± 0.18

Weak Strong Weak Weak Strong Weak None None Weak None None Weak None None None None None Weak Weak Weak Weak Weak Weak Weak Weak Weak Weak Weak Strong Weak Strong Weak Weak Strong Strong

Plantation agriculture

Natural forest

The number after the ± represents the standard deviation of the mean.

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Fig. 2. Box plots of (a) Hydrophobicity index (R), (b) Soil water sorptivity (Sw), (c) Soil ethanol sorptivity (Se) and (d) Hydraulic conductivity (k) of the sampled soils against Land uses. Boxplots showed the rectangular boxes representing the middle 50% (interquartile range) of the data, the median value indicated by the horizontal line inside the box, Lines (called “whiskers”) extending from the box representing the upper and lower 25% of the distribution (excluding outliers), and Outliers indicated by asterisks beyond the whiskers.

compound changes). The hydrophobic surface is therefore exposed to the air/water in soil pores creating a hydrophobic layer preventing the spread of water over the soil particles (Quyum, 2000; Goebel et al., 2011). Elsewhere, hydrophobicity has been observed in cropland soils in regions with seasonally dry environments. Blackwell (2000) and Ziogas et al. (2005) reported similar findings in regularly tilled agricultural soils in the west coast of Western Australia and southeast Greece. Several other studies on water repellency, most especially in other regions of the world reported difference in hydrophobicity between forest soils and croplands. Ritsema and Dekker (2000) discovered that many permanently vegetated soils exhibit repellent behaviour in The Netherlands. Woche et al. (2005) also examined soil profiles at six agricultural and eight forested sites and reported that none of the agricultural sites, but all of the forested sites exhibited water repellency. Zolfaghari and Hajabbasi (2008), in a study to test for water repellency in soils of arid regions observed repellency in forest soils, but none in pasture and cultivated lands.

Three classes (none (0 b 1); weak (1–3) and strong (N3)) of repellency index (R) were used in the study. Fig. 4 shows the proportional representation of the hydrophobicity classes of the different land uses (cropland, plantation and natural forest). Natural forest soils demonstrated the highest percentage of strong water repellency/hydrophobicity. Robichaud et al. (2008), in their study to assess post-fire soil water repellency at James Creek site (Boise National Forest) in Idaho identified two classes of water repellency: strong (0 to b3 mL min− 1) and weak (3 to b 8 mL min− 1) of soil water repellency. They discovered Mini Disk Infiltrometer (MDI) test values of 8 mL min− 1 or more to indicate no water repellency. Lichner et al. (2007) also documented value of R b 1 to indicates better wettability by water than ethanol in a perfectly wettable soil. This they confirmed will be due to the interaction between the liquid properties (i.e. surface tension, contact angle and viscosity) and complex pore structure in soil.

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3.2. Hydrophobicity/water repellency index (R) and other soil properties The leading and most documented contributor to soil hydrophobicity is organic matter (Doerr et al., 2000; Ellies et al., 2005; Karunarathna et al., 2010; Olorunfemi et al., 2014). Soil organic matter (SOM) ranges from 1.52 to 4.07% in all the sampled soils with an average value of 2.46%. Natural forest soils were more hydrophobic which may be due to their higher organic matter content. Conversely, there were inconsistencies in the relationship between SOM and R in the croplands. For instance, it was observed that CP 2 (Cropland 2) with an organic matter content of 1.76% has the highest water repellency index of 6.63, while CP 7 with SOM of 2.57% has the least water repellency index of 0.51. Despite that, index of soil hydrophobicity (R) correlated significantly with organic matter content (r = 0.485) and organic carbon (r = 0.492) at 99% significance level with a moderate correlation in all land use types but indicated no significant relationship with organic matter content in each land use type (CP, PA and NF) (Table 3). The positive correlation between soil hydrophobicity and organic carbon content is as a result of the complex organic acids produced during the decomposition of organic matter, which form coatings over particles of soil causing soil hydrophobicity (Franco et al., 2000; Zavala et al., 2009; Olorunfemi et al., 2014; Cesarano et al., 2016). Berglund and Persson (1996) and Taumer et al. (2005) also reported positive correlation between organic matter content and soil sample water repellency, while Jungerius and de Jong (1989); Doerr et al. (2005) and Doerr et al. (2006) reported little or no relationship. Out of the 35 sites investigated, hydrophobicity affected all major soil types as 25 sites were found to be hydrophobic. We observed that clay and sand content (g kg−1) vary from 200 to 600 g kg−1 and 240 to 640 g kg−1 among all the soil samples, respectively with no clear correlation with soil hydrophobicity index (Tables 2 and 3). When each land use was put into consideration, all the soil textural classes under PA (sandy clay loam, clay loam, sandy clay and clay) were found to be weakly repellent. Though, previous findings (Crockford et al., 1991; DeBano, 2000) hold the view that coarse-textured, sandy soils are mostly susceptible to hydrophobicity because of their relatively small surface area per unit of volume (Karnok and Tucker, 2002). Nonetheless and without doubting the validity of the previous researches, our findings support the fact that the occurrence of soil water repellency or hydrophobicity is not limited to any particular soil type. This observation also agrees with the findings of Scott (2000); Doerr et al. (2006) and Vogelmann et al. (2010). Soil hydrophobicity has also been found and documented in loamy, peaty clay and clayey peat soils (McGhie and Posner, 1980; Dekker and Ritsema, 1996a, 1996b), as well as in heavy clay soils with grass covers (Dekker and Ritsema, 1996c). This implies that high sand contents in soils may not actually be the only deciding Table 2 Pearson correlation coefficients and significance of coefficients between the means of index of hydrophobicity (R), sorptivity to water (Sw), sorptivity to ethanol (Se), hydraulic conductivity (K) and other soil properties of all sampled soils. Property −1

Sand (g kg ) Silt (g kg−1) Clay (g kg−1) Silt/clay pH SOC (%) SOM (%) CEC (cmol + kg−1) BS (%) ASP (%) BD (Mg m−3) PT (m3 m−3) MW (%)

R

Sw (cm h−1/2)

Se (cm h−1/2)

K (cm h−1)

ns ns ns ns ns 0.492⁎⁎ 0.485⁎⁎ 0.382⁎

ns ns ns ns ns ns ns ns ns ns ns ns −0.509⁎

ns ns ns ns ns 0.410⁎ 0.426⁎ 0.382⁎

ns ns ns ns ns ns ns ns ns ns −0.484⁎⁎ 0.488⁎⁎ −0.589⁎

ns ns ns ns ns

ns, not significant. ⁎ Correlation is significant at the 0.05 level. ⁎⁎ Correlation is significant at the 0.01 level.

ns ns ns ns ns

factor on the severity of soil hydrophobicity as fine particles soil could equally possess similar level of soil hydrophobicity. In all locations of the study sites, index of soil hydrophobicity (R) correlated significantly with cation exchange capacity (Table 2). Hydrophobicity of the natural forest soils equally showed significant relationship with CEC (r = 0.814) and percentage base saturation (r = 0.774) at p ≤ 0.05 (Table 3). There was also no significant relationship between water repellency and soil pH in all the sampled soils and the different land uses at the p ≤ 0.01 and p ≤ 0.05 in agreement with the work of Perez et al. (1999) and Vogelmann et al. (2010). Some location specific factors, like number of pH active functional group, which may be necessary to define hydro-repellency of soil (Bayer and Schaumann, 2007) must have led to such correlation between soil pH and hydrophobicity in conformation with the findings of Vogelmann et al. (2010). The bulk density and total porosity had no definite sequence in their distribution in all the sampled soils in the different land uses and showed no clear relationship with hydrophobicity probably due to the complex interactions between soil hydrophobicity and other soil properties. Over the years, decrease in annual rainfall and number of rainy days with corresponding increase in air and soil temperatures have been recorded in the study region. Such changes in climate of the study areas only shows that with the increasingly dry climate and a reduction in the availability of irrigation water, soil water repellency could emerge a serious issue in the sub-Sahara Africa. Prolong exposure to critically low water contents cause shape of the polar compound to change so that the hydrophobic surface is exposed to the air/water in soil pores. Applied water will pool on the surface of the dry soil rather than wetting it and will then either evaporates or run off. Once this occurs, the soil can be difficult to rewet again. 3.3. Soil water sorptivity (Sw) Soil water sorptivity (capacity of soil to ‘suck’ up water) varied among the land uses studied (Tables 4 and 5). On average, its follows the order: plantation (87.21 cm h− 1/2) N cropland (77.39 cm h−1/2) N natural forest (48.10 cm h−1/2). ANOVA results showed significant difference in Sw among the land uses (p ≤ 0.05). Statistical results indicate that plantations had significantly higher Sw than natural forest (t stat = 2.893; p b 0.011) (Table 7). Tukey test at 95% CIs for the difference of means between the treatments shows that the corresponding means between plantations – croplands and natural forests – croplands are not significantly different (Fig. 5). Low Sw at natural forests could be the result of soil hydrophobicity, which reduces the ability of the soil to “suck” water due to the effect on the retention curve (Diamantopoulos et al., 2013). In addition, higher antecedent volumetric moisture content coupled with the other soil properties may have impact on the values of Sw. This is not unconnected with the fact that sorptivity is dependent upon three factors: the liquid properties, the soil properties and the maximum liquid content behind the infiltration front when describing infiltration into initially dry material as reported by Schultel et al. (2007). There is little information on the comparison of soil water and soil ethanol sorptivity among natural forests, plantations and croplands. Soil sorptivity to water correlated negatively with antecedent moisture content among all 35 samples at p ≤ 0.05 as shown in Table 2. When considering each land uses, water content showed positive correlation with silt content and silt/clay relation, respectively (p ≤ 0.05) (Table 3) under the plantations, while there was a negative correlation between soil water sorptivity and soil moisture content (p ≤ 0.01) (Table 3) in the natural forests. The soil water sorptivity correlated negatively with soil moisture content, which showed that soil water sorptivity is dominated by the antecedent water content of the soil (Philip, 1957b; Chongs and Green, 1979; Hallet, 2007) without reference to gravitational effects (Philip, 1957a; Philip, 1957c). A dry soil typically has a much greater water sorptivity (higher matric pressure) than a wet soil (Hallet, 2007; Olorunfemi and Fasinmirin, 2011). The forces of adhesion

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Table 3 Pearson correlation coefficients and significance of coefficients between the means of index of hydrophobicity (R), sorptivity to water (Sw), sorptivity to ethanol (Se), hydraulic conductivity (K) and other soil properties under the land use types. Property

Sand (g kg−1) Silt (g kg−1) Clay (g kg−1) Silt/Clay pH SOC (%) SOM (%) CEC (cmol + kg−1) BS (%) ASP (%) BD (Mg m−3) PT (m3 m−3) MW (%)

Croplands

Plantation agriculture

Natural forests

R Sw cmh−1/2

Se

K (cm h−1)

R Sw cmh−1/2

Se

K (cm h−1)

R cmh−1/2

Sw

Se

K (cm h−1)

ns ns ns ns ns ns ns ns ns ns ns ns ns

ns ns ns ns ns ns ns ns ns ns ns ns ns

ns ns ns ns 0.498⁎ ns ns ns ns ns −0.702⁎⁎ 0.735⁎⁎

ns ns ns ns ns ns ns ns ns ns ns ns ns

ns ns ns ns ns ns 0.644⁎ 0.646⁎ ns ns ns ns ns

ns ns ns ns ns ns ns ns ns ns ns ns −0.857⁎⁎

ns ns ns ns ns ns ns 0.814⁎ 0.774⁎ ns ns ns ns

ns ns ns ns ns ns ns ns ns ns ns ns −0.921⁎⁎

ns ns ns ns ns ns ns ns ns ns ns ns ns

0.827⁎ ns −0.84⁎⁎ ns ns −0.926⁎⁎ −0.927⁎⁎

ns ns ns ns ns ns ns ns ns ns ns ns ns

ns

ns 0.689⁎ ns 0.625⁎ ns ns ns ns ns ns ns ns ns

ns ns ns ns ns ns

ns, not significant. ⁎⁎ Correlation is significant at the 0.01 level. ⁎ Correlation is significant at the 0.05 level.

establish the matric pressure, which usually dominates during the early stages of water entry into soil in an initially dry soil. On the other hand, the capillary component of matric pressure dominates when the medium is wet (Nimmo, 2005). Nonetheless, in an extremely water repellent soil, sorptivity and capillary rise will be zero (0) (Hallet, 2007). 3.4. Soil ethanol sorptivity (Se) The ethanol sorptivity of the different land uses ranged from 22.41 cm h−1/2 (±0.25) to 110.77 cm h−1/2 (±7.23) for the croplands; 37.72 cm h−1/2 (± 1.93) to 138.27 cm h−1/2 (± 8.59) for plantations and 54.26 cm h−1/2 (±2.52) to 132.93 cm h−1/2 (±5.28) for the natural forests (Table 1). Overall, the mean ethanol sorptivity of the different land uses differ significantly (p ≤ 0.001). Tukey HSD at 95% confidence interval shows that the mean ethanol sorptivity of natural forests (M = 93.7) and plantations (M = 77.6) are not significantly different but the mean ethanol sorptivity of croplands (M = 45.0) is significantly lower than that of plantations and natural forests (Fig. 2). However, we found that natural forest soils generally had a much higher Se than the other two land use types. This suggests that ethanol sorptivity is not influenced by soil hydrophobicity as ethanol wet all soil surfaces

Table 5 Means significant differences in surface soil properties between land uses.

Table 4 Analysis of variance of the surface soil properties between land uses. Properties

R K (cm h−1) Sw (cm h−1/2) Se (cm h−1/2) Sand (g kg−1) Silt (g kg−1) Clay (g kg−1) Silt/clay pH SOC (%) SOM (%) SON (%) CEC (cmol+ kg−1) BS (%) ASP (%) BD (Mg m−3) PT (m3 m−3) Mic (m3 m−3) Mac (m3 m−3) MW (%)

Mean square Trt (Land use type)

Error

40.671 3.1237 3392.5 7154.9 7833 8779 3558 0.08854 0.05943 0.6388 1.9036 0.00141 11.107 140.03 56.62 0.003108 0.000661 0.002279 0.000945 34.066

4.475 0.8233 915.6 764.8 9825 3571 9440 0.09627 0.10165 0.1126 0.3379 0.00032 3.591 90.07 21.97 0.00483 0.00087 0.000224 0.001181 3.345

with a contact angle equal to zero. Conversely, human modification of the soils under croplands, such as tillage and gardening by farm implements, might have led to reduce ethanol sorptivity. Soil ethanol sorptivity showed significantly positive correlation with organic carbon, organic matter content and cation exchange capacity among all the 35 soil samples at a significant level of p ≤ 0.05 (Table 2). Similarly, it correlated positively with soil organic matter and cation exchange capacity (p ≤ 0.05) under the plantation (Table 3), which implies that soil hydrophobicity which has a positive relationship with ethanol sorptivity might have been caused by organic compounds derived from living or decomposing plants or microorganisms (Olorunfemi et al., 2014). During the decomposition of organic matter and fatty waxes present in the soil, complex organic acids are produced causing soil hydrophobicity (Franco et al., 2000; Olorunfemi et al., 2014). The waxy substances penetrate into the soil as a gas and solidifies after it cools, forming a waxy coating around soil particles. These waxy skins effectively repel the water from the soil thereby limiting water availability to the crop (Olorunfemi et al., 2014).

Properties

F-value

Significance

9.09 3.79 3.71 9.35 0.8 2.46 0.38 0.92 0.58 5.67 5.63 4.49 3.09 1.55 2.58 0.64 0.76 10.18 0.80 10.18

*** * * *** ns ns ns ns ns ** ** * * ns ns ns ns *** ns ***

*** = p ≤ 0.001; ** = p ≤ 0.01; * = p ≤ 0.05 ns = not significant, Trt – treatment.

R K (cm h−1) Sw (cm h−1/2) Se (cm h−1/2) Sand (g kg−1) Silt (g kg−1) Clay (g kg−1) Silt/clay pH SOC (%) SOM (%) SON (%) CEC (cmol + kg−1) BS (%) ASP (%) BD (Mg m−3) PT (m3 m−3) Mic (m3 m−3) Mac (m3 m−3) MW (%)

Significance of the difference between land uses Croplands (CP)

Plantation agriculture (PA)

Natural forests (NF)

1.50A 2.32A 77.39AB 45.01A 438.80A 232.90A 328.20A 0.76A 5.93A 1.27A 2.19A 0.06A 5.98A 81.80A 11.34A 1.36A 0.49A 0.10B 0.41A 4.50B

1.77A 1.87AB 87.21A 77.55B 474.50A 181.80A 343.60A 0.60A 5.90A 1.43AB 2.50AB 0.07AB 6.38AB 81.50A 12.33A 1.36A 0.48A 0.10B 0.41A 3.94B

5.41B 1.21B 48.1B 93.7B 417.10A 217.10A 365.70A 0.68A 6.06A 1.78B 3.06B 0.09B 8.08B 88.75A 7.36A 1.32A 0.50A 0.13A 0.39A 7.72A

Means that do not share a letter are significantly different.

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3.5. Hydraulic conductivity (K) Mean unsaturated hydraulic conductivity values showed an increasing trend from the Natural forests to Plantations and croplands in the order: (1.21 cm h− 1 ± 0.53, 1.78 cm h− 1 ± 0.92, 2.32 cm h− 1 ± 0.96), respectively; unlike the mean soil hydrophobicity, which moves in opposite direction (Fig. 5). The comparison of the hydraulic conductivity between the three land uses showed that mean K is significantly higher in croplands (M = 2.32 ± 0.96) compared to natural forests (M = 1.21 ± 0.53), whereas mean K values of plantations (M = 1.87 ± 0.92) did not differ significantly from both (Croplands and Natural forests) (Table 4 and Fig. 3). The difference in the unsaturated hydraulic conductivity among the land uses is attributed to the combination of land use management and differences in microbial activities affecting soil texture and structure. The increase of soil hydraulic conductivity under the croplands was attributed to the continuous cultivation, which prevented the formation of hydrophobic surface. The manual tillage system adopted by farmers equally enhances soil loosening and therefore greater macropores. The reduced unsaturated hydraulic conductivity of the soil surface in natural forests and plantations might be due to the moss covered and crusted surface under those land uses (plantations and natural forests). We observed that during the dry season, there was shrinkage in the soils of the plantations and natural forests and subsequently, formation of hard upper layer, thus limiting the

infiltration of water in the surface layer. Slaking reduces infiltration because the aggregates block the macropores (Moores, 2004). Also, as the growing season progresses, hydraulic conductivity can decrease because of increased root growth (which is predominant in the natural forest and plantation agriculture soils) clogging pores and soil slaking (Lampurlanés and Cantero-Martínez, 2005). The vegetation covers in the natural forest soils increased the repellency and decreased both soil water sorptivity and unsaturated hydraulic conductivity. Hydraulic conductivity resulted to a negative correlation (r = − 0.403 at p ≤ 0.05) with hydrophobicity among all 35 sampled soils, as water repellent soils can resist or retard infiltration (Doerr and Thomas, 2000). Water repellent soils have decreased infiltration and increased runoff and erosion as compared to non-water repellent soils (Benavides-Solorio and MacDonald, 2001; Robichaud et al., 2008). Lichner et al. (2007) also found similar relationship in the glade (marshy grassland) and forest soil of southwest Slovakia. This according to them would be expected as the surface of the glade soil was observed to be covered with moss and the activity of microorganisms capable of producing hydrophobic compounds is greater at the surface than deeper down (Hallett et al., 2001). They also observed that the vegetation cover in the pine forest increased the repellency and decreased both water sorptivity and hydraulic conductivity. Doerr et al. (2000) and Wahl (2008) from their works equally concluded that hydrophobicity of soils can severely impede infiltration; promote surface runoff

Fig. 3. Differences of means for (a) hydrophobicity index (R), (b) Soil water sorptivity (Sw), (c) soil ethanol sorptivity (Se) and (d) hydraulic conductivity (k) of the sampled soils between the various land uses.

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texture influences the water retention capabilities of soils of the different locations. We observed a significant and positive relationship (r = 0.54, N = 35, p ≤ 0.01) between WHC (%) and Clay percentage in all the 35 sites. This shows that soils with high clay percentage tend to have high water holding capacity. Clay soils, due to very small size of the pore spaces (fine capillaries), retained more water in the capillary spaces as capillary water and as a result, water does not transmit easily. Likewise, soil with very high proportion of sand has very low water holding capacity due to large pore spaces between the particles, which enables the water to move freely (relatively higher hydraulic conductivity) into deeper layers leaving upper layers practically dry. Under the Croplands, the relationship between K and soil pH resulted to a positive correlation coefficient (p ≤ 0.05), while hydraulic conductivity displayed negative correlation with organic carbon and organic matter (p ≤ 0.01) in the natural forest soils (Table 3) as previously explained above. In the natural forest, hydraulic conductivity correlated positively with sand (%) content and negatively with clay (%) content (p ≤ 0.05) (Table 3). Increased sand and silt content in soil texture increases ratio of macro porosity to total porosity (Gulser and Candemir, 2008). Increasing macro porosity or decreasing micro porosity in soil structure causes increase in soil hydraulic conductivity as reported by Ahuja et al. (1984). A soil that is coarser and well aggregated will have a higher infiltration rate than that of a soil with a finer texture with poor aggregate arrangement (Troeh et al., 2004). Bulk density and total porosity showed significant negative and positive correlation (p ≤ 0.01) with hydraulic conductivity in all the sampled soils and in croplands, respectively (Tables 2 and 3). Hydraulic conductivity is usually high in soils due to high porosity (more open area for the flow of water) with the exception of clays, which typically have very low hydraulic conductivity also have very high porosities (due to the structured nature of clay minerals). This shows that clays can hold a large volume of water per volume of bulk material, but they do not release water rapidly (Messing and Jarvis, 1993). Increase in soil bulk density and a decrease in total pore space, significantly influences soil hydraulic properties (pore size distribution, water retention and hydraulic conductivity) (Lujan, 2003). Fig. 4. proportional representation of the hydrophobicity class of the different land uses (a) croplands; (b) plantation agriculture and (c) Natural forest.

and soil erosion. The infiltration capacity of water repellent soils can be markedly lower than those of wettable soils (Wang et al., 2000; Wahl et al., 2003, 2005). The unsaturated hydraulic conductivity (K) showed negative correlation with the antecedent moisture content (p ≤ 0.05) among all the soil samples (Table 3) as the antecedent moisture content affects the infiltration rates and hydraulic conductivity. An initially wetter soil profile generally lowers the initial rate of infiltration corresponding to reduced absorption (Furman et al., 2006). The data collected revealed that soil

3.6. Variations of the soil properties under different land uses Tables 5 and 6 shows that the differences among land uses were statistically significant for 11 of the 21 soil properties considered. One way ANOVA indicated that the variability of the following soil properties: sand, silt, clay, silt/clay, soil pH, base saturation (BS), aluminium saturation (ASP), bulk density (BD) and total porosity (PT) among the land uses were not statistically significant. The summary of the Post Hoc Tests (Table 5) is presented in Table 6. The Croplands differs significantly from the plantation agriculture in just one property (Se), and differs from the Natural forests in ten (10) properties. However, the plantation agriculture land use is significantly different from the Natural forests in five (5) soil properties. The changes in soil properties resulting from difference in land uses between Croplands and Natural forests are large; this is probably due to the distinct difference in their land use pattern. The result emphasized the impacts of land uses on soil properties, especially hydraulic properties, even if the land uses occur on similar soil

Table 6 Summary of post hoc tests: number and properties significant differences between land use pairs.

Fig. 5. Mean R and K (cm h−1) values against the different land uses.

Land use pair

No. of properties

Soil properties

Croplands - plantation agriculture Croplands - natural forests Plantation agriculture natural forests

1

Se

9 4

R, K, Se, SOC, SOM, SON, CEC, Mic, MW R, Sw, Mic, MW

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type and climatic conditions. This reveals that land uses influence soil properties differently and consequently establishes facts about the hydraulic and water repellency properties of soils and provide a better understanding of the soil characteristics and water management under the different land uses.

3.7. Hydrological implications of soil hydrophobicity Soil hydrophobicity has been observed to have clear impacts on soil water interaction. Although soils are generally considered to wet readily under rainfall or irrigation, some soils exhibit a reduced, or no affinity to water (water repellent) at the surface and within the root zone. The occurrence of soil hydrophobicity drastically reduced the amount of water available for plant growth and crop production as water repellent soils can resist or retard infiltration (Doerr and Thomas, 2000). The natural forest soil (NF 6) with the highest mean repellency index (R = 11.43 (±3.91) was found to have the least infiltration rate, reduced sorptivity to water and hydraulic conductivity (K = 0.54 cm h−1 (±0.01)) (Table 6). Our findings imply that hydrophobicity of soils can severely impede infiltration and reduce the amount of water available in plants' root zone, which often slows down seedlings emergence and plant growth rate thereby reducing productivity at field level. Consequently, hydrophobicity promotes surface runoff and soil erosion as explained by Doerr et al. (2000) and Wahl (2008). The infiltration capacity of water repellent soils can be markedly lower than those of wettable soils (Wang et al., 2000; Wahl et al., 2003, 2005), since croplands which were virtually unaffected by hydrophobicity have the highest hydraulic conductivity (CP 16, K = 4.82 cm h−1 ± 0.14). Water repellency in soils can have serious environmental implications, including accelerated soil erosion and enhanced leaching of agrochemicals through preferential flow. This can lead to bypassing of large parts of the soil matrix (Hendrickx and Flury, 2001) and faster displacement of chemical substances towards the groundwater i.e. this flow bye-pass can accelerate the transport of chemicals, thereby increasing the amount of harmful substances moving into the groundwater (Hendrickx and Flury, 2001; Taumer et al., 2005; Olorunfemi et al., 2014). Soil hydrophobicity can also result in poor distribution of water, fertilizer, pesticides, and a reduction in nutrients and water available for plant growth (Bauters et al., 1998).

Table 7 Difference of means of surface soil properties. Soil properties

R K (cm h−1) Sw (cm h−1/2) Se (cm h−1/2) Sand (g kg−1) Silt (g kg−1) Clay (g kg−1) Silt/clay pH SOC (%) SOM (%) SON (%) CEC (cmol+ kg−1) BS (%) ASP (%) BD (Mg m−3) PT (m3 m−3) Mic (m3 m−3) Mac (m3 m−3) MW (%)

Difference of means Cropland vs. plantation

Cropland vs. forest

Plantation vs. forest

t stat

p

t stat

p

t stat

p

−0.595 1.197 −0.855 −3.771 −0.999 2.298 −0.43 1.454 0.245 −1.324 −1.507 −1.106 −0.52 0.076 −0.5 0.229 −0.084 0.885 −0.404 0.905

0.557 0.242 0.401 0.001 0.327 0.03 0.67 0.158 0.809 0.197 0.144 0.279 0.608 0.94 0.621 0.821 0.934 0.384 0.690 0.374

−3.962 2.709 2.007 −4.785 0.504 0.647 −0.909 0.604 −1.113 −3.41 −3.407 −3.043 −2.75 −1.632 1.774 0.893 −0.973 −3.489 1.066 −3.480

0.001 0.013 0.057 0.001 0.619 0.525 0.373 0.552 0.278 0.003 0.003 0.006 0.012 0.117 0.09 0.381 0.341 0.002 0.298 0.002

−3.391 1.759 2.893 −1.619 1.048 −1.05 −0.412 −0.468 −0.983 −1.894 −751 −1.753 −1.766 −1.928 3.088 0.553 −0.69 −4.200 1.158 −4.217

0.004 0.098 0.011 0.125 0.31 0.309 0.686 0.646 0.34 0.077 0.099 0.099 0.096 0.072 0.007 0.588 0.5 0.001 0.264 0.001

4. Conclusion This research evaluated and characterized hydrophobicity, hydraulic conductivity and sorptivity on soils of similar climatic conditions but under different land uses (i.e cropland, plantation agriculture and natural forest) in Ekiti State, in the tropical rainforest of Nigeria. Bulk density and total porosity showed no definite sequence in their distribution across the different land uses. Organic carbon and Organic matter accumulation follows the order natural forest N plantation N cropland. The mean hydrophobicity index, R, showed a decreasing trend in the order as follows: natural forest ˃ plantation ˃ cropland. The hydrophobicity of soils was higher in soils with high organic matter content (natural forest and plantation agriculture soils), whereas the reverse was the case in croplands. There was no clear relationship between index of soil hydrophobicity and soil particle sizes under the different land uses. The soil water sorptivity is dominated by the antecedent water content of the soil without reference to gravitational effects. Soil ethanol sorptivity, which is the affinity of the soil for ethanol showed significant positive correlation with organic carbon, organic matter content and cation exchange capacity among all the soil samples. Hydraulic conductivity resulted to a negative correlation with hydrophobicity among all sampled soil. Increased hydrophobicity of soils can severely impede infiltration and reduce the amount of water available in plants' root zone, as increase in soil hydrophobicity strongly reduced the unsaturated hydraulic conductivity and soil water sorptivity. This often slows down seedlings emergence and plant growth rate thereby reducing productivity at field level; and consequently promote surface runoff and soil erosion. Land uses and soil management appear to be good predictor of soil hydrophobicity. Reliable knowledge on soil hydrophobicity and hydraulic properties under different land uses and evaluation of the soil properties affecting them can be of great interest in understanding the influences of human activities on soil water movement and possible implications for livelihoods in consideration of incessant climate and environmental changes.

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