ASSESSMENT OF SATURATED HYDRAULIC CONDUCTIVITY OF SOILS IN DIFFERENT LAND USE PRACTICES IN UYO CAPITAL TERRITORY, NIGERIA

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1 ASSESSMENT OF SATURATED HYDRAULIC CONDUCTIVITY OF SOILS IN DIFFERENT LAND USE PRACTICES IN UYO CAPITAL TERRITORY, NIGERIA ABSTRACT Obi*, C. I. and Esu, I. E. *Department of Soil Science and Technology, Federal University of Technology, Owerri, P.M.B 1526 Owerri, Nigeria Saturated hydraulic conductivity of soils is dependent on properties of the soils as a result of differences in land use. This study was conducted to investigate the saturated hydraulic conductivity of soils of Uyo, Akwa Ibom State in Nigeria under four different land uses practices. Composite samples were collected from each of the four land use practices which included a natural forest, fallow, arable land (cassava plot and vegetable garden) and waste management (dump site). Eight undisturbed core samples were randomly collected from each of the four land use practices. Four of the undisturbed core samples each were used for determining the bulk density and the saturated hydraulic conductivity. Routine analysis was carried out with the composite samples using standard laboratory procedures. Saturated hydraulic conductivity correlated insignificantly with bulk density (r = -0.18, P 0.05), porosity (r = 0.17, P 0.05) and organic carbon (r = 0.27, P 0.05). A better insignificant relationship (r = 0.18, P 0.05) existed between saturated hydraulic conductivity and the sand content than the clay (r = , P 0.05). Regression equations established that 21% of the total variation in saturated hydraulic conductivity could be accounted for by either bulk density or porosity of the soils. Keywords: Bulk density, hydraulic conductivity, porosity, land use, Ultisol INTRODUCTION Water is seldom at rest in soil and the direction and rate of its movement is of fundamental importance to many processes that take place in the biosphere. Although it is one of nature s simplest chemicals, water has unique properties that promote a wide variety of physical, chemical and biological processes especially in the pedosphere. Therefore, it is imperative to look into properties of the soil that facilitates the movement of water. The inter-related soil attributes such as texture, hydraulic conductivity, bulk density, and macroporosity influence hill slope and watershed hydrology (Farres, 1987; Rawls et al., 1993; Cerda, 1996). Land use practices have been shown to be of key importance to soil hydrological properties including hydraulic conductivity. Effects of land use practices are manifest in the forms of tillage, erosion, compaction, and pore structure evolution (Rasiah and Kay, 1995; Harden, 2006). In determining soil water movement, the aforementioned disturbances in some cases outweigh traits (genoform) inherited from parent material, topographic setting and other factors of soil formation (Schwartz et al., 2003; Zhou et al., 2008). The soil hydraulic conductivity (k) is one of the physical properties of the soil which helps in determining the structural stability of soils. It is to a large extent dependent on some other properties (chemical and biological) and energy gradients, water viscosity and temperature. Compared with soils impacted by human land use, soils under native vegetation (e.g. undisturbed forest) generally manifest low bulk density and high saturated hydraulic conductivity, total porosity, and macroporosity as a result of ample litter cover, organic inputs, root growth and decay, and abundant burrowing fauna (Lee and Foster, 1991). Studies investigating soil physical response to land use change have heavily emphasized comparison of cultivated cropland soils versus soils under native forest, shrubland, or grassland. It has been reported that soils impacted by land-use change may demonstrate marked disparities from the prior soil (Jimenez et al., 2006; Zhou et al., 2008). Replacement of natural vegetation with managed landcover is generally associated with decreased rooting networks and faunal activity, thereby reducing the potential for well-developed macropore networks (Reiners et al., 1994; Schwartz et al., 2003). As seen with cultivated soils, studies comparing soils under natural vegetation to pasture or lawn have also shown degradation of soil physical properties. It has been reported that forest cover has been associated with lower bulk density and greater saturated hydraulic conductivity than pasture in different climates and parent materials throughout the world (Reiners et al., 1994; Godsey and Elsenbeer, 2002; Jimenez et al., 2006; Li and Shao, 2006; Abbasi et al., 2007). Pizl and Jones (1995) reported that lawn grass typically demonstrates shallow rooting depth, low organic matter accumulation, and is generally associated with lower faunal activity than pasture or forest. Studies addressing the physical properties of soils underlying lawn grass have shown exceptionally low infiltration rates, low hydraulic conductivity and high bulk densities (Hamilton and Waddington, 1999; Oliviera and Merwin, 2001). NJAFE VOL. 11 No. 2,

2 Changes in soil physical properties associated with conversion of native to managed vegetation reduce soil infiltration, hydraulic conductivity and storage capacities, possibly resulting in increased overland flow and reduced subsurface storage. Given the increase in managed lawns associated with low and medium density urban growth occurring in many regions, it is important to understand the effects of such land-use change on soil physical properties. This research sought to determine the differences among soil saturated hydraulic conductivity under four different land use practices in the Uyo capital territory, Akwa Ibom State in southeastern Nigeria. This region was experiencing pronounced growth and the four land uses considered include-forest (natural forest and oil palm plantation), fallow, arable land (cassava plot and vegetable garden) and waste management (dump site). Understanding the differences in soil saturated hydraulic conductivity due to differences in land use is necessary for the development of a full understanding of how land-use changes in the study area is affecting watershed hydrologic processes. MATERIALS AND METHODS Description of the study area The research was carried out in six locations with different land use practices in the Uyo capital development territory of Akwa Ibom State in southeastern Nigeria. The four locations were- forest (natural forest and oil palm plantation) at Ntak Inyang, fallow located at Use Atai, arable land (cassava plot and vegetable garden) located at teaching and research farm of the University of Uyo, Use Offot and waste management (dump site) located at Uyo Village road. Soils of the four locations were derived from coastal plain sands. The natural forest in Ntak Inyang was a secondary forest that was untampered for several years made up of tress, shrubs and grasses, the fallow land use at Use Atai was about 10 years old while the arable land (cassava plot and vegetable garden have been used for 5 years, soils of the dump site at Uyo Village road were dominated by Amaranthus spinosus during the sampling period and had existed for 10 years. Uyo capital territory is located between latitude 4 50' and 5 07' N and longitude 7 45' and 8 05' E within a tropical climate characterized by rainy season (March November) and dry season (November March). The mean annual rainfall is about 2500mm. Soil sampling and laboratory analysis In the field, composite samples were collected from each of the six land use practices at 5cm depth with the aid of Dutch auger. Eight minimally disturbed core samples were randomly collected from each of the six land use practices. Four of those samples were used to determine bulk density (Grossman and Reinch, 2002) of the soil using the core method and porosity was calculated while the remaining four were used for hydraulic conductivity determination which was performed using the constant head permeameter method (Topp and Dane, 2002). The composite samples were processed and analyzed in laboratory for the soil properties as described below. Particle size analysis was performed using the Bouyoucous hydrometer method (Gee and Or, 2002). Exchangeable basic cations [calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na)] were extracted with 1 N NH 4OAc (ph 7) (Thomas, 1982). Exchangeable calcium and magnesium were determined by EDTA complexio-metric titration while exchangeable potassium and sodium were determined by flame photometry (Jackson, 1962). Exchangeable acidity (H + and Al 3+ ) was determined using 1 N KCl (Mclean, 1982). The effective cation exchange (ECEC) was calculated by summation of all the exchangeable bases and exchangeable acidity. Soil organic carbon was analyzed by Walkley and Black wet oxidation method (Nelson and Sommers, 1982). Available phosphorus was determined by the Bray I method (Olsen and Sommers, 1982). Total nitrogen was determined by micro Kjedahl digestion method (Bremner and Mulvaney, 1982). Soil ph was measured potentiometrically in water at the soilliquid ratio of 1:2.5. Electrical conductivity was determined in the same ratio using the conductivity bridge. Statistical analysis Data collected were summarized using descriptive statistics, and normality of distribution was tested with skewness and kurtosis. Correlation and simple regression analysis were carried out to find out the way variables relate with each other. All statistical analysis was carried out with the aid of SAS (1999). RESULTS AND DISCUSSION The mean and median were used as primary estimates of central tendency while standard deviations, coefficient of variation, skewness, kurtosis, minimum, and maximum were used as estimates of variability (Table 1). The soil properties were neither skewed nor kurtous in the entire sites with the exception of percentage total nitrogen which was found to be kurtous. The mean and median of most of the soil properties were similar with the median greater or less (with maximum of 10 points) than the mean for most of the soil properties, exceptions were organic carbon, available phosphorus and particle size fractions (Table 1). These showed that outliers did not dominate the measure of the central tendency but a true indication of the soil properties. Similarity of means and median of several physical, chemical and biological soil properties had been reported by Obi et al (2012), Shukla et al (2004) and Cambardella et al (1994). To enhance the pragmatic aspect of the study, no transformation was attempted although Parkin and Robinson (1992) had previously reported that many soil properties are log- NJAFE VOL. 11 No. 2,

3 normally distributed. The saturated hydraulic conductivity deviated from the mean by one negative power and varied highly (CV = 46%) in the study area. The variation in the saturated hydraulic conductivity is due to the different land uses to which the soils were subjected. From results of the laboratory analysis, the natural forest soils which had the least bulk density (1.02 gcm -3 ) had the highest saturated hydraulic conductivity ( cms -1 ). This agrees with the findings of Katie et al. (2009) which reported that forest soils demonstrated markedly lower bulk densities and higher saturated hydraulic conductivity, and water holding capacities, than lawn and pasture soils. The porosity, ph, Ca, Mg, exchangeable acidity, ECEC and silt content varied moderately while the C/N ratio, Na, K, percentage base saturation and the sand content varied a little in the study area. Table 1: Descriptive statistics of some physical and chemical properties of soils of the study area Variable SD Skewness Kurtosis Mean Median Min. Max. CV Ksat (cms -1 ) BD (gcm -3 ) f(%) ph OC (g kg -1 ) TN (%) C/N ratio Ca (cmol kg -1 ) Mg (cmol kg -1 ) Na (cmol kg -1 ) K (cmol kg -1 ) Al (cmol kg -1 ) H (cmol kg -1 ) ECEC (cmol kg -1 ) BS (%) Av.P (mg kg -1 ) Sand (g kg -1 ) Silt (g kg -1 ) Clay (g kg -1 ) Ksat: saturated hydraulic conductivity, BD: bulk density, f: porosity, OC: organic carbon, TN: total nitrogen, C/N ratio: carbon nitrogen ratio, ECEC: effective cation exchange capacity, BS: base saturation, Av.P: available phosphorus. Table 2 shows the Pearson correlation coefficients for soil properties of the studied sites. Saturated hydraulic conductivity correlated not significantly but negatively with bulk density (r = -0.18, P 0.05) in the entire sites. This agrees with the findings of Dorota et al. (2008) which stated that saturated hydraulic conductivity decreases with increasing bulk density. The saturated hydraulic conductivity increased as the porosity increased (r = 0.17, P 0.05). It had been reported that high saturated hydraulic conductivity in conventional tillage system was caused by increased porosity in soil (Radcliffe et al., 1988; Hill, 1990; Suwardji and Eberbach, 1998). Organic carbon was found to be negatively correlated with bulk density (r = -0.75, P 0.05) and positively correlated with saturated hydraulic conductivity (r = 0.27, P 0.05). Clay did not significantly correlate with saturated hydraulic conductivity (r = -0.02, P 0.05) while sand correlated positively with saturated hydraulic conductivity (r = 0.18, P 0.05). These showed that greater relationship existed between saturated hydraulic conductivity and the sand content than with the clay content. Simple regression analysis was carried out in order to establish respective contributions of predictors (bulk density and porosity) to saturated hydraulic conductivity. It had been reported earlier that bulk density and porosity controls the saturated hydraulic conductivity of soils. Bulk density (BD) was first used as the predictor (independent variable), while the saturated hydraulic conductivity (Ksat) was dependent and the relationship was established as: Ksat = (BD) , (r 2 = 0.21, P 0.05). Porosity (f) was also used as a predictor and its relationship with saturated hydraulic conductivity (Ksat) was established as: Ksat = (f) , (r 2 = 0.21, P 0.05). The regression lines explain 21% of the total variations in saturated hydraulic conductivity around the mean of bulk density and porosity, respectively. It is therefore worthy to note that saturated hydraulic conductivity of Ultisols under different land use practices will differ depending mainly on the soil properties that control it such as bulk density and porosity. NJAFE VOL. 11 No. 2,

4 CONCLUSION Outliers did not dominate the central tendency because the soil properties were neither skewed nor kurtous in the entire sites. Many of the soil properties (porosity, ph, Ca, Mg, exchangeable acidity, ECEC and silt content) varied moderately. There was no significant correlation between saturated hydraulic conductivity (Ksat) and bulk density, porosity and particle sizes. Bulk density and saturated hydraulic conductivity (Ksat) insignificantly correlated negatively while it was a positive insignificant correlation with porosity. The regression line explains 21% of the total variation in hydraulic conductivity around the mean of bulk density and porosity respectively. REFERENCES Abassi, M. K., Zafar, M. and Khan, S. R Influence of different land-cover types on the changes of selected soil properties in the mountain region of Rawalakot Azad Jammu and Kashmir. Nutrient Cycling in Agroecosystems 78, Blake, G. R. and Hartge, K. H Bulk Density. In: Methods of soil analysis. Part 1. A. Klute (ed.). ASA Monogr. No. 9. Madison, WI. Pp Bremner, J. M. and Mulvaney, C. S Total Nitrogen. In: C. A Black (ed.). Methods of Soil Analysis, part 2, Agronomy 9. American Society of Agronomy Inc. Madison Wisconsin. Pp Cambardella, C. A., Moorman, T. B., Novak, J. M., Parkin, T. B., Karlan, D. L., Turco, R. F. and Konopka, A. E Field scale variability of soil properties in Central Lowa soils. Soil Science Society of America Journal, Vol. 58, no.5, pp Cerda, A Soil aggregate stability in three mediterranean environments. Soil Technology 9(3), Dorota, D; Jose, D; Orsolya B and Rainer, H (2008). Effect of bulk density on hydraulic properties of homogenized and structured soils. J. Soil Sc. Plant Nutr. 8 (1), pp Farres, P. J The dynamics of rainsplash erosion and the role of soil aggregate stability. Catena 14, Gee, G. W. and Or, D Particle size analysis. In: Dane, J. H and Topp, G. C (eds). Methods of soil analysis, part 4. physical methods. Soil Science Society of America. Book series. No. 5 ASA and SSA Madison, W1, pp Godsey, S. and Elsenbeer, H The soil hydrologic response to forest regrowth: a case study from southwestern Amazonia. Hydrological Processes 16, Grossman, R. B. and Reinch, T. G Bulk density and linear extensibility in: Methods of soil analysis part 4. Physical methods. Dane, J. H and Topp, G. C. (eds.). Soil Science Society of America. Book series No.5. ASA and SSA Madison, W. I, Pp: Hamilton, G. W. and Waddington, D. V Infiltration rates on residential lawns in central Pennsylvania. Journal of Soil and Water Conservation 54 (3), Harden, C. P (2006). Human impacts on headwater fluvial systems in the northern and central Andes. Geomorphology 79, Hill, R. L Long-term conventional and no-tillage effects on selected soil physical properties. Soil Science Society of American Journal. 54, Jackson, M. L Soil Chemical Analysis. New York: Prentice Hall Inc. Pp Jimenez, C. C., Tejedor, M., Morillas, G. and Neris, J Infiltration rate in andisols: effect of changes in vegetation cover (Tenerife, Spain). Journal of Soil and Water Conservation 61 (3), Katie, P., Rhett, J. C. and Albert, J. P Variation of surficial soil hydraulic properties across land uses in the southern Blue Ridge Mountains, North Carolina, USA. Lee, K. E. and Foster, R. C Soil fauna and soil structure. Australian Journal of Soil Research 29(6): Li, Y. Y. and Shao, M. A Change of soil physical properties under long-term natural vegetation restoration in the Loess Plateau of China, Journal of Arid Environments 64: Mclean, E. O (1982). Aluminium. In: Method of Soil Analysis. Black C. A (ed.) Agron NO.9, part 2. American society of Agronomy, Madison, Wisconsin. Pp Nelson, D. W. and Sommers, L. E Total carbon, Organic Carbon and Matter. In: Page, A. L; Miller, R. H and Kenney, D. R (eds.) Methods of Soil Analysis, part 2, Chemical and Microbiological properties. Madison, Wisconsin. Pp Obi, C. I., Obi, J. C. and Onweremadu, E. U Modeling of permanent wilting from particle size fractions of coastal plain sands soils in Southeastern Nigeria. ISRN Soil Science, vol. 2012, article ID Pp 5. Oliviera, M. T. and Merwin, I. A Soil physical conditions in a New York orchard after 80 years under different groundcover management systems. Plant and Soil 234 (2), Olsen, S. R. and Sommers, L. E Phosphorus. In: Page, A. L; Miller, R. H and Keeney D. R (eds.). Methods of Soil Analysis, Part 2, Chemical and Microbiological Properties. Madison, Wisconsin. Pp NJAFE VOL. 11 No. 2,

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6 Table 2: Pearson correlation coefficients for soil properties of the study sites Ksat BD f ph OC TN C/N ratio Ca Mg Na K Al H ECE C BS Av.P Sand Silt Ksat BD f ** ph OC TN C/N ratio Ca ** Mg Na * 0.91* * K * 0.94** Al H * * ECEC * BS Av.P Sand * 0.89* * Silt ** -0.93** * * * Clay ** 0.72 Ksat: saturated hydraulic conductivity, BD: bulk density, f: porosity, OC: organic carbon, TN: total nitrogen, C/N ratio: carbon nitrogen ratio, ECEC: effective cation exchange capacity, BS: base saturation, Av.P: available phosphorus, *Significant at 5%, **Significant at 1% NJAFE VOL. 11 No. 2,