Physico-Chemical Properties of Soil Under Acacia Senegal (L) Willd Plantation in Sahel Zone of Nigeria

senegal (L) Wild xing and with to improve infertile planted in Sahel of for the of dearth of on the under Soil samples were collected from four 30 x 30m plots in the plantation and another plot in open woodland (control). In each plot, three replicate soil samples were taken from 0-15cm, 15-30cm and 30-60cm soil depths analyzed for micro and macronutrients, while soil acidity was determined both in water and 0.01M CaCl 2 . The general soil texture of the plantation is loamy sand but with slight textural differentiation under the open woodland. Results indicated that physicochemical properties of soils under the plantation were signicantly different at P < 0.05 compared with the control. The mean soil bulk density ranged from 1.6g/cm 3 in the plantation to 1.8g/cm 3 in the open woodland. The high values of soil bulk density might be ascribed to loss of organic matter as a result of soil erosion and animal grazing which are common occurrences.


Background
Trees perform a dynamic role of maintaining soil organic matter levels through the supply of litter and root residues, which in turn improve the fertility of the soil. Trees take up different amounts and different proportion of nutrients from a soil according to their species. However, the amounts taken depend on the soil conditions (Russell, 1973). Several studies have reported positive in uence of trees on soil fertility and conservation (Kessler, 1992 andBill, 2007). Acacia senegal for example, has special capacity to enrich poor soils, rehabilitate degraded land during fallow periods (Spore, 2001) using its root to break up hard clay soils, thus increasing aeration and drainage as a result, making essential nutrients more easily available and to mitigate deserti cation as well. Its use for associated food crops and its value for the production of gum arabic and good fuel wood even in areas with an extended dry season make it a very worthwhile species for large-scale exploitation as an agroforestry and a fuel wood species (NAS, 1979).
However, there is inadequate information on the soil physico-chemical properties of Acacia senegal (L) Wild trees in plantation in the Sahel zone of Nigeria where it is found in the wilds and plantations and that makes this study necessary.

Materials And Methods
Soil fertility study was carried out in Maifari Gum Arabic plantations in Maigatari Local Government Area of Jigawa State, Nigeria. The plantation covered an area of 20 hectares with an espacement of 6 × 6 m between and within the rows. The plantation lies between latitudes 12 0 48 and 18 0 20 N and longitudes 9 0 27 and 52 0 16 E with an elevation of about 350 m above sea level.

Experimental design and layout
The study investigated the effects of Acacia senegal on soil by comparing the soil properties of Acacia senegal plantation with that of a natural open woodland. The plantation was divided into plots of 30 m x 30 m with four plots randomly chosen with the adjacent natural open woodland of the same size selected as a control. In each plot, twenty ve quadrats of 6 m x 6 m were established and three quadrats were randomly chosen. The soil samples were collected from the depths of 0-15 cm, 15-30 cm and 30-60 cm. The study was conducted using a randomized complete block design.

Soil Analysis
The soil samples were collected in polythene bags and air-dried in the laboratory for several days. Samples were gently crushed with porcelain pestle and mortar and sieved through a 2 mm sieve to remove coarse fragments. The ne soils separated were stored in polythene bags and taken to the Department of Soil Science, Ahmadu Bello University, Zaria, Nigeria for laboratory analyses. Particle size distribution and bulk density were determined using the hydrometer method, while cores were oven-dried to constant weight at 105 0 C for two days and expressed as mass of dry soil per unit volume of moist soil. The soil pH was determined both in water and 0.01M CaCl 2 solution using a soil solution ratio of 1:2.5 (IITA, 1979;AWWA, 1992). Cation exchange capacity (CEC) of the soil was determined with 1M NH 4 OAC (1M ammonium acetate) buffered at pH 7.0 (Chapman, 1965, Rowell, 1994. Exchangeable Cation exchange capacity (ECEC) was calculated from the summation of exchangeable bases determined by 1M NH 4 0Ac extraction and the exchange acidity by 1M KCl extraction (Anderson and Ingram, 1998). Organic carbon was determined by chromic acid digestion method of Walkley and Black (1934). Total N was determined using the micro-Kjeldahl digestion technique (Bremmer, 1982), while available P was estimated colorimetrically by the molybdo-phosphoric-blue method using ascorbic acid as a reducing agent (Bray and Kurtz, 1945). Field and laboratory data generated were subjected to analysis of variance (ANOVA). Differences in mean values were tested at 0.05 level of signi cance with Duncan's multiple range tests (DMRTs).

Results And Discussion
The general soil texture of the plantation is loamy sand. However, under the open woodland used as the control (natural open woodland), the soil had a slight textural differentiation (Table 1). The texture of the soil is sandy, especially at the top soil (0 -15 cm depth). This is so as the study area is a part of Sahel savanna of Nigeria which is characterized by sandy to sandy loam soils (Odunze and Ogunwole, 2000).   The amount of silt and clay in the plantation was small compared to the value obtained for the sand. The silt contents were higher at 0 -15 cm depths both in the plantation and under the natural open woodland. However, the content of silt was much higher in the plantation than in the natural open woodland. This could be attributed to the vegetation cover of the Acacia senegal which helps to prevent soil erosion within the plantation. The clay content was also high at 0 result of inadequate plant materials (litter). In a similar study under Prosopis cineraria plantation in semi-arid areas in India, Shankarnarayan et al., (1986) observed that silt and clay content of soil increased up to a depth of 120 cm as compared to the surrounding open elds which are devoid of trees. These changes in soil physical properties resulted in the maintenance of soil moisture beneath the trees.
The mean soil bulk density ranged from 1.6 g/cm 3 in plot 1 to 1.8 g/cm 3 in the natural open woodland (Table 2). However, the results of analysis of variance indicated a positively high signi cant value at P < 0.05 (Table 3). The high mean values of soil bulk density obtained in both the plantation and the natural open woodland might be ascribed to the loss of organic matter through constant exposure of the plantation to sandstorms that usually blow away the rich ne particles of the top soil and the compaction of the soil by the roaming animals that were usually found grazing in the plantation. The case of the natural open woodland is a good example of a plot devoid of vegetation that could hardly prevent the sandstorms from blowing away the ne soil particles that are rich in organic matter.

Soil chemical properties
The general distribution of soil pH trend was a decrease from the top (0 -15 cm) to the lower soil depth (30 -60 cm). The results of analysis of variance tests for pH (H 2 0) indicated signi cant differences at different soil depths, while across the plots, there was no signi cant difference at p = 0.05. Also, for the pH (CaCl 2 ), there were signi cant differences among soil depths but not across plots (Tables 4 and 5). A similar trend was observed along soil depth by Lechisa et al. (2014) under different land use systems in Gindeberet district, Ethiopia. The reason attributed was the reduction of Ca and Mg ions along soil depth which lower soil pH from top to down the soil layers (Soto and Diaz erroz, 1993).
The amount of organic carbon was signi cantly higher in the topsoil and decreased with soil depth and following the same trend for total nitrogen (Table 4). This is in line with some studies (Jackson et al., 1996;Carter et al., 1997;Mohamed, 2005). Since A. senegal is a deciduous tree that sheds its leaves during the dry season and is adapted to harsh environmental conditions, the accumulation of organic carbon in the topsoil may be as a result of leaf litter decomposition. According to Schlesinger et al. (1996) and Burke et al. (1998), soil organic carbon storage and distribution are controlled by the balance of carbon inputs from plant production and outputs through decomposition.
The carbon-nitrogen ratio (C/N) in the plantation and natural open woodland decreased with increasing soil depth following the pattern of organic matter distribution (Table 4). The analysis of variance showed that differences in the C/N ratio were statistically signi cant across the soil depths. But the low C/N values of the 0 -15 cm soil depth may be ascribed to higher fresh litter accumulation, which lowers the rates of mineralization.
The mean value of available phosphorous was statistically signi cant both for the soil depth and across the plots. It should be noted that available P under the natural woodland was low and also decreased with soil depth. The decrease with depth is ascribed to decrease in organic matter levels as noted by Samndi (2005) under Tectona grandis plantation in Nimbia forest, Kaduna State, Nigeria. This phenomenon was observed by Ogunyebi (2008) in the study of decomposition of Gmelina arborea leaf litter in the lowland rainforest of Nigeria. Mean values with the same letters are not signi cantly different at 5% probability level by DMRT The Exchangeable cations investigated in the soil of the plantation and the natural open woodland are calcium (Ca), magnesium (Mg), potassium (K) and sodium (Na). While the calcium and potassium decreased as the soil depth increased, both magnesium and sodium increased as the soil depth increased. The results of decreasing calcium and potassium with soil depth were in agreement with the ndings of Oyun (1991) and could be a result of large uptake and storage of Ca and K by the trees. The increase of Mg and Na down the soil depths is in consonance with Tedela (2004) and might be attributed to less utilization of these nutrients by Acacia senegal. However, the increase of Na along the soil depth could cause soil salinity which is a common phenomenon in dry areas, such as the study area (Oyun, 1991).
The ECEC decreased as the soil depth increased and is low in values. According to Nwachokor et al (2009), a low ECEC less than 4 cmol/kg implies a low capacity for the soil holding cations against leaching. The CEC of soils commonly range from 3 to 50cmol/kg, comparatively therefore, the soil of the study area had low CEC. The low CEC values could probably be as a result of low organic matter in the soil. This could be attributed to the fact that the soil of arid zones is characterized by low organic matter and clay, oxides and hydroxides of iron and aluminium; all these could lead to poor growth of Acacia senegalensis in the plantation.

Conclusion
A comparison of soil nutrient contents of the study area showed that the Acacia senegal plantation nutrient contents were higher than what were obtained in the natural open woodland. The higher nutrient contents obtained under the plantation plots may be as a result of mineralization resulting from higher number of Acacia senegal trees which shed their leaves making the leaves available for decomposition thereby increasing the soil organic matter, nitrogen content, increase availability of micro-nutrient (Zn, Mn and Cu) in the soil surface and improved the soil structure. It can therefore be concluded that the decline in soil fertility in the natural open woodland is often as a result of less mineralization of litter which leads to the depletion of the nutrient pool of organic matter.
The study has also shown the in uence of trees in soil physical properties which are very important in augmenting the overall capacity of the land to produce.
The removal of the vegetative cover from the soil results in increases in bulk density, decreases in porosity and reduction in in ltration rate. This ultimately will reduce land productivity if not properly taken care of. In line with this, researches have demonstrated in uence of trees in reversing this problem. This effect was associated with the in uence of tree roots and biomass return in terms of mulching or tree litter on the structure and texture of the soils.

Recommendation
Judging from the above, the soil quality and productivity of the natural open woodland can be restored by planting the area up with Acacia senegal trees or any other tree(s) as trees will promote high biological activities of soil fauna, improve soil organic matter content and replacing the nutrients lost to soil or wind erosion and animal grazing on the land.