Land use changes from forest into cultivated ecosystems result in negative impact on soil structure and quality. The purpose of this study was to determine effect of land use and slope on selected properties and quality of an Alfisol in Afaka forest, Northern Guinea savanna of Nigeria. Land use systems, including natural forest and cultivated land were identified. Eighteen (18) composite disturbed and undisturbed samples were collected from depth of 0-5 and 5-10cm for analysis of pertinent soil properties in the laboratory using rigid grid procedure. Most physical and chemical properties show relative variations in response to land use types and geomorphic positions. Mean weight diameter (MWD) indicates higher water stable aggregates of 1.26 mm in forestland and less stable aggregates in cultivated land 1.19 mm. Textural classes of soils under all the land uses on different topographic positions were sandy loam. Results indicate that the soils had high degree of weathering potentials, low to moderate bulk density at 0-5cm depth values range between 1.42 to 1.49 Mg m-3 in forest and cultivated land, bulk density at 5-10cm depths had values ranging between 1.34 and 1.46 1.Mg m-3 for forest and cultivated land respectively. Total porosity ranged from 40.77 to 46.10 % for cultivation and forest land use while slopes have 45.00, 44.48 and 44.38 % porosity for upper, middle and lower slopes respectively. The highest soil moisture contents at 0-5cm depth were 4.20, 2.63 cm3/cm3 , while at 5-10cm depths values were 4.32 and 2.13 cm3/cm3 recorded under forest and cultivation land use. Middle slope shows higher soil moisture content of 4.35 and 4.38 cm3/cm3 for 0-5 cm and 5-10 cm respectively. The pH (H2O) ranged from 6.9 to 7.16 in the land uses. The electrical conductivity in the land uses was 0.13 dS/m and 0.12 dS/m was obtained for forest and cultivation land respectively, with highest EC at upper slope 0.16 dS/m. The organic carbon concentration of 10.2 g kg-1 and 8.2 g kg-1 was obtained in forest and cultivated land and highest carbon stock of 1522.1 C kg ha-1 was recorded at forestland with 1124.3 C kg ha-1 at cultivation land. The CEC of 8.60 cmol kg-1 and 8.54 cmol kg-1 was recorded on forest and cultivated land uses, whereas highest CEC value of 9.28 cmol kg-1 was recorded at middle slope. The exchangeable bases (Na, K, Mg and Ca) were medium rating in all land use and slope positions. The total nitrogen content of 1.21 g kg-1 and 1.11 g kg-1 for forest and cultivation land uses was recorded. The highest available phosphorus of 8.78 mg kg-1 and 5.47 mg kg-1 was recorded under cultivated and forest land use while lowest value 4.12 mg kg-1 of available phosphorus was recorded at middle slope. The highest carbon: nitrogen ratio 8.6:1 was obtained under forestland and 7.5:1 was obtained at cultivated land. Highest aluminum concentration of 0.14 and 0.26 cmol kg-1 was obtained at forest and cultivated land use. Alfisols at the upper forest land had better soil quality than those at other slope positions. Results indicate that soil fertility parameters were moderate to low in soils of cultivated land and all slope positions, suggesting that soil fertility management is required in order to make agriculture sustainable on Afaka area.
1.1 Background of study
Nigerian Sudan savanna zone is situate etween latitude 9 3 and 12 31 N and longitude
4 and 14 3 E whi h overs a out 22 8 million he tares representing a out one quarter of Nigeria‟s geographi al area (Odunze, 2006) The region is hara terized y high annual average temperature (28-32°C), short wet season and long dry season (6-9 months), abundant short grasses (<2 m) and a few scattered trees (Lal, 1997). Large expanse of arable land exists in the Sudan Savanna Zone of Nigeria with potential for the production of largely grain crops like maize, sorghum, millet, rice and wheat (FFD 2012). Most Nigerian Savanna soils are highly weathered and fragile with low activity clays, thus making their fertility decline under continuous arable cropping (Odunze 2006; FFD, 2012). Generally, soil productivity declines rapidly when vegetation cover is lost and inappropriate management practices are adopted (Lal 1997), thereby resulting in soil organic matter depletion and reduced agricultural productivity and food security.
Population increase and the need to achieve food security; especially in Nigeria, has given rise to clearing of forests for agricultural land use (Osakwe, 2014). Tropical soils are inherently fragile and therefore, sensitive to land use and management since removal of soil cover and subsequent tillage are activities that are likely to affect soil physical and chemical properties and micro aggregate stability (Osakwe, 2014).
Important soil property to consider for assessing soil quality under forest or cultivated land use is aggregate stability of soil. Soil aggregate is considered as soil quality indicator that provides information on soil‟s a ility to fun tion as a asi omponent of the ecosystem (Martinez et al., 2012). Micro aggregate stability may be a good indicator of erodibility of Tropical soils (Igwe and Obalum, 2013). Oguike and Mbagwu (2009) demonstrated that micro aggregate stability measured by aggregated silt plus clay (ASC), Water dispersible clay (WDC), Clay dispersion index (CDI) and Clay flocculation index (CFI) were affected by land use. Soils that have high water dispersible clay are vulnerable to surface sealing, crusting and limits infiltration, thereby increase runoff (Chang et al., 1994). Obi (1982), in an assessment of micro aggregate stability under different land use types in a tropical Nigerian soil, revealed a strong dependence of soil aggregation on land use (Igwe and Obalum, 2013). Gochin and Asgan (2008) investigated effects of land use (forest, pasture and cultivation) on soil quality, and reported 41-89 % less dispersible clay in the forest than in their cultivated counter parts. These studies noted that frequent cultivation caused deterioration of soil quality and enhanced erosion through decreased mechanically dispersed clay. Curtin et al. (1994) demonstrated that higher clay plus silt in water stable aggregates of forest than in
ultivated soils‟ mineralogy and soil organi ar on appear to influen e mi ro aggregate stability in Tropical soil but the major micro aggregating agent in tropical soil is iron (Fe) and aluminum (Al) oxides. However, the micro aggregating effect of iron sometime could be masked in soils with relatively high soil organic carbon content (1.6 – 6.9 %) as reported by (Opara, 2009); adding that soil organic carbon can also act as a micro aggregating agent or as a facilitator to micro aggregating effect of iron and Aluminum oxide (Igwe and Obalum, 2013). Soil organic carbon is an important soil property because of its hydrophobic characteristics that has the ability to reduce slaking which precedes dispersion. Mbagwu and Bazzoffi (1998) demonstrated that 70% of variations in
water dispersible particles were accounted for by organic matter. Brubaker et al. (1992) indicated that clay dispersion in water had been found to significantly correlate with total content of clay. Savanna Alfisols are low in inherent fertility, organic matter, cation exchange capacity, and dominated by low activity clays (Odunze, 2003). Savanna Alfisols also support production of crops such as maize, sorghum, millet, cowpea, groundnut, soya beans and cotton, and are cultivated continuously and intensively. This continuous and intense cultivation of soils in the zone has resultant effect of accelerated soil erosion, soil nutrient depletion and soil degradation (Bationo et al., 2003).
Another important physical and chemical soil property controlling solubility of many soil nutrients and influence soil fertility is pH of the soil. For example, Monges et al. (2013) did not find any significant variation in soil pH across land use types while Killic et al. (2012) showed slight change in pH with land use change. Land use affects soil fertility and productivity; this manifests as changes in soil properties such as macro nutrients (Nitrogen, Phosphorous, potassium, Calcium, Magnesium, Sulphur etc), pH, Organic matter, Cation exchange capacity, structure (Birkeland, 1984). Deforestation and cultivation of virgin tropical soils often lead to depletion of nutrients (N, P, and S) present as part of complex organic polymers (Birkeland, 1984).
Soil pH, total N, organic carbon, available P, exchangeable Ca, exchangeable Al, CEC and Al saturation significantly differed with the land use types. Lal (1996) and Shepherd et al. (2000) noted that variation of land use in tropical ecosystems could cause significant modifications in soil properties. In stressing the effect of this phenomenon on ecosystem, Schipper and Sparling (2000) posited that land use modifications are biologically and chemically more rapid than physical, as forest ecosystems are important
ecologically and economically. Forest soils are one of the major sequesters of carbon on earth due to their high OM status (Dixon et al., 1994)
1.2 Statement of the problem
Continuous and intense cultivation of soils has effect on soil; resulting to soil erosion and soil nutrient depletion. This is because continuous cultivation alters soil structure and increases loss of soil organic matter which leads to soil degradation. Increasing incidence of soil degradation and the consequent impoverishment of soils have generated interest in the need for proper understanding of soil characteristics, and adoption of suitable conservation practices by farmers for sustained crop production in the study area. This changes in soil quality can be assessed by measuring appropriate indicators and comparing them with desired values (critical limits or threshold level), at different time intervals, for a specific use in a selected agro-ecosystem (Odunze et al., 2012). Slopes have been shown to affect temperature of the soil, vegetation establishment and moisture levels. These factors in turn can affect the distribution of soil organic matter, pH and nutrient levels (Birkeland, 1984).Soil reaction (pH) trends and nutrient levels are usually associated with vegetation and can also be affected by slope aspect (Birkeland, 1984).
As population growth in Northern Nigeria increases, it may require additional farmlands for food production, leading to cutting and converting forest land to cropland. Clearing of forests for agricultural production and grazing is widespread, particularly in savannas of Nigeria. The prevailing semi-arid climate in Afaka area renders the ecosystems more vulnerable and less resilient to changes in land use. The destruction of natural forest and
pasture ecosystems and conversion to cropland can reduce soil productivity because of increased erosion, nutrient mining and decline in fertility, changes in aeration and moisture content, salinization or change in soil flora or fauna (Bossuyt et al., 1999). Effects on vegetation and species distribution have been documented in different ecosystems following anthropogenic activities (Peterken, 1974; Six et al., 2000; Bruun et al., 2001; Foster et al., 2003). Land use-induced changes in nutrient availability may influence secondary succession and biomass production (Foster et al., 2003) and reduce soil organic carbon (SOC); which plays a crucial role in sustaining soil quality, crop production and environmental quality (Doran and Parkin, 1994). Such changes directly affect soil physical, chemical and biological properties; such as soil water retention and availability, nutrient cycling, gas flux, plant root growth and soil conservation (Gregorich et al., 1994). Maintenance of SOC is especially important due to its effect on soil nutrient status and structural stability. Afaka study area is facing the problem of deforestation, over grazing, poor soil management and severe erosion. Combating and minimizing on-going soil degradation and enhance land productivity through sustainable use of soil resources requires understanding the soil physico-chemical characteristics under different land use systems. However, very little information is available about the study area. Therefore the present study was undertaken to evaluate effect of different land use systems and slopes on selected physical and chemical properties of different land use system in Alfisols of Afaka area.
I: Conversion of forest land to cultivation causes change in soil properties II: Subjecting of forest land to cultivation reduces soil carbon stock
The major objective of this work is to evaluate the effect of land use and slope on selected properties of an Alfisols in Afaka Forest Northern Guinea Savanna
1.5 Aim and objectives of the study
The general objective of this study is to evaluate the effect of land use and slope gradient on selected soil properties and quality of a Paleustalfs in Northern Guinea savanna of Nigeria. The specific objectives are:
1. To determine the effect of land use and slope gradient on selected properties and soil quality of an Alfisol in Afaka forest Area, Northern Guinea Savanna
2. To determine soil selected physical and chemical quality changes that occurred with slope position in the soil when forest land was converted to cultivated land use.
3. To assess the effect of forest and cultivation on soil organic carbon stock (SOC)