Status of selected physicochemical properties of soils under different land use systems of Western Oromia, Ethiopia

Land use change particularly from natural ecosystem to agricultural lands in general and to crop cultivation under poor management practices in particular are among the major causes of decline in soil fertility followed by land degradation and low agricultural productivity. Achieving scientific information thereof is vital for planning management strategies; this study assessed the effects of land use on soil physicochemical properties describing soil fertility under three land use types (natural forest, grazing and cultivated land) in Guto Gida District of Oromia Region, Western Ethiopia. The natural forest land was used as a control to assess status of soil properties resulting from the shift of natural forest to other land uses. Disturbed and undisturbed surface soil samples (0-20 cm) were collected from each land use type and examined for their analysis of soils physicochemical properties. The study pointed out the difference between different land use type on soil water content, pH, Cation exchange capacity (CEC), organic carbon (OC), total nitrogen (TN), available phosphorus (P), and exchangeable bases. Correlation analysis also showed highly significant and insignificantly positive relationship of soil pH with exchangeable Mg2+ and Ca2+ ions but significantly with extractable Fe3+, Mn2+ ions and PAS of the soils among the land uses. Relative to forest land, when the percent OC contents in cultivated and grazing lands depleted respectively, by 54.62 and 49.89%, the percent Al saturation in cultivated land increased by 65.62% and 28.57% in grazing land. Land use changes also caused a decline in CEC, PBS, exchangeable bases and increased BD and Clay content exhibited poor soil physical conditions and deterioration of soil fertility. *Corresponding Author: Achalu Chimdi  achalucc@yahoo.com Journal of Biodiversity and Environmental Sciences (JBES) ISSN: 2220-6663 (Print) 2222-3045 (Online) Vol. 2, No. 3, p. 57-71, 2012 http://www.innspub.net J. Bio. & Env. Sci. 2012 58 Introduction The management of tropical soils is crucial to address the issues of food security, soil degradation and environmental quality including the global carbon cycle. Sustaining soil and environmental qualities is the most effective method for ensuring sufficient food supply to support life (Soares et al., 2005). In an effort to this end, soil scientists developed the concept of soil quality to describe the fitness of soils to perform particular ecosystem functions and management. Maintaining soil quality mainly depends on the knowledge of the physicochemical properties of a given soil. The assumption of the sustainability of agricultural ecosystems also depends to a great extent on the maintenance of soil physicochemical properties. Its feasibility is based on the knowledge of the effects of management practices on soil properties, and how they affect soil-crop-water relations (Heluf and Wakene, 2006). Therefore, characterization and/or evaluation of soil properties is a master key for describing and understanding the status and qualities of the major nutrients in soils (Geissen et al., 2009). Assessing soil physicochemical properties are used to understand the potential status of nutrients in soils of different land uses (Wondowosen and Sheleme, 2011). This knowledge can ascertains whether the specified land use types are useful for a given production system and used to meet plants requirement for rapid growth and better crops production (Shishir and Sah, 2003). Over the past several decades, the conversion of native forest to agricultural land use has accelerated and featured in the development of Ethiopian landscapes and has apparently contributed to the widespread occurrence of degraded land across most part of the country. However, different land use practices have a varied impact on soil degradation and on both physical and chemical property of soil. Study by Wakene and Heluf (2003) have examined the impacts of different land uses on soil qualities and the study indicated, the rate of soil quality degradation depends on land use systems, soil types, topography, and climatic conditions. Assessing land-use-induced changes in soil properties is essential for addressing the issue of agro ecosystem transformation and sustainable land productivity. The selection of suitable indicators with well established ecological functions and high sensitivity to disturbances is of paramount importance. In this regard, soil organic carbon(SOC) is important for the overall carbon reservoir of the biosphere and plays a preponderant role in the global biogeochemistry cycle of major nutrients and it has been used extensively by authors to monitor landcover and land-use change patterns (Koutika et al., 2002; Sisti et al., 2004). Though they are sensitive to land-use changes, soil organic C and N have been proposed by some worker as indicators for assessing the effect of land use management (Alvarez and Alvarez, 2000). Thus, it is a key source of soil nutrients for plant growth and soil structural stability, as well as carbon stock levels (Sisti et al., 2004). However, its dynamics and composition are influenced by land-use changes, agricultural and management practices (Stevenson, 1994 and Barthes et al., 1999). Knowledge about an up-to-dated status of soil physical and chemical properties of different land use systems plays a vital role in enhancing production and productivity of the agricultural sectors on sustainable basis. However, practically oriented basic information on the status and management of soil physicochemical properties as well as their effect on soil quality to give recommendations for optimal and sustainable utilizations of land resources remains poorly understood. Therefore, this study was conducted with specific objectives to assess and explore the status of soil physicochemical characteristics of three different land use systems of representative area of Western Oromia Region. The results of this study are expected to add value to the up-to-date scientific documentation of the status of soil fertility and soil quality of different land uses of J. Bio. & Env. Sci. 2012 59 the study area and other similar agro-ecological environments in the country. Materials and methods The study area Geographically Guto Gida District is located in the Oromia Regional State, Western highlands of Ethiopia (Fig. 1) lying between 080 59’and 090 06` N latitude and 370 51`and 370 09`E longitude at an altitude of 1650 meters above sea level (masl). The study site is suited at a road distance of 310 km from the capital, Addis Ababa. According to the Ethiopian agro-climatic zonation (MOA, 1998), the study area falls in the highland (Baddaa) and mid altitude (Badda Darree). The ten years (1996-2007) climatic data from Nekemte Meteorological Station recorded an average annual rainfall 1780 mm which is characterized by unimodal rainfall pattern and its annual mean minimum and maximum monthly temperatures lies between 13.75 and 27.65 0C (Fig. 2). According to FAO, (1990) classification, the soil class of the study area is Nitosols and topographically characterized by mountainous and gentle sloping landscape. Subsistence agriculture is the main livelihood of the community and crop-livestock mixed farming system is predominant. The major crops commonly grown in the study area are coffee (Coffee arabica), teff (Eragrostis tef), barley (Hordeum vulgare), maize (Zea mays), potato (Solanum tubersoum) and hot pepper (Capsicum frutescence). Crop production are based on rainfed agriculture and harvested usually once in a year. Data source and analysis of soil samples In order to have general information about the land forms, land uses, topography and vegetation cover, a preliminary survey and field observation using the topographic map (1:50,000) of the study area was carried out during the year of 2010. Accordingly, three major representative land uses (natural forest, grazing and cultivated) lands were selected based on their history and occurrence. The natural vegetation of the study area is characterized by Indigenous natural forest and canopies, where as rainfed crop cultivation bounded by scattered settlements in the cultivated land and communal and private grazing land were the characteristic features of the land use types. The composite top soil (0-20 cm) samples from representative site of each land use in three replicates were collected, air dried, ground and passed through a 2 mm sieve for analysis. Analysis of soil samples were carried out at Chemistry laboratory of Ambo University and Holleta Soil Research Laboratory Center based on their standard laboratory procedure. Soil particle size distribution was analyzed by the Bouyoucos hydrometer method as described by Day (1965). The soil-water holding capacity (WHC) values were measured at -1/3 bar for field capacity (FC) and -15 bar for permanent wilting point (PWP) using the pressure plate apparatus method (Klute and Dirksen, 1986). Specific surface area was determined using ethylene glycol equilibrium method as described by Dipark and Sarkar (2003). Soil bulk density (Db) was measured from undisturbed soil samples collected using a core sampler which was weighed at field moisture after drying the pre-weighed soil core samples to constant weight in an oven at 105 0C as per the procedures described by (1965). Particle density (Dp) was determined by the pycnometer method (Devis and Freitans, 1984). Total porosity was estimated from the bulk and particle densities as described as: Total porosity (%) = (1Db/Pd) x 100 Where Db bulk density in (g cm-3) and Pd particle density (g cm-3) The pH of the soil was measured potentiometrically with a digital pH meter in the supernatant suspension of 1:2.5, soil: liquid ratio (Baruah and Barthakur, 1997). Organic carbon (OC) content was determined by the dichromate oxidation method (Walkely and Black, 1934). Total N was determined using the micro-Kjeldahl digestion, disti