Background to the Study
Human activities exert tremendous effects on land cover through a variety of land uses. Human use of land has altered the structure and functioning of the ecosystem (Turner et al., 1994). Land, whether in tropical or temperate regions land is being used for the purpose of growing trees, crops, and animals for food, as well as building sites for houses and roads, or for recreational purposes among others (Hartemink et al., 2008).
Globally, land use/land cover has changed considerably in the past decades – mostly reflecting the enormous growth in human population and their need for food, shelter and other necessities of life. Changing demography and particularly the changing age structure of populations, a high rate of urbanization, and a faster rate of population growth in relation to economic growth (Braimoh, 2003; Ouedraogo, et al., 2010), are major drivers of environmental change with significant negative impacts on the natural resource base. Demographic change therefore constitutes a major driver of land use/land cover change (LULCC): Its primary and most direct impact is through opening up of new land for agriculture, settlement and infrastructural development, although other extractive activities such as logging and mining are also significant (FAO, 2003). The world’s population has doubled smce 1960 (Hartemink, 2006). The developing world accounts for about 95% of the population growth with sub Saharan Africa as the world’s fastest growing region (Hartemink, 2006). The growing population has many implications but most of all it requires an increase in agricultural production to meet food demand. This demand can be met by expansion of agricultural land or by intensification of existing systems. Clearing of new lands, for whatever purpose has long been common in areas with rapidly growing populations (Kates, 2000; Braimoh, 2003 ; Ouedraogo, et al., 2010).
Land resources have been altered by rapid land use and cover changes accelerated by changeable socio-economic factors including high population growth, rapid urbanization (Fabiyi, 2006), agricultural intensification and government policies (Entwisle et al., 1998; Mather et al., 2000; Braimoh, 2003 ; Ouedraogo, et al. , 2010). Human pressure upon land resources and interactions between varying climatic characteristics facilitate changes in land use/land cover (Hartemink et al., 2008). In the absence of alternative livelihood opportunities and proper management of the environment, this rapid population growth and urbanization has resulted in environmental degradation and resource depletion. Between 1990 and 2000, sub-Saharan Africa lost 52 million hectares of forests which amounts to a decrease of 0.8% per year and 56% of the global total. It is estimated that 60% of the tropical forest areas cleared in Africa as a whole between 1990 and 2000 were converted to permanent agricultural smallholdings (FAO, 2003).
Rapid Population growth affects the natural resource base through increased demand for food, water, arable land and other essential materials, such as firewood among others, thereby facilitating encroachment into forests and woodlands as well as increasing demand for fossil fuels and other resources. The poor or low soil quality base arises due to two major factors. With few exceptions, growth in human population have led to a reduction in the per capita land availability and a breakdown of the erstwhile traditional natural fallow system that used to be the means of replenishing soil fertility. The methods used to restore the fertility of soils and to2sustain agricultural productivity under traditional shifting agriculture have become ineffective, and in some cases, have disappeared altogether. As high potential land becomes less available and the rural human population increases, farming is extending into more fragile lands, undermining the natural resource base as well as undermining the continued ability to produce food for the teeming populations. Degradation of the natural resource base in tum impinges on the livelihoods of all, but particularly rural communities. More small farmers are forced to work harder, often on shrinking farms on marginal land, to maintain household incomes.
Soil degradation, and in particular the decline of soil chemical fertility, is a major concern in relation to food production and the sustainable management of land resources. It also affects land use/land cover but the spatial and temporal effects of soil fertility change and its interaction with land use/land cover change remains to be investigated.
Land use involves the manner in which the biophysical attributes ofthe land are manipulated and the intent underlying such manipulation for which the land is used, whereas land cover implies to the biophysical state of the earth’s surface and immediate subsurface including biota, soil, topography, surface and groundwater, human structures etc (Meyer et al., 1994; Lambin et al., 2003). Land use change implies the conversion of land use due to human intervention for various purposes such as agriculture, settlement, transportation, e t c (Williams, 1994; Meyer, 1994; Turner et al., 1995). While land cover change on the other hand, refers to modification of the existing land cover or complete conversion of biophysical cover of the land to a new land cover type (Solomon, 2005).
Though people have been using and modifying land to obtain food and other essentials for thousands of years, current rates, extents and intensities of Land use/land cover change are far greater than ever in history, resulting in unprecedented changes in ecosystems and environmental processes at local, regional and global scales (Lambin et al., 2003).
The area under crop cultivation in the world has increased globally from an estimated 3 00-400 million ha in 1 700 to 1 500-1800 million ha in 1 990, a 4.5- to fivefold increase in three centuries and a 50% net increase just in the twentieth century. The area under pasture increased from around 500 million ha in 1700 to around 3 1 00 million ha in 1990 (Ramankutty et al., 2002b). These increases imply to changes, clearances or conversions of forests cover and the transformation of natural grasslands, steppes, and savannas. Forest area decreased from 5000-6200 million ha in 1 700 to 4300-5300 million ha in 1 990. Steppes, savannas, and grasslands also experienced a rapid decline, from around 3200 million ha in 1700 to 1 800-2700 million ha in 1990 (Ramankutty et al.,
2002b) (Table 1. 1 ).
Table 1. 1: Global historical changes in LULC over the last 300 years
(Million ha) 5000-6200
(Million ha) 3200
(Million ha) 3 00-400
(Million ha) 400-500
|1990||4300-53 00||1 800-2700||1500- 1 800||3 1 00-3300|
Source: Lambin et al. (2003) based on Ramankutty et al. (2002b).
Nigeria was once covered by extensive vegetation varying from humid tropical forests in the south to savannah grasslands in the northern parts (Federal Department of Forestry, 1998). A greater percentage of this luxurious vegetation cover has been removed in the course of various human activities such as crop cultivation, urbanisation and other forms of developments (FDF, 1998). Presently, forest reserves cover about 10% of the national territory, mostly of the savannah woodland type (PDF, 2001 ). The first indicative forest inventory project completed in Nigeria in 1 977 put reserved forest at approximately 1 0% of the total land area. Between 1 981 and 1 985, deforestation, being a fundamental land cover change, was estimated at an average rate of 400,000 ha (3 .48%), per annum; while between 1 986 to 1 990, it increased at an estimated average rate of 3 .57%, including the loss of some forest reserves (PDF, 2001 ). Recent studies using Remote Sensing and GIS in the country show that undisturbed forest covers only 121 14 km2 representing about 1 .3% of the Country’s total land area. By the year 2005, Food and Agricultural Organisation (FAO), had concluded that if this rate is maintained, the remaining forest area in Nigeria will disappear by the year 2020.
As has already been established, most tropical soils have characteristically low inherent fertility as well as several soil related constraints to sustain intensive cropping (Lal et al., 1 997). Soils classes in northern parts of Nigerian are predominantly alfisols and ultisols, which cover about 60% of the region (Odunze et al: 2002). The agricultural value of these soils usually ranges
between poor to average and the nutrients levels are most usually low (Singh, et al., 1 996). The chemical status of the soils was proved to be poor with low total nitrogen, available phosphorous and Cation exchange capacity (Singh, et al., 1 996). The soils have low fertility status, poor water holding capacity and low organic matter content (Odunze et al; 2002, Jaiyeoba,, 1995; Yakubu). These soils also have a dominance of low activity clays in the surface horizon (Jaiyeoba, 1995) with accompanying low exchange capacity. The soils have poorly developed structures, which deteriorate with cropping and compaction (Singh, et al., 1996) and are fragile and degrade rapidly under continuous cultivation (Singh, et al., 1 996).
Several researches and experiments have demonstrated a rapid decline in the soil quality following land use/vegetative cover conversions either from forest or grasslands to agriculture and related uses of varying intensities (Jaiyeoba, 1995; Arshad et al., 1996; Lal, 1 996; Hajabbasi et al., 1997; Hartemink, 1998; Lumbanraja et al., 1998; Saikh et al., 1998; Wang et al., 1998; Arshad et al., 2002 ; Doran et al., 2002). Improved understanding of the processes related to land use/land cover change, therefore, provides the foundation for evaluating the interactions among factors influencing human activities and feedbacks within the coupled human environment system (FAO/UNEP, 1997; Ostrom et al., 1999). Overall concern with the global changes and their effects on existing and future environment requires a better understanding and quantification of the processes and effects of land use and cover changes on soil which serve as the major source of human livelihood (Sa’ eb, et al., 2008; Sanchez, 2002; S anchez et al., 2002).
Therefore, assessing the effects of Land use/land cover change (LULCC) in the face of the current tempo of essential resources production and consumption has therefore assumed a major research and policy priorities particularly in marginal areas of the semi-arid region which are naturally characterised by delicately balanced ecosystem (Gadzama, 1991).
Statement of Research Problem
Due to repeated annual droughts and/or desertification in this area, which reduce the natural regeneration rate of land resources, as well as rapidly changing socio-economic opportunities and varying policy priorities occasioned and aggravated by state and local governments creations in the late 1 980s, land use/land cover transformations of unprecedented proportion have remarkably occurred in greater parts of grassland semi-arid ecosystem of northern Nigeria (Federal Department of Forestry, 2001 ). Consequently, the rate of annual natural vegetation depletion in this region related to biogeophysical factors of poor soil condition, drought and desertification leading to changes in land use/land cover in the 1 990s has been estimated at one kilometre (Forestry Management and Coordinating Unit, 1 996; FDF, 2001 ). The expansion of agriculture, desertification, deforestation, the shortening or elimination of fallows, inappropriate farming practices, and low agricultural inputs have resulted into changes in land use and cover (Lal, 2008� Hartemink, 2008}.
Research findings in many tropical countries have established that changes in land use/land cover, particularly from natural vegetation to crop cultivation and related uses have resulted in depletion of soil fertility (Stoorvogel et al., 1990; Reganold et al., 1 993 ; Nardi, et al., 1 996; Islam, et al., 1 999; Solomon et al., 2002; Parisi et al., 2005; Ursula et al., 2006; Shamsollah et al., 2007; Hartemink, 2008). Since the late 1980s, declining soil fertility has been recognized as an important cause for low agricultural production in this region (Stoorvogel et al., 1990; Verheyen et al., 1 999; Brejda et al., 2000; Lal, 2008; Hartemink, 2008). A significant factor in soil degradation is the soil chemical fertility decline caused by insufficient nutrient inputs and replacements as a result of changes in land use/land cover, which serve as the main reason why farmers continue to clear more lands (Arshad et al., 1 992; Boehn et al., 1997). Soils in this region are being depleted at an estimated annual rates of 22 kg/ha for nitrogen, 2.5 kg/ha for phosphorus, and 15 kg/ha for potassium (Smaling et al., 1997). Organic matter content of the soils is also rapidly declining in a similar rate.
However, as knowledge of the ecological consequences of land use/land cover conversions on soil quality is increasingly becoming available (Papendick, 1991; Ghuman et al., 1991; Veldkamp, 1994; Lal, 1994; Juo et al., 1996; Hartemink, 2008), the dimension, extent and magnitude of these changes and particularly their effects, which vary from one region to another, needs to be properly estimated on spatial and temporal basis especially in view of the existing diversity of physical and socio-economic development interventions in the region.
Studies on the edaphic effects of conversions of land use/land cover from grassland to croplands and related uses appeared limited, particularly when compared with rather more familiar and detailed studies of conversions from forest to croplands (Hartemink, 2008). Majority of such studies were more skewed in favour of the forest biomes to the detriment of tropical grassland biomes. Even the limited studies that have so far been conducted on the effects of land use /land cover change on soil quality are themselves limited in scope as many have not quantified the extent and nature of the changes in spatial and temporal perspectives first before examining the effects of such changes on soils quality (Bowman et al., 2000; Wilson et al., 2003 ; Mulugeta, 2004; Braimoh et al., 2006; Chen et al. , 2006; Fasina et al., 2007; Shamsollah et al., 2007; Moustafa et al., 2008 etc). S ince information on the type, extent and nature of land use /land cover change is important in assessing and properly understanding the effects of the change on soil quality, an integrated study involving a combined use of sound techniques such as Remote sensing (RS), Global Positioning System (GPS), and Geographic Information System (GIS), as well as laboratory analysis is required to both identify the type, extent and pattern/trend of the changes over spatial and temporal scales and their effects on soil quality. The need for such a study constitutes the problem of research interest here and hence this study intends to advance understanding in this direction by utilising the same methodological approach in examining human and biophysical aspects of land use and cover change in semi-arid parts of northern Katsina state.
The study seeks to provide answers to the following questions that constitute the research questions:
- What is the spatial extent of Land use/Land cover change within the study period?
- What is the pattern (change scenario/trajectory) of Land use/Land cover change observed in the study area?
- What are the effects of various changes observed on the fertility of the soil in the study area?
Have the observed changes in Land use/Land cover similar or uniform effects on the quality of soil in the study area?
1 .3 Aim and Objectives
The study seeks to assess the effects of Land use/Land cover changes on soil quality in parts of Katsina state, Nigeria. In specific terms, the objectives of the study are to:
Examine the nature and processes of Land use/land cover changes.
- Examine the temporal and spatial changes in land use /land cover in the study area.
iii. Examine the rate and extent of changes in land use/land cover in
the study area.
1v. Generate and examine change scenarios/trajectories using sub sets of areas that have manifested LULC changes across the study area.
- Collect and analyse soil samples from identified change scenario areas in order to assess the effects of the various change trajectories/scenarios on soil quality.
1 .4 Justification of the Study
In virtually all semi-arid regions of the world, development is dependent heavily on natural resources (FAO, 2001; S anchez, 2002), and hence regular assessments through monitoring of changes in environmental components such as land use and land cover and their effects on such environmental variables as soil quality should constitute an integral part of resource management planning. The whole thrust of soil quality research arose from the recognition that soils are a vital component of and provide necessary services to the ecosystem (Daily et al. 1997), and that the ability of soils to continue to provide those services is threatened by degradation (Sanders, 1992). Thus, a research that shall promote understanding of the consequences of changes in land use and land cover on soil quality appears to have scientific relevance.
However, examining the effects of land use/land cover change on soil quality is not a goal in itself, but could however be used for land use planning or to analyse spatial patterns of soil fertility change or for regional nutrient budgets (Hartemink, 2005a). The relevance of soil as an important denominator in the overall functioning of the environment and human well-being cannot be overemphasized (FAO, 1 998) and thus assessments of the consequences of land use/land cover changes on its quality becomes a fundamental requirement of sustainable environmental planning and management (Grant, 1 995). This is particularly so considering the enormous vulnerability of the semi-arid region to environmental changes such as drought, desertification, and deforestation and subsequent changes in climate which are the most serious environmental problems occasioned by excessive environmental resources exploitation (Gadzama, 1991; Mortimore et al., 1 999). Even though, governments at local, state and federal levels are aware of some of the consequences of changes in land use/land cover, not much is known about the dynamic nature, pattern and rate/magnitude of changes and their effects on soil quality at detailed local scale levels, particularly in this ecologically fragile area (Gadzama, 1991) where resilience ability of the soil has seriously been contemplated (Lal, 1995). Thus, the need arises for land use/land cover changes and their effects on soil quality be assessed in order to achieve sustainable environmental resources planning and management.
Though many studies have been conducted on land use and land cover change either at national, regional, state or local levels in different parts of the country (FORMECU, 1 996; Omojola 1997; Okude, 2006; Daniel et al., 2007; Abbas, 2008) none had specifically investigated the effects of such changes on soil or any other environmental component in a continental savannah areas such as the semi-arid region of Katsina state. Most previous researches in LULCC in Nigeria were too general in nature with a much wider regional or national spatial scopes and perspectives (FORMECU, 1 996; Omojola et al., 1997; Abbas, 2009; NLULCP/NCRS, 2008; Ademiluyi et al., 2008) instead of a specific local level scenario. Even the work of Omojola, 1997, which attempted to assess LULCC in the semi-arid region of northern Nigeria, was relatively general considering the fact that northern part of Nigeria as a region comprises of various distinctive ecological sub regions. Besides this, most previous researches lacked a clear pattern description of LULC change scenarios. Moreover, previous studies are lacking in this agriculturally important region of Nigeria that integrate field, laboratory and geospatial technologies to assess the effects of land use/cover changes on soil quality. Yet, such a systematic analysis of local-scale land use/land cover change conducted over a range of timescales helps to uncover general principles that provide explanation and prediction of LULCC phenomena. Therefore, this study is well justified to be conducted and more particularly in the study area.
1 .5 Scope and limitation of the Study
The whole thrust of soil quality research arose from the recognition that soils are a vital component of and provide necessary services to the ecosystem (Daily et al. 1 997), and that the ability of soils to provide those services now and in the future is threatened by degradation (Sanders, 1992). The concept of soil quality includes soil fertility, potential productivity, resource sustainability, and environmental quality (Singer and Ewing, 2000). As rightly argued by Doran and Parkin (1996) soil quality guidelines are intended to protect the ability of ecosystems to function properly. In this study, the scope is limited to consideration of fertility component of soil quality because knowing it serves the most important pre-condition for knowing the other components of soil quality. Other attempts in research in strongly recommended that will dwell into other areas, such as soil quality and environmental resource sustainability and potential productivity.
Aspects of soil fertility components of soil quality to be considered in this study include physical, chemical and biological properties (Blair et al., 1996; Andrews et al., 2002; Andrews et al., 2004).
The physical properties to be determined include aggregate stability (soil textural classes: sand, silt and clay) and soil depths (as in variations between top and sub surface soils characteristics). The chemical properties to be determined comprised of pH (H20) and (CaClz), Available phosphorous (AP), Cation exchange capacity (CEC) and exchangeable bases (Calcium, Magnesium, Potassium, and Sodium). Being the major bedrock of soil fertility, the biological properties to be considered comprised of the organic matter and total nitrogen (Ghunam et al., 1991; Arshad et al., 1992; Arshaad et al., 2002; Hartmink, 2008). The selection of these properties is based on the assertion that they form the basic determinants of soil fertility which serve as bedrock for agricultural and plant productivity (Blair et al., 1996; Hartmink, 2008). Moreover, the spatial entities (land use/land cover types) to be considered in this research include Woodland, Scrubland, Scattered cultivation, B are sand surface/ Rock out crop, Water body/Reservoir, Settlement/Built-up areas and Riparian vegetation as adapted from the Federal department of forestry (1 993 ) and National Land use/land cover Project carried out by the National centre for remote sensing (2008).
In terms of temporal and spatial scopes, the study utilized satellite image data for 1986, 1 996, 2006 and 201 1 on a subset portion of the semi-arid area of northern parts of Katsina state (see figure 3 .1 ). This appears to be one of areas suffering from the most noticeable trends in land degradation affecting the state.
Both soil fertility and land use/land cover changes are complex environmental issue consisting of several attributes that interact over spatial and temporal scales. Land cover conversion has a large short term (1-5 years) on-site impact
on soil properties such as soil organic C and bulk density, whereas land use intensification has much longer term (10 to 80 years) effects on soil properties
(Hartemink, 2006). Measurements and/or monitoring of soil fertility levels require long-term (five to ten years) research commitment as well as detailed knowledge about spatial and temporal variability of soil fertility levels (Hartemink, 2006). Unlike satellite image data for such a long time frame that almost always appear readily available; soil fertility data for such a local scale level could hardly be available. Absence of this constitutes the major research obstacle for this study. Relevant soil fertility data of the study area for previous years on which current and future researches could be based appear extremely unconsolidated and thus unreliable.
1.8 Organization of the Thesis
This research was organized into six chapters with chapter one being the introduction serving as background to the study. The chapter introduced the concept of LULC change in relation to soil quality in continental savannah, statement of research problem, aim and objectives, research questions as well as justification for the study and choice of study area.
Chapter two comprised review of relevant literature on LULC, RS and GIS, change detection, Landsat satellite sensor, soil quality and soil fertility. It also reviewed international and local applications of satellite remote sensing data and GIS in LULC mapping and change detections.
Chapter three comprised of the geographical description of the study area right in the heart of semi-.arid environment. The description included its location, climate, size, relief, soil, drainage, vegetation, population and land uses.
Chapter four described both primary and secondary data sources used in the research, method of data analysis. The materials used included satellite imageries, Ilwis Academic GIS software, GPS and camera among others.
Chapter five presented the study results and discussions which centred on the pattern and rate of change based on spatial and temporal scales in the study. Change scenarios areas were observed from which soil samples were collected for change effects assessment.
Chapter six contained summary, conclusions and recommendations based on the findings of the study.
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