ABSTRACT
The study was conducted to determine carbon and nitrogen sequestration and dynamics in soils developed on different parent materials (Coastal Plain Sand, Falsebedded Sandstones and Shale) under different land use types (forest, fallow and cultivated lands) in South-eastern Nigeria. Geology maps were used to guide the location of sampling sites. Three parent materials and three different land use types in each of the parent materials were randomly selected. The study was a three factor experiment laid in a randomized complete block design (RCBD). A total of 27 profile pits were studied. Soil samples were collected from each of the profiles according to their horizons. Undisturbed soil samples for determination of bulk density were collected in core samplers. Small portions of the samples were air dried, crushed and sieved using a 2-mm sieve in preparation for laboratory analyses. Carbon and Nitrogen forms and sequestration, morphological and physico-chemical properties of soils were determined. Data were subjected to ANOVA , multiple regression, coefficient of variation and correlation analyses. From the results, carbon sequestration ranged from 3229 gCm2 in Falsebedded sandstone-derived soils to 3648 gCm2 in Shale-derived soils, and did not differ significantly across the soils. Nitrogen sequestration differed significantly (p < 0.001) with soils derived from Coastal plain sands having higher quantity (248.00 gNm-2) while the least was recorded in soils formed from Shale (91 gNm2). The C sequestration capacity of the soils of the different land use types varied significantly (p < 0.05) with fallow soils derived from Falsebedded Sandstone and Coastal plain sand containing the highest quantities (4753 gCm-2 , 4222 gCm-2). Carbon and nitrogen sequestration increased with horizon thickness in all the profiles across the soils studied. The mean total carbon contents ranged from 39.20 to 82.80 gkg-1 across the soils and did not follow uniform pattern of distribution down the profiles, except in fallow soils of the Falsebedded Sandstone where it increased with depth. Soils derived from Shale had the least quantity of total carbon while those of Falsebedded Sandstone had the highest value. Forest soils had higher quantity of total carbon (109.20, 42.40 gkg-1) compared to those of fallow and cultivated soils of the Falsebedded sandstone and Shale . Organic carbon constituted about 58% of total carbon in Shale-derived soils, 20.81% in Coastal plain sand and 27.66 % in Falsebedded sandstone – derived soils. In soils of the different land use types, forest soils contained significantly higher proportion of organic carbon, followed by fallow and lastly by those of the cultivated lands. Organic carbon correlated significantly with clay (r = 0.513, 0.578) (p < 0.001), WSA (r = 0.506, 0.626, 0.646) (p < 0.001) and BD (r = – 0.537, – 0.900, -0.736 ) (p < 0.001) respectively. The mean total nitrogen contents of the soils varied from 5.49 – 8.24 mg/kg in soils of dissimilar parent materials, 3.60 – 14.33 mgkg-1, 7.33 – 8.87 mgkg-1, 2.01 – 10.49 mg/kg in soils of the different land use types. Soils formed from Coastal plain sand and Falsebedded sandstone contained significantly higher (p < 0.01) quantity of total nitrogen than those of Shale. In soils of the different land use types, forest soils contained significantly higher (p < 0.001) proportion of total nitrogen (14.33, 10.49 mgkg-1) than fallow and cultivated soils. Soils formed from Falsebedded sandstone and Shale had significantly higher (p < 0.001) proportions of available N compared to those of Coastal plain sand. In soils of the different land use types, forest soils had significantly higher proportion of available N compared to fallow and cultivated soils. Soils developed on different parent materials under different land use types had varying colour matrix ranges. Soil texture ranged from sand, loamy sand to sandy loam in soils derived from Coastal plain sand and Falsebedded Sandstone, loam, sandy clay loam, silt clay loam to clay in Shalederived soils with soils formed from Shale containing significantly (p < 0.001) higher proportion of clay (263 gkg-1) than those of Falsebedded sandstone (77 gkg-1) and Coastal plain sand (90 gkg-1). The mean bulk density (BD) values ranged from 1.06 to 1.22 gcm-3 in soils derived from the three parent materials. Forest soils had the least bulk density values (0.98, 1.09, 1.08 gcm-3) compared to other land uses. In soils of different parent materials, Shale derived soils had highest percentage moisture content (12.49%) while those derived from Falsebedded sandstone had the least amount (9.09%). In soils under the three land use types, forest and fallow soils had significantly higher (p < 0.001) quantity of soil moisture than the cultivated soils. Shale-derived soils had significantly higher (p < 0.001) stable aggregates (29.23%, 1.35 mm) than those derived from Coastal plain sand and Falsebedded sandstone. In soils of the varying land uses, soils of the forest had significantly higher (p < 0.001) stable aggregates compared to those of fallow and cultivated lands. Soils were slightly acidic across the parent materials and land use types. Significantly (p < 0.01) least proportion of Calcium (Ca) (2.76 cmolkg-1) was recorded in soils of the Falsebedded sandstones while those developed on Shale had the highest quantity (4.28 cmolkg-1). Significantly higher percentage base saturation was obtained in soils derived from Shale (91.4%) while the least value was obtained in Falsebedded sandstone-derived soils (68.71%). Taxonomic classification was done to the Subgroup level. The soil classes derived from soil taxonomic classification of the USDA was correlated with the World Reference Base. Soils were classified as Grossarenic Kandiudults (USDA), Chromic Acrisols (WRB), Typic Kandiudults (USDA), Rhodic Acrisols (WRB), Lithic Kanhapludults (USDA), Rhodic Acrisols (WRB), Arenic Kandiudults (USDA), Chromic Acrisols (WRB), Vertic Paleudults (USDA), Haplic Acrisols (WRB), Entic Paleudults (USDA), Haplic Acrisols (WRB), Psammentic  Hapludults (USDA), Arenic Acrisols (WRB).
Keywords: Carbon, Nitrogen in Soils, Lithologies , Nitrogen
TABLE OF CONTENTS
Title page                               i
Certification                             ii
Dedication                              iii
Acknowledgements                        iv
Table of contents                          v
List of Tables                             viii
List of Figures                             x
Abstract                                  xii
CHAPTER ONE    1.1 Introduction         1
CHAPTERÂ TWO
Literature Review 5
2.1 Carbon sequestration 5
2.1.1 Quantification of soil carbon sequestration 7
2.1.2 Potentials of different agricultural practices to sequester carbon in soils 9
2.1.2.1 Conservation tillage practices and carbon sequestration 12
2.1.2.2 Use of agricultural chemicals and nutrient recycling 16
2.1.2.3 Afforestation and/or reforestation 17
2.1.2.4 Fallow system and age of fallow 19
2.1.2.5.Recycling and application of Organic waste 20
2.2 Carbon dynamics in soil and atmosphere 21
2.3 Soil carbon and climate change 24
2.4 Carbon forms and sequestration in Soil 26
2.4.1 Soil organic carbon (SOC) 28
2.4.2 Effects of soil organic carbon on soil properties 30
2.4.2.1 Increased soil moisture and infiltration rate 31
2.4.2.2 Reduced bulk density and increased aggregate stability 31
2.4.2.3 Nutrient release and improvement in Soil biological properties 32
2.4.3 Factors controlling soil organic matter levels in soil 34
2.4.4 Components of organic matter; significance and function 36
2.5 Nitrogen behavior, roles and dynamics in soil and plant 40
2.5.1 Chemical reaction of ammonia and nitrite with organic matter 44
2.5.2 Sources of Nitrogen in soil 44
2.5.3 Nitrogen losses in soil 46
2.6 Carbon-nitrogen (C/N) ratio in soils 47
2.7 Parent materials and their influences on soil properties 49
2.7.1 Lithological composition of the three-parent materials studied 51
2.8 Land use 54
2.8.1 Common Land use practices in Nigeria 55
2.8.1.1 Continuous Cultivation 59
2.8.1.2 Fallow land 61
2.8.2 Effects of land use on soil quality and land degradation 63
2.8.2.1 Land and soil degradation 63
2.8.2.2 Land use impacts on Soil physical, biochemical and morphological properties 64
2.9 Soil classification 66
2.9.1 Classification of Nigerian Soils 69
CHAPTER THREE
Materials and Methods 73
3.1 Study area 73
3.1.1 The Physical Environment of various Study Areas 75
3.2 Field study 84
3.3 Experimental design 86
3.4 Laboratory analysis 86
3.5 Forms of carbon and carbon sequestration 87
3.6 Forms of nitrogen and nitrogen sequestration 88
3.7 Soil classification 88
3.8 Data analysis 88
CHAPTER FOUR
Results and Discussion 90
4.1 Morphological Properties of Soils 90
4.2 Physical Properties of Soils 104
4.3 Chemical Properties of Soils 116
4.4 Carbon forms and dynamics in Soils 123
4.5 Nitrogen forms and dynamics in Soils 135
4.6 Carbon and Nitrogen Sequestration in Soils 141
4.7 Multiple Linear Regression Models of Nitrogen and Carbon Forms and Sequestration against Selected Soil Properties 152 4.8 Classification of Soils derived from different parent materials 157
CHAPTER FIVE
5.1 Summary and Conclusions 163
5.2 Recommendations 165
REFERENCESÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 166
 APPENDIX
Appendix 1: Profile distribution of Physio-chemical properties of soils derived from different parent materials                200
Appendix 2: Profile distribution of Carbon and Nitrogen forms and sequestration in soils derived from different parent materials      218
Appendix 3: Soil profile description        227
Appendix 4: Ratings of selected soil properties 255
Appendix 5: Temperature and rainfall data for Imo and Abia states  257
Appendix 6: Photo plates of soil profile pits   261
CHAPTER ONE
1.1Â INTRODUCTION
Soils are important reservoirs of active organic components (such as carbon, nitrogen) and play a major role in the global cycle of these elements. As such, soil can be either a source or sink for atmospheric carbon dioxide (CO2) depending on land use and management of soil and vegetation (Lal, 2005). Over 60% of the world‘s carbon is held in both soils (more than 40%) and the atmosphere (as carbon dioxide; 20%) (Stevenson, 1994).The conversion of native ecosystems such as forests, grasslands and wetlands to agricultural uses, and the continuous harvesting of plant materials, have led to significant losses of plant biomass and carbon (Davidson and Ackerman, 1993), thereby increasing the carbon dioxide (CO2) level in the atmosphere. However, soil disturbance is redistributing the carbon, augmenting the atmospheric carbon pool. Thus, a part of carbon increase in the atmosphere is thought to have come from agriculture, affecting not just climate change but also productivity and sustainability of agriculture and natural resources (Robbins, 2004). The value of soil for agricultural and other uses is majorly determined by the concentration of organic components of the soil and parent materials from which the soils are formed.
Carbon sequestration involves the process of transferring atmospheric CO2 into the soil through crop residues and other organic solids, and storing it securely so it is not immediately reemitted into the atmosphere (Lal, 2004). Thus, soil carbon sequestration means increasing soil organic carbon (SOC) and soil inorganic carbon (SIC) stocks through judicious land use and recommended management practices (Akamigbo, 2010a). This transfer or ―sequestering‖ of carbon helps off-set emission from fossil fuel combustion and other carbon-emitting activities while enhancing soil quality and long term agronomic productivity. Soil carbon sequestration can be accomplished by management systems that add high amounts of biomass to the soil, cause minimal soil disturbance, conserve soil and water, improve soil structure, and enhance soil fauna activity. A major challenge facing scientists and policy makers is how to increase the amount of carbon sequestered in soil in order to mitigate climate change. Global climate change is a long-term energy and environmental challenge requiring major investments in targeted research and development. Gaining a greater knowledge of how carbon cycles through ecosystems is a critical element of the national strategy to understand climate and potential changes that might occur due to anthropogenic greenhouse gases and to develop solutions to reduce future increases in CO2 (the most important Green House Gas (GHG)) and other GHGs (Forge,2001). Understanding how climate affects both natural and managed ―pools‖ (e.g., forest, agriculture lands) of carbon stored in global ecosystems and how these carbon ―sinks‖ influence atmospheric concentrations of CO2 will be important in reducing uncertainty in climate models and in understanding the long-term sequestration capacity of those pools.
The rapidity of soil carbon decline in tropical soils is worrisome as it is a principal factor in soil quality of the biome which results to soil structural deterioration. Poor carbon sequestration has been attributed to shortened fallow cycles, poor management practices, changes in microbial chemistry, bush burning, deforestation, conventional tillage, mining, climate change and poverty (Onweremadu etal., 2008a). Of all the causes of poor carbon sequestration in the soil, deforestation takes a great toll in sub-Saharan Africa, and indeed the tropical world. In Southeastern Nigeria, there is increased deforestation and resultant erosion damages of soil resources (Oti, 2007). However, in the tropics, erosive activities have led to a decline in organic matter (Mbagwu and Obi, 2003), resulting to adverse changes in physical properties of the soils, low nutrient status and build-up of toxicities such as excessive concentration of heavy metals in soils. In the light of the above, several soil fertility enhancing practices such as prolonged fallow, conservation tillage and improved agro-forestry systems have been suggested with little success due to increasing population and poverty which consequently result to pronounced degradation of soil resources (Mbagwu and Obi, 2003).
Human induced soil degradation is an important cause of the decline in productivity of many soils. Experience in Europe and most developing countries like Nigeria has shown that the vulnerability of soils to specific type of degradation differs widely according to land use. Some soils are vulnerable to erosion by water, ice or wind, others to physical compaction or chemical degradation (Batjes and Bridges, 1993).  In large parts of the tropics, chemical soil degradation is caused by the depletion of plant nutrients (Hartemink and Bridges, 1995), poor carbon sequestration and low soil organic matter content of the soils resulting from unsustainable agricultural activities. Unless nutrients removed by agricultural crops are replaced, either naturally through weathering and bio-geocycling, carbon sequestration or by the use of fertilizers, the soil nutrient reserve will gradually be depleted. Human-induced loss of nutrients was estimated to be affecting 45 million hectares in Africa and 135 million hectares worldwide (Oldeman etal., 1991).
The contributions of soil carbon on physical, chemical and biological properties of soils and thus in sustaining their productivity have been appreciated since the dawn of human civilization. Important factors that control soil carbon levels include climate, hydrology, soil fertility, biological activity, vegetation patterns and land use (Bahattacharyya etal., 2000). Soil organic carbon is sensitive to human activities such as deforestation, biomass burning, land use changes and environmental pollution. Rapid degradation of arable soils in Nigeria due to anthropogenic factors and the attendant decline in farm productivity have brought about renewed focus on the need to cultivate the hitherto little-exploited arable soils (Izak etal., 1990; Eshett, 1993). In most cases, soils are mined, leading to low organic matter content (Onweremadu, 2006). There is abundant literature in the humid tropics on changes in soil physico-chemical properties following deforestation, subsequent land cultivation and soil parent materials (Akamigbo, 1999a; Sisti etal., 2004; Walker and Desanker, 2004; Onweremadu etal., 2007a). However very few are concerned with the dynamics of organic components and their sequestration in soils of dissimilar lithology under forested, continuously cultivated and fallow lands. Information on nutrient dynamics is not only necessary to make good land use decisions in agriculture, but also it is a pre-requisite to understanding gaseous exchanges between soil and atmospheric systems, especially in this era of global warming and climate change. Recent concerns about greenhouse gases (carbon dioxide (CO2) and nitrous oxide (N2O)) and damage to the ozone layer have resulted in more studies on the inputs, outputs and storage of carbon and nitrogen in different terrestrial systems. Carbon and nitrogen are in dynamic equilibria in soil and an increase or decrease in one may result in an increase or decrease in the other. To sustain the quality and productivity of soils, a good knowledge of soil organic components in terms of their concentrations, dynamics and qualities is indispensable. Assessments of the distribution of carbon within and among soil types are critical to developing an understanding of the cause and effect relationships between climate, land use change and release of CO2 to the atmosphere (Schimel etal., 1985). However, proper knowledge of carbon sequestration is critical when developing carbon budgets and explaining the cause and effects of climate change, and for basic ecosystem characterization (Alleta etal., 2004).
In addition, classification of soils of any given location helps in generating soil and soilrelated data which are useful in predicting soils for their sustainable uses (Onweremadu etal., 2007b). Appropriate and proper use of soils depends upon the characteristics of such a soil. There is therefore a need to characterize and classify them in a manner that will ease communication and transfer of knowledge about such soils to farmers and other land users (Nuga etal., 2008). On the basis of organic matter content, soils are classified as mineral (inorganic) or organic soils (Food and Agricultural Organization (FAO), 2005). Mineral soils form most of the worlds cultivated land and may contain from trace to 30% organic matter (Soil Survey Staff, 2003). Organic soils are naturally rich in organic matter containing more than 30% organic matter principally for climatic reasons and are not vital cropping soils.
There is need to understand the impact of land use management on soil carbon stocks regionally and globally, because this stock is not only twice the total amount of CO2-C in the atmosphere, but it is also sensitive to land use changes (Wu, 2011). More so, restoration of soil quality through soil organic carbon management has remained a major concern for tropical soils. To make this successful, the comprehensive knowledge of the sequestration and dynamics of carbon and nitrogen in the tropical soils should form an essential pre-requisite in future land resource management programmes. However, knowledge of the amounts, forms and distribution of these elements is essential in understanding nutrient dynamics in soils of a densely populated central south-eastern Nigeria and its relationship to global nutrient cycles. In view of this, the major objective of this work was to study the sequestration, forms and dynamics of organic components (carbon and nitrogen) of soils of dissimilar parent materials under three different land uses.
The specific objectives were to:
- characterize and classify soils of the study area using USDA Soil Taxonomy and World Reference Base (WRB).
- determine carbon and nitrogen forms in the soils derived from different parent materials under different land use types.
- determine the quantities and variations of carbon and nitrogen sequestered by the soils derived from different parent materials under different land use types
- estimate the degree of association between soil organic fractions and selected properties of the soils under study.
SEQUESTRATION AND DYNAMICS OF CARBON AND NITROGEN INÂ SOILS OF DISSIMILAR LITHOLOGIES UNDER DIFFERENT LANDÂ USE TYPES IN SOUTHEASTERN NIGERIA
