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 The fundamental basis of carbon (C) sequestration and its effect on global climate change and agriculture have become a major concern in recent years. Emissions of greenhouse gases (water vapour, carbon dioxide (CO2), methane, and nitrous oxide) as a result of human activities continue to alter the atmosphere in ways that are expected to affect change in climate. Anthropogenic activities produce CO2, which is the primary greenhouse gas that contributes to climate change to be released to the atmosphere at rates much faster than the earth’s natural processes can cycle. To help alleviate or possibly reverse the trend, a variety of means of enhancing natural sequestration processes are being explored. Increasing CO2 sink (C sequestration) has been acknowledged and accepted as a major possible mitigation to these effects. This is buttressed by the report of Rice and McVay (2002) indicating that through C sequestration, atmospheric CO2 levels are reduced as soil organic carbon (SOC) levels are increased”. Among the three natural sinks for C (ocean, forest and soil), soils contain more C than is contained in vegetation and the atmosphere combined (Swift, 2001). The SOC pool which forms the largest sink after sedimentary rocks and fossil deposits however is the most vulnerable to disturbance (Schlamadinger and Marland, 2000) especially because of the competition between the various types of land use. Six et al. (2000) reported that tillage operations promote the loss of SOC through macroaggregate disruption and exposure of soil organic matter (SOM) to microbial decomposition. Also, Blum (1997) indicated that the decomposition and alteration (mineralization and metabolization) of organic compounds produces trace gases which can be harmful to the global atmospheric cycle. The impact of organic carbon (OC) losses in soils may have a variety of serious environmental consequences. Lal (2004) reported that several depletion of SOC degrades soil quality, reduces biomass productivity, and adversely impacts water quality. Lal et al. (1998) observed that organic matter (OM) losses from soil worldwide contribute to increased atmospheric CO2 concentration. Lugo and Brown (1993) indicated that the net losses of SOC due to land use changes may occur as a result of decreased organic residue inputs and changes in litter composition, and increased rates of soil organic decomposition and soil erosion. The contribution of soil erosion to global C emission has also been recognized by Tans et al. (1990) as equally important to that of deforestation and fossil fuel burning. Lal (1995) estimated that the total SOC displaced by water erosion globally as 57 Pg yr-1 [Pg = Petagram. Where, 1 Pg = 1 Gt (Gigaton) = 1015 g = 1 billion tons]. Houghton et al. (1996) predicted that CO2 emission to the atmosphere would increase from 7.4 Gt C yr-1 in 1997 to approximately 26 Gt C yr-1 by 2010. Furthermore, the annual CO2 flux from the soil to the atmosphere (68 Pg yr-1) is 11.3 times the emissions from fossil fuel combustions (6 Pg yr-1) (Raich and Schlesinger, 1992). However, the Inter-Government Panel on Climate Change (IPCC) recognised three main options for the mitigation of atmospheric CO2 concentrations by the agricultural sector: (i) reduction of agriculture-related emissions, (ii) creation and strengthening of C sinks in the soil, and (iii) production of bio-fuels to replace fossil fuels (Batjes, 1998). Hence, the need to evaluate the role of soil as one of the natural C sinks that secludes organic C as stable humus for enhancing soil fertility and stability of soil microaggregates. Therefore, soil C pool and its dynamics play vital role and the knowledge of their spatial distribution is important for understanding the pedosphere in the global C cycle for the overall management of C. It is with this background that several attempts have been made to access the potential of cropland (Lal et al., 1999; Lal and Bruce, 1999), grazing systems (Follet et al., 2000), and forest ecosystem (Birdsey et al., 1993) to sequester C as possible strategies to curtail the rate of increase of atmospheric concentration of CO2. Carbon sequestration refers to the removal of C, from the atmosphere through photosynthesis and dissolution, and storage in soil as OM or secondary carbonates (Lal, 2001). Through this process, C storage in soil is enhanced and its loss minimized, thereby reducing the chances of global warming by the reduction of atmospheric concentration of CO2. Recognizing the soil as one of the important potential sinks for C requires understanding of the processes that influence C sequestration. Soil aggregation has been observed as an important process of C sequestration and hence a useful strategy for mitigating increase in concentration of atmospheric CO2 (Shrestha et al., 2007). Igwe et al. (2006) stressed the importance of the study of the role of SOC in restoration of soil fertility and stability of soil microaggregates. The impact of C sequestration on greenhouse gases and agricultural sustainability has not been well elucidated at regional, national or global scales. Some available statistics are generally based on extrapolation. Lal (2004) reported that the rates of SOC sequestration in agricultural and restored ecosystems range from 0 to 150 kg C ha-1 yr-1 in dry and warm regions, and 100-1000 kg C ha-1 yr-1 in humid and cool climates. He also estimated the total potential of C sequestration in world soils as 0.4-1.2 Gt C yr-1, all of which were derived from national resource inventory. Improvement in the data base on the concentration of SOC needed to be validated with ground truth measurement/assessment, as the use of reliable data is essential for developing techniques of soil management and identifying policy options needed for promoting appropriate measures. Despite several studies carried out on the quantification of soil sequestered C in different geographical regions of the world (Cruz-Rodriguez, 2004; Denef et al., 2004; Lal et al., 1998; Lal, 2001; Shrestha et al., 2007), there are limited knowledge about SOC pool dynamics in the tropical humid agroecosystem of southeastern Nigeria. Quantification of SOC within aggregate size classes permits evaluation of aggregation under different soil management systems and its contribution to the accumulation and loss of OM (Sotomayor-Ram´ırez et al., 2006). The relevance of this study is to generate reliable information which is essential for developing techniques of soil/land management systems and for recommendation of agricultural practices that promote C sequestration for sustainable agriculture leading to advancement in food security and consequently, mitigate global warming. The hypothesis is that SOC sequestration is a function of soil texture and soil aggregation; and that SOC is similar between soil phases (cultivated and uncultivated) of the same soil series. Therefore, the main objective of the study was to assess the potentials of various aggregate size fractions of varying soil textures and depths to sequester C in cultivated and uncultivated soils. The specific objectives included to; (i) Determine the soil physico-chemical properties of cultivated and uncultivated soils. (ii) Quantify SOC and total soil nitrogen (TSN) stocks and assess their distribution across aggregate size fractions as stratified by location, land-use, soil texture and soil depth. (iii) Determine the effect of SOC and TSN on soil aggregation and other soil properties. (iv) Understand the SOC pool dynamics among different soil textures and depths, and between cultivated and uncultivated soils.
Project detailsContents
Number of Pages138 pages
Chapter one Introduction
Chapter two Literature review
Chapter three  methodology
Chapter  four  Data analysis
Chapter  five Summary,discussion & recommendations
Chapter summary1 to 5 chapters
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