ABSTRACT
A study was carried out on soils sampled at 0-10, 10-20, and 20-30 cm depths from both cultivated and uncultivated soils at four different locations (Awgu, Okigwe, Nsukka I, and Nsukka II), to evaluate the potentials of various aggregate size fractions of varying soil textures and depths to sequester carbon under different land uses. A 4 x 2 x 3 factorial experiment was conducted in a completely randomized design (CRD). Factor A was location at four levels, while factor B (land use) had two levels. Factor C (soil depth) comprised of three levels. Results showed that in both land uses, soil texture varied with depth in each location and included clay, loam, clay loam, sandy loam and sandy clay loam. Generally, all the soil properties varied with soil depth across the locations and land uses. Land use significantly (P = 0.05) affected pH in
KCl, Ca2+, Al3+, CEC, 0.50-1.00 mm water stable aggregates (WSA), total soil nitrogen (TSN) in
1.00-2.00 mm WSA, and soil organic carbon (SOC) in 1.00-2.00 mm and < 0.25 mm WSA. Cultivation at 0-30 cm depth significantly reduced SOC in 1.00-2.00 mm WSA by 19.30 %, and TSN in 1.00-2.00 mm WSA by 2.50 %. Land use effects on SOC in WSA at 0-30 cm depth of the various locations followed no consistent trend, except that SOC was higher in cultivated than in uncultivated soils of Nsukka II location. The SOC pool significantly decreased with soil depth. The SOC pool at 0-10 cm, 10-20 cm, and 20-30 cm depths averaged 17.62, 16.40 and 13.05 Mg
C ha-1 respectively, in cultivated soils; and 19.59, 17.86 and 12.03 Mg C ha-1 respectively, in
uncultivated soils. The SOC pool to the depth of 30 cm differed distinctly amongst the study sites in both land uses; however, cultivation had no significant effect on SOC pool. The effect due to soil texture on SOC pool indicated that C sequestration was significantly greater in clay loam > clay > sandy loam > loam > sandy clay loam. In all, SOC pool was most secluded at 10-20 cm depth, and least at 20-30 cm depth. Whereas SOC pool significantly correlated with dispersion ratio (DR), aggregated silt and clay (ASC), water dispersible clay (WDC), microporosity (Pmi),
0.50-1.00 mm WSA, mean weight diameter (MWD), soil pH, K+, and C/N ratio in cultivated
soils; it correlated significantly with ASC, Na+, and CEC in uncultivated soils. Apart from Pmi, whose variability was largely due to the effect of SOC that significantly predicted up to 76 %, SOC significantly accounted between 34 % and 54 % of the variability in MWD, WDC, and WSA classes of > 2.00 mm, 1.00-2.00 mm and 1.00-0.50 mm of the cultivated soils.
CHAPTER ONE
INTRODUCTION
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.
This material content is developed to serve as a GUIDE for students to conduct academic research
SOIL ORGANIC CARBON SEQUESTRATION POTENTIALS IN AGGREGATE FRACTIONS OF CULTIVATED AND UNCULTIVATED SOILS OF SOUTHEASTERN NIGERIA
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