ABSTRACT
A study was conducted to evaluate the physical changes that occur in the soil due to conversion of forest to arable land. Six locations in Enugu State that were selected for this purpose include Ugbo-Okpara (Li), Ugbo-nabo (L2), Ugwogo-Nike (L3), Iyi- Ukwu (L4), Edem (L5) and Ugwo (L6). Soil parameters determined include bulk density, saturated hydraulic conductivity, macro porosity, micro porosity, total porosity, organic carbon, pH, Dithionate citrate bi-carbonate (DCB) Iron oxide, particle size distribution, and micro aggregate stability indices. With respect to the water stable aggregate, the indices used for evaluation include particle size distribution, aggregate size distribution of WSA, mean weight diameter, aggregate stability water retention by WSA at both -33kpa and -1500kpa, available water capacity, organic carbon, aggregated silt plus clay (ASC), water dispersible clay (WDC) and clay dispersion index (CDI). Soils of both land use types were coarse to moderately fine texture, with an average pH of 3.95 and 4.1 in the cultivated and forest land use respectively. In the soil, cultivation significantly (P ≤ 0.05) increased bulk density by 17 %, reduced hydraulic conductivity, macro porosity, micro porosity and total porosity by 38 %, 23
%, 19 % and 21% respectively. Soil organic carbon and Dithionate citrate bicarbonate iron oxide was reduced by 34 % and 6.7 % respectively. Land use had significant effect on aggregated silt plus clay but no significant effect on water dispersible clay, clay flocculation index and clay dispersion index. However significant effect due to the interaction of land use and location was observed in these micro-aggregate stability indices which imply that variation due to land use was dependent on location. With respect to the water stable aggregates, cultivation reduced the macro aggregates and increased the proportion of the smaller (< 0.25mm) sized aggregates. The mean weight diameter and aggregate stability declined by 21 % and 50 % respectively. Water retention at -33kpa and -1500kpa were not affected by land use while a slight but significant (P ≤ 0.05) influence on available water capacity was observed. Particle size distribution of the WSA was changed with cultivation of forest soils. aggregated silt plus clay and water dispersible clay and SOC declined by 20 %, 15 % and 25 % respectively while the clay dispersion Index increased by 2%. The interaction effects of location, aggregate size and land use demonstrated that cultivation improved the physical properties of the WSA in L1, L2 and L3 while L4, L5 and L6 were adversely affected by forest conversion to crop land. Moreover the magnitude of effect in soil properties due to cultivation varied with location as was observed that the highest decline in soil physical property was indicated in L6 compared to other locations which was attributed to its textural characteristics. Therefore the change or magnitude of change in soil properties was location specific.
CHAPTER ONE
1.0 INTRODUCTION
Aggregate stability expresses the resistance of aggregates to breakdown when subjected to potentially disruptive processes (Hillel, 1998). The structural stability of soil aggregates upon wetting has been the subject of a lot of research around the world (Boucher, 2006). Eynard (2004) indicated that measurement of stability of soil aggregates in water is generally used to estimate structural changes due to cultivation, as water is the main agent of aggregate breakdown in cultivated soils of the humid tropics.
The quality of any soil depends on its degree of aggregation. Soils that are well aggregated, which remain stable when wetted, are important for erosion resistance, water retention and availability and root growth. Mbagwu (2003) reported that factors that influence aggregate stability are important in evaluating the ease with which soils erode, the potentials of soils to crust or seal, soil permeability and quasi-steady state infiltration rates, seedling emergence and in predicting the capacity of soils to sustain long term crop production. Some researchers observed that soils with low Mean Weight Diameter (MWD) have the potential to erode faster than those with high MWD because detachability is lower with the later (Igwe and Ejiofor 2005).
The stability of soil aggregate is affected by soil properties that change relatively little, that is, the inherent composition (texture, type of clay, calcium, sodium, aluminum and iron content) and properties that change in response to vegetation and management (Carter, 2002). According to Seta and Karanthanasis (1996), dispersibility of clay and silt sized particle fractions when immersed in water has been found to affect a number of soil physical properties, such as shrink-swell, water retention characteristics, hydraulic conductivity and hard-setting characteristics of the soils. Also Igwe and Nwokocha (2005) reported that soils with high water dispersible clay (WDC) have high potential to disperse. They added that when such soil disperses on saturation with water, it leads to soil aggregate breakdown, forming seals and crusting and reduces permeability of water. Such soils with heavy rainfall of high intensity will disperse, erode and constitute some problems down stream while both the fertility of the soils will be drastically reduced. Furthermore, Tisdall and Oades (1982) indicated that calcium carbonate as well as iron and aluminum oxides can impart considerable stability to otherwise weak soils. This could explain the reason for stability of aggregates in tropical soils even when they contain little organic matter.
However, there have been numerous correlations between the content of organic carbon in soils and water stable aggregates. Mbagwu and Piccolo (1990), observed that soils amended with organic wastes showed a reduction in dispersibility of aggregates. Also, Brady and Weil (2008) were of the view that even though flocculation of clay was necessary for aggregation, yet it was not a sufficient condition. Tisdall and Oades (1982) reported that stability of larger aggregates involves cementing or binding agents which may be inorganic, organo-mineral or organic. They added that roots and hyphae stabilize macro-aggregates while water stability of micro aggregates depends on the persistent organic binding agents which appear to be linked to the inherent characteristics of the soil, independent of management.
Near surface or use-dependent soil properties are relatively dynamic and can change over a few years or time. Martel and Mac Kenzie (1980) compared the effect of different land use practices on soil quality and showed that conversion of forest to agricultural soils was accompanied by loss of soil organic matter and structural stability and under some conditions an increase in soil compaction. This loss might be as a result of soil exposure, tillage and lack of residue input (Pirhaero et al. 2004 and Six et al.
2004). Many researchers have found that cultivation is exploitative and causes decline in soil organic matter content, which plays a major role in binding aggregates to withstand stresses caused by rapid wetting which prevails in the humid tropics (Guerif et al. 2001).
The major objective of this research is to use some physical characteristics of water stable aggregates from some forest and cultivated soils of Enugu State, Southeastern Nigeria to assess the level of physical changes that occur in the soil due to forest conversion to arable land.
The specific objectives are
a) To determine some physical and chemical properties of the soil.
b) To quantify the soil organic carbon, water retention and available water capacity associated with WSA in forest and cultivated land use.
c) To determine particle size distribution across the different size aggregates .
d) To determine some micro aggregate stability indices across the aggregate sizes.
This material content is developed to serve as a GUIDE for students to conduct academic research
COMPARATIVE ANALYSIS OF THE PHYSICAL CHARACTERISTICS OF WATER STABLE AGGREGATES IN SOME FOREST AND CULTIVATED SOILS OF ENUGU STATE SOUTH EASTERN NIGERIA.
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