COMPARATIVE ANALYSIS OF THE PHYSICAL CHARACTERISTICS OF WATER STABLE AGGREGATES IN SOME FOREST AND CULTIVATED SOILS OF ENUGU STATE SOUTH EASTERN NIGERIA.

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.

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