ABSTRACT
A study was conducted in the runoff plots at the University of Nigeria Nsukka Teaching and Research Farm, in 2010 and 2011 to monitor the changes in aggregate stability,  and some selected physicochemical properties of Nkpologu sandy loam soil under different cover and soil management  systems.  The management  systems were bare  fallow (BF), grass fallow (GF), legume (CE), groundnut (GN), sorghum (SM), and cassava (CA) cultivation. Following the characterization  of the soil of the study site,  three samplings were carried out at five- month interval marking the end of first cropping season, and the start and end of the second cropping season respectively. There was no change in soil texture due to treatments. The soil was acidic throughout the period of the study with pH values ranging from 5.1 (under BF) to 5.5 (under GF) in 2010 and from 4.8 (BF) to 6.1 (SM) in 2011. The aggregate stability (AS), mean  weight  diameter  (MWD),  water  dispersible  silt  (WDSi),  bulk  density  (BD),  total porosity (TP), macroporosity (MACP), aggregate size distributions (> 2 mm, 1- 0.5 mm and <  0.25  mm)  and  Ksat  showed  significant  (P  =0.05)  changes  under  different  cover management  practices.  The  Ksat  varied  (CV  =  52%)  significantly  (P<  0.05)  under  the different cover management practices over the sampling period. Generally, the highest values for Ksat, AS and MWD were obtained in the first sampling period whereas the lowest values were obtained in the last sampling period. There were significant effects (P<0.05) of cover management  systems  on  AS,  MWD  and  Ksat.  The  highest  values  for  AS,  MWD  and aggregate size fraction > 2 mm (80.3, 2.22 and 55.6 % respectively) were obtained under GF whereas the highest Ksat(16.8cm/hr)  was obtained  under GN. The lowest values for these parameters throughout the sampling periods were obtained under BF. The preponderance of aggregates < 0.5 mm under BF showed that raindrop impact and other agents broke down macroaggregates into microaggregates. The interaction of cover management and sampling period was not significant (P<0.05) for the structural and hydraulic parameters determined. The cover treatments generally increased organic matter (O.M.) content compared with the BF. Soil pH increases with increasing O.M. content and vice versa.  The Fe and Al oxides were significantly  (P<0.05) affected by the different cover and soil management  systems. The concentration of Fe oxides was high relative to the concentration of Al oxides. The O.M. had significant (P<0.05) correlation with two aggregate size ranges; 1-0.5mm (r = – 0.276* at P<0.05) and 0.5-0.25mm (r = – 0.245*at P <0.05) and Fe oxides. The cover management systems affected the infiltration characteristics measured. The highest infiltration rates (1,317 mm  h-1)  and  cumulative  infiltration  (72,390  mm)  were  obtained under  the  GF  and  CE respectively  whereas  the  lowest  values;  287  mmh-1  (infiltration  rate)  and  14,455  mm
(cumulative infiltration) were obtained under the BF.
The study has shown that cover and soil management systems affected the organic  matter content,  soil  pH,  Fe  and  Al  oxides,  infiltration  characteristics,  aggregate  stability  and structural  properties  of the  Nkpologu  sandy  loam  soil  differently  over  time.  Continuous addition of organic manure is encouraged. Legume and other crop fallows which protect the soil and guarantee regular additions of organic materials are ecologically sound components of  sustainable  management  of  Nkpologu  sandy  loam  soil  for  improved  agricultural productions.
CHAPTER ONE
1.0 INTRODUCTION
Evaluating  the impact of agricultural  practices on agroecosystem  functions  is essential  to determining the sustainability of management systems which cover the Productivity (Liebig et al., 2001), and environmental  components  of land use systems (Smyth and  Dumanski, 1995).  Soil  structure  is  the  physical  characteristic  most  vulnerable  to  soil  management practices. Structure describes the state of aggregation of the solid material in soils and their arrangement into what are called either aggregates, structural units or  peds (Doerr, 2007). Soil structure exerts important  influences on the functioning  of  soil, its ability to support plant and animal life, and its control on environmental quality with special emphasis on soil carbon sequestration,  nutrient  and gas fluxes and water  quality.  A good soil structure  is important for sustaining long-term crop production in agricultural soils because it influences water status, workability,  resistance  to erosion,  nutrient  availability  and crop growth and development (Piccolo and Mbagwu, 1999). Soil structure is often expressed as the degree of stability  of  aggregates  being  a  major  factor  which  moderates  physical,  chemical,  and biological  processes  leading the soil  dynamics  (Bronick  and Lal, 2005).  Thus; aggregate stability is a measure of the structural stability of soils (Mbagwu, 2003).
Soil  aggregate  stability,  defined  as  the  ability  of  the  aggregates  to  remain  intact  when subjected to a given stress, is an important index of structure that affects the movement of and storage of water, aeration, erosion, biological activity and the growth of crops (Amezketa et al., 2003).Soil aggregate stability is an important indicator of soil physical quality (Castro Filho  et  al.,  2002)  and  maintenance  of  high  aggregate  stability in  soils  is desirable  for sustainable  land  use,  as  it  is  essential  for  the  preservation  of  agricultural  production, minimizing  soil erosion and degradation and  reducing environmental  pollution (Amezketa 1999). This process, dynamic and complex, is influenced in turn by the interaction of several biotic and abiotic factors (Kovistra and Tovey, 1994; Topp et al., 1997;  Bronick and Lal, 2005) including environmental components such as soil temperature and moisture (Chen, et al., 1998), soil management, plant effects through the activities of plant roots (root exudates), but largely by soil properties such as soil organic matter, clay mineralogy, and oxide contents (Oades, 1984). Soil aggregation is the result of aggregate formation or development and their stabilization (Allison, 1968). The aggregates are primarily formed by physical processes; but biological  and chemical  processes  are mainly  responsible  for  their  stabilization  (Allison, 1968; Lynch and Bragg, 1985). Flocculated clay particles, their complexes with humus and soil organic matter act as main cementing agents in soil aggregate development (Kodesova and Rohoskova, 2009). The stabilization of soil aggregates takes place due to the cementing action of organic and inorganic materials. Silicate clays, calcium carbonate and sesquioxides (Kooreva, et al., 1983) cement particles together but their binding effect is much smaller than that of humus. The mechanism  involved in soil aggregation  is  complex but in general is viewed as microaggregates are formed from organic molecules tied to clay and polycations, which  in  turn  are  linked  with  other  microaggregates  to  form  macroaggregates.  Briefly, aggregation  is the result  of  rearrangement,  flocculation  and cementation  of soil particles where soil organic carbon, polycations, clay minerals and, especially biota play a key role (Fernando et al., 2008).
The contributions of soil organic matter (SOM) to soil aggregate stability have been studied (Tisdall  and  Oades,  1982;  Six  et.al.,  2000;  Six  et.al.,  2004).  Generally,  the  level  of aggregation  and stability  of aggregates  increases  with  increasing  organic  matter  content, surface  area  of  clay  minerals  and  cation  exchange  capacity  (Bronick  and  Lal,  2005). Mbagwu and Bazzoffi (1989) found that the mineralogy of the clay fraction was an important factor in microaggregation, with low activity clays being more stable than high activity clays. Oades and Waters (1991) remarked that for soils high in 2:1 clay mineral, soil organic matter is a major binding agent because polyvalent  metal-organic  matter complexes form bridges between the negatively charged soil organic matter and 2:1 clay platelets. However, part of the stability in 1:1 clay mineral dominated soils is induced by the binding capacity of oxides and 1:1 clay minerals.
Most tropical soils are low in soil organic matter. They are predominated by 1:1 clay minerals and are rich in oxides and hydroxides of Fe and Al, which, according to  Rampazzo et al (1993)  are  closely  related  to  the  structural  status  of  these  soils  and  contribute  to understanding  of the structure and its functionality  within a soil  profile. Six et al., 2002, noted that the cementing effect of free Fe and Al oxides was  important in soils with low organic matter content. Igwe et al. (2004) noted that the stabilizing role of various forms of Fe, Al and Mn oxide was as a result of their large surface area, abundance, and favourable environment for their formation.
Several  management  systems  can improve  soil  productivity  such  as conservation  tillage, addition of organic and inorganic manure e.t.c, and modification of some soil attributes such
as  soil  structure  by  root  growth  can  be  used  to  evaluate  their  impact  on  soil  physical properties. Plant roots are known to influence soil aggregation. As a result of  variation in biomass production and root systems, crops have different ability to promote aggregation and stabilization of soil aggregates due to carbon supply (Wohlenberg, et al., 2004), mechanical effect (Tisdall and Oades, 1979; Silva and Mieliriczuk, 1997, 1998; Campos et al., 1999), exudates  production  or  mycorrhizal  association(Tisdall  and  Oades,  1979;Tisdall,  1991; Degens, 1997), and can contribute to seasonal variation of aggregate stability over a growing season (Campos  et.al.,1999).  Soil aggregate  distribution  has been used as a conservation index for clayey Oxisols, cultivated with wheat (Triticum aestivum), maize (Zea mays) and soybean (Glycine max) in Panama state (Castro Filho, et al., 2002). Although a number of studies  indicate  positive  effects  of  plant  roots  on  aggregate  formation,  a  reduction  in aggregate size and stability of soil by root growth has also been recorded (Reid and Goss, 1981). It is widely recognized that cropping with soybean reduces soil aggregate size  and stability (Fahad et al., 1982; Bathke and Blake, 1984; Alberts and Wendt, 1985; Nakamoto and Suzuki, 2001) and consequently increases a risk of soil erosion.
The susceptibility of soil to erosion, or soil erodibility, is linked to soil aggregate stability, which characterizes resistance to soil breakdown. Aggregates stabilize the soil and maintain productivity while preventing erosion and deterioration. Many studies have shown the effects of organic constituents on the size and stability of soil aggregates. However, few studies have considered the contributions of sesquioxides and different crop management practices on the stability of soil aggregates. The knowledge of how the stability of aggregates of our fragile soil is influenced by not only SOM, but also sesquioxides and cropping systems is especially important  in  the  tropics  where  high  rainfall  erosivity,  variable  biomass  production  and intensive tillage can accelerate soil’s susceptibility to water erosion on agricultural land.
The objectives of the study are;
1.  to determine the role of soil organic matter, aluminium and iron oxides in the stability of soil aggregate of the Nkpologu sandy loam soil, and
2.  to determine the relative contributions of different crops towards the stability of soil aggregates in the Nkpologu sandy loam soil.
This material content is developed to serve as a GUIDE for students to conduct academic research
AGGREGATE STABILITY OF NKPOLOGU SANDY LOAM SOIL UNDER DIFFERENT SOIL AND CROP MANAGEMENT SYSTEMS
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