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PROJECT TOPIC – Aggregate Stability of Soils of Coastal Plain sand in Owerri, Southeastern Nigeria in Relation to their Contents of Carbohydrates and Human Substances

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Aggregate Stability of Soils of Coastal Plain sand in Owerri, Southeastern Nigeria in Relation to their Contents of Carbohydrates and Human Substances


Profile soil samples and 25 additional top(0-20cm)soil samples from five land use types from Owerri, Southeastern .Nigeria, were used to study the role of carbohydrates (R-CHO) and humic substances (HS) including fulvic (FA) and humic (HA) acids in aggregate stability of soils of coastal plain sands. The land use types were four year fallow (FY), grass vegetation (GV), continuously cultivated (CC), forest vegetation (FV) and one year cassava farm (CF) lands.

Aggregate stability was measured at the macro level by mean weight diameter (MWD) of water stable aggregates (WSA) and at the micro level by dispersion. ratio (DR), clay dispersion index (CDI), aggregated silt plus clay (ASC) and clay flocculation index (CFI). The topsoils
were sandy loam to loamy sand.

Clay content increased with depth while sand decreased. Saturated hydraulic conductivity (Ksat) showed an inverse relationship with bulk density (BD) with the former. decreasing as the later increased with depth. Values for the Ksat were fairly rdpid (0.96 to 1.95 cm min-l) except for the GV where BD values were higher (1.5 to 1.589 cm3). Topsoil BD were moderate, varying from 1.20 to 1.50 g cm-3 while the
subsoil values where higher, starting from 1.41 in one horizon to 1.58 g cm-3 in another. Except for the exchangeable Al, which was fairly high, the soils were low in exchangeable cations.

The exchangeable cations and CEC, with the exception of Na did not influence aggregate stability to any appreciable extent. Comparatively, HS, FA and HA influenced aggregate stability both at the macro and micro levels, whereas, R-CHO did not. As far as organic matter (OM) fractions were concerned, FA exerted a stabilizing influence on soil aggregates under CC while for soil under FV, FA together with HA
influenced aggregate stability.

Humic substance (HS) showed its influence on aggregation of soil under CF and FY whereas no OM fraction was involved in aggregate stability of soil under GV. For the soils studied, humic substances (FA and HA) rather than R-CHO were implicated in aggregate stability, as
revealed by the Kaiser Varimax Rotated Components of all the properties of the land use types.




Organic matter (OM) exerts some influence on soil structural stability. It is a significant component of soil, fundamentally contributing to the overall soil quality. Spaccini et a/. (2002) reported that changes in land use, for example, conversion of natural forest to crop land,  contributed to land degradations such as losses of soil organic matter (SOM) and aggregate stability. However, the authors could not relate contents of carbohydrates to aggregate stability in the forested and cultivated soils, suggesting that the aggregate stability was mediated by such other aggregating agents as humic substances (HS) and iron and aluminum oxides.

Continuous cultivation degrades soil structural properties due to the diminished OM content. Nevertheless, in such arable lands, inputs of organic residues ameliorate soil physical properties as much as OM accumulation does in forest, grass or planted fallow lands. Assessment of changes in soil properties which are associated with different land use systems is vital to conclusions on appropriate soil conservation and management practices to be adopted.
Information on components of SOM influencing flocculation and dispersion is required to understand which component in particular affects stability of the aggregates. Despite the use of several indices for the characterization of soil structural stability, it is still doubtful whether or not the components of OM have the same and equal influence on such indices. Therefore, a knowledge of these components and their effects on aggregation will be necessary in deciding which is more useful as soil amendment.
It has been reported that fractions of SOM rather than the total alnount per se, are more important in modifying the structural stability of soils (Hamblin and Greenland, 1977; Dormaar, 1983; Dutarte et a/., 1993; Piccolo, 1996). Humic substances (HS), namely, humic acid (HA), fulvic acid (FA) and humin (H) as well as non-humic substances, for example, carbohydrates, peptides, resins and waxes are major SOM components involved in soil aggregate stability (Tisdall and Oades, 1982; Piccolo and Mbagwu, 1999). At the macroaggregate level, some organic biopolymers, for example, polysaccharides and proteins cover the soil particle surfaces, transiently forming physical linkages
between them and hence temporarily acting as binding agents.
However, Adesodu et a/. (2001), had shown that carbohydrate pools were not very effective in stabilizing soil aggregates whereas Mbagwu and Piccolo (1998) and Piccolo and Mbagwu (1990) had reported that HS, at the colloidal level of soil aggregation, binds the particles together through formation of hydrogen bridges, covalent linkages and complexes between clay particles and polyvalent metals. Most of the studies with HS and R-CHO were carried out in the temperate regions (Kerndorff and Schnitzer, 1979; Senesi, 1992; Hanschmann et a/., 1997; Markarov, et a/., 1997) mainly focusing on their chemistry and structure ( ~ e r d e n and Berggren, 1991; Ricca et a/., 1993; Piccolo, 1997). In the tropical region, the studies were, however, focused on exogen0u.s applications of HS and R-CHO as amendments to soils to improve their physicochemical properties and productivity (Mbagwu e: a/., 1993; Piccolo and Mbagwu, 1994; Ekeh et a/.’ 1997).

There is therefore, the need for further research on tropical soils involving the role of endogenous HS and R-CHO fractions of SOM in structural stability. Hence. this study, which is generally an investigation into the relative contributions of R-CHO a’nd HS to structural
stability of soils under different vegetation and management systems. It is also an attempt to harmonize the controversial results from studies on the role and actual fractions of OM implicated in soil aggregate stability.
It is expected that adding high molecular weight organic materials, such as HS and R-CHO, to structurally fragile soils would be a useful management practice for improving their aggregate stability. The specific objectives of the study include:
1) Identification of variations in physico-chemical properties of soils under different land use types,) and
2) Evaluation of the relative and/or complementary roles of HS and R-CHO in aggregate stability of these soils in Owerri, Southeastern Nigeria.


Aggregate Stability of Soils of Coastal Plain sand in Owerri, Southeastern Nigeria in Relation to their Contents of Carbohydrates and Human Substances


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