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.
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;
- 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
to determine the relative contributions of different crops towards the stability of soil aggregates in the Nkpologu sandy loam soil.