In fact, bulk density is significantly varied with land-use types due to difference in land management and land-use history Moges and Holden The highest mean 1. Pondo et al. Compaction resulting from intensively grazing animal might have caused a relatively high bulk density in grazing land than that of cultivated land.
In contrast, the research conducted in Ethiopia by Husien et al. The mean soil bulk density value in the steep slope was the highest 1. This indicates that the removal of fine particles from higher slope leaves higher concentration of coarser particles behind that causes higher bulk densities in such sites. The correlation indicates that the presence of relatively higher clay fraction and SOC lowers soil bulk density.
In line with this, Achalu et al. The average values of total porosity of grazing land, cultivated land, shrubland and forestland were 48, The highest total porosity This result indicated that the conversion of forestland to cropland accompanied by intensive cultivation without appropriate soil management caused a declining pattern of total porosity of the soil. Considering the interaction effect of land use by slope gradient, the highest interaction mean value of total porosity As a result, the total porosity of the soil in the Gumara watershed increases with the decreasing of slope gradient.
This is related to the accumulation of contents of organic matter and clay that maximize the level of the total porosity of soil in the gentler slopes of the watershed. Higher total porosity can make the soil workable and will have favorable plant rooting conditions. The reduction in total porosity may have a negative impact on the infiltration capacity and moisture content of the soil. As a consequence, sandy soils of the steeper slope gradient of the study area will have lower contents of moisture availability for plant growth.
Moreover, soils with high total porosity e. From these points of view, soil and water management technology selection and implementation should consider the texture of the soil and the slope gradients of the study area. The texture of the soil in the study watershed showed variation in terms of land use and slope gradient. The clay content increased as one moves from the steeper to gentler slope areas of the watershed.
The result could be due to deposition from up land areas, resulting into enrichment of the low-lying area. The silt content varied from This shows a removal of silt content from the moderate and steeper slopes and its counter deposition in the gentler slope gradients. This indicates that the geomorphic process in the study catchment is strong enough to detach and remove both finer and medium-sized particles to lower slope gradients.
As a consequence, relatively high concentration of contents of sand This confirms that erosion is a selective process that can remove clay and silt particles and leave the coarse fragments behind. Li and Lindstrom also observed a linear decrease in clay content and a corresponding increase of sand content in the steeper slope due to the selective removal of fine particles by water. Table 4 shows that soil pH values varied from 5. The pH value observed in the study area is within the ranges of moderately acidic to slightly acidic.
Transformation of land from forest to cultivated land caused a reduction of soil pH value in the study watershed. As a result, the highest 6. The lowest value of pH under the cultivated land reflects a presence of intensive cultivation and the removal of the basic cations during crop harvest and consequent drainage to the nearby streams due to runoff caused by accelerated erosion.
Considering the slope gradient, the highest and lowest mean value of pH 6. The study in Canada by Hendershot et al. The higher soil pH values in the gentler slope gradient could be attributed to the accumulation of bases that were presumed to have been removed from the top to the bottom slope position Garcia et al. The other reason for the reduction of soil pH in steeper slopes might be related to the difference in the management practices such as crop rotation and fertilization rate.
Contents of soil organic carbon SOC ranged between 1. This result is similar to Bovine et al. Amount of soil organic carbon was relatively high 3. The decline in SOC contents in the cultivated land following deforestation and conversion to cultivated lands might have been aggravated by the insufficient application of organic carbon from the farming system due to residue removal and absence of crop rotation.
This idea was substantiated by different scholars Duff and Rasmusen-Smiley ; Grace et al. In addition, leaching problem that can be attributed to the relatively high sand content in the steeper slope and the resultant light texture of soil in the gentler slope areas might have contributed to the reduction of SOC in the steeper slope compared to the gentler slope of the watershed.
With regard to the interaction between SOC and slope gradient, the highest 3. The highest concentration of the SOC in the gentler slope gradients is caused by differences in geomorphic processes of erosion in steeper slopes and deposition in gentler slope gradients in such sites.
Thus, the geomorphic processes conditioned by slope gradient factors must be considered in the practices of land management strategies. It accounted a maximum of 0. A similar conclusion was made for the soils of Bako area of central Ethiopia Wakene The considerable reduction of total nitrogen in cultivated land could be related to the rapid mineralization of the organic substrates derived from root biomass following intensive cultivation McDonagh et al.
The highest nitrogen content under forestland resulted from nutrient recycling as the amount extracted returned to the soil as leaf litter. The average nitrogen content decreased considerably from 0. The gentler lands of the study watershed are not susceptible to soil erosion thereby leading accumulation of nitrogen and other soluble elements as discussed earlier.
On the other hand, the steeper slope lands are highly susceptible to runoff and erosion that caused removal of nitrogen from such sites. At the same time, the gentler slope land was more commonly used as pastureland which is also important to maintain the concentration of nitrogen. As a general fact, the concentration and distribution of nitrogen in the study watershed showed a similar trend with organic carbon concentration among land-use type and slope gradient.
The low contents of total nitrogen Landon and Landon in the study watershed resulted from the low level of soil organic carbon content and mineralization of the existing soil organic matter following cultivation and communal free grazing. The limited use of nitrogen-containing inputs such as commercial fertilizer and plant and animal residues in such continuous cropping land and communal grazing land has contributed to the loss of total nitrogen throughout the study watershed.
In the study watershed, crop residues are collected for animal feed, fuel, thatching and temporary construction purposes. Similarly, animal dung and waste are collected for fuel. These could be probably the major factors contributing to low contents of soil organic carbon and consequently the decline of total nitrogen in the soil system. The result for available P showed that it varied from 2. It was the highest in forestland and lowest in grazing types of land use.
Accordingly, the highest 5. Although level of available P can be generally rated as low Landon and Landon in the study watershed, there is a decline of Contents of available phosphors were higher 4. The high concentration of phosphorus in the gentler slopes is related to its removal from steeper and deposition in gentler slope gradients. As is discussed earlier, geomorphic processes of erosion—deposition are responsible in the spatial variations of available P in the study watershed.
The average value of exchangeable value of cultivated land, forestland, grazing land and shrubland were 0. The highest exchangeable 1. This result indicated that the conversion of forestland to cultivated land cause a decline in the amount of exchangeable magnesium. The mean value of exchangeable magnesium varied between 1. The lower concentration of magnesium was observed in the steeper and moderate slopes than in the gentler slope.
The increment of concentration of exchangeable magnesium from the steeper and moderate slope to the gentler slope indicated that there is a downward leaching of basic cations from the steep slope area and accumulated in the gentler slopes. Exchangeable potassium K varied between 0. Soil under cultivated land and grazing land had lower exchangeable K than other LC types Table 4.
Lindstrom, M. W, Schumacher, T. Quantifying tillage erosion rates due to moldboard plowing. Soil and Tillage Research 24, DIS 4 ME. Desertification Indicator System for Mediterranean Europe. Definition Name. Brief definition. Sloping land in which vegetation has been removed subjected to high erosion rates photo C. Contribution to a the definition and mapping of ESAs and b assessment of the desertification risk of an area. Importance with respect to desertification. Soil erosion is considered as the main process of land degradation and desertification in hilly Mediterranean areas.
Generally speaking, soil sediment loss can be estimated by the product of the amount of surface water run-off times the slope gradient times a constant related to soil surface characteristics. As the slope becomes steeper, the runoff coefficient increases, the kinetic energy and carrying capacity of surface water flow becomes greater, soil stability and slope stability decreases, soil sediment loss increase.
Therefore, slope gradient is undoubtedly considered as one of the most important determinants of soil erosion and desertification. The effects of overgrazing on soil microbial community and fertility in the Chaco dry savannas of Argentina. Soil Ecol. Aprill, W. Evaluation of the use of prairie grasses for stimulating polycyclic aromatic hydrocarbon treatment in soil.
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Phospholipid fatty acid - A bioindicator of environment monitoring and assessment in soil ecosystem. Kourtev, P. Exotic plant species alter the microbial community structure and function in the soil. Leifeld, J. Control of soil pH on turnover of belowground organic matter in subalpine grassland. Melero, S. Chemical and biochemical properties in a silty loam soil under conventional and organic management. Plants that are drought-friendly should be planted in areas that experience more heat.
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