Characterization of Degraded Soils in the Humid Ethiopian Highlands

Abstract

Hardpan is a major cause of land degradation that affects agricultural productivity in developing countries. However, relatively, little is known about the interaction of land degradation and hardpans. The objective of this study was, therefore, to investigate soil degradation and the formation of hardpans in crop/livestock‐mixed rainfed agriculture systems and to assess how changes in soil properties are related to the conversion of land from forest to agriculture. Two watersheds (Anjeni and Debre Mewi) were selected in the humid Ethiopian highlands. For both watersheds, 0–45 cm soil penetration resistance (SPR, n = 180) and soil physical properties (particle size, soil organic matter, pH, base ions, cation exchange capacity, silica content, bulk density and moisture content) were determined at 15 cm depth increments for three land uses: cultivated, pasture and forest. SPR of agricultural fields was significantly greater than that of forest lands. Dense layers with a critical SPR threshold of ≥2000 kPa were observed in the cultivated and pasture lands starting at a depth of 15–30 cm but did not occur in the undisturbed forest land. Compared with the original forest soils, agricultural fields were lower in organic matter, cation exchange capacity, and exchangeable base cations; more acidic; had a higher bulk density and more fine particles (clay and silt); and contained less soluble silica. Overall, our findings suggest that soil physical and chemical properties in agricultural lands are deteriorated, causing disintegration of soil aggregates, resulting in greater sediment concentration in infiltration water that clogged up macro‐pores, thereby disconnecting deep flow paths found in original forest soils. Copyright © 2016 John Wiley & Sons, Ltd.

Modeling discharge and sediment concentrations after landscape interventions in a humid monsoon climate: The Anjeni watershed in the highlands of Ethiopia

“…Degraded hillslopes differ from permeable hillslopes mostly due to the decreased water storage capacity of the soil so that saturation and overland flow are more readily produced in patches of thin permeable soil either over rock (Kirkby & Chorley, ) or a slowly permeable layer formed due to land degradation (Tebebu et al, ; Tebebu et al, ) than in the deeper soils with high infiltration capacities. Some field observation (exposed bedrock, profiles showing hardpans, and compacted soils) or field measurements (e.g., soil‐depth mapping in Kejela, and Zeleke, , and soil penetrometer tests in Tebebu et al, ) can be used initially to roughly identify the difference between such areas. However, soils exist on a continuum from a zero infiltration capacity to a very high infiltration capacity (Kirkby & Chorley, ) and also have variable soil depths meaning that this distinction cannot always be practically measured in the field for larger catchments; similar to distributed physically‐based models implicitly lumping processes for effective parameters at larger scales (Beven, ).…”

“…That is, a high level of antecedent soil moisture promotes runoff generation (Legates et al, 2011), and after these storage elements are filled, water will be released from the watershed (Sayama et al, 2011). hillslopes mostly due to the decreased water storage capacity of the soil so that saturation and overland flow are more readily produced in patches of thin permeable soil either over rock (Kirkby & Chorley, 1967) or a slowly permeable layer formed due to land degradation Tebebu et al, 2016) than in the deeper soils with high infiltration capacities. Some field observation (exposed bedrock, profiles showing hardpans, and compacted soils) or field measurements (e.g., soil-depth mapping in Kejela, 1995and Zeleke, 1998 and soil penetrometer tests in Tebebu et al, 2016) can be used initially to roughly identify the difference between such areas.…”

Impact of Soil Depth and Topography on the Effectiveness of Conservation Practices on Discharge and Soil Loss in the Ethiopian Highlands

“…To discuss the differences in discharge and runoff coefficients between the two watersheds, we note that prior experimental studies have found that the main runoff generation mechanism in the (sub) humid Ethiopian highlands is saturation excess runoff [15,20,22,24,[45][46][47][48]. Saturated excess overland flow occurs at locations either where the soils become saturated above an impeding layer or where the regional groundwater reaches the surface.…”

“…As a result, sediment concentrations have increased in the runoff water, greatly increasing the natural rate of hardpan formation. The hardpan is a typical characteristic of degraded soils [20,21]. While the hardpan affects the watershed hydrology by restricting the downward water movement, the surface soil remains highly permeable and has infiltration rates greater than the prevailing rainfall intensity.…”

Gully Head Retreat in the Sub‐Humid Ethiopian Highlands: The Ene‐Chilala Catchment

“…When accelerated by inappropriate anthropogenic practices, it adversely affects in situ water availability and soil fertility, and thereby forest and agriculture productivity Sop & Oldeland, 2013;Dai et al, 2015;Prosdocimi et al, 2016), increases silting up of reservoirs (Mekonnen et al, 2015), causes water quality deterioration (Bogen et al, 1994;Poesen, 2011), and obstructs agricultural practices and social interactions between communities (Bewket & Sterk, 2002;Ayele et al, 2016). In addition, the sediment-laden water infiltrates into soil and accelerates hardpan formation by filling up the pores in the soil, thereby reducing deep percolation of water (Shiferaw & Holden, 1998;Taddese, 2001;Costa et al, 2015) and according to Tebebu et al (2016) is one of the major causes of land degradation.…”