Impacts of rangeland degradation on soil physical, chemical and seed bank properties along a rradient in three rangeland vegetation types in Somali region, eastern Ethiopia

Abstract

The Somali region of Ethiopia is a typical pastoral area, that occupy >50% of the 0.7 million km² rangeland in the country. However, all forms of rangeland degradations have become major threats to enhance a sustainable pastoral-livestock production in Ethiopia. But except for very few studies on rangeland condition assessments, no or too little research has been conducted to understand the different impacts of soil degradation on the range bio-physical ecology. As a result, there exists a gap in knowledge to plan present and future rangeland improvement interventions in the country. The purpose of this study is therefore, to understand the impacts of different rangeland degradation conditions on the soil physical, chemical and seed bank properties along a gradient. Three rangeland vegetation types were selected in Erere wereda, Shinille zone in north eastern Somali region of Ethiopia, classified as arid (60%) and arid and semi-arid (40%). The experimental sites included: the Asbuli grassland (9–11.80N and 40–41.80E), the Aydora open savanna (8–10.80N and 40–4180E) and the Hurso closed savanna (4–6.80N and 40–42.80E), with an altitude ranging between 300 and 1200 m.a.s.l,. Each site was laid out in to four sites of degradation condition classes, namely: excellent (benchmark), good, moderate, and poor along the gradients. Accordingly, some important soil physical, chemical and soil seed bank parameters were analyzed using scientific procedures. Based on the results, soil texture showed a shift from clay type to silt clay, while soil erosion and compactions intensities were increased as rangeland conditions declined from excellent in to poor classes along the gradient. Further, increases in conditions of degradation simultaneously increased bulk density while vegetation covers showed a significant (p < 0.05) reduction. This was explained in terms of increased bare ground with inverse proportion of basal cover. On the other hand, soil moisture, organic matter and organic carbon made a significant (p < 0.05) reduction as degradation condition increased from excellent into poor classes. Patterns in soil Nitrogen, available Phosphorus and Potassium also reduced significantly (p < 0.05) as rangeland degradation increased, may be due to higher leaching induced by higher runoff. Nevertheless, Cation exchange capacity, electrical conductivity, pH and percent base saturation (i.e. Ca, mg and Na) highly decreased as degradation was increased. To this end, deteriorations in rangeland condition has an overall negative impact on the soil physical and chemical characteristics, demanding for more efforts to improve the health of range vegetations. Likewise, number and density of regenerated plants declined in the soil seed banks from excellent to poor condition classes. Nevertheless, the soil seed banks still give an opportunity for rangeland improvement using appropriate rehabilitation, conservation and utilization techniques.

Keywords: Rangeland ecology, condition classes, soil texture, xchangeable ions, pH, soil nutrients.