Soil seed bank characteristics in relation to land use systems and distance from water in a semi-arid rangeland of southern Ethiopia

The size and species composition of the soil seed bank of southern Ethiopia rangelands were examined under three land use systems (communal land, a government ranch and traditional grazing reserves) and along a distance gradient (near, middle and far) from water sources. Soil seed bank sampling was carried out at the end of the growing season from 0.25 m− 2 quadrates 30 mm deep. The aboveground species composition was also compared. Of the total plant species identified in the soil seed bank in the greenhouse, 25% were grasses and 75% non-grasses. The near, middle and far sites from water was dominated by Leucas glabrata, Verbesina encelioides and Lintona nutans, respectively. Lepthotrium senegalensis made up 67% of the total composition of the soil seed bank in the traditional grazing reserve. Graminoid seedling density at the traditional grazing reserve (798.8 seedlings m− 2) was higher (P < 0.01) than that of the ranch and communal sites, but did not vary significantly along the distance gradient from water (average 239.2 seedlings m− 2). Non-graminoid seedling density did not vary much (P > 0.05) between the land use systems (average 128.9 seedlings m− 2) and along the distance gradients from water (average 183.7 seedlings m− 2). Sorensen's similarity index of the aboveground vegetation and the associated seed bank was low at all the sites. Difference in grazing pressure between the land use systems was one of the most likely causes of variations in the soil seed bank density and botanical composition of graminoids. The non-significant differences in the graminoid seed bank density along the distance gradients from water were due to similar population of grasses and may not explain the levels of grazing impact and disturbance. Regeneration from the soil seed bank may have a profound effect in restoring the composition of the grass layer in the degraded Borana rangelands if sustainable management practices are applied and sufficient rest periods followed. By translocating seed rich topsoil from the grazing reserves to degraded grazing areas, the soil seed bank can be used to develop valuable grass species at small community level.