Geodiversity effects on soil quality and geo-ecosystem functioning in drylands

Abstract

Geodiversity is defined as the natural variability of geologic, geomorphic, and soil features. It has been acknowledged to positively affect biodiversity and species richness. A long-term drought occurring in the semi-arid northern Negev of Israel has led to the mass mortality of shrubs, with the particularly adverse impact on the predominant Noaea mucronata species. Recent observations in these shrublands have suggested that this mass mortality is not uniformly distributed over the landscape, but is confined to hillslopes characterized by low-geodiversity. This type of hillslope is defined by a deep (>1 m) soil layer, lacking rock fragments in its profile and on its surface. Also, the herbaceous vegetation cover of these hillslopes is very high (>90%), but shrubby vegetation cover is very low (<10%) and for the most part, non-vital. At the same time, shrubby vegetation in high-geodiversity hillslopes – defined by a shallow (~10 cm) soil layer, and high rock fragment content (>30% volume) and cover (>20%) – is rather dense (>25%) and vital, but their herbaceous vegetation cover is quite sparse (<30%). Soil was sampled from the 0–10 cm depth in two micro-habitats – shrubby patches and inter-shrub spaces – on both the homogeneous and heterogeneous hillslopes. Overall, soil quality was found to be considerably higher in the heterogeneous hillslopes compared to the homogeneous hillslopes. This included the contents of hygroscopic moisture, total organic carbon, particulate organic carbon, and carbon lability, whose means were 22%, 36%, 27%, and 38%, respectively, greater in the heterogeneous hillslopes. Further, mean clay dispersion index was 18% lower in the heterogeneous hillslopes. At the same time, mean soil electrical conductivity was 13% greater in the heterogeneous hillslopes. Yet, this could be attributed to the greater clay content in the former type (19.1%) than that in the latter type (12.7%) of hillslope, with the assumingly higher salt-adsorption from the soil solution. Overall, the results show that the effect of micro-habitat on the soil properties was relatively moderate, and therefore, suggest that the impact of geodiversity might override that of vegetation patchiness. The results highlight the importance of assessing patch-scale and/or hillslope-scale geodiversity in future studies of dryland ecosystems around the world.