The effects of loess erosion on soil nutrients, plant diversity and plant quality in Negev desert wadis

Severe erosion, initiated by climatic changes during the Late Pleistocene-Early Holocene period and resultant declines in dust deposition, causes the formation of waterfalls during the winter floods in many wadi systems in the central Negev desert of Israel, In some areas, erosion of the original loess substrate has been complete, so that the underlying rock has been exposed. We examined the effects of this erosion in four wadis in the central Negev desert on soil nutrients, plant community structure and plant quality. We predicted that erosion has caused reductions in soil nutrients. Reductions in soil nutrients should result in reductions in plant cover. Furthermore, reduced soil nutrient availability should cause reductions in the nutrient status and quality of the plants growing there. In addition to the loss of biodiversity that may result, this erosion may result in economic hardship for the Bedouin peoples whose herds depend on these resources. In this study, there were significant negative effects of erosion on soil organic carbon, nitrate nitrogen and water-holding capacity, but not on soil phosphorus, conductivity or pH. Furthermore, there was a negative effect of soil erosion on an overall measure of soil quality derived from a principal components analysis in three of the four wadis we studied. Erosion resulted in an increase in plant species richness and significantly altered plant community structure in eroded areas of wadis. Increased plant species richness in eroded sites is consistent with the intermediate disturbance hypothesis of plant community structure. Plants growing in eroded areas did not differ in two quality indices (nitrogen content and digestibility), although plants typical of eroded areas had significantly lower levels of common digestion inhibitors (total polyphenols) and toxins (alkaloids) than plants from undisturbed sites. These last-mentioned results are contrary to our prediction and are consistent with the notion that plants growing in disturbed (e.g. eroded) sites maximize growth at the expense of investments in defense. (C) 2001 Academic Press.