Vegetation Change and Soil Nutrient Distribution along an Oasis-Desert Transitional Zone in Northwestern China

Many studies have focused on soil nutrient heterogeneity and islands of fertility in arid ecosystems.However,few have beenconducted on an oasis-desert transitional zone where there is a vegetation pattern changing from shrubs to annual herbs.The goal of the present study was to understand vegetation and soil nutrient heterogenity along an oasis-desert transitionalzone in northwestern China.Three replicated sampling belts were selected at 200 m intervals along the transitional zone.Twenty-one quadrats(10×10m)at 50m intervals were located along each sampling belt.The vegetation cover wasestimated through the quadrats,where both the soil under the canopy and the open soil were sampled simultaneously.The dominated shrub was Haloxylon ammodendron in the areas close to the oasis and Nitraria tangutorum dominated theareas close to the desert.In general,along the transitional zone the vegetation cover decreased within 660 m,increasedabove 660 m and decreased again above 1020m(close to the desert).The soil nutrients(organic matter,total N,NO3~- andNH4~+)showed significant differences along the zone.The soil nutrients except the soil NH4~+ under the canopy were higherthan those in open soil,confirming"islands of fertility"or nutrient enrichment.Only a slight downward trend of the level of"islands of fertility"for soil organic matter appeared in the area within 900m.Soil organic matter both under canopy andin interspace showed a positive correlation with the total vegetation cover,however,there was no significant correlationbetween the other soil nutrients and the total vegetation cover.We also analyzed the relationship between the shrubs andannuals and the soil nutrients along the zone.Similarly,there was no significant correlation between them,except soilorganic matter with the annuals.The results implied that annual plants played an important role in soil nutrient enrichmentin arid ecosystem.