Effect of Caragana tibetica nebkhas on sand entrapment and fertile islands in steppe-desert ecotones on the Inner Mongolia Plateau, China

Phytogenic mounds (nebkhas) formed by shrubs are a common phenomenon in arid and semi-arid areas and play important roles in preventing soil erosion and nutrient loss. One feature of nebkhas is the development of fertile islands. We investigated whether fertile islands were present inside and underneath Caragana tibetica-formed nebkhas in the northwest portion of the Ordos on the Inner Mongolia Plateau, China, and if such a fertile island effect increased with the age of the nebkhas. We also analyzed the spatial heterogeneity of the soil properties and quantified the amount of sand trapped by C. tibetica. The morphometric characteristics of C. tibetica nebkhas were investigated in a 4.5 ha area. Soil samples were collected inside, underneath and outside of the nebkhas and the soil organic matter (SOM), total phosphorus (TP), soil moisture (SM) and soil texture were determined. The SOM, TP and SM inside and underneath nebkhas at later stages of growth were higher than those at smaller, younger growth stages, which in turn were higher than those outside the nebkhhas. Inside the nebkhas at the establishment and early growth stages, SOM and TP first increased and then decreased with increasing soil depth, but SM steadily increased. The amount of sand trapped by C. tibetica per unit area was 0.0313 m(3)m(-2). Nebkhas of C. tibetica primarily accumulated fine sand, which accounted for 74% of the soil, and is significantly higher than that outside the nebkhas. Caragana tibetica significantly increased sand entrapment, and fertile islands are formed inside and underneath C. tibetica nebkhas.