Fertile island effect in the sedimentary process of Tetraena mongolica Maxim nebkhas 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. The formation of nebkhas is accompanied by the appearance of the fertile island effect. Quantitative evaluation of the shrub sand compost island effect is a key link in preventing soil erosion and nutrient loss. This study took the typical shrub Tetraena mongolica in desert areas as the research object and quantified the sand-trapping capacity of the shrub. We revealed the influence of sediment texture and volumetric soil water content on fertile islands during the development of T. mongolica nebkhas. The results showed that (1) the single shrub intercepted large amounts of sediments due to the high density of branches of T. mongolica , forming nebkhas that were positively correlated with the shrub size. (2) The overall soil nutrient content below the shrub was greater than the soil nutrient content outside the shrub, forming a typical fertile island effect. The soil organic carbon (SOC), alkaline hydrolytic nitrogen (AHN), available phosphorus (AP), and available K (AK) content all increased gradually with increasing nebkha volume. Compared with the deep soil of nebkhas, the nutrient content of the surface soil was generally higher. (3) There was a positive correlation between the volumetric water content and nutrient content in nebkhas. (4) The semi-ellipsoid shape of T. mongolica enabled it to intercept large amounts of coarse-grained material. Fine sand (100–250 µm) was the main particle size in the sediment aggregates inside the nebkhas. The fine sand content generally increased with increasing shrub size. Redundancy analysis (RDA) revealed that the fine sand content of the nebkha sediments had a strong positive correlation with the soil nutrient content. This paper provides an example for evaluating the fertile island effect during the deposition process of nebkhas in the desert transition zone.