The term soil seed bank (SSB) refers to the sum of all viable seeds present on and in the soil (Roberts, 1981). As a potential population, it provides provenance for plant species renewal, community succession, and dispersal processes (Bakker et al., 1996; Zhang et al., 2021b). SSBs are fundamental to the process of plant population settlement, survival, reproduction, and diffusion. They play an important role in reshaping and maintaining biodiversity and can indicate the vegetation on the ground (Bakker et al., 1996; Peterson and Baldwin, 2004; Schwienbacher et al., 2010; He et al., 2018). For this reason, analyzing the regularity and succession of vegetation growth and development in SSBs is a critical component of vegetation reconstruction and restoration studies (Yang et al., 2010).

Nebkhas are aeolian landforms produced by sand deposited near to and obstructed by shrubs, and their development is mainly affected by plants, aeolian sand deposition, and hydrogeological conditions (Li and Ravi, 2018). In China, nebkhas are widely distributed in desert–oasis ecotones, desert steppe marginal zones, interlaced agricultural–pastoral zones, and other regions (Wang et al., 2006). Natural factors and human activities interfere with their formation and development to a large extent, and their appearance has a great impact on the ecological environment (Tengberg and Chen, 1998; Langford, 2000; Li and Ravi, 2018). A large number of Nitraria tangutorum thickets grow in the transition zone between the desert oases in the northeastern part of the Ulan Buh Desert, and nebkhas of different sizes are formed by the annual accumulation of sand. These form a natural “sand prevention wall” around the desert oasis, effectively preventing quicksand from invading the ecological environment of the oasis and playing a positive role in the stability of its environment (Dang et al., 2019). As important sand-fixing vegetation in the transition zone of desert oases, N. tangutorum shrub communities, however, have gradually declined, and some parts have even been activated and disintegrated (Xu et al., 2008a). This has greatly weakened the ecological function of the community, increased the vulnerability of the oasis, and jeopardized associated industrial and agricultural production of the oasis (Jia et al., 2002).

There has been considerable research into the development and evolution of nebkhas: morphological changes, soil physicochemical properties, vegetation biomass predictions, reasons for vegetation degradation, restoration techniques, conservation measures, and wind erosion (Zhang et al., 2021a; Abdurimithi and Yusuf, 2016; Du et al., 2010; Li et al., 2010). However, a majority of these studies have ignored the ecological function of nebkhas. Additionally, studies of species composition and size, species diversity characteristics, spatial distribution characteristics, and plant succession of the underground SSB of potential plant provenance on the horizontal scale of N. tangutorum nebkhas are also limited. Here, we examined a suite of N. tangutorum nebkhas with different sizes in the field and conducted seed germination experiments in the laboratory to analyze the characteristics of SSB and their relationship with aboveground vegetation. The goal of this study is to reveal the relationship between SSBs and the establishment, evolution, and degradation of nebkhas in a desert–oasis ecotone, which is critical for vegetation restoration and land degradation prevention in desert areas.

We found that annual herbaceous plants, primarily Bassia dasyphylla, Salsola collina, Agriophyllum squarrosum, and Aristida adscensionis, accounted for the largest proportion of species on nebkhas in a desert–oasis ecotone in northwestern China. This finding was consistent with the studies of SSBs in the Alxa Desert region (Zeng et al., 2003), the central desert of the Hexi Corridor (Lu et al., 2019), and the Gurbantunggut Desert (Zhang and Li, 2018), which also reported that the species life forms were mainly annual herbs. This is not surprising as annuals have strong drought tolerance and larger niche breadth (Jia et al., 2017). Additionally, the rainfall in our study area was low and mostly concentrated in July–September, allowing most annual herbs to complete their entire life cycle during this period and to produce a large number of seeds.

In this study, the SSB density is relatively high and is consistent with studies conducted in a South American warm desert (Luis et al., 1998) and a desert–oasis marginal zone in middle reaches of Heihe River, China (Li and Fang, 2008). The density of SSB plants is different from the SSB density observed in other arid and extremely arid deserts (Yu et al., 2015; Zhang and Li, 2018; Hadinezhad, et al., 2021). This difference may be caused by the fact that nebkhas, with various shrub species, increase buried seeds in soil by directly trapping seeds or by indirect mechanisms through an intermediary animal or plant species (Giladi et al., 2013). More importantly, abiotic factors such as wind regime, landform, and soil condition also affected SSB density (Chambers and MacMahon, 1994). There were also significant differences in the spatial distribution patterns of SSB in different nebkha sites because of the strong habitat heterogeneity in the study area. Previous studies have shown that at the bottom of nebkhas, the light intensity is relatively weak, the soil water storage capacity is relatively strong, and thus soil moisture and nutrient conditions are better, which favor seed germination, survival, and establishment of plants (Xu et al., 2008b; Liu et al., 2011; Wang, 2021). However, compared with the middle and bottom of nebkhas, the light intensity is higher at the top, and the plants cannot obtain the soil moisture and nutrients needed for their growth, so there are relatively few species. Likewise, in the southern margin of the Gurbantunggut Desert, the density of each dune slope was found to decrease in sequence at the bottom, lower middle, upper middle, and top (Jia et al., 2017). Plant seeds generally fall on the soil surface or enter the surface layer after they mature, and only a small proportion enter deeper soil layers as a result of wind erosion and sand burial (Yang et al., 2016; Liu et al., 2021).

The study area is located in a windy desert area, and a majority of the windy conditions occur in spring (Cui et al., 2010; Gao et al., 2014). When the wind passes through the nebkhas, areas of low wind speed form in the lower-middle parts of the nebkhas because of the influence of the shrubs on the wind–sand flow. When the wind speed decreases, the ability of the wind–sand flow to carry seeds decreases, so that more seeds will accumulate in the lower-middle parts (Qian et al., 2016; Ma et al., 2021). Consequently, at the bottom of nebkhas, the SSB density in the surface layer is relatively high. Due to the fluctuation of nebkha height, soil moisture, light radiation, soil nutrients, and sand buried by wind erosion would change slightly in small local areas (Jia et al., 2021); at the bottom of nebkhas, wind erosion was not too severe. More plant litter accumulates, which reduces the loss of soil water and nutrients (Shao et al., 2001; Haas, 2004), and the increase in soil fertility improves vegetation growth, resulting in an increase in the number of species in aboveground vegetation (Zhang et al., 2006; Hadinezhad et al., 2021).

Previous studies have shown that in desert regions, more than one-half of all species were found in SSB as well as in the aboveground vegetation (Ma, et al., 2015; Teng et al., 2020; Lv et al., 2021). In this study, 14 species appeared both in the aboveground vegetation and in the SSB, three species appeared only in the SSB, and no species appeared only in the aboveground vegetation. These observations were similar to those reported in a previous study (Teng et al., 2020). The main reason for this was that the study area was located in the transition zone of desert and oasis and there were few plant species, which increased the probability of the same species appearing both in SSB and in aboveground vegetation in the different parts of the nebkhas. Bassia dasyphylla, Salsola collina, and N. tangutorum appeared in the aboveground vegetation and SSB. Thus, B. dasyphylla and S. collina could be the dominant species on nebkhas because their reproductive ability was very strong. In addition to seed germination, the main propagation method of N. tangutorum, as a constructive species in this area, is to bury its own branches by intercepting sand by itself and carrying out clonal propagation with branches or root tillers under suitable water and heat conditions.

The succession process of N. tangutorum nebkhas is strongly related to the process of plant growth, development, and change. With sufficient sand supply under the same wind conditions, vegetation with different ecological characteristics creates different wind speed and wind flow fields, thus developing different forms of nebkha (Yin et al., 2017). Plant morphology, quantity, structure, and litter affect the development of nebkhas through their impact on the environment and ability to block sandstorms (Miao et al., 2020; Han et al., 2021). Some studies have found that the changes in nebkha morphology are determined by the coverage, distribution, abundance, and height of plants (Hesp and McLachlan, 2000; Hesp, 2002). A study of nebkha morphology in Ulan Buh Desert by Gao et al. (2015) found that crown size and plant height affected the development of nebkhas. The nebkha slope may also affect the distribution of nebkha SSB. Seeds on a steep slope are more likely to slide to the bottom of the slope under the action of wind and rainfall (Pa et al., 2016; Li et al., 2018). Our results showed that the morphological characteristics of N. tangutorum nebkhas can affect the number of aboveground vegetation and SSB species and the vegetation height. During the development of N. tangutorum nebkhas, the growth and development of vegetation can inversely affect nebkha morphology. This reflects the decisive role that vegetation plays in the initial development of nebkhas. When they develop to a certain extent they react to the vegetation, thus intensifying the difference in the vegetation species composition in different parts of the nebkhas and further affecting the SSB.

We recorded 17 species of plants in six families in the SSB of N. tangutorum nebkhas in a desert–oasis ecotone, dominated by Chenopodiaceae. The life forms were mainly annual herbs, accounting for 58.82%–71.43%. The number of species in the SSB followed the sequence large nebkha > medium nebkha > small nebkha, and there was no significant difference in SSB density. However, the SSB densities in different parts of each nebkha were in the sequence bottom > middle > top. The SSB densities were significantly different and showed a decreasing trend with the increase in soil depth. In the range of 0–10 cm soil layer, most of the plant seeds were distributed in the 0–5 cm layer, which decreased with the increase of soil depth. The similarity coefficient between the SSB of each nebkha and the aboveground vegetation was high. From the bottom to the top, the number of common species and similarity coefficient between the aboveground vegetation and the SSB decreased, and the number of species in the SSB was more than that of the aboveground vegetation. Vegetation plays a decisive role in the initial development of a nebkha. When a nebkha develops to a certain extent, it reacts to the vegetation, thereby intensifying the differences in the composition of vegetation species in its different parts and further affecting the SSB.