**4. Discussion**

One of the primary goals in restoring degraded areas is to improve species diversity and environmental stability. Consequently, much attention has been paid towards understanding the process of restoration succession and mechanisms of ecological recovery. Numerous studies have thus focused on developing effective means to rehabilitate and reconstruct ecosystems by conducting species diversity surveys in context of community recovery based on SSBs [26, 27]. In particular, the species diversity of SSB is an important component to understand restoration of degraded desert steppe environments [28]. SSB species richness and seed density are important metrics that underpin the recovery of degraded desert steppe [18]. Seed density within the topsoil (0–5 cm) was five times higher in passive restored plots than that in control treatment [29]. In this study, a total of 33 species were identified in the SSB from six plots spanning an age gradient of *C. microphylla* plantation. The increases in Gramineae grass abundances suggests improvement of grass quality that coincides with an increased species diversity in the SSB and consequently, a potentially positive effect on the recovery of ground vegetation due to the planting and maturation of *C. microphylla* plantation. Among these, annual and biennial herb species comprised 20 of the species and accounted for 58.8% of the total richness. This may be due to annual and biennial plants primarily propagated by seeds with a short life cycle, contrary to perennial plants with a longer life cycle [30]. Perennial plants commonly use asexual reproduction during drought conditions, and although they form seeds, there are far fewer than during sexual reproduction. Consequently, perennial herbs will generally occupy a smaller proportion of the SSB compared to annual and biennial herbaceous plants. A total of 35 species were identified in the aboveground vegetation, whereas 18 species were identified in the SSB and in the aboveground vegetation that included *Euphorbia humifusa*, *Setaria viridis*,*Tribulus terrestris*, *Sophora alopecuroides*, *Cynanchum komarouii*, *Corispermum mongolicum*, *Gueldenstaedtia stenophylla*, *Lespedeza potaninii*, *Euphorbia esula*, *Agriophyllum squarrosum*, *Stipa glareosa*, var. *graminifolia*, *Cuscuta chinensis*, *Bassia dasyphylla*, *Eragrostis poaeoides*, *Polygala tenuifolia*, *Artemisia scoparia*and *Salsola beticolor.* Thus, these results indicate that *C. microphylla* plantation had a significantly positive effect on the enrichment of seeds in the SSBs of the steppe. A total of 15 species only occurred in the SSB, while 17 species only occurred in the above ground vegetation, and 53% of the species were mutual to the ground vegetation and SSB. These results indicated that species composition differed between the ground vegetation and SSB communities in the Yanchi County.

Planted *C. microphylla* was correlated to a significant enrichment of seeds in the SSB. For example, the average density of seeds was 11,248.75 seeds<sup>m</sup><sup>2</sup> in the *C. microphylla* plantation, while the seed bank density was 661.25<sup>m</sup><sup>2</sup> in the semifixed sand dunes, representing a 17-fold decrease compared to the *C. microphylla* plantation soils. *Artemisia scoparia* seeds accounted for 82.27% - 93.81% of all seed density among plots in the different aged *C. microphylla* plantation SSB. Abundant rain lead to high levels of *Artemisia scoparia* germination in the early ground vegetation community, while late rainfall lead to the production of large amounts of seeds. These results further indicated that the SSB composition and density were related to the temporal and quantitative distribution of rainfall. The change in water spatial gradients was one of the primary factors that affected the composition,

*Evaluation of Soil Seed Banks in Different Aged* Caragana microphylla*… DOI: http://dx.doi.org/10.5772/intechopen.98712*

quantity, and spatial distribution of SSB, the correlation between seed banks and surface vegetation, and the seed yield of plant [31].

Seeds were significantly enriched in the 0–2 cm depth interval of the SSB of *C. microphylla* plantation, which is consistent with other studies [27, 32]. In contrast, there was a greater density of seeds below 2 cm in the semi-fixed sand dune SSB that could be due to increased porosity in the surface of the semi-fixed sand dune relative to the *C. microphylla* plantation, thereby allowing deeper movement of seeds into soils [33]. The number of seeds in the 0–2 cm SSB first increased, and then decreased with forest age, wherein seed densities followed the pattern: 25 years >16 years >7 years >37 years. This may be associated with the initial growth of *C. microphylla* plantation gradually flourishing that would help lower surface wind speed and promote the accumulation of soil nutrients, in which increased the richness of ground plant species and the seed amount in the SBB. After the growth of *C. microphylla* plantation for 20 years, the large absorption and consumption of soil water and nutrients caused the decline of the communities of other aboveground species, leading to the decrease of the number of seeds in the SBB. At present, there are few researches in this topic and the coupling analysis among SBB, aboveground vegetation and soil physical and chemical components should be strengthened in the future.

Many studies focused on analyzing SSB species diversity in order to explain the community succession and recovery mechanism, in which could be useful to guide the rehabilitation [34, 35]. Understanding the characteristics of species diversity in SSBs was an important pathway to restoration of the degraded desert steppe [36]. This study suggested that SSB diversity and evenness changed with age in *C. microphylla* plantation, which was inconsistent with the results of Bao [37]. The species density and diversity of SSB in typical steppe in Inner Mongolia of China were conducted under four season nomadic grazing (FSNG), two season rotation grazing (TSRG), settlement grazing (SG). The results of similarity between SSB and vegetation revealed that similarity coefficients under FSNG, TSRG,'FSRG and SG were 0.323, 0.351, 0.511 and 0.500 and that similarity coefficient between SSB and vegetation was higher when vegetation was more degraded [37].

The persistent seed bank only comprised three species among the plots analyzed here, indicating a lower species diversity in the persistent seed bank of desert steppe ecosystems. However, SSB species diversity was lower in the natural steppe ecosystem than in each plots of the *C. microphylla* plantation, which indicated a positive effect on SSB diversity. In order to explain the responses of SSB and vegetation to the increasing intensity of human disturbance in a semi-arid region of Northern China, four land use types were selected (native grassland, abandoned artificial grassland, artificial grassland and farmland). The results showed that native grassland had a significantly higher soil seed density and species richness than the other land use types. Moreover, the common species both in the SSB and vegetation between the native grassland and other land use types gradually decreased as disturbance intensity increased [6]. *Caragana korshinskii* of different ages was planted in degraded steppe of Mu Us Sandy Land in Ningxia. The results showed that planting *C. korshinskii* forest had a positive effect on vegetation restoration of degraded grassland. The diversity of plant community increased with the increase of the age of *C. korshinskii* [38]. Our results were similar to those of the two previous studies.

#### **5. Conclusions**

Our findings suggested that (1) the species composition of the SSB and the aboveground vegetation communities differed among *C. microphylla* plantation with different ages in Yanchi County. A total of 33 species were observed in the SSB and 35 species in the aboveground vegetation communities, moreover, 18 mutual species. Among the plots, 15 species only appeared in SSB, while 17 only appeared in the aboveground vegetation communities. (2) The *C. microphylla* plantation had a significantly positive effect on the enrichment of seeds in the SSBs. The average seed density in the SSB of *C. microphylla* plantation was 17 times than that in the semi-fixed sand dunes The SSB were primarily concentrated in the 0–2 cm soil depth interval, which accounted for 80% of the seeds in the 0–10 cm range. (3) Different aged *C. microphylla* plantation exhibited different SSB enrichments, i.e., the highest number was in 25 years plantation, then the 7 years plantation, thirdly the 16 years plantation, finally the 37 years plantation. (4) Overall, *C. microphylla* plantation resulted in positive ecological effects on the SSB species diversity in degraded desert steppe.
