Biological soil crusts enhance the role of soil seed banks in semi-arid ecosystems

Soil seed banks provide materials for plant distribution, dynamics, and ecosystem evolution. In semi-arid sandy regions, biological soil crusts (BSCs) exist widely and play a significant role in the early stages of the life history of vascular plants. Seed banks are regulated by the interactions between BSCs and vascular plants, and are also affected by the spatial distribution of BSCs. While many studies have focused on how BSCs influence seed germination and seedling survival, their impact on soil seed banks has been less explored, even though seed banks are crucial for plant community assembly and ecological resilience in the semiarid regions.

We collected soil samples of the patches of bare sand, physical, algal, and moss crust at the typical distribution site of BSC in the Mu Us Sandy Land, northern China, over two periods. Germination experiments were conducted to analyze the composition and distribution of transient and persistent seed banks, respectively. We also investigated the BSC patches and plant communities to assess the similarity between seed banks and above-ground vegetations.

In the transient seed bank, the seed density increased 289%, 422%, and 1032% in the physical, algal and moss crust, respectively, compared to bare sand. The persistent seed bank showed no significant differences among different patches. The moss crust improved significantly on the transient seed bank by collecting many annual herb seeds. The algal crust significantly increased the proportion of perennial herbs and xerophytes in the persistent seed bank, although there were no significant differences in diversity among different patches.

Synthesis and applications: BSCs exert a storage effect (particularly the moss crust) and a filtering effect (particularly the algal crust), enhancing the role of both transient and persistent seed banks in semi-arid lands. The patchy mosaic distribution pattern triggered "seed islands" within communities, which harbor high seed densities by the storage effect or preserve rare species by the filtering effect. These seed islands improve ecosystem regeneration and resilience and maintain heterogeneity in plant communities, highlighting the importance of BSCs in combating desertification and maintaining biodiversity and ecosystem function in semi-arid regions.