Vertical distribution of soil seed bank in alkaline grasslands - seeds can survive deeper than we expected

Our new paper - the first first-author paper of my PhD student, Ágnes Tóth - has been recently accepted for publication in the journal PeerJ.

The citation of the paper is:

Tóth, Á., Deák, B., Tóth, K., Kiss, R., Lukács, K., Rádai, Z., Godó, L., Borza, S., Kelemen, A., Miglécz, T., Bátori, Z., Novák, T.J., Valkó, O. (2022): Vertical distribution of soil seed bank and the ecological importance of deeply buried seeds in alkaline grasslands. PeerJ 10: e13226 doi: 10.7717/peerj.13226 [IF2020: 2.984]

The paper is freely available at the journal homepage, and can be downloaded from here.

In this study, we were interested in the vertical distribution of soil seed bank in alkaline grasslands. Why is it interesting? In this habitat type, the environmental conditions - such as the very high soil salinity and special physical structure of the soil - are harsh and as an adaptation, persistent soil seed bank can help plants to bridge the unsuitable environmental conditions. Due to the special soil characteristics of Vertisols, crack formation is a typical phenomenon in the arid summer conditions, when the seeds can easily penetrate to deep soil layers. Vertisols occur all over the globe, especially in tropical and subtropical areas in total on more than 335 million hectares extension.

Halophyte species - such as Salicornia prostrata - often have long term persistent seed banks that can help them to bridge unsuitable environmental conditions.

 

We studied the vertical distribution of soil seed bank in high resolution and to a depth that - up to our knowledge - was never studied before. We sampled cores down to 80 cm and divided each sample to sixteen 5-cm-deep segments. Well, it is not easy to describe the extraordinary sampling conditions and challenges... The soil was very hard so sometimes we needed to use special solutions and creativity for drilling... You can see some videos in our past blog post here.

One of the study sites in early spring, at the time of the soil sampling.

 

Processing of the drilled samples.

 

After sample concentration we started the experiment in a greenhouse using the seedling emergence method. We found that the 75% of the total seed density was concentrated in the uppermost (top 5cm) soil layer, but seeds of 5 species were found even at the deepest layer (80cm deep). We found that seed shape was the most important factor that determined burial depth: round shaped seeds could penetrate deeper than the elongated ones. The deep soil layers contained mainly seeds of alkaline grassland species, and only very few weeds.

The vertical distribution of seedling number and species richness, starting from Layer 1 (0-5 cm) to Layer 16 (75-80 cm).

Why are these deep buried seeds important? Can they ever have a role in vegetation dynamics, can they ever germinate? Of course, more studies investigating this question are needed. However we assume that there are several natural and anthropogenic processes that can bring the deep buried seeds close to the soil surface. For example, the ecosystem engineering activity of burrowing mammals can bring the seeds close to the soil surface (see our related blog post here). Also, seeds can be brought to the surface during construction works, or in the soil preparation phase of restoration projects. As an indirect evidence, we often recorded halophyte species not present in the extant vegetation after large-scale soil preparation works, like in channel elimination projects. You can read about our previous studies on the topic here: Deák et al. (2015), Valkó et al. (2017). In conclusion, it was fascinating to find germinable seeds in such deep soil layers which shows that we might reconsider the spatial dimensions of the distribution of the soil seed bank. This propagule reservoir can be more important that previously thought: deep buried seeds can also contribute to vegetation dynamics if some natural or anthropogenic disturbance help them to reach the soil surface.

Halophyte species - probably originating from deep soil layers - on the surface of a Steppe Marmot burrow in Kazakhstan.

Halophyte species growing on large open soil surfaces created in a landscape-scale restoration project.