Recently, there have been significant advancements in seed enhancement technologies designed to overcome specific challenges associated with seed-based restoration (Brown et al. 2021; Davies et al. 2018; Erickson et al. 2019b; Gornish et al. 2019). However, the potential of incorporating fresh topsoil in pellets for the restoration of sites with degraded soils, such as mine sites, remains largely unexplored in the literature. To address this knowledge gap, we investigated the effect of pellets containing varying amounts of fresh topsoil on seedling emergence, seedling height, and microbial activity on degraded post-agricultural land where topsoil had been removed. We found variable effects on seedling emergence depending on the species investigated and the proportion of topsoil in the pellet.
The field emergence of non-enhanced seeds was generally low for most of the species investigated, with levels below 15%, except for R. caespitosum. This low level of field emergence is not uncommon in direct seeding practices (Grossnickle and Ivetić 2017). For one species, Chrysocephalum apiculatum, we found increased seedling emergence in the pellet treatments, this was in the topsoil 30% and 50% treatments as well as the base pellet, which did not contain topsoil. Hence, for C. apiculatum pelleting improved emergence regardless of the inclusion of topsoil, but the highest emergence occurred in the topsoil 50% pellets, with a 6.8 times greater likelihood of emergence compared to non-enhanced seeds. However, for A. milleflorum and G. tabacina, seedling emergence was significantly reduced by pelleting in most of the treatments. The field emergence for these species, even in non-enhanced seed, was very low, suggesting that they may not be suitable candidates for restoration, at least with the seed batch we used. Future studies should investigate methods to enhance their field emergence. Linum marginale and R. caespitosum seedling emergence was not affected by any of the treatments, with L. marginale having overall low emergence, and R. caespitosum having reasonable emergence of around 40%.
Many studies exploring seedling emergence in SETs have found species-specific results (Alfonzetti et al. 2022; Dadzie et al. 2022; Stock et al. 2020), and it is unclear what attributes affect performance in pellets. Indeed, the other study exploring fresh topsoil in pellets found adverse effects on the emergence of one species but null effects on another (Alfonzetti et al. 2022). Here, our primary focus was to establish a proof of concept regarding the potential of topsoil pellets to improve emergence. Species from the community were selected based on availability, and in the hope that at least one species would emerge successfully enabling us to examine treatment effects. Hence, the improved emergence in C. apiculatum supports the notion that pellets containing topsoil of up to 50% can improve seedling emergence. The two species that pellets negatively affected emergence had very low emergence across all treatments and are currently not suitable for restoration.
Mechanical or direct seeding is the most common approach to reintroduce species to degraded sites where topsoil has been removed (Gibson-Roy et al. 2010; Gibson-Roy et al. 2010). Rytidosperma caespitosum is typically not used in restoration due to its non-deep physiological dormancy and a floret structure that makes it difficult to disperse effectively via mechanical seeders (Berto et al. 2021; Grice et al. 1995). These findings suggest that pellets could be an effective method for dispersing R. caespitosum, as no negative effects of pelleting were found.
We also explored seedling height as a measure of plant growth and health in the treatments. Here, we only had enough individuals for two species at the end of the field trial to explore this. For both species, we found significant reductions in height in the pellet treatments, for Chrysocephalum apiculatum seedlings the reduction was found in the topsoil 30% treatment, and for R. caespitosum seedlings reductions were found in all pellet treatments. Previous studies have found delays in seedling emergence in pellets which reduces time aboveground for plant growth compared to non-enhanced seeds (Brown et al. 2019; Ritchie et al. 2020), which could explain the reduced height in our study. Plant height can be related to plant survival as larger plants are more likely to survive the summer drought period (Gardiner et al. 2019). Here, we finished our trial in the second autumn after commencement and the seedlings of the species investigated in height survived through the first summer drought period, a critical time when mortality typically occurs (Morgan 2001). Furthermore, although pellet treatments had a negative effect on seedling height, there was no significant difference in aboveground biomass for both species across all pellet treatments (Fig S2, Table S1). This suggests a shift in growth allocation, where shorter seedlings exhibited a higher leaf production. Further research is needed to determine the optimal topsoil pellet composition for different species to maximise their growth and establishment on degraded sites.
Soil microbes play a crucial role in maintaining soil function, fertility, and plant growth and survival (Begum et al. 2019; Singh et al. 2016). Yet topsoil scalping can negatively impact soil microbial diversity and health by removing the upper layers where microbes are often found (Eilers et al. 2012; Gibson-Roy et al. 2010; Seuradge et al. 2017). Although the SET treatments did not have a significant effect on the emergence of R. caespitosum seedlings, we did observe an increase in microbial activity for this species in the topsoil 50% treatment, as indicated by elevated measurements of CO2 evolved per g of C in the soil. While we were unable to identify the specific microbial taxa responsible for this increase in activity, the findings suggest that pelleted topsoil could potentially improve microbial activity and support plant growth on degraded sites. However, we did not find increased activity for any other species or treatments.
The transfer of soil from an intact reference site after the removal of degraded topsoil has been identified as one of the best methods for restoring soil microbial activity, enhancing physicochemical properties of soil, and for facilitating native species recovery (Bulot et al. 2017; Wubs et al. 2016). Based on our findings, it can be inferred that including 50% of topsoil in pellets is necessary to benefit microbial activity, and this also had the greatest improvements in seedling emergence. However, the results in this study suggest that when fresh topsoil is contained within the microsite of seeds, the effects are more species-specific or less effective, possibly due to the reduced topsoil quantities compared to topsoil relocation studies (or quality). The effects of topsoil may be compromised by the presence of plant and soil pathogens from the topsoil source or the scalped soil, which could have a negative effect on seedling emergence and plant health (Alfonzetti et al. 2022; Emam 2016). Furthermore, any beneficial bacterial or fungal communities contained in the fresh topsoil may be depleted or damaged as a result of the wetting and drying process during pellet production (John et al. 2010; McIntyre et al. 2007). It is unclear from this study and others whether it is the handling of topsoil during the pellet production procedure or the presence of plant and soil pathogens that drive these variable responses. We know that when topsoil components such as fungi, bacterial communities and cyanobacteria are isolated and then incorporated into SETs, they can improve seedling emergence, growth, and properties of degraded soils (Colla et al. 2015; Dadzie et al. 2022; Román et al. 2020). However, this isolation process is much more involved and costly for restoration practitioners and may not be feasible for large-scale restoration.
Further investigations are needed to fully explore the potential of fresh topsoil in pellets for seed-based restoration. It is crucial to consider the impact of the wetting and drying process on microbial and fungal communities during pellet production (John et al. 2010; McIntyre et al. 2007). Our study and other SET research highlight the need for species-specific responses to be considered, which may require a large-scale study with a diverse range of species. This study should consider species factors such as dormancy, life form and seed size to establish patterns and drivers for species-specific responses. Despite these uncertainties, our findings indicate that fresh topsoil pellets can be a valuable method for facilitating native species recovery on degraded sites. Further research is required to optimise the use of fresh topsoil in pellets to promote seedling emergence of many species and long-term plant health.