Stockpiling disrupts the biological integrity of topsoil for ecological restoration

Abstract

Purpose Biotic and abiotic properties of soils can hinder or facilitate ecological restoration, and management practices that impact edaphic factors can strongly influence plant growth and restoration outcomes. Salvaged topsoil is an invaluable resource for mine-site restoration, and a common practice is topsoil transfer from mined areas to restoration sites. However, direct transfer is often not feasible, necessitating storage in stockpiles. We evaluated the effects of topsoil stockpiling on plant performance across diverse ecosystems impacted by mining throughout Western Australia. Methods We conducted a bioassay experiment using a widespread native Acacia species to assess how topsoil storage might impact plant growth, physiology, and nodulation by N-fixing bacteria using soils from native reference vegetation and stockpiled soils from six mine sites across Western Australia. Results Plant responses varied across mine sites, but overall plants performed better in soils collected from native vegetation, exhibiting greater biomass, more root nodules, and higher water-use efficiency compared to those grown in stockpiled soils. Soil physiochemistry showed few and minor differences between native soils and stockpiles. Conclusion Results strongly suggest observed differences in plant performance were biotic in nature. This study highlights the negative effects of topsoil storage on the biological integrity of soil across diverse ecosystems, with important implications for mine-site restoration; our results show that topsoil management can strongly influence plant performance, and stockpiled soils are likely inferior to recently disturbed topsoil for restoration purposes. We also use this study to illustrate the utility of bioassays for assessing soil quality for ecological restoration.

Evaluating biological properties of topsoil for post‐mining ecological restoration: different assessment methods give different results

“…These findings are further supported by previous studies using the same bioassay species, Acacia saligna , to evaluate effects of topsoil stockpiling on plant growth and root symbionts. A study using the same topsoils as used here assessed plant growth, physiology, and nodulation by nitrogen‐fixing bacteria (Valliere et al 2021) and recorded minimal differences in soil physiochemistry in stockpiled versus native reference topsoils, suggesting negative effects of topsoil stockpiling on plant performance were biotic in nature. Further, Valliere et al (2021) observed less root nodulation (lower rhizobial activity) in A. saligna seedlings grown in stockpiled topsoils than reference topsoils, which is in line with the Gorzelak et al's (2020) finding that key rhizobia taxa were lacking in stockpiled topsoils when compared to fresh topsoils.…”

“…Additionally, soil physiochemistry showed few and minor differences between native soils and stockpiles (Table S2; Valliere et al 2021).…”

“…The use of plant bioassays, while time and resource intensive, is critical to calibrate any measures and may be necessary for fully understanding the ability of some soils to support restoration (e.g. Valliere et al 2021).…”

“…A plant bioassay (see Valliere et al 2021) was conducted in a glasshouse at the University of Western Australia (Crawley, Australia) from September 2019 to February 2020 using Acacia saligna (Labill.) H.L.…”

“…For example, assessing microbial activity yields no information on community composition while community composition provides no indication of whether microbes are functionally competent; and molecular approaches are expensive and complex (Hart et al 2020). However soil functionality can also be determined by its ability to support plant growth, achieved through bioassay experiments measuring plant–soil responses under controlled conditions (Dietrich et al 2013; Valliere et al 2021). While such experiments may provide early warning of critical deficiencies in soil functionality prior to restoration, they may require lengthy greenhouse studies.…”