Could Biochar and Peat Moss Help Address Reclamation Challenges of Using Shrub Species on Overburden Dumps from a Subarctic Iron Mine?

Iron ore mining produces large quantities of residual substrates not suitable for natural plant colonization. Overburden, the substrate of interest in this study, consists of the superficial layer of soils and rocks lying on top of the exploitable iron ore (Paradise 2017). It contains lower concentrations of heavy metals (such as iron and manganese) than the gangues (i.e. commercially worthless material that surrounds the mineral of interest in the ore deposit) or the tailings (material already stripped of valuable minerals; Aebischer et al. 2015). Still, overburden dumps can be an important source of contamination for the environment if exposed to wind, rain, and snow (Lv et al. 2020).

The restoration of a plant cover on overburden dumps would greatly reduce their contamination potential. Plant roots would help stabilize the overburden, thereby attenuating the impact of wind and water erosion (Wong 2003, Tordoff et al. 2000, Madejón et al. 2010, Srivastava et al. 2014). Revegetation would also increase organic matter content and nutrient availability of overburden and even promote the diversity and structure of microbial communities (Madejón et al. 2010, Jambhulkar and Kumar 2019). Moreover, the use of native plant species (Tordoff et al. 2000), an environmentally friendly and low-cost method to reclaim such substrate (Srivastava et al. 2014, Jambhulkar and Kumar 2019), could trigger the process of plant succession and lead to the development of functional ecosystems (Bradshaw 1997, Tordoff et al. 2000).

Overburden revegetation in subarctic environments is, however, challenging since plant productivity is constrained by climatic conditions and by the limited amount of nutrients, mycorrhizal propagules and organic matter, and the high concentrations of heavy metals (Bois et al. 2005, Srivastava et al. 2014, Jambhulkar and Kumar 2019). One approach often used for overburden revegetation aims to foster their ability to support plant growth by improving their physicochemical and biological properties (Bradshaw 1997, Madejón et al. 2010, Sheoran et al. 2010, Beesley et al. 2011; Srivastava et al. 2014, Al-Lami et al. 2019). Therefore, our objective was to evaluate the impact of two organic amendments (biochar and peat moss) on the survival and growth of seedlings of Betula glandulosa (Dwarf birch) and Alnus viridis ssp. crispa (Green alder) on overburden collected from an Iron Mining Site, near Schefferville (Québec, Canada; Latitude: 54°48′ 11.39″ N, Longitude: –66°48′ 11.39″ W).

For this greenhouse experiment, we amended the overburden with biochar and peat moss. We used biochar made from the pyrolysis of hardwood chips (< 75% sugar maple) at a temperature of 500°C for 48–72 hours (Produits forestiers Basques Inc, Québec, Canada. See Supplementary Materiel Table S1 for its physicochemical properties) while peat moss was from Fafard (Québec, Canada. See Supplementary Materiel Table S2 for its physicochemical properties). For each of the 32 treatments (4 levels of biochar × 4 levels of peat moss × 2 shrub species), we replaced either 0%, 5%, 10% or 15% of the overburden volume by either biochar or peat moss, or by both for the treatments combining biochar and peat moss addition. Experimental units, consisting of individual pots of 3.79 L with a single seedling, were randomly assigned to one of the treatments. Our design was therefore a 4 × 4 × 2 factorial experiment with each treatment replicated five times (i.e. five blocks), for a total of 160 experimental units. Seedlings of B. glandulosa and A. crispa were grown from seeds extracted from catkins collected near the mining sites in Schefferville in 2016. Seeds were germinated in seedling trays (45 cells, four seeds per cell) filled with sand and peat moss (50:50) and topped with a thin layer of silica to prevent excessive drying. Cells were thinned out to one seedling after two weeks. At eight weeks, seedlings of B. glandulosa and A. crispa (average height of 35 and 33 cm, respectively) were transplanted in the experimental pots.

For this four month long experiment, we set the temperature at 22.5°C, the minimum ambient...