Synergistic Effects of Biochar and Hydrogel on Growth of Three Herbaceous Species Hydroseeded onto Gold Mine Tailings

The Eldorado Gold Lamaque mine has operated since 1935 on the outskirts of Val-d'Or, Quebec (Canada). It produces large quantities of fine tailings and waste rock, which cover over 150 ha at the city limits (Nadeau et al. 2018). The restoration of these mine tailings is essential for two reasons. First, it would pay off ecologically to considerably reduce their risks of environmental contamination. Secondly, it would make it possible to meet the obligation imposed by Quebec legislation on mining companies to restore mine tailings, which have been generated by their operations (Nadeau et al. 2018, Tardif-Drolet et al. 2020). These mining substrates could make their restoration a difficult challenge because they are depleted in plant organic matter and nutrients. Moreover, fine tailings have, as the term suggests, a very fine particle size (22–75 micron), while waste rock consists of large crushed and unstructured pebbles (above 75 micron) (Callender et al. 2016, Nadeau et al. 2018).

Using hydrogel and biochar could alleviate these substrate problems by creating better conditions for plant growth. Hydrogels are three-dimensional hydrophilic networks that can absorb large quantities of water, up to 400 times their mass. These products would make moisture gradually available to plants (Orikiriza et al. 2009). Amending agricultural soils or mine tailings with hydrogel is one way of overcoming water availability problems in arid and semi-arid ecosystems (Al-Humaid and Moftah 2007, Chirino et al. 2011). By increasing the availability of water in mining substrates, hydrogel amendments promote germination and establishment of plants (Bigot et al. 2013, Guterres et al. 2013, de Souza et al. 2021). In contrast, biochar is a product of the pyrolysis of plant biomass under limited oxygen conditions. Incorporating biochar into mine tailings would stimulate plant growth (Fellet et al. 2014, Rees et al. 2015) by acting directly on the physicochemical and biological properties of the latter (Fellet et al. 2014, Rees et al. 2015, Allaire and Lange 2013). Particularly, it improves their aeration, density, water retention and nutrient retention (Allaire and Lange 2013).

Until now, most work that has been conducted on biochar and hydrogel as amendments for revegetation of mine tailings has hardly considered their synergistic effects. On mine tailings, this combination could improve the germination and growth conditions of plants. The objective of this study was to determine, under greenhouse conditions, the combined effects of biochar and hydrogel on the growth of a mixture of three herbaceous species (Avena sativa L. [common oat], Festuca rubra L. [creeping red fescue] and Trifolium repens L. [white clover]), which were hydroseeded on gold mining tailings. The selection of these herbaceous plants is based upon their widespread use in initial revegetation of mine tailings, both in Canada and elsewhere (Green and Renault 2009, Guittonny-Larchevêque et al. 2016, Juge et al. 2021). These herbaceous plants could ensure the colonization of gold mining tailings from Val-d'Or since they are pioneer species that facilitate the introduction of woody species.

We tested biochar and hydrogel in greenhouse experiments (temperature: 22.5°C, minimum ambient humidity: 60%, the radiation intensity: 50 micromoles/s/m²) on fine tailings and waste rock from Eldorado Gold Lamaque mine (fine tailings: 48°06′38.4″ N and 077°44′44.7″ W, waste rock: 48°06′20.7″ N and 077°45′43.1″ W See Nadeau et al. 2018 for their physico-chemical characteristics). Mesocosms containing the substrates (54 cm long × 34 cm wide × 18.5 cm high) were amended with biochar or hydrogel, or both. BQ-Érable-500-3 biochar was applied for this test at a rate of 15% v/v (a ratio of 85% of residues to 15% of biochar v/v) (see Allaire et al. 2015 for its principal characteristics). We manually applied biochar to the mine tailings surface and mixed in using a small rake. We also applied hydrogel (Solid Rain, San Diego, CA, USA) as solid granules. Rainwater is retained within the granules as a molecular solid with potassium acrylate. Three levels of...