Addition of recyclable biochar, compost and fibre clay to the growth medium layer for the cover system of mine tailings: a bioassay in a greenhouse

Mine closures require landscape reclamation to reduce the environmental risks of tailings fields. However, information about the feasibility of recyclable waste materials as a growth medium layer for the cover systems of mine tailings and their effects on vegetation restoration and reforestation success is scant especially in the boreal climate. This study examines the use of various recyclable by-products in improving vegetation success on reclaimed mine tailings. The physical and chemical properties of two wood biochar types, fibre clay, compost, tailings soil and forest till soil as well as their effects as growth media on the growth of several plant species during one growing period in a greenhouse were examined. Marked differences in the properties (e.g. pH, element concentrations, water retention) as well as in plant growth among the growth media were found. Fresh non-oxidized tailings soil showed high salt contents and electrical conductivity which together with fine soil texture provided the poorest or nonexistent plant growth. Fibre clay was the coarsest and driest material and also showed poor plant growth. Root and shoot growth was greatest in pure compost. All media without compost additive showed relatively poor growth which indicates the lack of nitrogen. The results suggest that forest till soil and biochar are the most suitable growth media for the cover systems of mine tailings when added with compost or another nitrogen source. Scots pine container seedlings, willow cuttings and sown red clover showed to be the most feasible plant species to be grown on boreal tailings covers.


Introduction
After post ore-extracting activities, mine closures require landscape reclamation to reduce the environmental risks of tailings fields. As tailings fields store large amounts of mining waste, they can pose threats to the environment such as uncontrolled spills of tailings or the release of hazardous substances. In Europe, such devastating effects on humans and the environment have been seen following past accidents such as the aluminium sludge spill in Kolontar, Hungary, in 2010 (Thorpe 2010) or the accident at the Talvivaara Mining Company in Finland in 2012 (Kotilainen 2015;Leppänen et al. 2017).
Typically mine tailings are covered with low-permeability materials to prevent sulphide oxidation and minimize outflow of harmful effluents from reactive mine wastes into the surrounding environment (O'Kane and Wels 2003;MEND 2012;Kauppila et al. 2013). Commonly the most hazardous wastes are covered with geosynthetic clay liners (GCLs) to limit the oxygen flow through the cover layer into the waste material and to increase the alkalinity of the leaching water (Bouazza and Rahman 2007;Lange et al. 2010). As bentonite clay has to be imported to Northern Europe, there is a need to develop cheaper local materials and practices for tailings covers.
For a successful reclamation of abandoned mine sites effective revegetation is also required, in which the soil fertility is often the determining factor (Redente et al. 1997;Sydnor and Redente 2002;Dere et al. 2011). A good cover provides a growth medium for plants, retains water, prevents wind and water erosion, increases evapotranspiration and reduces oxygen flow to mine waste layers and oxidation of sulphide minerals (Tordoff et al. 2000;O'Kane and Wels 2003;Lottermoser 2007;INAP 2009;MEND 2012). In the long run, growing vegetation modifies microclimate and provides a source of carbon (C) to the soil profile thereby increasing the likelihood of natural vegetation succession. However, unfavourable substrate structure and nutrient composition require commonly soil amendments for successful vegetation restoration (Anawar et al. 2015).
There is a global augmentative need for recycling increasing amounts of organic by-products and wastes from households, energy plants and industries. Many of the recyclable waste materials, such as biochar (Bc), composted sewage sludge and waste paper sludge (fibre clay) have been suggested and are increasingly being used for landscaping, soil improvement and carbon sequestration (Méndez et al. 2015;Ali et al. 2017). Economic and ecological pressures have also led to the utilization of waste materials as growth media in horticulture (Kern et al. 2017). Recently, various waste materials have been tested and used in highly degraded lands and also in landscaping of mine reclamation areas (Fellet et al. 2011;Gwenzi et al. 2015;Peltz and Harley 2016). For instance, Bc addition into degraded soils has been indicated to improve soil fertility by supplying and retaining nutrients and water for plant growth and by improving physical, chemical and biological properties of the soil (Glaser et al. 2002;Lehmann et al. 2003;Verheijen et al. 2010;Arif et al. 2017). However, further research is required on the potential of Bc and other recyclable waste materials in reclamation and revegetation of mine tailings. Information about the feasibility of various recyclable waste materials as a growth medium layer for the cover systems of mine tailings and their effects on vegetation restoration and reforestation success is scarce especially in the boreal zone having a cool climate, a short growing season and limit vegetation growth.
The aim of this preliminary study was to examine the properties of various recyclable organic by-product and waste materials which are locally available and have potential to be used as cover material on Northern Finnish mine waste areas. In addition, the feasibility of the materials as growth media for vegetation restoration was evaluated.
Thus, we examined the physical and chemical properties of two biochar (Bc) types (from wood of Norway spruce [Picea abies L. Karst.] and common silver birch [Betula pendula Roth], fibre clay (dried waste paper sludge), compost and forest till soil as well as their effects as growth media in varying mixing proportions on plant growth. Tailings soil was also examined as a control medium as such and mixed with other amendments. Some of the most typical northern Scandinavian forest plant species were tested during one growing period in a greenhouse:

Media description
Tailings soils were dug in summer 2017 from the tailings field at the Rautuvaara iron enrichment plant (Kolari, Northern Finland 67°29′43.9″ N, 23°55′17.3″ E, mean yearly temperature 0.3 °C and precipitation 450-550 mm). The upper part (about 20 cm thick) of the soil was brownish and oxidized (Ta1), and the lower part grey and nonoxidized (Ta0). The forest till soil (Ti), which is commonly used in tailings covers, was dug from forest soil nearby the tailings field.
Biochars used in the experiments were received from RPK Hiili Oy (Mikkeli, Finland). The first biochar type (Bc1) was made from Norway spruce wood (porosity ca. 1.6 cm 3 g −1 ) and second (Bc2) from common silver birch wood (porosity ca. 1.1 cm 3 g −1 ). Due to the delivery difficulties the used biochar types in plant growing experiments 1 (Bc1a and Bc2a) and 2 (Bc2a and Bc2b) were obtained from different batches and has thus various physical characteristics (particle size distribution and specific surface area) (Table 1). Instead, due to the similar feedstock materials and process conditions the chemical characteristics were similar between batches (Tables 2, 3).
The used fibre clay type (FC) was light, dried fibre sludge from the effluent treatment plant of a paper mill (Stora Enso Oyj, Veitsiluoto, Finland). The paper mill produces fibre clay 35,000-40,000 t (in fresh mass) annually. The fibre clay contains mainly clay, filling ingredients and a slight amount of wood fibres (dry matter 50-55%). The filling substances are mainly calcium carbonate and less kaolin and talc (Finncao 2001;Niutanen and Korhonen 2002;Raivio 2013). The used compost (Co) was sewage sludge composted with peat, wood chips and sand (Levin vesihuolto Oy; Sirkka, Finland; Evira acceptance code FIC009-05135/2008NA).

Physical analyses
The particle size distribution for individual media components was measured (n = 3) using dry sieving with a series of sieves (from 20 to 0.06 mm). The bulk density (Db) was determined as the ratio of dry mass (dried at 105 °C) to near-saturated volume (n = 3). The particle density (Ds) was estimated using an average density of 2.65 g cm −3 for mineral and 1.5 g cm −3 for organic components. Loss-onignition at 550 °C for 2 h provided an approximate estimate of the organic matter for each medium (n = 3). The total porosity (TP) was estimated as ((Ds − Db)/Ds) * 100.
Desorption water retention characteristics of the media were measured as water content (WC) at decreasing matric    (Klute 1986;Dumroese et al. 2011). Metal cylinders (height 60 mm, diameter 58 mm) were filled with each media, saturated, allowed to drain freely (to about − 0.3 kPa), and then exposed to successive matric potentials from − 1 to − 100 (n = 3). WC at wilting point potential (− 1500 kPa) was measured from parallel loose samples. Saturated hydraulic conductivity was measured by applying the constant-head method (n = 3) (Klute 1986;Dumroese et al. 2011). Specific surface areas of biochars were analysed. Samples (200 mg) were pretreated at low pressures and high temperatures to clean the surfaces. Sample tubes were immersed in liquid N (− 197 °C) and N gas was added to the samples in small steps and the resulting isotherms were obtained. Specific surface areas were calculated from adsorption isotherms according to the BET method (Brunauer et al. 1938).

Chemical analyses
Total C and N were measured (ISO 10694 1995;ISO 13878 1998) from sieved and air-dried samples on a CHN analyzer (LECO-1000, LECO Corp., St. Joseph, MI, USA). Samples for other elements (SFS-ISO 11466, 2007) were digested by the closed wet HNO 3 -HCl digestion method in a microwave (CEM MDS-2000; CEM Corp., Matthews, NC, USA) and the extract was analysed on an iCAP 6500 Duo ICP-emission spectrometer (Thermo Scientific Ltd., Cambridge, UK).
To assess soluble sulphur (S), acid ammonium acetate (pH 4.65) extractant was used (Vuorinen and Mäkitie 1955). The quantification was done using the previously described ICP-emission spectrometer. Soil ammonium (NH 4 -N), nitrate (NO 3 -N), and total N were determined from a KCl-extract on a FIA-analyzer (Lachat QuickChem 8000, Lachat Instruments, Milwaukee, WI, USA) (Kalra and Maynard 1991). Using a microwave (CEM MDS-2000 described above), we used the hot water refluxing method to extract easily soluble boron, quantified using the previously described ICP-emission spectrometer.
For cation exchange capacity (CEC), substrates were prepared as for soluble nutrients (ISO 11260 2018). We used a 0.1 M BaCl 2 solution to extract exchangeable cations, and their total concentrations in the filtrate were determined using the previously described ICP-emission spectrometer.
To determine exchangeable acidity, the 0.1 M BaCl 2 extract was titrated with a 0.05 M NaOH solution up to pH 7.8. Effective CEC [ECEC(cmol kg −1 )] was then calculated as ECEC (in cmol kg −1 ) = Na + K + Ca + Mg + ACI_E, where ACI_E is exchangeable acidity from BaCl 2 extract. Percentage base saturation was calculated as the sum of the bases (Na, K, Ca, Mg) divided by ECEC.
In addition, Bc samples were analysed (Fagernäs et al. 2015) by Eurofins Nab Labs Ltd (Finland) for 16 PAH (polyaromatic hydrocarbons) compounds usually monitored by the US Environmental Protection Agency EPA Extraction (ISO 16703 2004). The PAH components were analysed from n-hexane extract of the sample. The sample was transferred to the extraction funnel and the internal deuterated PAH standards were added. The extract was then cleaned using DMSO liquid-liquid partitioning. The PAH compounds were analysed from the cleaned n-hexane extract using gas chromatography-mass spectrometry (GC/MS) running in SIM-mode. The quantitation was made using the Internal Standard Method. Four deuterated PAH compounds were used (d8-naphthalene, d10-anthracene, d12-chrysene and d14-dibenzo[a,h]anthracene. The limit of detection for the method is 10 μg kg −1 sample and the repeatability ± 20-40% depending on the concentration of the individual component present in the sample.

Plant growing experiment 1
Forest till soil, tailings soil (oxidized and non-oxidized), two various biochar types, compost and fibre clay were used either alone or as a component of various growth media mixtures (Table 1). The growth media were mixed and packed into 3 L pots (Soparco, Condé-sur-Huisne, France) by hand. The depth of the media in the pots during the experiment was 18 cm and the volume 2700 cm 3 . There were in all 17 different growth media combinations with 4 replicates for 5 different plant species in the pots (total of 340 pots).
Seeds of some of the most common grass species were selected for the experiment. Red clover, red fescue and maiden pink were sown into the pots on 22nd November 2017 in the greenhouse (Luke, Suonenjoki, Finland). 15 seeds were sown per pot to determine the germination rate at 7, 14 and 21 day after sowing with frequent irrigation from above. Two germinants per pot were then left to grow. Two common tree species were also used: dark-leaved willow cuttings with two clones (K2183, E6778) and 1-year-old containerized Scot pine seedlings (from local central Finnish seed) were gradually thawed after cold storage, acclimatized to ambient conditions and then transplanted into the pots on 24th November (one clone per pot, i.e. two willow cuttings per pot; one pine seedling per pot).
Plants were grown in a greenhouse, where the natural light was supplemented with artificial diurnal light for 18 h (high-pressure sodium lamps were on from 5 to 23 h). During the night, one interruption lamp was lit to prevent too early bud formation. The temperature was set to 20 °C (day) and 15 °C (night). Actual daily temperatures at seedling shoots were 20-32 °C due to light warming. Full light yielded photosynthetically active radiation at the seedling shoot level of 380-450 μE m -2 s -1 . Relative humidity varied between 30 and 70%, being about 10% higher with no light. The interruption light was shut off after 1.5 months and temperature was lowered 4-5 degrees and light cycle was shortened to 16 h and 14 h after 2 and 2.4 months from the start, respectively. During the experiment, pot positions were changed once a week to reduce spatial variation in growing conditions.
Irrigation was done manually first for 2 weeks by spraying pure tap water from above and then using tap water added with 700 nitrogen (N) and 30 µg l −1 phosphorus (P) (using urea and NaH 2 PO 4 ) to mimic the average N and P concentrations in rain water in northern Finland (Järvinen and Vänni 1998;Vuorenmaa et al. 2001). No other added nutrients were used. Irrigation need was estimated twice a month by weighing the pot masses. The target pot mass after irrigation was determined for each pot as the volumetric WC of about 0.55*TP, where TP is the total porosity of the medium (Heiskanen 2013;Dumroese et al. 2018). Each medium was aimed to be irrigated before the WC limit of 0.35*TP was achieved. Once a month, pots were watered similarly over the target masses so that a percolated water sample of equal amount could be taken from the bottom of the pots for monitoring pH and electrical conductivity (EC) (from three samples per treatments).
Plant quality and height were measured every second week. Plant shoot quality was estimated using a 5-category visual classification (0 dead, 1 dying, 2 weakened, 3 slightly symptomatic, 4 vigorous). On 2nd February 2018, plants were harvested and measured for morphological attributes: height, visual vigour (i.e. greenness and chlorotic tone), and dry mass of shoot. The greenness tone was estimated using a chlorophyll content meter (CCM-300, Opti-Sciences, Hudson, New Hampshire, USA). Root volumes were estimated visually using a 5-category classification (0 dead or nonexistent roots, 1 little viable roots, 2 some long roots, 3 many long, spiralled roots, 4 pot full of long and thick roots) and measuring the maximum root length per pot.

Plant growing experiment 2
The second growing experiment was carried out with red fescue to find out suitable application proportions of Bc1, Bc2 or FC in growth media because in Exp. 1 the effect of Co was found to be the most pronounced. Each new mixture contained a Co proportion of 0 or 20 vol% (Table 2); for example, a medium code Ti80Bc10Co20 denotes mixture of till soil (80 vol%), Bc1 (0 vol%) and Co (20 vol%). Bc1, Bc2, Co and FC were of different batch and particle size than in the growing experiment 1 ( Table 3). The media were packed into 2-L pots (OS Plastic A/S, Denmark) by hand. The medium volume is 1770 cm 3 when the surface is 1 cm below the upper rim of the pot. Red fescue seeds were sown on 5th March 2018, grown and measured as in the first plant growing experiment. There were 68 pots in all (17 media × 4 replicates). The experiment was ceased, and the plants harvested on 3rd May 2018.

Statistical analyses
Data were analysed with IBM SPSS Statistics (Version 25.0). Two-way (Exp. 1) and one-way (Exp. 2) analysis of variance were used to compare the effect of plant species and growth medium on the shoot dry mass. Homogeneity of variance was tested using Levene's test of equality of error variance. The least significance difference pairwise method (LSD) at a significance level of p < 0.05 was used for post hoc multi-comparison tests of the differences between means. Because of the small number of replicates (4), categorical variables (0-4; plant vigour and root volume) could not be analysed or tested with ordinal or binary logistic regression.

Physical properties
In Exp. 2, Bc1 and Co were somewhat coarser while Bc2 and FC were finer in particle size than in Exp. 1 (Table S3). Highest bulk density was in Ti and Ta0, and lowest in FC (Table 4). Largest available water content released between matric potentials − 0.3 (near saturation) and − 9.81 kPa (field capacity) was in FC-Co and lowest in Ta0 (Table 4). Bc1 and Bc2 had intermediate water release, but Bc2 released negligible water in drier conditions between − 9.8 and − 98.1 kPa. Saturated hydraulic conductivity was variable being highest in FC-Co (mean 0.441 cm min −1 ) and lowest in Ta0 (0.002 cm min −1 ). In Bc1 it was lower (0.092) than in coarser Bc2 (0.179 cm min −1 ). The specific surface area as BET-area of Bc1 (in Exp. 1 and 2) and Bc2b (in Exp. 2) were 180, 61 and 0.26 m 2 g −1 . BET-area of Bc2 (in Exp. 1) could not be detected due to the tars in the sample.

Chemical properties
Tailings soil contained much different elements and especially Ca, Mg, Fe, S, Cr, Cu and Ni (Table 5). Ta1 contained the highest amounts of Cd and As. Co contained much P as well as Cu, Pb and Zn. Co (compost) contained the most Al. Ca was the highest in FC. C/N ratio was highest in Bc1, Bc2 and FC. N was the highest in Co. Extractable elements were clearly on a lower level compared to the total amounts (Tables 5, 6). FC contained Ca and P the most. Bc1 had clearly the highest K content. Co contained Ntot and NO 3 and the most. Ti had the lowest ECEC. Total PAH concentrations of biochars varied from 0.7 to 2.4 (Bc1) and 40 to 57 mg kg −1 (Bc2) (Supplementary Table 1).

Plant growing experiment 1
pH and EC of the percolate from pots were all within 5.0-9.7 pH and 0.04-16.4 mS cm −1 during the experiment, respectively (Fig. 1). EC tended to lower during the experiment except in Ta0, where it elevated to as high as 16.4 mS cm −1 .  Acidity was highest at the start of the growing in Co percolate (pH 5.0) where it lowered later on (pH 6.7). Acidity was lowest in Bc1 percolate, where it decreased from pH 9.7 to 9.3 during the growing. All the measured plant variables, germination/burst rate, development of plant vigour, shoot and root growth, differed by species and growth media but showed parallel results. Less than 50% of red clover germinated in 21 days in Ta0, Ta1 and Bc2 (Fig. 2). On red fescue, the germination was below 50% in Ta0, Ta1, Ti and Ti-Co, while it was as high as 98% in Bc1. Maiden pink germinated poorest on the average, but was best in Ti (78%) and FC (80%), and poorest in Bc2 (0%). At least 50% of willow cuttings were burst in all growth media in 21 days. On pine seedlings, the bud burst was gradual and similar in all media except in Ta0, where it was delayed about one week. Development of plant vigour (classes 0-4) was best in pine and willow although it decreased during growing. Overall, the poorest vigour was in Ta0 while it was best in Bc1-Co, Bc2-Co, Co and Ti-Co especially in the grass species (Supplementary Fig. 1).
Shoot dry mass after the growing season differed highly significantly by the growth medium and plant species but most by the growth medium (Table 7). There was also a significant interaction effect of species and medium. Shoot growth was the greatest in Co and good in Bc1-Co, Bc2-Co, and Ti-Co (Fig. 3). In Bc1-Co growth was equal or slightly better than in Bc2-Co. Pine and willow were able to grow also in the other growth media, but the grass species were not. However, no shoot growth occurred in Ta0 except on pine. Shoot growth of plant species decreased, on average, in the order pine, willow, red clover, red fescue and maiden pink. There was no root growth in Ta0 (Fig. 4). The grass species showed no roots in Bc1 either. Roots grew best in Bc1-Co, Bc2-Co, Co and Ti-Co. Mean root mass after one growing period deceased in the order willow, pine, red clover, red fescue and maiden pink.

Plant growing experiment 2
pH and EC of the percolate from pots were all within 6.2-8.0 pH and 0.05-1.75 mS cm −1 during the experiment, respectively. EC tended to lower during the experiment (to less than 0.8) except in Ti0FC80Co20 where it reached 1.75 mS cm −1 . Acidity tended to decrease slightly during the experiment (mean pH from 7.2 to 7.5).
Red fescue germinated fairly in all media. The germination rate was within 63-95% after 21 days in the media. Also shoot growth (as shoot mass) differed significantly by the growth medium (Table 8). Growth was good in media amended with Co except in FC, where the growth was minimal (Fig. 5). Ti soil with Co but without Bc grew intermediately. Bc2 mixtures yielded somewhat better growth than Bc1 mixtures. Similarly, root growth (as root volume class) was poor in media with FC and media without Co (Fig. 6). In Bc2 mixtures also root growth was somewhat better than in Bc1 mixtures. Both shoot and root biomass tended to increase when Bc concentration increased from 10 > 20 > 40 to 60% of growth media.

Discussion
The studied pure tailings soils (Ta0 and Ta1) had high metal/metalloid in the order Fe > Mg > S > Al > Mn > Ni > Cr > Cu and low N concentrations. Especially EC was high in Ta0 and concentrations of Cd and As were high in Ta1. However, only the marginal proportions of the concentrations were in water-extractable form which suggests low leaching risk of these metals in the neutral or alkaline conditions at Rautuvaara tailings. Bc addition can further increase soil pH and alkalinity and reduce phytoavailability of Cd, Zn, and Pb (Martins et al. 2018). Compost (Co) originating from sewage sludge exhibited high concentrations of Al and P, which indicates that P in the sewage sludge has been precipitated using Al. Also, concentrations of P as well as Cu, Pb and Zn were high in compost medium. However, concentrations of metals in the used compost and biochar types were all below the limit values of Finnish Fertilizer product Act (539/2006; Cd 1.5, Cr 300, Cu 600, Ni 100, Pb 100 and Zn 1500 mg kg −1 ) showing that the products are suitable to be used as a part of growth media.  The overall seed germination with abundant irrigation was best in treatments Co, Bc1, FC and Ti, while it was poorest in pure Ta0, Ta1 and Bc2 (Exp. 1). The need for amended growth layer in the tailings soil to plant growth is evident as all the tested plant species showed poor or no germination and growth in Ta0 and Ta1. This is in accordance with field observations in Rautuvaara tailings area, where the early success of vegetation is negligible in the tailings soil without any amendments. Even though the total concentrations of toxic metals as Cd and As were much lower in Ta0 than in Ta1, Ta0 showed much higher phytotoxicity on plants i.e. lower germination and no root growth. Poor growth in Ta0 was presumably also contributed by very limited oxygen availability owing to the soil compactness and wetness. Growth prerequisites of Ti soil were significantly improved by the amendment due to the increased amount of nutrients (esp. N) and improved soil structure, which enhanced growth of all tested plant species. Instead, FC addition gave no advantages on plant growth.
Previously, also in temperate Appalachian region, afforestation of coal-mined areas is found successful with topsoil covers (Zipper et al. 2011(Zipper et al. , 2013. Successful afforestation of non-acidic (low S level) mine waste areas have been found with at least 20-50-cm-thick topsoil layer in boreal conditions in Canada (Larchevêque et al. 2014). Bc and other additives such as peat and sewage sludge mixed in mine covers have also been used to enhance vegetation restoration (Drozdowski et al. 2012;Dietrich et al. 2017;Miller and Naeth 2017). Compost, clay and new recyclable materials such as dried waste paper sludge (FC) have been somewhat studied as growth media components in mine cover systems and bottom sealing layers (Nurmesniemi and Pöykiö 2006;Nurmesniemi et al. 2007;Pöykiö et al. 2007;Kuokkanen et al. 2008;Kukkonen et al. 2011;Raivio 2013;Tuomisto et al. 2016). Addition of organic materials to coarse or compact mineral soil has generally shown to improve soil CEC, water retention and aeration and thus availability of water and nutrients to plants (Bunt 1988). Varying effects of composted sewage, paper mill sludges and other composts have been found on vegetation restoration and tree seedling growing (Hernández-Apaolaza et al. 2005;Heiskanen 2013;Chong and Lumis 2017). In this study, the poor plant growth in FC media after successful germination, was probably due to the poor wettability and rapid subsurface drainage caused by its coarse particle size. FC has generally shown a good liming effect increasing soil pH and concentrations of P, K, Ca and Mg but, in high quantities, high pH and EC may first limit growth of some plants. However, the properties of FC depend on the manufacturing process of the paper (raw material either mechanical mass, chemical pulp or recycled papermass from de-inking sludge) and thus various products and their long-term effects should be studied case by case.
Effect of Bc on plant germination and growth varied between the plant species but no improvement occurred when Bc was mixed with mere Ti (Exp. 1). Instead, when mixed to both Co and Ti (Exp. 2), the addition of Bc enhanced the shoot mass and root volume class of red fescue; the higher Bc concentration (0-60%) the higher was the growth. Also previously it has been found that the low vegetation growth caused by nutrient deficiency in Bc can be avoided by adding compost or manure to the Bc medium ). Both Bc types (50% rate) inhibited here remarkably germination of maiden pink compared to seeds sown in pure Co or Ti soil. Instead, red fescue and red clover grew better in soils with Bc or Co added either separately or mixed. In Exp. 1, Bc1 showed better plant growth than Bc2 but in Exp. 2 the effect was opposite. As the particle size distribution of Bc was the only difference between the batches, the main reason for growth variation was probably the coarser structure of Bc2 in Exp. 1 and Bc1 in Exp. 2, which lead to faster drying. This was also shown in their water retention characteristics as Bc2 (in Exp. 1) contained no plant available water between the matric potentials − 9.8 and − 98.1 kPa compared to Bc1. Nevertheless, both Bc1 (birch) and Bc2 (spruce) were relatively coarse and dry materials, which had to be irrigated frequently during the growing experiments. On the other hand, Bc tended to disintegrate and compact somewhat during the growing. Overall, the irrigation need varied a lot among the different growth media.
Differences in the concentrations of the PAH compound of Bc1 and Bc2 were high. In the case of Bc2, the sum of PAHs exceeded the recommendations for good quality biochar for land use (6 mg kg −1 ) (EBC 2020) and can thus not be recommended to be used in tailings covers. Even though high PAH concentration had no effect on plant growth in our study, the long-term effects and bioaccumulation cannot be excluded. The reason for a high PAH content remains unresolved but together with tars observed in BET analysis it may indicate incomplete pyrolysis processes. In general, the growth and vigour of the plant species grown in the studied growth media decreased in the order, pine, willow, red clover, red fescue and maiden pink. From the tested plant species, pine container seedlings, willow cuttings and sown red clover are presumably the most feasible to be grown on boreal tailings covers. The short duration of the growing experiments (one growing season) cannot, however, predict how the plants can grow later on. It may be, for instance, that pine and willow grew well in the first season using their photosynthate reserves but may grow less in the next season with reduced reserves. Furthermore, red clover possesses a high biological N fixing ability which could not be seen in growth in the first season, but may later enhance growth. On the other hand, red clover and red fescue yielded dense root systems in the first season, which may accomplish better nutrient and water availability in the next seasons and thus increased growth.
In addition to the plant species studied here, also response of other boreal plant species and forest floor communities to biochar and compost containing growth media may be worth for further examination. According to the review of Dhar et al. (2018), the studies conducted in boreal ecosystems of northern Alberta (Canada) showed that the use of forest floor mineral mix in oil sands reclamation results in greater overall plant cover, richness of native species, and lower non-native species cover relative for example to peat-mineral soil mix. Also in the synthesis paper of Mcdonald et al. (2015), the use of natural regeneration and direct placement of forest floor material combined with seedings of native species is recommend gaining the rapid forest re-establishment of mining sites. As our results are based only on one preliminary experiment with growing media from one or two batches only, they can provide only general indications. Repeated studies with varying materials from several production batches should be examined in the future before valid formula recommendations can be drawn.
As Bc is yet not a standardized product, its properties and also application targets may differ greatly depending on the used feedstock material and production conditions (Youn et al. 2019). Even though the pyrolysis conditions of Bc1 and Bc2 were exactly the same, their chemical and physical characteristics varied depending on feedstock material. Effects of Bc on soil depend on the properties of the Bc and the soil (Nelissen et al. 2014;Burrel et al. 2016;Hagner et al. 2016;Hansen et al. 2016;Lim et al. 2016;Liu et al. 2017;Kaudal et al. 2018). In general, Bc has shown to be a potential growth medium component for agri-and horticulture and for vegetation restoration as well as for C sequestration (e.g. Steiner and Harttung 2014;Nemati et al. 2015;Thomas and Gale 2015). Bc addition to coarse or compact soil tend to increase soil crumb structure, porosity and water retention capacity and decrease bulk density Nelson et al. 2011;Basso et al. 2013;Heiskanen et al. 2013;Peltz and Harley 2016;Tuomisto et al. 2016;Aller et al. 2017). Bc usually increases also soil pH, CEC and concentrations of C, P and K, but it provides no actual liming effect (Robertson et al. 2012;Tuomisto et al. 2016;Li et al. 2018).
In forest soil, Bc addition can increase N net mineralization and NH 4 concentration during two growing seasons as found in Northern Sweden (Gundale et al. 2016). According to Palviainen et al. (2018), Bc addition of 5-10 t ha −1 to mid-Finnish Scots pine forest soils may increase N mineralization slightly in 2 years. Bc can also alter soil microbe structure and greenhouse gas emissions (usually reduces N 2 O emission and increases CH 4 sink and has mixed effect on CO 2 emission) (Li et al. 2018). Adding Bc to soils is usually found unharmful for forest tree seedlings or may enhance their growth prerequisites Robertson et al. 2012;Pluchon et al. 2014;Thomas and Gale 2015). Bc is a potential growth media for tree seedlings especially when pH, irrigation and fertilization are adjusted to the properties of the Bc media Dumroese et al. 2018;Lange and Allaire 2018;Sarauer and Coleman 2018). Addition of Bc into a highly weathered acidic soil has been found to increase seed germination, plant growth, vegetation cover, as well as N and P use efficiency (Zhu et al. 2014;Anawar et al. 2015). According to Pluchon et al. (2014), Bc with high P concentration and CEC increase growth of tree seedlings in Northern Sweden, but deciduous tree seedlings (Silver birch and European aspen) seem to be more sensitive to Bc additions than coniferous seedlings  (Scots pine and Norway spruce). However, in this study, no differences were found in pine and willow shoot and root growth, and vigour class. This discrepancy is most probably due to the differences in the used Bc types and high Bc application dose (50%) as well as to the fact that the pine seedlings were containerized in this study.
Bc addition has shown potential for revegetating mine tailings and decreasing the bioavailability of organic and inorganic pollutants in heavy-metal-concentrated soils (Beesley and Marmiroli 2011;Fellet et al. 2011Fellet et al. , 2014Peltz and Harley 2016;Ali et al. 2017;Yuan et al. 2019). In the actual boreal mining reclamation sites, the possible longterm effects, such as alterations in fertility and phytotoxicity of the growth medium layer on tailings soil and flux of metals to cover plants, remain to be assessed by later studies. At the moment, the main hindrance to practical application of Bc is its high production costs. To minimize transportation costs, Bc pyrolysis should be performed close to local raw material reserves and be mixed with other materials (e.g. composts, manures) for synergic effects (Petz and Harley 2016). Also initially nutrient-rich Bc (e.g. derived from sewage sludge and animal manures) might provide long-term fertility for plant growth.

Conclusions
Studies investigating the feasibility of recyclable waste materials as a growth medium layer for the cover systems of mine tailings and their effects on vegetation restoration are scarce especially in the boreal climate. In this study, the potential of various recyclable by-product amendments (biochar, composted sewage sludge and fibre clay) in improving the success of boreal plant species on reclaimed mine tailings was investigated in two well-controlled greenhouse experiments. Our results showed marked differences in the properties among the growth media mixtures as well as in plant growth. Used fibre clay was too coarse and a dry material and showed only poor plant growth. All media without compost additive showed relatively poor growth which indicates the lack of nitrogen. From tested plant species, pine container seedlings, willow cuttings and sown red showed the best growth.
The results suggest that forest till soil is biologically and economically the most suitable growth media for the cover systems of boreal mine tailings when added with compost or another N source. However, when considering also the possible life-cycle costs and longer term effects on soil fertility, plant growth and C sequestration, biochar with N addition may be better medium for tailings covers if its production costs can be reduced. The long-term effects of nutrients and metals on fertility and phytotoxicity of the media and on plant growth as well as the life-cycle costs of media components and their effects for C sequestration on mine reclamation sites remain to be assessed by further studies.
Funding Open access funding provided by Natural Resources Institute Finland (LUKE).
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