Sweden is the world’s second largest exporter of pulp, paper and wood products; this amounted to ≅ 125 billion Swedish krona (SEK)in 2017 [1
]. There is about 28 million hectares of forest land in Sweden and pine trees constitute 40% of the total standing volume [2
]. Although approximately 80 million m3
of Swedish forest stands are harvested annually, the total standing volume has considerably increased in the last century [3
]. In fact, a total of 400 million containerized tree seedlings are produced by Swedish forest nurseries to restock forests each year [3
]. However, intensive annual forest harvests remove essential soil nutrients, which may cause problems for forest productivity.
In Sweden, container-grown seedlings are dominantly produced in peat and peat-based growth media [4
]. Peat-based substrates have many advantages such as long-term drainage ability, good aeration for tree seedling roots, good fertilizer absorbance and release capability. However, peat-based media are considered non-sustainable as their extraction have adverse environmental impacts [5
]. Hence, researchers tend to evaluate various growing substrate alternatives that could fully or partially replace peat [7
]. Particularly, nutrient-rich growing media have become a special area of interest for research, as the growing global demand for wood harvest will cause further depletion of soil nutrients. Therefore, sustainable approaches towards forest production and plantation management are urgently needed.
Biochar has been introduced as an environmentally sustainable option to replace peat in nursery conditions [8
]. Biochar is the carbonaceous residue of rapid biomass combustion in the absence or partial supply of oxygen (pyrolysis) [12
]. Biochars as soil amendment not only contribute to storing carbon in the soils but also act as fertilizers [13
], which subsequently reduces the environmental impact [15
] and economic cost of plant production [17
]. Moreover, biochar can increase substrate pH, improve water-holding capacity and enhance phyto-available nutrients by decreasing nutrient leaching and increasing cation exchange capacity (CEC) and consequently, enhance plant growth and productivity [18
]. Biochar from paper sludge has also been characterized with high specific surface area [22
] and has shown promising effect on simulating growth of plants such as Lolium perenne
]. Biochar from pulp and paper mill sludge has been reported to have higher environmental performance relative to conventional disposal methods for sludge such as landfilling or incineration [24
]. In many studies, the term ‘biochar’ is generally understood to mean ‘pyrochar’, which is a coproduct of fast pyrolysis or gasification. Another type of biochar is hydrochar, which is produced through hydrothermal carbonization (HTC). Therefore, it may have different physical structure and chemical composition, and consequently cause different effects on nutrient availability and plant growth compared to pyrochar [26
]. Hydrochar is characterized by a low pH, high carbon content, high nutrient levels, good heavy metal absorption capacity and a carbon supply for microorganisms in the soil [18
]. Generally, the effects of biochar on plant growth and physicochemical properties of the growing media vary depending on biochar feedstock, particle size and production process, e.g., temperature and heating duration [27
]. Potentials of pyrochars and hydrochars as substrate components for production of tree seedlings in nurseries have not been well documented. Pyrochar has been evaluated in forms of either pellet or powder and shown promising results as nursery substrate for pine tree seedlings as well as sequestering carbon as part of normal reforestation [10
]. Few empirical studies investigated the effects of hydrochar on tree seedling and soil nutrient cycling. Some studies reported adverse impacts of hydrochar derived from beet root chips on plant productivity and seed germination even when applied as low as 20–25% of volume of the growth mixture [33
]. However, other studies offered contradictory findings about the effects of hydrochar on a fast-growing tree species: Baronti et al. [35
] and George et al. [27
] showed that biomass productivity and nitrogen use efficiency increased in poplar tree seedlings treated with hydrohar derived from maize (Zea mays
L.) silage feedstock. These contradictory results concerning the effects of hydrochar on plant productivity call for careful choice of feedstock, application rate and the target species to ensure optimum growth benefits.
As hydrochar is usually friable and dusty, pelletization decreases dust formation, unifies its shape and size and facilitate transportation and distribution. Other benefits of reducing the loss of nutrients (e.g., nitrate and phosphate) and water, reducing bulk density and providing a beneficial environment for microbes as well as improved total porosity and aeration porosity in containers have been also reported by Di Lonardo et al. [20
] and Dumroese et al. [10
]. Thus far, however, there has been no discussion about the use of hydrochar pellets in nursery substrate constituent. Moreover, despite the potential significance of hydrochar, the effects of this byproduct on plant and microorganism interactions, e.g., mycorrhizal associations, is poorly understood. Ectomycorrhizal (ECM) associations play a key role in nutrient uptake in many woody plants, e.g., pine trees, such that growth and survival of these plants in forest ecosystems considerably rely on ECM fungi [36
]. The existing body of research on biochar suggests positive effects of pyrochar on mycorrhizal symbiosis of the host plants, however, some substantial differences between hydrochar and pyrochar require assessment of hydrochar for any potential negative effects [33
Therefore, the aim of this paper is to study the effects of hydrochar, derived from paper mill biosludge, on growth, quality, mycorrhizal associations and nutrient/heavy metal uptake of pine tree seedlings. We analyzed whether effects varied significantly between hydrochar forms (powder or pellets) or hydrochar proportions mixed with peat (10% or 20% hydrochar v/v). The effects of hydrochar addition on pine tree seedling was evaluated under three fertilization regimes (no fertilizer, 50% fertilizer and 100% fertilizer). We hypothesized that the growth, quality and mycorrhizal colonization of pine tree seedlings grown in substrate mixed with hydrochar would improve. We also expected pine tree seedlings grown with hydrochar to require less fertilizer to achieve similar or higher growth, mycorrhizal colonization and associated nutrient uptake relative to seedlings grown without hydrochar but with optimum rates of fertilizer (100% fertilizer). To the best of our knowledge, this current study is the first paper to explore the potentials of hydrochar powder and pellets for being used as a growing media component in production of containerized pine tree seedlings.