Abstract
Peatlands require soil improvement to be suitable for cultivation. Creating eco-friendly and cost-effective carbon sinks in peatlands originated from peat production has several benefits. For this purpose various valuable biomass can be used by utilizing industrial by-products also as soil conditioners and fertilizers. For example, the addition of such materials has potential to transform peat bogs, which otherwise would slowly release methane, into productive cultivated areas. The rehabilitation of peat bogs from unused land into various agricultural and forestry areas is also a viable business activity. The examined industrial by-products could have many agricultural applications in non-food potato production, wherein monoculture causes problems such as condensed soil, lost humus or soil organic matter, and reduced nutrient retention capacity, leading to increased leaching of nutrients and negative impacts on the environment. Five industrial by-products were examined in this study as soil conditioners and fertilizers: fiber sludge, biocarbon, hygienic biodigestate, paper mill sludge, and gypsum waste. Based on the results of a nutrient content analysis, hygienic biodigestate and fiber sludge were the most effective fertilizers.
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The Utilization of Industrial By-products as Soil Conditioners and Fertilizers in Non-food Potato Production
Matti Kuokkanen,a,c,* Jussi Tuomisto,b Hanna Prokkola,a Pekka Tervonen,c and Ulla Lassi a
Peatlands require soil improvement to be suitable for cultivation. Creating eco-friendly and cost-effective carbon sinks in peatlands originated from peat production has several benefits. For this purpose various valuable biomass can be used by utilizing industrial by-products also as soil conditioners and fertilizers. For example, the addition of such materials has potential to transform peat bogs, which otherwise would slowly release methane, into productive cultivated areas. The rehabilitation of peat bogs from unused land into various agricultural and forestry areas is also a viable business activity. The examined industrial by-products could have many agricultural applications in non-food potato production, wherein monoculture causes problems such as condensed soil, lost humus or soil organic matter, and reduced nutrient retention capacity, leading to increased leaching of nutrients and negative impacts on the environment. Five industrial by-products were examined in this study as soil conditioners and fertilizers: fiber sludge, biocarbon, hygienic biodigestate, paper mill sludge, and gypsum waste. Based on the results of a nutrient content analysis, hygienic biodigestate and fiber sludge were the most effective fertilizers.
Keywords: Soil conditioner; Fertilizer; Fiber sludge; Paper mill sludge; Biocarbon; Biodigestate; Gypsum; Non-food production; Peat bog; Carbon sink
Contact information: a: University of Oulu, Research Unit of Sustainable Chemistry, P.O. Box 3000, FI – 90014, Oulu, Finland; b: Potato Research Institute, Alapääntie 104, FI – 61400, Ylistaro, Finland; c: University of Oulu, Research Unit of Industrial Engineering and Management, P.O. Box 3000, FI – 90014, Oulu, Finland; *Corresponding author: matti.kuokkanen@oulu.fi
INTRODUCTION
The Earth’s population is increasing by up to 200,000 people per day, while the amount of available land for food production is decreasing. When salinization and soil desertification occur, as well as rising water levels in coastal areas, formerly arable land is converted to bioenergy production. However, the current global trend is to move from fossil raw materials towards a bio-economy, increasing the need for fields and woodlands. Thousands of hectares of peatlands are removed from peat production annually, and in Finland, an estimated 50,000 hectares of land have become non-productive. Most of the areas that no longer produce peat are located in southern, central, and northern Ostrobothnia. However, peatlands require soil improvements to be suitable for all kinds of cultivation uses (Myllys 1996).
Fig. 1. The circular economy cycle of non-food potato production
Peatlands cover only 3% of the Earth’s land surface, but boreal and subarctic peatlands store about 15% to 30% of the world’s soil carbon as peat (Limpens et al. 2008). Despite covering less than 3% of the Earth’s land surface, boreal and subarctic peatlands store between 270 TgC and 370 TgC (1 TgC = 1012 g of carbon) as peat (Turunen et al. 2002), which would also amount to 34% to 46% of the 796 TgC currently held in the atmosphere as CO2 (Solomon et al. 2007). These massive deposits are the legacy of peatlands acting as sinks of atmospheric carbon dioxide (CO2) for millennia, but they also illustrate the potential for large CO2 and methane (CH4) fluxes into the atmosphere or dissolved carbon (DC) into rivers if peatland carbon stores were to be destabilized by global warming, and changes in land use are not made. Until now, peatlands have contributed to global cooling on the millennium scale (Frolking and Roulet 2007), and undisturbed peatlands are likely to continue functioning as net carbon sinks despite the large interannual variability of individual peatlands (Moore et al. 1998).
Soil improvement aims to improve water conductivity and water retention, improve nutrient retention, adjust the soil’s pH by liming, and utilize recycled nutrients. In non-food production (Fig. 1), recycled nutrients could be used increasingly in the future according to the circular economy principle. The intensification of agriculture by the use of high-yielding crop varieties, fertilization, irrigation, and pesticides has contributed substantially to the tremendous increases in food production over the past 50 years. Land conversion and intensification, however, also alter biotic interactions and patterns of resource availability in ecosystems and can have serious local, regional, and global environmental consequences. The use of ecologically sound management strategies can increase the sustainability of agricultural production while reducing off-site consequences (Matson et al. 1997).
A new innovation in this ongoing project is to introduce eco-friendly and cost-effective materials that can serve as carbon sinks. These are added to peatlands that no longer produce peat. Various types of valuable biomass that are available from industrial by-products can be used as soil conditioners and fertilizers. The carbon sink potential of peatlands depends on the balance of carbon uptake by plants and microbial decomposition. Both of these processes’ rates will increase with global warming. The present-day global sink will increase slightly until around the year 2100 but decline thereafter. Peatlands will remain a carbon sink in the future, but their response to warming will switch from a negative to a positive climatic feedback (i.e., decreased effectiveness as a carbon sink with warming) at the end of the twenty-first century (Gallego-Sala et al. 2018). The potential benefits of carbon sink activity are as follows:
1. It changes peat bogs that slowly release methane into productive cultivated areas while creating valuable new carbon sinks by also increasing the biomass that is required for a long growth period (Lai 2009).
2. Pre-treating soils for cultivation is easy, as they have no trees, roots, or stones. The areas are also large. Thus, splitting them into different uses is easy, including experimental areas that could simultaneously involve both agricultural and forest cultivation.
3. It is potentially an eco-friendly, materially efficient, and cost-effective way to utilize various industrial by-products in accordance with the current European Union (EU)/Finland National Waste Strategy, as well as granulated products (e.g., bio ash, bio-sludge, and gypsum waste-based materials). By using legislation that will evolve in the future, this list will also include materials that are currently classified as waste (Karvonen et al. 2011). Consequently, the topic is central to circular economic thinking.
4. Through the conversion of former peatlands, more farming areas will become available for the needs of a strongly evolving bio-economy and expanding bio-refinery plants that are partially foreign-owned. These needs include different fibrous and potato starch-based products, of which new ones include textiles and plastic substitutes. Additionally, the quality of fiber- and starch-based products depends decisively on, among other things, what kind of wood the products are made of, where the trees are grown, and what kinds of fertilizer are used to grow the tree. Wood formation is attributed to many factors, including site, environmental, and stand conditions, as well as management, genetics, and age (Zobel and van Buijtenen 1989; Saranpää 2003). Therefore, one must strive to use the correct amount and type of fertilizer. This innovation presumably has a notable impact on the quality of the fiber material used in the manufacture of future products.
5. The rehabilitation of peat bogs from unused land to various agricultural and forestry areas will also be a viable business activity due to the increased value of the soil. Peatlands can provide income and other benefits to local communities by supporting forestry and agricultural cultivation under wet conditions, and cultivation can occur wherever there are marketable plants and animals living in wet conditions. This can produce biomass for bioenergy, feed for livestock, fibre, building materials, and even food (Joosten et al. 2016).
6. Rehabilitating peat bogs for the sole production of thermal energy, for example by rapid growth or the current cultivation of willows, does not create carbon sinks in the affected areas, as was noted last year in Finland’s successful negotiations on intensifying the felling of forests in the EU.
7. The creation of new carbon sinks can also be considered as being important for general climate policy reasons, in accordance with the EU’s climate policy strategy and other related objectives.
8. The implementation of this new carbon sink objective requires a wide range of experts to succeed. The design and implementation of projects must be performed by various university and vocational institutions and research institutes, as well as by large energy companies, municipal parties, and, especially, new environmental firms that deal with granulation, drying technology, and humus removal from natural water bodies.
In potato production, monocultures are common in Finland, with the main reasons being economic: Potato production farms specialize in potato production, farms have the entire machine chain, and markets are built solely for potato production (Tuomisto 2011). Furthermore, the mass of potatoes transported from the fields is large, so potato cultivation has been concentrated close to economic centers (Myyrä 2001; Tuomisto and Huitu 2008; Tuomisto 2011). However, monocultures create problems, including increased plant pest and disease activity, condensed soil, loss of humus or the soil’s organic matter, and weakened soil nutrient retention, resulting in nutrient leaching, environmental damage, and economic losses (Lemola et al. 2000). Hence, the concentration of localized environmental damage can be considerable.
Environmental hazards can be reduced by using appropriate soil conditioning agents, such as paper mill sludge, native fiber sludge, and biotechnologically modified fiber sludge, along with other fiber sludge-based products in general, because their chemical purity can be considered a viable option. In the soil structure, between the soil particles, remaining bonds (such as hydrogen bonds), play an essential role in helping potato plants access water and nutrients. Usable water for plants is bound to the medium-sized pores, while macroscopic pores are filled with air and allow for plant root growth. In the saturated state, pores are filled with water. When soil is drying, the large pores empty first, followed by the small pores. In small pores, water movement is weak (Boone et al. 1978; van Loon and Bouma 1978). In sand, the groundwater capillary rise is strong. Potatoes are grown mainly in coarse soils with an abundance of large pores, and in potato production, the soil needs organic matter. Therefore, one alternative with a strong likelihood of success in potato production is applying biotechnologically modified fiber sludge enhanced with appropriate nutrients to the soil, where it operates as a soil conditioning agent and fertilizer. Additionally, fiber sludge degrades slowly and functions as a breeding ground (Kuokkanen et al. 2015). The application of biotechnologically modified fiber sludge is currently being researched, and results will be published in the near future. In this study, however, only native fiber sludge was tested as a soil conditioner. Five different industrial by-products were tested in this study as soil conditioners: fiber sludge, biocarbon, hygienic biodigestate, paper mill sludge, and gypsum waste.
Soil Conditioners
Gypsum is commonly used in the manufacture of plasterboard, soil conditioner, fertilizer, cement, building coatings, alabaster (glass-like material frequently used for ornamental purposes), medical-grade plaster, and other products. Approximately 20 million m2 or 200,000 tons of plasterboard is produced in Finland each year. Between 14,000 tons and 20,000 tons of gypsum waste (Fig. 2) is generated in Finland each year; presently, only a small fraction of it is recycled.
Fig. 2. Gypsum waste
The rest was previously placed in landfills (the current waste tax is 70 €/t). At the beginning of 2016, the situation changed as gypsum could no longer be placed in landfills in Finland. As a soil conditioner, gypsum works on the ground so that it dissolves in the earth’s barrier layer, where it changes the soil’s properties. Gypsum does not bind to phosphorus, but phosphorus remains in use for various plants (Ekholm et al. 2010). Gypsum increases the solubility of the soil due to soluble sulfate (SO42-), binds earth particles with Ca-bridges, and enhances phosphate retention in ground particles. In this case, the soil remains in the field, erosion decreases, and less phosphorus is flushed from the fields.
Wood fiber sludge is generated as a by-product of the pulp and paper industry in Finland at the rate of approximately 750,000 tons per year. In the past, wood fiber sludge was either combusted or landfilled. The amount sent to landfills has decreased because wood waste is currently subject to a waste tax (now 70 € / wet waste ton), which also applies to industrial landfills (Kuokkanen et al. 2018). For energy production, fiber sludge is not a very efficient fuel due to its high moisture content (typically 80% to 90%). Wood fiber contains fewer nutrients, but it has a considerable amount of slowly biodegradable organic matter. By spreading this by-product on the fields, more organic matter is absorbed into the soil, improving the structure of low-loam clay land. In field soil, fiber improves microbial conditions, retains moisture, and increases biological activity. In the autumn, carbonaceous wood fiber binds nitrogen to the soil and, thus, reduces nitrogen leaching. An innovative approach is being developed in which the fiber sludge by-product of the pulp industry is utilized as a new kind of soil conditioner for potato production, which would have a fertilizing, liming, and aerating effect (Kuokkanen et al. 2018). The effect of this biotechnologically modified fiber sludge (Fig. 3b) is based on a cost-effective enzymatic treatment of native wood fiber sludge (Fig. 3a) in which the fiber cluster is chemically opened, providing a greater reaction surface for various applications via binding (Kuokkanen et al. 2018). Various recent fiber sludge applications include binding agents for combustion pellets used in the energy industry, bedding pellets used on horse farms, a binding component of granulated soil conditioners in agricultural and forestry farming, and as such in soil conditioners. In addition there are efficient and ecological dust binding agents for various purposes, for example, road building and horse fields (Kuokkanen et al. 2018) and horse stables. The suitability of new eco-efficient materials as raw materials for these granulated symbiosis products in non-food potato cultivation with peat bogs has also been studied in the ongoing project. Furthermore, this research will be extended in the future to corresponding forestry projects.