Sweet sorghum - Biomass production and energy balance at different levels of agricultural inputs. A six-year field experiment in north-eastern Poland
Introduction
Economic development has increased the demand for energy in the agricultural sector. Technological progress has increased energy consumption many fold, and primary energy use is highest in agriculture (Escobar et al., 2009). Growing concerns about energy security and the environmental impacts of fossil fuels have prompted the search for renewable energy sources (Midilli et al., 2006). Lignocellulosic biomass grown in dedicated farms can be converted into bioproducts, bioenergy and biofuels (bioethanol, biogas, etc.) (Lin and Tanaka, 2006; Kumar et al., 2009; Silva et al., 2018; Diallo et al., 2019; Hassan et al., 2020; Rezania et al., 2020).
The greatest controversy surrounding energy crops stems from the fact that these plants have similar land and water requirements as food and feed crops (Dalla Marta et al., 2014; Iqbal et al., 2015; Schorling et al., 2015). In the future, the key challenge in the agricultural sector will be to satisfy the growing demand for biomass without compromising food production (Muylle et al., 2015). Crops with a shorter growing season, which are generally not suitable for food production, could contribute to the achievement of that goal (Theuretzbacher et al., 2013).
Biomass feedstocks are used mainly in the production of bioenergy (electricity and/or heat) and second generation fuels, mainly biogas (Borkowska and Molas, 2013). Biogas produced from dedicated energy crops is a promising source of renewable energy in Europe. Maize (Zea mays L.) silage is the most popular feedstock in agricultural biogas plants in Europe (Weiland, 2006; Schittenhelm, 2008), and it accounts for up to 73 % of all plant-derived feedstocks in biogas production in Germany (Barbosa et al., 2014). Maize is often grown in the proximity of biogas plants, including in monoculture, to guarantee continuous and reliable feedstock supply. However, maize monocultures for biogas production can contribute to environmental concerns, including reduced biodiversity, higher pressure from pests and greater susceptibility to diseases (Schittenhelm, 2010). Various energy crops are being researched to address these issues, including miscanthus (Miscanthus spp.), sunflower (Helianthus annus L.), switch grass (Panicum virgatum L.), poverty oat grass (Helictotrichon pretense L.), Jerusalem artichoke (Helianthus tuberosus L.), small-sedge poor-fen meadows, tall herb meadows, montane hay meadows and sorghum (Sorghum spp.) (Beck et al., 2007; Richter et al., 2009; Schittenhelm, 2010; Mahmood and Honermeier, 2012).
Maize should be grown in rotation with high-yielding annual plants to improve the structure of cropping systems. Unlike perennial plants, annual plants do not disrupt crop rotation practices in farms and enable producers to swiftly respond to market changes (Jankowski et al., 2016).
Energy crops should also be resistant to drought. In water-deficient regions of the world, irrigation of energy crops could compromise the production of food crops (Dalla Marta et al., 2014). In Europe, the above requirements are met by sweet sorghum (Sorghum bicolor (L.) Moench), an annual grass with a C4 photosynthetic pathway. Sweet sorghum plants have long stems that can be used in the production of feed (Ceotto et al., 2014; Kozłowski et al., 2006; O’Hara et al., 2013). Sweet sorghum biomass can be directly burned to generate bioenergy, it can be converted to bioethanol and used as feedstock in biogas production (Monteiro et al., 2012; Velmurugan et al., 2020). The post-harvest biomass of sweet sorghum contains 50- and 8-times less sulfur and ash than hard coal, respectively, and its energy value has been determined at 16.8 MJ kg−1 (Rennie et al., 2011). The discussed crop has low agricultural requirements, is drought tolerant (Velmurugan et al., 2020), produces high yields in a wide range of environmental conditions (Wight et al., 2012; Ceotto et al., 2014; Umakanth et al., 2019) and is abundant in fermentable carbohydrates (Han et al., 2011; Li et al., 2018). Sweet sorghum stems accumulate soluble sugars (Li et al., 2018; Diallo et al., 2019) which can be extracted mechanically and fermented to produce first generation ethanol (Umakanth et al., 2019). The remaining lignocellulosic biomass can be converted to second generation ethanol (Dalla Marta et al., 2014; Damay et al., 2018; Silva et al., 2018). The sugar yield of sorghum biomass is estimated at 6−8 Mg ha−1, and ethanol production from sorghum juice reaches 3000 dm3 ha−1 (Almodares and Hadi, 2009). Sugar and ethanol yields can be improved through the introduction of hybrid cultivars of sweet sorghum (Silva et al., 2018; Umakanth et al., 2019). Ethanol and sugar yields are considerably higher in sugarcane (3000−5000 dm3 ha−1 ethanol and 7−8 Mg ha−1 sugar) and sugar beets (5000−6000 dm3 ha−1 ethanol and 8−10 Mg ha−1 sugar), but these crops have a longer growing season, require higher quality soils and higher rates of mineral fertilizers (Almodares and Hadi, 2009).
Sorghum is widely grown in Africa, Asia and North America (Dweikat, 2012; O’Hara et al., 2013; Gill et al., 2011; Velmurugan et al., 2020). In the temperate climate of Central Europe, sweet sorghum is cultivated mainly for feed production and energy generation, often in rotation with maize (Kołodziej et al., 2015). The dry matter yield (DMY) of sweet sorghum biomass ranges from 9 to 12 (Mahmood et al., 2013; Bonin et al., 2014; Szempliński et al., 2014; Houx and Fritschi, 2015; Smith et al., 2015) to 12−19 (Jankowski et al., 2016, 2020) or even 23−29 Mg ha−1 (Księżak et al., 2012; Mahmood et al., 2013; Garofalo et al., 2016; Appiah-Nkansah et al., 2019). Similarly to maize, sorghum is characterized by high biomass yield, high content of starch, sugar and cellulose (Amaducci et al., 2004a, 2004b; Rooney et al., 2007), but is more resistant to drought (Zegada-Lizarazu et al., 2012; Farré and Faci, 2006) and requires less favorable soil and environmental conditions than maize (Reddy et al., 2005; Zegada-Lizarazu and Monti, 2012). In a study conducted by Amaducci et al. (2016) in northern Italy, the average biomass yield of sorghum was approximately 37 % higher in comparison with maize. Irrigation minimized the difference between the biomass yield of maize and sorghum, which indicates that maize is a more energy-intensive crop. Sorghum is more drought tolerant than maize, and its production requires less energy (Grassini and Cassman, 2012; Banaeian and Zangeneh, 2011; Garoma et al., 2012; Ren et al., 2012; Pishgar-Komleh et al., 2012; Amaducci et al., 2016). Sorghum can be cultivated on compact soil within a wide range of pH (5.0–8.5) (Smith and Frederiksen, 2000) and salinity values (Almodares et al., 2007). The discussed crop can be grown on both heavy and sandy soils (Smith and Frederiksen, 2000). It is well adapted to warm and dry regions of Greece (Sakellariou-Makrantonaki et al., 2007; Vasilakoglou et al., 2011), Spain (Farré and Faci, 2006), Italy (Dalla Marta et al., 2014), Nigeria (Jantar et al., 2018), Iran (Goshadrou et al., 2011), Australia (Muchow, 1989) and India (Singh and Singh, 1995). It should also be noted that sorghum is one of the most genetically diverse crops, which facilitates the development of new cultivars that are well adapted to various climates around the world (Zhang et al., 2010).
One of the greatest advantages of sweet sorghum is that it has low agricultural requirements (Almodares et al., 1997; Bonin et al., 2016). Sorghum is characterized by high biomass yields in low-input farming systems, which reduces greenhouse gas emissions, increases soil organic carbon levels and is consistent with the concept of sustainable agriculture (Berndes et al., 2003; Sims et al., 2006). Climate change is inextricably linked to agriculture: agricultural production is a significant source of greenhouse gas emissions, but is also affected by weather fluctuations resulting from climate change (Maracchi and Sirotenko, 2005). To achieve sustainable development goals, agriculture has to cater to the growing demand for high-quality crops while minimizing fertilizer and fuel use and promoting rational use of crop protection agents (Nakamura and Ohashi, 2008; Graff et al., 2012). The higher inputs associated with farming operations are justified only if they increase profits, maximize energy efficiency and deliver environmental benefits (Szeptycki, 2005).
The energy efficiency of biomass is determined by the energy inputs and energy output associated with the specific farming technology (Budzyński et al., 2015; Elsoragaby et al., 2019). According to Almodares and Hadi (2009), sweet sorghum is a cost-competitive energy crop relative to sweet beets (Beta vulgaris L.) and sugarcane (Saccharum officinarum L.). Some sweet sorghum genotypes have a short growing season (4 months), they have lower soil, water and nutrient requirements, and are more resistant to pathogens. Energy-efficient crops are excellent sources of feedstock for bioenergy generation (Budzyński et al., 2014). In Europe, the cultivation of annual plants of the family Brassicaceae, whose seeds are converted to first generation biofuels, has an energy efficiency ratio ranging from 1.1 to 2.4 (Cardone et al., 2003; Venturi and Venturi, 2003; Firrisa et al., 2014) to 3.6–5.4 (Jankowski et al., 2015) with straw excluded, and 9–12 with straw included (Jankowski et al., 2015). High-yielding non-food crops used for the production of second generation biofuels are significantly more energy efficient than Brassica oilseed crops. The energy efficiency ratio of maize and sweet sorghum has been estimated at 8.4–18.6 and 8.8–12.5, respectively (Ren et al., 2012; Budzyński et al., 2014; Tworkowski et al., 2014; Amaducci et al., 2016; Jankowski et al., 2016, 2020).
This paper presents the results of a study comparing the biomass yields of sweet sorghum and the energy inputs and energy efficiency of production technology at two levels of agricultural inputs. A field experiment was conducted in a large-area farm in north-eastern Poland. The compared farming technologies differed in raw material and energy inputs. The aim of the study was to determine the most energy-efficient technology for the production of sweet sorghum biomass in large-area farms that use high-performance machines and equipment.
Section snippets
Field experiment
A field experiment was conducted in 2013–2018 in the Agricultural Experiment Station in Bałcyny in north-eastern Poland (53°35′46.4′' N, 19°51′19.5′' E, elevation 137 m). The station covers around 2000 ha of arable land, and it is owned by the University of Warmia and Mazury in Olsztyn. The experimental variables were two levels of agricultural inputs in the production of sweet sorghum biomass (Sorghum bicolor (L.) Moench): (i) a high-input and (ii) a low-input. The experimental field had an
Weather conditions
Total annual precipitation during the 6-year field experiment (2013–2018) was determined at 454−940 mm. The long-term average (1981−2015) was 588 mm. In four years of the study (2013, 2014, 2015, 2018), total precipitation was below the long-term average. Precipitation levels exceeded the long-term average by 28 % and 60 % in only two years of the study (2016 and 2017, respectively), mainly due to heavy precipitation in June and August of 2016 and in June, July and September of 2017. In most
Energy inputs
The demand for energy in biomass production is determined mainly by the amount of energy invested in the farming technology and agricultural operations (fertilization, tillage, weed control, pest and disease management, harvest) (Budzyński et al., 2015; Muylle et al., 2015) as well as the region of cultivation (irrigation requirements) (Garoma et al., 2012). According to Jankowski et al. (2016), energy inputs in the cultivation of energy crops can be minimized by increasing the effectiveness
Conclusions
In north-eastern Poland, the DMY of sweet sorghum can reach 19.7 Mg ha−1. Productivity was influenced by weather conditions and agricultural intensity. Biomass yields were lowest (11.4.−13.7 Mg ha−1 DM) in dry years when precipitation levels were below the long-term average. The highest yields (14.5-19.7 Mg ha−1 DM) were noted in a year characterized by heavy rainfall which exceeded the long-term average. In all experimental years, the DMY of sweet sorghum was 18 % higher in the high-input than
CRediT authorship contribution statement
Krzysztof Józef Jankowski: Conceptualization, Methodology, Validation, Resources, Writing - original draft, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Mateusz Mikołaj Sokólski: Writing - original draft, Writing - review & editing. Bogdan Dubis: Conceptualization, Methodology, Investigation, Resources, Writing - original draft, Writing - review & editing, Project administration, Funding acquisition. Dariusz Załuski: Software, Formal
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The results presented in this paper were obtained as part of a comprehensive study conducted at the University of Warmia and Mazury in Olsztyn (grant No. 20.610.020-110). Project financially supported by Minister of Science and Higher Education in the range of the program entitled ‘Regional Initiative of Excellence’ for the years 2019–2022, project No. 010/RID/2018/19, amount of funding 12.000.000 PLN. The authors would like to thank the staff of the Agricultural Experiment Station in Bałcyny
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