Research articleAssessment of microbial diversity associated with CH4 emission from sugarcane vinasse storage and transportation systems
Introduction
The use of liquid biofuels in substitution of fossil fuels is a feasible strategy to mitigate greenhouse gas (GHG) emissions. Sugarcane ethanol is considered one of the sustainable options since several studies have indicated that this biofuel can mitigate around 85% of GHG emissions in comparison with gasoline (Börjesson, 2009; Chagas et al., 2016). However, the effective contribution of sugarcane ethanol to mitigate GHG emissions are associated with management strategies and the disposal and use of the main residues generated in agricultural and industrial operations (Bordonal et al., 2018).
The production of ethanol from sugarcane generates several residues, like straw (crop residues), bagasse, filter cake, ash and vinasse. Vinasse, also called stillage, is the main residue of ethanol production since for 1 L of ethanol an amount ranging from 10 to 15 L of vinasse are generated. Vinasse is produced during the distillation of ethanol (Fuess and Garcia., 2015) and is characterized as a residue with dark color, acid pH and rich in nutrient contents like calcium, nitrogen (N) and mainly potassium (Moran-Salazar et al., 2016).
In the last decades, the main destination of vinasse in Brazil has been the application into the sugarcane field as liquid fertilizer, promoting vast benefits to crop production (Parnaudeau et al., 2008). However, once produced in huge quantities, before recycling back into the field, the vinasse is stored and distributed in different systems, which according to Oliveira et al. (2015, 2017) could emit significant quantities of CH4 to the atmosphere. In Brazil, the most common systems of vinasse storage and transportation are open channels (either uncoated or coated) and closed pipes and tanks (Oliveira et al., 2017). Open channels represent an older system and are formed basically by a furrow at sugarcane fields around the mill. This system is generally comprised of coated and uncoated sections in which vinasse is stored and transported by gravity and active pumping before its application in the field by fertirrigation. The system composed of tanks and closed pipes have been used more recently in substitution of the channel system. In this system, the vinasse is temporally stored in coated tanks and transported by closed pipes until the application in sugarcane fields by fertirrigation.
In a pioneer study, Oliveira et al. (2017) measured the CH4 emissions by vinasse storage and transportation in Brazil and highlighted that depending on the system, the vinasse can emit large amounts of CH4 to the atmosphere. CH4 is a powerful GHG, 34-fold higher global warming potential than carbon dioxide (CO2) (IPCC and Stocker, 2013) and is produced by microbial communities in the final step of anaerobic digestion of organic substrates, like vinasse. Biological methanogenesis is the main pathway to CH4 production from a limited number of substrates, including hydrogen (H2), acetate, and some carbon compounds (Zinder, 1993), representing the primary biogenic source of CH4 in the atmosphere and a key contributor to climate change (Vanwonterghem et al., 2016).
It is well known that CH4 emissions generated by the disposal of agricultural residues are determined by many factors. With regard to vinasse, they include extreme anaerobiosis, high temperatures, substrate availability and the direct contact between vinasse, residue sediment and soil (Oliveira et al., 2015). There are many studies associating CH4 production with microbial community composition and abundance (Freitag and Prosser, 2009; Zeleke et al., 2013; Nazaries et al., 2013; Wilkins et al., 2015). However, despite the process of vinasse storage and transportation be an important source of CH4 emissions, there is a lack of studies evaluating the microbial community and abundance in these systems and also about the correlations between the microbes and CH4 emission. Thus, in situ measurements of environmental parameters, CH4 production and microbial community activities are challenging and crucial to better understand the underlying processes that drive this GHG production and so, to propose mitigations strategies.
Based on previous studies in which CH4 emissions in the systems composed by open channels were 620-fold higher than those observed from tanks and closed pipes (Oliveira et al., 2017), we suggest that these emissions are dependent on the structure and abundance of microbial communities and hypothesized that different environments in each system of vinasse storage and transportation, like substrate availability, redox potential and temperature, would harbor a distinct microbial community composition. To test this hypothesis, we combined high-throughput 16S rRNA sequencing with real-time PCR to evaluate how composition and abundance of microorganisms respond to the different systems of vinasse storage and transportation. Additionally, an incubation study was conducted to isolate the effects of sediment and vinasse on the microbial community. The data generated herein provides a description of the methanogenic community in an agricultural environment and can provide important information to mitigate CH4 emissions resulted from vinasse storage and transportation systems, thereby making sugarcane-based bioethanol a cleaner biofuel.
Section snippets
Study areas and field measurements
This work evaluated case studies covering the two most widespread systems of vinasse storage and transportation in Brazil. The study areas were located at two commercial sugarcane mills in São Paulo State, Brazil and represent the following systems: (i) system 1 - open coated and uncoated channels; and (ii) system 2 - coated tanks and closed pipes (Fig. 1). System 1 consisted of vinasse storage and transportation by channels, in which the first 40 km is coated with cement while the last 20 km
CH4 emissions
CH4 emissions observed at the field studies were positively affected by the type of vinasse storage and transportation system. In system 1, the CH4 emissions were consistently higher in the uncoated section of the channel (Table 1), with values in average 11-fold higher than that observed in the coated section. The CH4 emission in system 2 was significantly lower than in the system composed by open channels. Regarding the emissions observed in the incubation experiment, the treatment V + S
Discussion
In this study, we assessed for the first time the microbial community associated with CH4 emissions in the two most widespread vinasse storage and transportation systems used by the Brazilian sugarcane industry. It is known that anaerobic decomposition of vinasse emits large quantities of CH4 to the atmosphere, mostly when it is stored and transported using low-level technology such as open uncoated channels (Oliveira et al., 2017). Here, we provide a better understanding on the effect of
Conclusion
We concluded that different vinasse distribution systems shape the structure and diversity of the microbial communities, with direct effects on methanogen-related groups and, consequently, CH4 emission. We showed that the interaction of vinasse with the soil increases microbial diversity and abundance of methanogens, increasing CH4 emission. Based on our results, we suggest that better vinasse disposal management, such as the use of coated channels and tanks, has the potential to reduce CH4
CRediT authorship contribution statement
Bruna G. Oliveira: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft, Writing - review & editing. Lucas W. Mendes: Formal analysis, Writing - original draft, Writing - review & editing. Eoghan M. Smyth: Formal analysis, Writing - original draft. Siu M. Tsai: Conceptualization, Resources. Brigitte J. Feigl: Conceptualization, Methodology, Resources, Supervision, Funding acquisition. Roderick I. Mackie: Conceptualization, Resources, Writing - original draft,
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 authors would like to acknowledge professor Dr. Carlos Clemente Cerri “in memoriam” for important contribution in this research. They also would like to thank the São Paulo Research Foundation (FAPESP) for financial support (Grants: 2013/05597-5; 2012/05735-6; 2013/09377-0) and for the graduate scholarship (Process 2012/05735–6; 2013/09377–0) provided to B. G. Oliveira. This research was also supported by the Energy Biosciences Institute (EBI) (Grants: Project 002J14) at the University of
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