Adding value to home compost: Biopesticide properties through Bacillus thuringiensis inoculation
Graphical abstract
Introduction
In the last years, home and community composting have arisen as strategic tools for the self-management of organic waste generated in households becoming a tool for the urban sustainable development (Karkanias et al., 2016). Home composting is especially encouraged in low population or rural areas, where through efficient home composting programmes has had a positive impact on promoting the active participation of citizens in waste management also increasing environmental awareness (Vázquez and Soto, 2017). In fact, into the integrated and hierarchic waste management strategy at local territorial levels, the combined option of segregating domestic biodegradable waste at the source and directly destining it to home composting has been pointed as a prevention action contributing to reduce the generation of household waste (Tatàno et al., 2015). The suitability of home composting has been demonstrated from technical and environmental point of view (Storino et al., 2016, Colón et al., 2010). Studies analysing the most environmentally friendly waste management systems have included home composting in the technologies with lowest environmental impact (Edwards et al., 2018) as home composting reduce transport total cost and gaseous emissions by diverting the organic fraction from the general waste and environmental savings from substituting fertiliser and peat with compost. Generally, compost obtained from source-selected organic waste is considered high-quality compost, particularly by their low heavy metal content (Huerta-Pujol et al., 2011). Technically, home composting produces stable and mature compost compared with compost from industrial facilities (Barrena et al., 2014).
The positive effects of compost application on soil (soil improvement, fertility, plant nutrition, etc.) are well understood. However, the value of this end product could be improved by enhancing its properties, such as the natural ability to suppress plant pests and diseases. Compost is rich in fungal and bacterial species and it has been reported having a suppressive capacity due to the antagonistic action of their microbial communities (Suárez-Estrella et al., 2013). Its use in biological control of plant pathogens has been widely studied (Doyle, 2017). However, the use of compost as a suppressive agent has not been extended because this capacity is complex and not completely understood, hindering to control the ability to predict its effectiveness in controlling plant pest or diseases (Doyle, 2017). The enrichment of composts with specific biological control agents (BCAs) has been also reported to enhance the suppressive effect of compost (López-Mondéjar et al., 2011) becoming a good alternative to peat in greenhouse nursery.
Compost with added biopesticide properties can be applied to the soil to prevent pests affecting plants at root level (López-Mondéjar et al., 2011) or through extraction as compost tea to deal with other insect pests affecting plant leaves (St Martin, 2014). However, neither the use of compost tea nor the compost itself as a biocontrol agent is completely satisfactory. The variable levels of biopesticides’ efficacy of disease control difficult their use; however, there is still a need to find sustainable and environmentally friendly alternatives to chemical pesticides for pest control and, in this approach, the use of compost tea has been suggested as part of an integrated disease management system (St Martin, 2014).
Bacillus thuringiensis (Bt) is a gram-positive bacterium naturally present in soil and it is the most widely used microbial biopesticide (Chandler et al., 2011). Bt produces a crystal protein during bacterial spore formation, which presents toxicity by ingestion to many insect pests of the orders Diptera, Coleoptera and Lepidoptera (Chandler et al., 2011). Bt is traditionally produced by submerged fermentation (SmF) but an emergent alternative is solid-state fermentation (SSF). SSF is a technology that allows the valorization of organic solid substrates for their biotransformation into bioproducts, turning waste into resources in line with the concept of circular economy (European Commission, 2019). Ballardo et al. (2017) showed the viability to produce biopesticide using non-sterile organic fraction of municipal solid waste (OFMSW) as the main substrate by SSF. The cost of production by this method could represent an improvement to the SmF process because it allows the use of an organic solid waste as a growing media. Rodríguez et al. (2019) used digestate obtained from OFMSW to growth Bt through SSF in different reactors under controlled aeration conditions and different temperature regimes with volumes ranging from 10 to 100 L.
The application of molecular biology techniques to investigate the diversity of compost microbial communities has shown the potential of the compost as a source of interesting and useful microorganisms (Jurado et al., 2014). These techniques can help widening the knowledge on microbial communities providing a better understanding of the relationships between microbial abundance and diversity.
Previous works (Ballardo et al., 2017) demonstrated the feasibility of Bt growth on non-sterile OFMSW in 10L scale adiabatic SSF reactors. An inoculation and operation strategy was proposed to overcome the effect on Bt of the temperature increment occurring as a consequence of heat released due to the metabolic activity of the biomass present. In brief, the reactors were inoculated with Bt just after the thermophilic period of the SSF process, when the system returned to temperatures around 30 °C. At the end of the SSF process, the final material in the reactor was used to inoculate other reactors by mixing it with OFMSW (in a 1:1 w:w ratio). On the light of these results, the objective of the present work was to determine the viability of producing Bt-enriched quality home compost from biowaste using an inoculum produced by SSF under non-sterile conditions thus defining a novel procedure for the local management of biowaste obtaining a product with a double benefit, resulting in a simultaneous reduction of the use of fertilizers and chemical pesticides. The present work also aims to answer two significant questions: (i) will the inoculated Bt affect the microbial communities present in the home composting bins and in the final products? (ii) can compost quality be ensured despite the presence of the added microorganism?
Section snippets
Materials
Fruit and vegetables leftovers (FVL) from different shops and markets were used to feed home composting bins mixed with pruning waste (PW, bulking agent) in a 1:1 volumetric ratio. FVL were collected and used the same day or the day after (in that case, they were stored at 4 °C). Pruning waste was collected at the beginning of the experiment and stored outdoors. It consisted of crushed tree pruning residues (sieved to 2–4 cm) from green areas of the university campus.
A commercial strain of Bt
Inoculation phase
Solid Bt inoculum was obtained following the operational strategy described in Ballardo et al. (2017) briefly described in the Introduction section. Bt inoculum was prepared in 50L adiabatic reactors using the final solid product as inoculum for a 400 L home composting bin (see Materials and Methods chapter above and Fig. 1). Viable Bt cell content of 1.3 × 107 ± 2.2x106 CFU g−1 DM and spore count of 5.7 × 107 ± 3.0 × 105 CFU g−1 DM were the values determined in this inoculum.
Process evolution
Table 1 and Table 2
Conclusions
An effective strategy has been applied to produce Bt-enriched compost from home composting of green and vegetable waste in a commercial home composting bin using a solid Bt inoculum. In parallel, a home composting bin without inoculation was run as control process. No differences among compost from both composters were found, fulfilling quality standards, being humidity between 33 and 38%, organic matter content of 60% and C/N ratio of 13. Appropriate levels of stability (DRI < 1 g O2 kg−1 OM h
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.
Acknowledgments
The authors thank the Spanish Ministerio de Economía y Competitividad, which gave financial support (Project CTM2015-69513-R) to this work, the Consorci del Bages per a la Gestió de Residus for the waste supply, and Dra. M. Àngels Calvo Torras from the Department of Animal Health and Anatomy of the Universitat Autònoma de Barcelona for her advice on Bt identification. Cindy Ballardo thanks the Peruvian Government for the President of the Republic Scholarship (PRONABEC).
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