Effect of microbial inoculation on physicochemical properties and bacterial community structure of citrus peel composting

Highlights

• The microorganism inoculant was inoculated in the citrus peel composting.

• The fertilizer efficiency of pilot-scale composting was better than lab-scale.

• The inoculation improved the richness and diversity of the bacterial community.

• Bacillus and Sphingobacterium in inoculum were dominant genus during the composting.

• Key factors accounting for bacterial community dynamics were identified by redundancy analysis.

Abstract

In this study, microorganisms were inoculated during citrus peel composting for citrus waste recycling and valorisation. The physicochemical properties and the bacterial community structure of citrus peel composting inoculated microorganism were studied. The thermophilic stage of pilot-scale composting (T2) was 20 days longer than lab-scale composting (T1). C/N, organic matter, moisture, pectin and cellulose content decreased along with composing process, but the pH, soluble protein and total nutrient showed an opposite trend. The inoculation improved the richness and diversity of the bacterial community and the diversity index reached maximum on 21 days. As composting progress, Bacillus, Sphingobacterium and Saccharomonospora in inoculum became the dominant genus. Redundancy analysis showed that C/N, pectin degradation rate and temperature could explain 30.1%, 24.9% and 15.6% of the variation in bacterial genera, respectively.

Introduction

Citrus is one of the most important and abundant economic crops in the world (Lv et al., 2015). Chongqing is the main citrus producing area in China, and the annual production of citrus is approximately 2.43 million tons. After processing, the citrus industries produce a large amount of citrus peel (CP), which represent over 50% of the total processed fruits (Sharma et al., 2017). CP typically shows high organic matter (around 95% of total solid), high water content (around 80–90%) and low pH (3.5–5.8), which result in CP highly perishable and easily fermentable (Bernal-Vicente et al., 2008). However, the large amounts and particular characteristics of CP cause serious environmental and economic problems, due to unauthorised disposal and the high disposal cost (around 200–600€tonne⁻¹ for non-hazardous management (Ruiz and Flotats, 2014). In general, the main destination of CP is incineration, dump storage, cattle feed (Torre et al., 2019). But those traditional CP disposal strategies have heavy impacts on environment (air, water and soil) because of uncontrolled fermentation, boosting the production of greenhouse effect gases and the leachate rich in organic matter (Arukwe et al., 2012). Nowadays, CP also can be processed in biorefinery industries, but still hindered by economic constraints (Zema et al., 2018). Therefore, with the increasing amount of CP, to develop an environmentally sound way of CP management has become a very urgent and difficult issue.

Composting is an useful utilisation for transforming CP into agricultural products (Gelsomino et al., 2013). Several studies reported that the addition of CP compost can increase the organic load of soils and avoid plant infection by pathogens (Siles et al., 2016, Van Heerden et al., 2002). Recently, co-composting of CP with other lignocellulose containing agricultural waste has been proposed to create products replacing the chemical fertilizer (Gelsomino et al., 2013, Lopez-Mondejar et al., 2010). However, because the quantity and biodegradability of the indigenous community in CP are insufficient and poor (Xi et al., 2015), a longer composting time is required to obtain mature product. Furthermore, large amount of organic acid is produced in the initial stage of composting which decreases the pH of the substrate (<5.0), leading to the inhibition of aerobic mincroorganism and undesired fermentation (Ruiz and Flotats, 2014). Currently, considerable attention have been focused on the bioaugmentation of microorganisms involved in the composting for accelerating composting process and promoting the compost quality (Jurado et al., 2014). Several studies reported that the inoculation can promote the agricultural and municipal solid waste biodegradation and increase the yield and quality of organic fertilizer (Sánchez et al., 2017, Xi et al., 2015). Nevertheless, to our knowledge, the study for microbial inoculation in CP composting is few.

Complex microbial communities play a key role in the composting (Xi et al., 2015). However, due to the competition between inoculation and indigenous microorganisms, the efficiency of the inoculation composting decreased (Nakasaki et al., 2013). Therefore, understanding the dynamic changes of microbial communities in the composting process is extremely important for successful inoculation composting. With the development of molecular tools, the high-throughput sequencing technologies (HTS) have been widely applied in the fields of composting, especially to analysing the composition and dynamics of bacterial communities during the composting (Chu et al., 2015, Silva et al., 2016, Tkachuk et al., 2014). On the other hand, the composting microorganisms are markedly influenced by temperature fluctuation, physicochemical components of composting substrate (such as pH, C/N ratio) and dissolved organic carbonand nitrogen (López-González et al., 2015, Xi et al., 2016). Nowadays, redundancy analysis (RDA) have been widely used to evaluate the correlation between microbial community composition and environmental factors, which can identify the composting factors that could influence the dynamic change of microbial communities (Zhang et al., 2011). However, the HTS and RDA analyses for composting CP are few, and the correlation between microbial community composition and environmental factors is unclear. It is important to study the relationship between microbial community structure and environmental factors for the recycling of CP composting inoculated microorganism, which making for recycling CP.

In this study, the microbial inoculum was isolated from thermophilic composting piles and inoculated in lab-scale and pilot-scale composting pile. The changes of physicochemical parameters between the two-size of piles were compared during the CP composting process. Secondly, HTS was used to analyze the diversity and structure change of bacteria in compost of the inoculated with microorganism. In order to optimize the process parameters and provide a theoretical basis for the mechanism of action of microorganisms during composting, the correlation between microbial community succession and physicochemical parameters was analyzed by RDA.