Development of compatible lignocellulolytic fungal consortium for rapid composting of rice straw
Introduction
Rice is the main staple crop in the world, where 661.811 million tons of rice was produced from 155.711 million hectares of land in 2008 (USDA 2009). Annually a large amount of straw is accumulated as a byproduct from rice cultivation, as straw makes up about 50% of the dry weight of the rice plant. Farmers do not incorporate rice straw in the crop field because of its slow degradation rate, disease infestation, unstable nutrients, and reduced yield caused by the short-term negative effect of nitrogen immobilization (Pandey et al. 2009). They usually dispose of it through open field burning. As a consequence, carbon dioxide, carbon monoxide, methane, nitrous oxide, and sulphur dioxide are emitted into the atmosphere. This process also emits harmful air pollutants such as polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), which have toxic properties and are, notably, potential carcinogens that can cause severe impacts on human health (Gadde et al. 2009).
Thus, proper management and disposal of bulky rice straw is a serious concern all over the world. Attention has been focused on nonhazardous, environment friendly, and sustainable techniques for safe disposal of rice straw in a short period of time. Microbial composting is an effective environmentally sound alternative for the recycling of rice straw into compost. It promotes sustainable agriculture and environmental protection, improving the soil’s physical, chemical, and biological properties (Perez-Piqueres et al., 2006, Rasool et al., 2008, Mylavarapu and Zinati, 2009), which ultimately results in better plant growth and yield.
Composting of lignocellulosic rice straw requires a process that ensures rapid biodegradation despite the fact that the lignin matrix shields cellulose and hemicelluloses from biodegradation. Naturally a few microbes have the potential to depolymerize lignin. Fungi have an advantage in the composting of lignocellulosic waste because they are filamentous and have the ability to produce prolific spores, which can invade substrates quickly. Moreover, mixed cultures can better influence colonization of the substrate through increased production of enzymes as well as resistance to contamination by other microbes. The most important determinant in mixed cultures is strain compatibility, which influences the organization, distribution, and density of the microhabitat population and the ecological balance of the communities (Gutierrez-Correa and Tengerdy, 1997, Molla et al., 2001). Hence, a compatible lignocellulolytic fungal consortium might play a vital role in the rapid disposal of rice straw. In order to address the foregoing issues, this study was undertaken to isolate, screen, and evaluate the compatible lignocellulolytic fungal consortium from ecologically related habitats for rapid and environmentally friendly composting of rice straw.
Section snippets
Isolation of lignocellulolytic fungi
In-situ samples were collected from several sources: decomposed rice straw and soils from dairy and goat farms and rice fields of the Universiti Putra Malaysia (UPM) and Kuala Selangor, Malaysia. In-vitro samples were obtained from different phases of a composting process conducted at Composting Unit UPM, Malaysia. Isolation was performed by the dilution plate method on potato dextrose agar (Difco, USA) and Trichoderma Medium E (TME) incubated at room temperature (28 ± 2 °C) for 7 days. The single
In-vitro lignocellulolytic activity
All of the 49 fungal isolates exhibited a different ability to degrade lignin and cellulose when tested on tannic acid and CM-cellulose-amended media, respectively (Table 2).
Isolate F44 produced a significantly (p ≤ 0.05) higher dark brown zone (84.71%) on tannic acid media. Development of a dark brown zone on tannic acid medium confirmed the polyphenol oxidase (PPO) activity of the fungal isolate. Polyphenol oxidase—a mixture of monophenol oxidase and catechol oxidase—catalyzed the reaction
Conclusions
A total of 49 fungal isolates were isolated from several rice straw compost sources. Ten isolates were selected based on their enzymatic degradation of lignin and cellulose and further screened on rice-straw-powder-amended media. Four isolates (F26, F28, F29, and F44) showing optimum lignocellulolytic activities based on their adaptability and ability to degrade lignin and cellulose in rice-straw-powder-amended media were evaluated for their in-vitro compatibility. A consortium of A. niger
Acknowledgements
We thank Universiti Putra Malaysia and the Ministry of Science, Technology and Innovation for providing the financial support for this Ph.D. project.
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