Fungi for improving the nutritional value of rice straw

Rice (Oryza sativa) is a staple food widely consumed by Indian community. To meet the ever increasing demand of growing population, its production needs to be increased by another 40 %. This will further add to the accumulation of rice straw, rice stubbles and rice husk. Presently, paddy residue constitutes about 51% of the 350 m.t/A of total crop residue produced in India. The management of this voluminous biomass is a great challenge experienced by farming community. Due to its bulky nature, farmers generally get rid of this residue by burning it in the field it self. The burning of rice straw not only leads to health hazards but also pollutes the environment due to emission of carbon dioxide, carbon monoxide, methane, nitrous oxide and aerosols. Burning paddy straw also leads to loss of useful microorganisms and insects present in soil thereby adversely affecting the soil properties. In addition, burning causes huge losses of N (up to 80 %), P (25 %), K (21%) and S (4-6%), thereby depriving the soil of its organic matter content. In order to stop the straw burning practice, there is need to adopt some alternative environment friendly strategy that may not only result in its effective management but also add some value to this undesired component.

Composition of rice straw

Rice straw contains high content of cellulose (31-35 %), hemicellulose (29-32 %) and lignin. Forty three percent of the total cellulose is in the crystalline form. The content of silica varies from 5-15 % depending upon the rice variety. The high content of silica and lignin bound cellulose reduces the biodegradability of rice straw as well as its digestibility by ruminants. Moreover, the poor crude protein content of 2-7 % (on dry weight basis) and high oxalate that reduces the absorption of calcium, makes it a less desired cattle feed. Rice straw differs from other cereals straw such as wheat, oat and barley in that it contains a relatively high portion of leaf (60 %) that has less neutral detergent fiber than stem and more ash and acid insoluble ash. This also leads to low in vitro dry matter digestibility. The low nitrogen, poor phosphorus and high C: N ratio, low sugar, amino acids and minerals content don't favor the growth of autochthonous microorganisms. The in situ incorporation of rice straw in to soil is also not preferred due to its slow degradation, disease infestation and reducing the availability of important mineral nutrients to growing plants through immobilization. All these attributes make it an undesirable residue.

Role of fungi in improving degradability of rice straw
To break down the cellulose, hemicellulose and lignin component of rice straw, the latter needs to be treated either with the specific enzymes (cellulases, hemicellulases and lignases) or inoculated with these enzyme producing microorganisms. Extracellular lignin modifying enzymes include lignin peroxidase, manganese dependent peroxidase, laccase (phenol-peroxidase), hydrogen peroxide producing peroxidase and glyoxaloxidase. There are certain white rot fungi that have the potential to decompose phenolic monomers and break the bonds with which lignin is cross linked to polysaccharides. These fungi include Pleurotus sajor caju, Cyanthus stercoreus, Trichoderma reesei, Penicillium fumicalosum, Phanerochaete chrysosporium, Phlebia radiata, Trametes versicolor, Polyphorus varsicolor, Aspergillus niger and Fusarium moniliforme. The complete oxidation of lignin leads to the formation of syringaldehyde, vanillin and ferulic acid. The final cleavage of these aromatic compounds yields organic acids, carbon dioxide, methane and water. Fungal treatment with Cyanthus stercoreus enhances the fiber digestibility by increasing the availability of cellulose. Rumen fungi Piromyces rhizinflata and Orpinomyces joyonii have the high potential to degrade rice straw. After the cleavage of lignin barrier, cellulose and hemicellulose are transformed to simple sugars by the action of cellulase and hemicellulase. These enzymes are complimentary to each other whose action facilitates the degradation of rice straw.

Composting of rice straw
Transforming the rice straw into high quality product as compost through fungal inoculation provides a prospective solution to its disposal problem. Aerobic decomposition of rice straw after its supplementation with nitrogen source and inoculation with lignocellulolytic fungi under controlled conditions of moisture results in the compost that is rich in nitrogen, phosphorus and humus content. Humic and fulvic acid (humic substances) act as permanent source of energy for microbial proliferation and regulate the carbon cycle. During composting, complex organic matter gets transformed into simple ones with the evolution of CO2 and energy. The mature compost on application to soil nit only adds organic matter to soil but also acts as slow release biofertilizer.
Application of lignin degrading enzymes originating from ligno-cellulolytic fungi is a promisingl biological treatment for improving the nutritive value of straw by selective delignification, but direct inoculation of fungi is much easier and more economical practice to convert straw into good quality manure. Composting of rice straw is not only the best way to recycle nutrients but also the simple and cost effective technology to protect the environment from pollution.

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