Rice Straw Degradation with Mixed Cultures of Microfungi and Their Enzymes

Abstract

Huge quantities of agricultural residues are generated every year but it is neither converted into energy nor allowed to go back to the soil and sometimes burnt that leads to air pollution and loss of soil biology and fertility. Cellulolytic microfungi secrete extracellular enzymes that degrade lignocellulosic biomass in nature and this ability of microbes may be exploited to enhance the rates of degradation of agriculture residues to recycle carbon, nitrogen and minerals in the soil. This study aimed to find out the effect(s) of co-cultivation of high cellulolytic microfungi in various combinations on rice straw under field conditions to enhance its microbial decomposition to discourage the farmers from burning it in the fields. The effectiveness of five fungal cultures in different combinations was tested for efficient degradation of rice straw. Five dominant species of fungi that have been earlier shown to secrete high amounts of cellulases in our own laboratories, were cultured on medium containing yeast powder (2g/L), jaggery (5g/L) and urea (1g/L) at room temperature. Release of reducing sugars from 1 g of rice straw treated with 10IU/mL of fungal extracellular enzymes showed that Penicillium chrysogenum released highest amount of mono and oligomers (96 mg/g) followed by Aspergillus flavus (80 mg/g) and A. oryzae (78 mg/g), A. fumigatus (72 mg/g) and Trichoderma viride (70 mg/g) within 24 h that increased with increasing temperature and increasing period of incubation. Treatment of rice straw with fungal cultures of Penicillium chrysogenum, Aspergillus flavus and A. oryzae revealed that the co-inoculation of all these species decomposed approximately 75% of the total rice straw as assessed by weight loss method. The application extracellular secretory enzymes on rice straw, though, revealed to release of reducing sugars, but the rate of reducing sugars was not sufficient enough to be used for degradation of rice straw under field conditions, hence, the used of mixed cultures of microfungi was planned and tested experimentally, which allowed the decomposition of rice straw much faster than control which were treated with heat-killed dead cultures of test microfungi. Hence, it is recommended that additional spray of mixed cultures of microfungi on rice straw may facilitate its degradation under field conditions.