Abstract:
Several commercial available namely Agromax, B-2, F, Kilodor have been tested for their efficiency of decomposing the rice straw. The difference of the results showed statistically insignificant in rate of decomposing (C/N ration) the rice straw those treated with the biofertilizers and the control, which implied that the microorganisms present in those biofertilizers are not suitable for decomposing rice straw under the conditions tested. In this study, we intended to isolate and selects some fungi with high ability to decompose the rice straw under local conditions. Two hundred and eight strains of fungi have been isolated from various samples of decomposed rice straw. Among those isolates, Aspergillus sp. (A-8), Aspergillus sp. (B-25) and Humicola sp. (H-30) were selected. The selection based on their ability to produce cellulose and xylanase when cultivate in rice straw at 45℃ for 5 days while Trichoderma viridae QM 9414 (cultivated at 30℃) was use as standard strain. The comparative studies of the three strains showed that Aspergillus sp. (A-8) produced the highest amount of cellulose and carboxymethy1 cellulase (1.22x10⁶ and 1.88 x 10⁶ units/gm rice straw) while Aspergillus sp. (B-25) was the best B-glucosidase producer (3.08x10⁴ units/gm rice straw) and Humicola sp. (H-30) produced the highest amount of xylanase (1.99x10⁷ units/gm rice straw). The morphological and physiological characteristics of Aspergillus sp. (A-8) was found similar to Aspergillus fumigatus Fresenius, while Aspergillus sp. (B-25) similar to Aspergillus flavipes, and finally, Humicola sp. (H-30) similar to Humicola lanuginosa. When the fungi were cultivated on 1: 1 ratio of 5 mm. rice straw and the fine rice bran, the maximal number of spore produced by Aspergillus sp. (A-8) and Aspergillus sp. (B-25) were 4.98x 10² and 2.02 x 10¹¹ spores/gm of substrate respectively while the maximal number of spore produced by Humicola sp. (H-30) was 1.55 x 10¹¹ spores/gm of substrate. In case of Humicola sp. (H-30) the optimal ratio fo rice straw : fine rice bran in the substrate was 1: 9. The experiments performed in glass jar (7 cm. in diameter, 24 cm. in hight) showed the difference (statistically significant) in the rate of decomposing rice straw treated with either single or the mixer of two strains to the untreated control group with the best result from those of single strain treatment. Among various sources of inorganic nitrogen tested, ammonium nitrate was proved to be the best, thus, the optimal concentration of ammonium nitrate in the range between 1.5 5.0 % will enhance the rate of decomposing and the CO₂ evolution of rice straw treated. Further studies of large scale decomposition when straw were allowed to decompose in round cement tanks (80 cm. in diameter, 70 cm. in hight), again a higher rate of decomposition when rice straw were allowed to decompose in the presence of either Aspergillus sp. (A-8) or Aspergillus sp. (B-25) than untreated sample. In addition, it was found that number of fungal cell in the decompose straw increase with time of incubation suggesting that these fungi are capable of growing and competing for nutrition with other microorganisms presence in culture. The results reported above suggest that Aspergillus sp.(A-8), and Aspergillus sp. (B-25) might be the promising strains to be used as biofertilizer for decomposing the rice straw. Further investigations especially using these fungi for larger scale decomposition and the field tests need to be continued.