Decomposition of cellulose and chitin substrates plays an important role in ecosystem functions. The relationships between the microbial diversity and the ecosystem functions of decomposition and antagonistic processes were examined in this study using rice straw. A microcosm was designed for the purpose of investigating complexity in microbial interactions of 16 different microorganisms belonging to 6 different functional groups and were designated as generalists or specialists depending on the number of hydrolases produced. Multiple regression analysis showed that increasing number of isolates, the presence of three fungal and one bacterial generalists as well as two fungal specialists contributed significantly to the decomposition of rice straw. Straw decomposed by one fungus and one bacterium classified as specialists, three fungal and two bacterial generalists and the 16-isolate mixture significantly  inhibited growth of the phytopathogenic Rhizoctonia solani on the decomposed straw.The growth of R. solani was reduced significantly by 62% - 93% compared to that in the non-inoculated controls. There was a negative correlation between rice straw weight loss and the growth of R. solani. The results from this study suggest that generalist species may play a more important role in ecosystem functions of decomposition and antagonistic processes compared to that by the specialist microorganisms.