CHANGES IN MICROBIAL BIOMASS AFTER CONTINUOUS APPLICATION OF AZOLLA AND RICE STRAW IN SOIL

A model experiment was conducted under tropical conditions with a view to evaluating the changes in microbial biomass and nutrient dynamics in upland soil through the continuous application of azolla and rice s traw (2 g C kg(-1) soil per each application). Flush decomposition of C was observed immediately after each application and the rate of mine ralization did not change appreciably during this period. After hush d ecomposition, the rate of C mineralization from azolla was higher than that from rice straw until 9 to 13 weeks after each application and t hereafter the mineralization rate was similar, The amount of inorganic N released from azolla increased following each application, whereas inorganic N in rice straw plot was immediately immobilized and the rat e of immobilization increased until the 3rd application and did not in crease further after the 4th application, The amounts of biomass C and N increased immediately after residue incorporation, reached the maxi mum level one week after each application and declined thereafter. Max imum biomass formation increased until the 2nd application and then th e level remained constant, Maximum biomass N formation was higher in a zolla than in rice straw after the Ist application, but after repeated applications, the difference became less pronounced, Continuous incre ase in biomass in a certain week after each application was observed, probably because of the cumulative effects of the previous application s. The increase suggests that continuous application of organic materi als may enable to improve the amount of soil microbial biomass.