MICROBIAL BIOMASS NITROGEN IN ANAEROBIC SOIL AS AFFECTED BY N-IMMOBILIZATION AND N_2-FIXATION

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Wet paddy soil was preincubated under anaerobic condition with glucose and either (^<15>NH_4)_2SO_4 or ^<15>N_2 for 4 weeks and the changes in the amount of microbial biomass N were estimated by the chloroform fumigation method, followed by submerged incubation for 4 weeks. Added ^<15>NH_4^+ was actively immobilized within the first day of preincubation. ^<15>N_2 was also actively fixed within the first day, followed by a slight fixation till 3 week of preincubation. In both ^<15>N-treatments, the contents of available N (N mineralized after chloroform fumigation during 28 days of submerged incubation) and labelled available N increased in the early stage within the first week of preincubation, then decreased until the end of the preincubation. On the other hand, the amount of labelled min-N (N mineralized in unfumigated soil during submerged incubation) remained almost constant throughout the preincubation period. Remineralized NH_4^+ during the 4th week of preincubation had a similar ^<15>N abundance to that of available N in the early stage of preincubation. The kN factor (proportion of microbial biomass N mineralized after fumigation-incubation) in anaerobic soil was calculated as 0.43.
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