Microbial Biomass Nitrogen in Anaerobic Soil as Affected by N-Immobilization and N2-Fixation

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Abstract
Wet paddy soil was preincubated under anaerobic condition with glucose and either (15NH4)2SO4 or 15N2 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 15NH4 + was actively immobilized within the first day of preincubation. 15N2 was also actively fixed within the first day, followed by a slight fixation till 3 week of preincubation. In both 15N-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 NH4 + during the 4th week of preincubation had a similar 15N abundance to that of available N in the early stage of preincubation. The K N factor (proportion of microbial biomass N mineralized after fumigation-incubation) in anaerobic soil was calculated as 0.43.

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