MICROBIAL BIOMASS NITROGEN POOL IN SOILS FROM A WARM TEMPERATE GRASSLAND, AND FROM DECIDUOUS AND EVERGREEN FORESTS IN CHIBA, CENTRAL JAPAN

We evaluated the status of the microbial biomass N pool in grassland, and in deciduous and evergreen forest soils in Chiba, central Japan. M icrobial biomass N, a labile fraction of total N in the soil, ranged f rom 6.96 g N m(-2) (15 cm depth) in the grassland to 24.8 g in the dec iduous and 20.7 g in the evergreen soils, on a landscape basis. Thus t he pattern in the grassland and in the forest soils differed. The N fl ush measured by a fumigation-incubation method indicated that in the g rassland soil microbial biomass N was underestimated by a factor of 2. 6 compared with the results from a fumigation-extraction method, becau se of heavy N immobilization in the microbial biomass. This was in con trast to results from the forest soils, which did not immobilize N. Th us, the forest soils were in a steady-state condition compared with th e grassland which formed a seral phase in the ecological succession. S imple correlation coefficients indicated a significant positive relati onship between biomass N and organic C in the soil and the N concentra tion in the litter, the main component of organic matter in the soils of the three ecosystems.