ESTIMATING MICROBIAL BIOMASS NITROGEN USING THE FUMIGATION INCUBATION AND FUMIGATION EXTRACTION METHODS IN A WARM-TEMPERATE FOREST SOIL

Microbial biomass-N was measured in the top 60 cm of acid soils on a forested slope using the fumigation-incubation and fumigation-extraction methods. Average microbial biomass-N was 54.4 and 68.5 g N m-2, respectively. Statistical differences between methods and depths were detected, but not between sites along the slope. Estimates of microbial biomass-N measured by the two methods were correlated only in samples taken from 0-20 cm (r = 0.77, P < 0.01). With both methods microbial biomass-N decreased rapidly with depth following a multiplicative model. Organic carbon showed a significant curvilinear relationship with microbial biomass-N measured in the 0-20 cm by the fumigation extraction method (r = 0.81, P < 0.001) and by fumigation-incubation (r = 0.72, P < 0.001). In a multiple linear regression with organic C, the pH of the samples improved the relationship with fumigation extraction values (multiple r = 0.82, P < 0.001) and with fumigation-incubation values (multiple r = 0.79. P < 0.001). but pll only explained 4.9 and 5.2% of the variance. When only the 0.20 cm samples are considered, only organic C explained a significant portion of the variance in the distribution of microbial biomass. The N in microbial biomass accounted for 3.3 and 2.6% of the total N in the soil when using the fumigation extraction and the fumigation incubation techniques, respectively. The results obtained using the easier fumigation extraction procedure seem more reliable and predictable by environmental factors than those obtained by fumigation incubation. © 1990.