Relationship between microbial biomass nitrogen and nitrogen availability index

This study examined the microbiological basis for the anaerobic N mineralization assay, a technique that has been widely used in studies of N availability in forest soils. It was hypothesized that N mineralized using the anaerobic incubation method came mainly from killed microbial cells, similar to the flush of N obtained from the chloroform fumigation-incubation method for determining microbial biomass. Soil samples, representative of a wide range of forest types in Oregon, were analyzed for the amount of NH4+ released after a 7-d anaerobic incubation and by the standard chloroform fumigation-incubation method. A second experiment was done using the same soils treated with 15NH4Cl, and the isotopic labeling of the NH4+ released by both methods was determined. There was a highly significant correlation between both the amount and atom % 15N of NH4+ released by the two procedures. These results suggest that the anaerobic N mineralization assay measures N released primarily from the microbial biomass