Relationship Between Microbial Biomass Nitrogen and a 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 NH + 4 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 15 NH 4 Cl, and the isotopic labeling of the NH + 4 released by both methods was determined. There was a highly significant correlation between both the amount and atom % 15 N of NH + 4 released by the two procedures (r 2 = 0.81 and 0.82, respectively). These results suggest that the anaerobic N mineralization assay measures N released primarily from the microbial biomass.