Relationship between soil microbial biomass and gross N mineralisation

The gross rate of N mineralisation was measured under field conditions over 12 mo using the 15N pool dilution technique. Significant differences in the mineralisation rate were found during the year, with an average rate of 610 ng N g−1 soil d−1. Changes in the gross mineralisation rate were not reflected in the size of the soil microbial N pool, which remained essentially stable over 12 mo. Attempts to determine the contribution of microbial N to gross mineralisation using 15N isotope methods proved unsuccessful as we could not uniformly label the microbial N pool with 15N in the field. The data suggested that only a fraction of the microbial biomass was involved in N mineralisation. Soil moisture and temperature were the most important factors influencing the gross rate of mineralisation.