Insight into the Active Organic Nitrogen Pool Estimated by Isotopic Equilibrium Approaches

A method that reliably estimates the size of the active organic N pool has been desired for predicting N mineralization dynamics. Among the methods proposed to date, the isotopic equilibrium approaches characteristically use N mineralization-immobilization turnover processes to label the biologically labile organic N. In the present study, the implications and usefulness of the active organic N pool estimated with this approach were assessed in a long-term incubation. 15N-labeled NH+4 and glucose were added to soil, and the dynamics of the labeled N in the organic and inorganic phases were monitored by periodic sampling during the incubation. After appropriate revisions, two equations from the literature were used to estimate the active organic N in soil at Time 0 (OT#) and at each sampling during incubation (OT*), respectively. Both OT# and OT* tended to increase with incubation time and were affected by the amounts of labeled N and available C addition. Considerable amounts of mineralized N came from other than OT* fraction of soil organic N, but N mineralization from the OT* pool was much faster than from the bulk soil organic matter. The results supported the concept that soil organic N exists in a continuum of degradability. If the operational procedure is standardized, however, OT# can be used as a relative measure of the active organic N in soil.