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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

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

^a NR&M, 80 Meiers Rd, Indooroopilly, Brisbane, QLD 4068, Australia
^b CSIRO Land and Water, GPO Box 1666, Canberra 2601, ACT, Australia
^c Dep. of Resource Management and Horticulture, the Univ. of Melbourne, Parkville 3052, VIC, Australia

^* Corresponding author (weijin.wang{at}nrm.qld.gov.au).

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.

Abbreviations: AL, labeled NH⁺₄–N (i.e., fertilizer N in NH⁺₄) • AT, total NH⁺₄–N = AL + AU • AU, unlabeled NH⁺₄–N of soil origin • NL, labeled NO^-₃–N • nm_OT*, net N mineralization from the OT* pool • nm_OT, net N mineralization from the OT pool • NT, total NO^-₃–N = NL + NU • NU, unlabeled NO^-₃–N of soil origin • OL, labeled organic N (i.e., added fertilizer N in OT) • OT, total organic N • OT*, active organic N in soil at the time of each sampling during incubation • OT^#, active organic N of the original soil • Q_e, ¹⁵N abundance (atom %) of a N pool at the time of sampling during incubation • Q_f, ¹⁵N abundance (atom%) of labeled fertilizer N • WHC, water-holding capacity