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Estimating Gross Nitrogen Mineralization and Immobilization Rates in Anaerobic and Aerobic Soil Suspensions

CSIRO Division of Soils, G.P.O. Box 639, Canberra 2601, ACT, Australia

P. M. Chalk and D. M. Crawford

School of Agriculture and Forestry, Univ. of Melbourne, Parkville 3052, Victoria, Australia

J. T. Wood

CSIRO Biometric Unit, G.P.O. Box 1666, Canberra 2601, ACT, Australia

* Corresponding author.

ABSTRACT

Zero-order models for estimating gross rates of N mineralization (m) and immobilization (i) of NH⁺₄ in soils were characterized by expressing the analytical solutions in a standard format. Three models of m were identified out of seven evaluated. One model was based on the solution of differential equations, and two on arithmetic mean data of ratios of labeled and total NH⁺₄ pools. Data from well-mixed anaerobic and aerobic soil suspensions amended with rice straw and labeled with (¹⁵NH₄)₂SO₄ were used to compare the three models of m and four models of i for 56 d, with measurements every 7 d. Estimated rates of m declined during incubation, particularly during the first 21 d. Correction of m for remineralization of immobilized ¹⁵N in the anaerobic system using the total organic N pool (OT) had little effect, but the correction was more pronounced when the estimated active organic N pool (OT*) was used. Remineralization did not account for the observed decline in m during anaerobic incubation. Numerical modeling and nonlinear parameter estimation were also used to estimate m and i, with values being similar to analytical estimates for Days 0 to 7 (m) and Days 7 to 14 (i). The OT* comprised 10 to 11% of OT in both suspensions. Estimates of OT* were derived from the numerical analysis (anaerobic), or by assuming approximate isotopic equilibrium between the NH⁺₄ (anaerobic) or NO^–₃ (aerobic) pools and the organic N pool following remineralization of immobilized isotope.

Received for publication July 1, 1993.

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