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Factors Affecting Nitrogen Immobilization in Soil as Estimated by Simulation Models

Inst. of Soils and Water, ARO, The Volcani Center, Bet Dagan 50250, Israel

J. A. E. Molina

Dep. of Soil Science, Univ. of Minnesota, St. Paul, MN 55108

C. E. Clapp

USDA-ARS, St. Paul, MN 55108

* Corresponding author.

ABSTRACT

Factors that control immobilization of inorganic N in soils not amended with organic C-rich materials have not been fully established. Different N flow schemes for N immobilization were tested by comparing the kinetics of CO₂ evolution, NH⁺₄-N, NO^-₃-N, inorganic ¹⁵N, and organic ¹⁵N excess, measured in soils incubated with 200 mg kg^–1 of ¹⁵N-labeled NH⁺₄-N, with data simulated by the computer model NCSOIL structured to representseveral pathways of C and N turnover. Estimates of those parameters most likely to induce or enhance N immobilization, namely the mineralizable soil organic matter content, its C/N ratio, and the efficiency with which C is assimilated by the soil microbial biomass were obtained by optimization. Cumulative N immobilization was up to 10% of the N added and was greater when NH⁺₄ was added to dry soils than when added to preincubated soils. The immobilization was primarily due to the microbial biomass drawing N from the inorganic pool, rather than utilizing N directly from the decomposing soil organic matter. Nitrate formed by nitrification seemed to have also been immobilized. A microbial efficiency factor of 0.4 and a C/N ratio of 20 or more for mineralizable soil organic matter were needed to account for the observed rate of N immobilization.

Contribution from the ARO, The Volcani Center, Bet Dagan, Israel. no. 3320-E 1991 series. This research was supported by a grant from the U.S.-Israel (Binational) Agricultural Research and Development fund (BARD).

Received for publication August 6, 1991.

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