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Nitrogen Incorporation and Flow Through a Coniferous Forest Soil Profile

Inst. of Arctic Biology, Univ. of Alaska, Fairbanks, AK 99775-0180

Mary K. Firestone

Dep. Plant & Soil Biology, Univ. of California, Berkeley, CA 94720

* Corresponding author.

ABSTRACT

One of the major controls on N cycling in forest ecosystems is the dynamics of N in the forest floor. Uptake and movement of NH⁺₄ by the microbial component of a mixed conifer forest soil in central California were examined by injecting ¹⁵NH⁺₄ into either the O2 or the A horizon. Distribution of the tracer was constrained by 15 x 30 cm cylinders placed in situ 1.5 yr prior to the experiment. The ¹⁵N was followed over 1- and 31-d periods to measure both the short-term uptake and the longer-term fate of N. Recovery of ¹⁵N was determined in coarse roots, coarse woody detritus, fine detritus, fungal strands, and the bulk soil in each horizon; microbial biomass ¹⁵N was determined in the A horizon only. Nitrogen-dynamics in the forest floor were characterized by a period of rapid microbial NH⁺₄-uptake after which transformations were slower. The rate of NH⁺₄ immobilization was approximately 200 mg m^–2 d^–1, giving a turnover time for the NH⁺₄ pool of less than 1 d. In the A horizon, there was no conversion of microbial ¹⁵N into soil organic ¹⁵N, but there was extensive lateral translocation of ¹⁵N from microbial biomass in the bulk soil into coarse dead roots and coarse detritus. There was little vertical translocation up the profile from either the O2 or the A horizons. This work indicates the rapidity of N turnover in the forest floor and suggests that N sequestering in woody residues may be an important fate of N in this forest soil.

Contribution of Dep. Plant & Soil Biology, Univ. of California, Berkeley. This work was supported by NSF grant BSR-83-06181 and McIntire Stennis project 4262 of the Univ. of California Agric. Exp. Stn.

Received for publication July 1, 1988.

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