Nitrogen Incorporation and Flow Through a Coniferous Forest Soil Profile

Abstract

AbstractOne 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 × 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.