Document: JON-3-59-109

Impacts of increased nitrogen deposition on nitrogen cycling and leaching losses of dissolved nitrogen from lodgepole pine and trembling aspen stands.

CARRASCO, J.J.* and C.A.WESSMAN

University of Colorado, Boulder, CO 80309 USA ¹

Abstract:
An on-going fertilization experiment is being conducted in a trembling aspen, Populous tremuloides, and lodgepole pine, Pinus contorta var. latifolia, stand in the Colorado Front Range. This project focuses on the impacts of increased N deposition within and between forest types and is relevant to understanding ecosystem and landscape level nutrient cycling dynamics. Background anthropogenic deposition to these stands is approximately 5 kg N ha^-1 yr^-1 and an additional 25 kg N ha^-1 yr^-1 is being applied to simulate increased deposition. Our objectives are to determine the impacts of ammonium nitrate fertilization on (1) nitrogen cycling within and between forest types and (2) leaching losses of nitrogen from the two forest cover types. Net N mineralization and nitrification measurements during the growing season were highest in the aspen organic horizon (net N mineralization: 9.6 ug g^-1 30 d^-1 and net nitrification: 4.1 ug g^-1 30 d^-1) while the lodgepole pine organic horizon immobilized nitrogen (-6.2 ug g^-1 30 d^-1m and 0.0 ug g^-1 30 d^-1). These differences are supported by significantly lower C:N ratios in the aspen soil (14.7, partly due to presence of nitrogen fixing species in the understory) than in the lodgepole pine soil (30.7) which leads to greater mobility of N in the aspen stand. Initial results from fertilization indicate that the aspen stand has a decreased capacity for N retention/immobilization than the lodgepole pine stand, resulting in a varied response of the montane forests in the Colorado Front Range to increasing N deposition.

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This abstract is being presented at: 2:30 PM in session:
Oral Session #28: Effects of N Deposition in Ecosystems.