Comparison of methods for estimating soil nitrogen transformations in adjacent conifer and alder–conifer forests

Abstract

Nitrogen transformations were examined in two pairs of adjacent, 55-year-old forests dominated by conifers (primarily Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco)) and by conifers and nitrogen-fixing red alder (Alnusrubra Bong.). Nitrogen availability was examined in aerobic and anaerobic incubations in the laboratory, and with resin-core and buried-bag incubations in the field. Rates of nitrogen mineralization and immobilization were examined in the field incubations using [15N]ammonium. Net nitrogen mineralization in a year-long series of resin-core incubations of forest floor plus 0–0.15 m depth soil was about 9 kmol•ha−1•year−1 for the two alder–conifer stands, but almost nil in the conifer stand at the low-productivity Wind River site, and 2.1 kmol•ha−1•year−1 in the conifer stand at the high-productivity Cascade Head site. The 15N pool dilution experiment showed that buried-bag incubations demonstrated more differences among stands than did resin cores; resin cores typically gave greater rates than buried bags. Previous estimates of nitrogen budgets were coupled with net mineralization estimates to examine how well the estimated fluxes balanced at an ecosystem scale. This tabulation of the complete nitrogen cycles showed substantial discrepancies, prompting caution in interpretation of some of the differences among the stands.