Soil Nitrogen Dynamics following Harvesting and Conversion of Red Alder and Douglas Fir Stands

Drastic reductions in NO 3 ‐ leaching have been observed after harvesting of mature red alder ( Alnus rubra Bong.) stands. Our objective was to examine whether these reduction were linked to changes in soil N dynamics. Adjacent alder and Douglas fir [ Pseudotsuga menziessii (Mirbel.) Franco] stands on young glacial soils (Alderwood; a loamy‐skeletal, mixed, mesic, ortstein Aquic Haplorthod) in western Washington were harvested and replanted with either alder or Douglas fir seedlings; reference plots were established in nearby undisturbed stands. Three years after site conversion, when NO 3 ‐ leaching declined most drastically in the harvested alder plots, net N mineralization and net nitrification in the upper soil were determined seasonally by in situ incubation using the buried‐bag technique. There were no significant increases in soil NH 4 ‐N during incubation, indicating that nitrification rates closely followed net N mineralization in all plots. In the N‐poor soils of the harvested and uncut Douglas fir plots, nitrification was consistently <2 µg N g −1 per 30 d. In the N‐rich alder reference plot, nitrification was highest in summer (52 µg N g −1 per 30 d), lowest in winter (4 µg N g −1 ), and intermediate in spring (20 µg N g −1 ). Seasonal fluctuations in nitrification diminished in the harvested alder plots. Nitrification rates were significantly lower than in the uncut alder plot in summer only (5–9 µg N g −1 per 30 d), but this difference was insufficient to account for the lower NO 3 ‐ leaching rates in the conversion plots. Soils in the harvested plots were drier and experienced greater temperature extremes, but these changes did not correlate well with the variability in N‐mineralization rates between uncut and harvested alder plots. Nitrification was positively correlated to soil temperature in the uncut alder plot only. The establishment of red alder seedlings did not appear to have influenced the N dynamics in either N‐poor or N‐rich soils.