Respiration, nitrogen fixation, and mineralizable nitrogen spatial and temporal patterns within two Oregon Douglas-fir stands

Substrate respiration, mineralizable nitrogen, and nitrogen fixation rates, substrate moisture content, and temperature were measured in trenched and undisturbed plots within two western Oregon Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands. The stands represent two different environments and ages. Woods Creek, the site of the lower elevation mature 70-year-old stand, is located in the Willamette Valley near Philomath, Oregon. The H.J. Andrews Forest, site of the higher elevation old-growth stand, lies on the western slopes of the Oregon Cascades. Mineralizable nitrogen rates were 1.3 times higher at Woods Creek than at the H.J. Andrews Forest; nitrogen fixation was 1.3 times greater at Woods Creek than at the old-growth H.J. Andrews stand. Litter evolved more CO2 and yielded more than 3 times the mineralizable nitrogen rates of logs and soils. Woods Creek logs had significantly higher nitrogen fixation than mineral soils (p < 0.001); there was greater nitrogen fixation in logs and soils sampled at 0n4 cm than at 4n20 cm (p < 0.03). At the old-growth H.J. Andrews Forest site, nitrogen fixation was significantly greater in logs than in soils (p < 0.001). Nitrogen fixation tended to be higher in samples from 0 to 4 cm within logs and mineral soils than in samples from 4 to 20 cm. There were no statistical differences in mineralizable nitrogen at the H.J. Andrews Forest between logs and mineral soils either by type or by depth at the p < 0.05 level. Neither site yielded a statistically significant difference in mineralizable nitrogen between trenched log and soil plots and nontrenched log and soil plots. Mineralizable nitrogen declined with depth, but this pattern was only statistically significant at Woods Creek (p < 0.005).