Four centuries of soil carbon and nitrogen change after stand-replacing fire in a forest landscape in the western Cascade Range of Oregon

Episodic stand-replacing wildfire is a significant disturbance in mesic and moist Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests of the Pacific Northwest. We studied 24 forest stands with known fire histories in the western Cascade Range in Oregon to evaluate long-term impacts of stand-replacing wildfire on carbon (C) and nitrogen (N) pools and dynamics within the forest floor (FF, Oe and Oa horizons) and the mineral soil (0-10 cm). Twelve of our stands burned approximately 150 years ago ("young''), and the other 12 burned approximately 550 years ago ("old''). Forest floor mean C and N pools were significantly greater in old stands than young stands (N pools: 1823 +/- 132 kg.ha(-1) vs. 1450 +/- 98 kg.ha(-1); C pools: 62 980 +/- 5403 kg.ha(-1) vs. 49 032 +/- 2965 kg.ha(-1), mean +/- SE) as a result of significant differences in FF mass. Forest floor C and N concentrations and C/ N ratios did not differ by time since fire, yet potential N mineralization rates were significantly higher in FF of old sites. Old and young mineral soils did not differ significantly in pools, concentrations, C/ N ratios, or cycling rates. Our results suggest that C and N are sequestered in FF of Pacific Northwest Douglas-fir forests over long (similar to 400 year) intervals, but that shorter fire return intervals may prevent that accumulation.