Accumulation of Organic Matter and Soil Nitrogen Beneath A Plantation of Red Alder and Douglas‐Fir

Abstract Soil nitrogen accumulation beneath a plantation of red alder and Douglas‐fir was compared with that under a pure fir segment of the same 30‐year‐old stand on the Wind River Experimental Forest in southwestern Washington. Beneath the mixed plantation, there were significantly greater amounts of nitrogen in the forest floor and in the upper 24 inches of the mineral soil. Organic matter content in the upper 12 inches of mineral soil was also greater and bulk density at 0 to 3 inches was significantly less. Beneath the alder‐fir stand, the carbon‐nitrogen ratio was less, both in the forest floor and at 0 to 3 inches. There were 938 pounds more N per acre to a depth of 36 inches under the alder‐fir plantation. Consequently, an average of 36 pounds more soil N per acre per year has accumulated beneath the mixed stand than under pure fir.