Denitrification enzyme activity of Douglas-fir and Red Alder Forest soils of the Pacific Northwest

Nitrogen-fixing red alder (Alnus rubra Bong.) increases soil organic matter and N content of forest soils. This study compared denitrification enzyme activity (DEA) to related N-cycling and microbial indicators in adjacent stands of alder and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in two Pacific Northwest U.S.A. research forests over 16 months. Laboratory denitrification rates were measured in non-amended soils and soils amended with combinations of water, NO3−, and glucose. The NO3−-and glucose-amended soils provided estimates of DEA. DEA in alder soils was greater than or equal to that in corresponding Douglas-fir soils. Denitrification in alder soils was occasionally limited by energy source (glucose) but not by NO3−, whereas in Douglas-fir soils, it was frequently limited by both NO3− and glucose. For a given soil, DEA was generally not well related to respiration potential, anaerobic mineralizable N, or exchangeable ammonium over time, but it was well related to nitrification potential across different soils and over time within two soils.