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 m atter and N content of forest soils. This study compared denitrificati on enzyme activity (DEA) to related N-cycling and microbial indicators in adjacent stands of alder and Douglas-fir (Pseudotsuga menziesii (M irb.) France) 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 Dougla s-fir soils. Denitrification in alder soils was occasionally limited b y energy source (glucose) but not by NO3-, whereas in Douglas-fir soil s, it was frequently limited by both NO3- and glucose. For a given soi l, DEA was generally not well related to respiration potential, anaero bic mineralizable N, or exchangeable ammonium over time, but it was we ll related to nitrification potential across different soils and over lime within two soils. (C) 1998 Elsevier Science Ltd. All rights reser ved.