CONTROL OF DENITRIFICATION ENZYME-ACTIVITY IN A STREAMSIDE SOIL

Progress curve analysis of NO3- and NO2- reduction in surface soil samples from a streamside soil gave K(m) values of 4.24 and 6.33 muM, and V(max) values of 2.16 and 1.83 mumol l-1 min-1, respectively. Recoveries of reduced NO3- and NO2- as gaseous N averaged 82 and 108%. The unrecovered NO3-N was presumably dissimilated to NH4+-N. The denitrification enzyme activity (DEA) was examined throughout a year and showed seasonal and spatial variabilities of only 10% to 26%, suggesting a high persistency of denitrifying enzymes. Soil moisture and DEA correlated significantly (r = 0.7671; P < 0.01). The DEA in saturated subsoil also showed a relatively little variation, with spatial variabilities of between 28 and 38%. Amendment with NO3- rarely enhanced the activity more than two-fold at either depth. Addition of glucose increased the activity 2.3 and 2.5 times in the surface soil and subsoil respectively, indicating a moderate carbon limitation of denitrification. The activation energy of DEA was found to be 64.9 kJ mol-1 and Q10 values for the 2-12-degrees-C and 12-22-degrees-C temperature ranges were 2.71 and 2.53, respectively. Extrapolation suggested there would be a 4.4.fold increase in DEA if the temperature was changed from 0 to 15-degrees-C. Substrate diffusion limited the denitrification 10 to 25 fold. Thus, under anaerobic moist conditions it appears that changes in denitrification might primarily be due to varying diffusion of substrates into the anaerobic soil centers. Over a year, fluctuations in DEA, temperature changes and fluctuations of electron-acceptor and -donor supply will only have a minor effect on natural denitrification activity.