Denitrification potential and reduction enzymes dynamics in a Norway spruce plantation

As part of a programme aimed at understanding controls of the N cycle and possible effects of increased N depositions in a soil under a Norway spruce plantation, the denitrification potential and the kinetics of NO3−, NO2−, N2O and N2 were investigated. Intact soil cores were conditioned for 14 days at 25°C, sieved and amended with 20 mg NO3−N kg−1. Treatments were with or without C2H2 and with or without chloramphenicol (found to inhibit de novo synthesis of reduction enzymes). Samples were purged of O2 and anaerobically incubated at 25°C for 96 h while CO2 and N2O production and NO3− and NO2− concentrations were monitored. Chloramphenicol did not affect the CO2 production which decreased by nearly 50% under anaerobic conditions. Differences in NO3− concentrations between chloramphenicol and unamended soil were only detectable after 48 h while differences in NO2− concentrations were only measurable after 70 h. Nitrous oxide production in the chloramphenicol-amended and unamended soils was comparable, 1.3 and 2.6 mg N kg−1, respectively, for the first 38 h while no N2 was produced in either treatment over the first 15 h. The production of N2 was only 0.6 mg N kg−1 after 96 h in the chloramphenicol-amended soil but it was the sole gaseous product of denitrification after 70 h in the unamended soil. The kinetics of NO3−, NO2− N2O and N2 were satisfactorily described with a model, DETRAN, assuming a competitive Michaelis-Menten kinetic and an uptake of NO3− by micro organisms that was not directly matched by a reduction of NO3−. The uptake of NO3− was higher (ca. 2 mg NO3−N kg−1) than the release of reduction products NO2− and N2O for the entire incubation in the chloramphenicol-amended soil but only for the first 70 h in the unamended soil. Concentrations of NO3−, NO2− and N2O reductase appeared to increase by 4-fold between 20 and 30 h after the onset of anaerobiosis when the affinity for NO3− was set at 1 and the affinity for NO2− was between 120 and 160 and for N2O between 1.2 and 2.6. Several factors of C and N dynamics in these experiments are compared with those made in soil from a poorly drained pasture. It is concluded that the microbial community in the well-drained forest soil described here was not well pre-adapted to periodic anaerobiosis.