Comparison of denitrifying communities in organic soils: kinetics of NO−3 and N2O reduction

Our aim was to determine whether intrinsic differences in the denitrifier communities existed in farmed organic soils from three different sites (Germany, Sweden and Finland) on which field fluxes of N2O had been measured continuously over 2 y. To estimate enzyme kinetic parameters (i.e. Vmax and Km) for N2O reductase, NO−3 was first removed by a combination of soil washing and anaerobic incubation. Then the samples were incubated anaerobically (as slurries) with or without added NO−3, N2O and C2H2. The estimated half saturation constants for N2O reductase were similar for all soils, and very low (Km=0.1–0.4 μM) compared to other investigations. In response to a prolonged anaerobic conditioning incubation (48 h), the Km values increased significantly and similarly for all soils, suggesting a shift in the dominant members of the denitrifier communities. The ratio between the estimated Vmax values for NO−3 reduction-to-N2O and N2O reduction-to-N2 was much lower for the Swedish than for the other two, indicating that the community of the Swedish soil would be more efficient in reducing NO−3 all the way to N2. This difference is congruent with differences in annual field fluxes. The results thus suggest that intrinsic differences in community composition of soils exist, with consequences for the emission of N2O. Prolonged anaerobic incubation (48 h at 20°C) resulted in a convergence of the communities towards similar ratios between the two Vmax values, suggesting that the apparent intrinsic differences may disappear in response to such severe treatment. Thus, the persistence of such patterns may depend on the drainage capacity of the soils. The results indicate that qualities of the denitrifying community must be taken into account when trying to understand and to model field fluxes of N2O from soils. They also illustrate how global trace gas emissions can be affected by changes in the community compositions of soils.