Denitrification rates in organic and mineral soils from riparian sites: a comparison of N2 flux and acetylene inhibition methods

Denitrification rates were measured in relatively dry mineral soils and water-saturated organic soils by both direct N2 flux and acetylene inhibition methods to compare the relative sensitivity of the two methods and their applicability to different soil types and saturation conditions. The direct N2 flux method, previously used only in submerged sediments, was modified for use in terrestrial soils. Our experiments demonstrated that the N2 flux method is adaptable for both mineral and water-logged organic soils, and that denitrification rates based on direct N2 fluxes can be measured after only 2 days of incubation in most cases. We used a diffusive flux model to separate the portion of total N2 flux attributable to denitrification from that due to passive diffusion of atmospheric N2 from soil porespaces. Denitrification rate measurements were taken during spring, summer, and fall seasons using replicate cores from four different riparian sites (two with mineral soil, two with organic soil). Rates determined by N2 flux were 35–130 and 10–245 μmol N m−2 h−1 for mineral and organic soils, respectively. Those by acetylene inhibition, for the same sites were one, and sometimes two, orders of magnitude less with rates of 0.1–10 μmol N m−2 h−1 for mineral soil and 0.2–3 μmol N m−2 h−1 for organic soil. This study has demonstrated that denitrification measured by acetylene inhibition in riparian sites may be underestimated by at least an order of magnitude in both mineral and organic soils.