Laboratory evaluation of a new method for in situ measurement of denitrification in water-saturated soils

In this study, we present and test a new method to measure in situ denitrification in water-saturated subsoils. The procedure consists of applying a test solution containing 15NO3− or acetylene to the subsoil and subsequently monitoring dissolved denitrification products to quantify denitrification activity. Computer simulations predicted a period of linear increase of dissolved denitrification gas concentrations during several hours, which was influenced by total and entrapped air porosity, volume of gas producing soil and gas diffusion coefficient, but was independent from denitrification activity. A series of monolith experiments was performed in order to evaluate the method under laboratory conditions. Subsoil probes were used to apply test solutions to water-saturated soil cylinders and to collect liquid samples that were analyzed for the dissolved denitrification products N2O and 15(N2+N2O), and for NO3−. Measured concentrations of dissolved denitrification gases increased during at least 23 h. In the treatments without acetylene maximum rates of dissolved 15(N2+N2O) were observed in the beginning of the experiments, whereas treatments with acetylene exhibited a time lag of several hours before maximum rates of dissolved N2O and 15(N2+N2O) occurred. Denitrification rates calculated from maximum rates of 15(N2+N2O) and N2O as well as from NO3− disappearance agreed satisfactorily in the treatments with acetylene. It was concluded that N2O reductase was completely blocked and that this treatment quantified denitrification activity reliably. In the treatments without acetylene, however, the maximum rates of 15(N2+N2O) were approximately five times those of NO3− disappearance. This discrepancy was attributed to incomplete equilibrium of dissolved denitrification gases between liquid and entrapped gas phases. It was concluded, that the time course of dissolved 15(N2+N2O) concentrations in the absence of acetylene is not suitable for determining denitrification activity in soils containing entrapped air.