A Proposed Method for Measuring Subsoil Denitrification In Situ

Abstract

We present and test a new approach for measuring denitrification in unsaturated subsoils in situ. The procedure consists of applying a test solution containing 15NO−3 to the subsoil, measuring steady-state 15(N2 + N2O) concentration in the pore space of the 15N-labeled soil, and determining denitrification rates by fitting measured and simulated concentrations. We adapted and evaluated a numerical, spherical diffusion model to simulate production and diffusion of 15(N2 + N2O) evolved from a 15(N2 + N2O)-producing subsoil volume surrounded by an infinite nonproducing soil volume. The effects of gas diffusivity, gas production rate, and spatial distribution of 15NO−3 on steady-state 15(N2 + N2O) concentrations were investigated with computer simulations, which proved the methodological principle theoretically. Field experiments were done using two different probe designs. Initially, multipurpose probes were used for applying 15NO−3 solution to the subsoils and subsequent collection of gas samples, but denitrification rates were unrealistically high, probably because of depressed soil gas diffusivity caused by the probe installation procedure. Nondisturbing capillary probes were used for two subsequent in situ experiments. Here, in situ 15(N2 + N2O) concentrations were near the detection limit and resulted in denitrification rates between 0 and 7 g N ha−1 d−1 10 cm−1, which were comparable to rates determined by laboratory reference methods. We concluded that the in situ 15N gas emission method is a promising tool for measuring subsoil denitrification because it overcomes some of the limitations of the alternative techniques.