Denitrification Rates Predicted by Nitrogen-15 Labeled Nitrate Microcosm Studies, In Situ Measurements, and Modeling

Denitrification rates in the range of 359 to 599 niniol N m⁻² d⁻¹ were found in the waterlogged soil of a minerotrophic fen. Three different methods were used to estimate denitrification rates. The fen revealed a zone of enhanced denitrification and within this zone the spatial variation of denitrification rates was elucidated using two different approaches: microcosms and field observations. The denitrification rates in the microcosms were measured, either by use of ¹⁵N-N₂ production rates (Method 1) or by setting up a mass balance for nitrate (Method 2). Denitrification rates in the peat horizon at depths of 0 to 69 cm were in the range 182 to 243 mmol N m⁻² d⁻¹ (Method 1) and 213 to 217 mmol N m⁻² d⁻¹ (Method 2). Denitrification rates measured in the deeper sandy soil horizons at depths of 69 to 156 cm were 46 to 103 mmol N m⁻² d⁻¹ (Method 1) and 33 to 137 mmol N m⁻² d⁻¹ (Method 2). Thus, both experiments revealed a strong and comparable decrease in denitrification rate with depth, which also corresponded well to soil carbon content. Denitrification rates were calcultaed from field data using Daicy's law and the conservation equation for one-dimensional, steady state, advective transport of nitrate in an incompressible porous medium. The same significant decrease in denitrification rates with depth was observed as in the microcosm setup. The calculated denitrification rates were: 330 mmol N m⁻² d⁻¹ at depths of 0 to 75 cm (peat), 20 mmol N m⁻² d⁻¹ at depths of 75 to 115 cm (sandy soil), and 9 mmol N m⁻² d⁻¹ in the sandy soil horizon at depths of 155 to 205 cm. This study shows that detailed spatial information about pedological and hydrological parameters in combination with detailed spatial mapping and measurement of denitrification rates is fundamental for properly understanding nitrogen removal in waterlogged soils with ground water through-flow. Further, the results confirm that provided this information is present, it is possible to compare detailed laboratory experiments with field studies of denitrification.