Evaluating nitrate removal and travel times in a bare deciduous forest soil through a column tracer experiment

In response to increased atmospheric nitrogen deposition, understanding the fate of nitrate within forest soils is crucial for predicting the ecological trajectories of soil microbial communities. This study leverages a controlled soil column experiment to investigate nitrate removal in bare soil from a Swiss deciduous forest in the Cugy region near Lausanne. Though not fully replicating natural hydrological conditions, our experimental approach examined dynamics near the soil’s field capacity—a critical threshold for redox-driven processes affecting nitrate and labile dissolved organic carbon (cellobiose) transformation. We deployed a tracer-based approach (bromide and nitrate) with HYDRUS-1D to validate our soil characterization, estimate transport parameters, and determine nitrate consumption rates. Using a simplified kinetic model, we effectively captured nitrate dynamics despite its limitations. By tracing spatiotemporal nitrate removal through nitrate-to-bromide comparisons, we explored the influence of travel time on nitrate removal overlaid by redox and soil moisture conditions. Our results suggest that the saturation–redox relationship exhibited a gradual, rather than abrupt, shift from anaerobic to aerobic conditions, with the two becoming decoupled during prolonged periods of either state. Additionally, our findings indicate that the Swiss deciduous forest soil in the Cugy region near Lausanne can buffer against increased nitrogen inputs and remains in a transitional phase, likely benefiting from elevated nitrate depositions. This experimental design provides insights into developing refined models to predict the transport, transformation, and fate of nitrate and organic carbon in ecosystems.