Tracking the fate of nitrate through pulse-flow wetlands: A mesocosm scale ¹⁵N enrichment tracer study

Quantitative information about the fate of applied nitrate (NO₃-N) in pulse-flow constructed wetlands is essential for designing wetland treatment systems and assessing their nitrogen removal services for agricultural and stormwater applications. Although many studies have documented NO₃-N losses in wetlands, controlled experiments indicating the relative importance of different processes and N sinks are scarce. In the current study, ¹⁵NO₃-N isotope enrichment tracer experiments were conducted in wetland mesocosms of two different wetland soil types at two realistic agricultural NO₃-N source loads. The ¹⁵N label was traced from the source NO₃-N into plant biomass, soil (including organic matter and ammonium), and N-gas constituents over 7–10 day study periods. All sinks responded positively to higher NO₃-N loading. Plant uptake exceeded denitrification 2–3 fold in the low NO₃-N loading experiments, while both fates were nearly equivalent in the high loading experiments. One to two years later, soils largely retained the assimilated tracer N, whereas plants had lost much of it. Results demonstrated that plant and microbial assimilation in the soil (temporary N sinks) can exceed denitrification (permanent N loss) in pulse-flow environments and must be considered by wetland designers and managers for optimizing nitrogen removal potential.