Quantifying nitrogen process rates in a constructed wetland using natural abundance stable isotope signatures and stable isotope amendment experiments

This study describes the spatial variability in nitrogen (N) transformation within a constructed wetland (CW) treating domestic effluent. Nitrogen cycling within the CW was driven by settlement and mineralization of particulate organic nitrogen and uptake of NO₃ ⁻ The concentration of NO₃ ⁻ was found to decrease, as the δ¹⁵N-NO₃ ⁻ signature increased, as water flowed through the CW, allowing denitrification rates to be estimated on the basis of the degree of fractionation of δ¹⁵N-NO₃ ⁻ Estimates of denitrification hinged on the determination of a net isotope effect (η), which was influenced by processes that enrich or deplete ¹⁵NO₃ ⁻ (e.g., nitrification), as well as the rate constants associated with the different processes involved in denitrification (i.e., diffusion and enzyme activity). The influence of nitrification on η was quantified; however, it remained unclear how η varied due to variability in denitrification rate constants. A series of stable isotope amendment experiments was used to further constrain the value of η and calculate rates of denitrification, and nitrification, within the wetland. The maximum calculated rate of denitrification was 956 ± 187 μmol N m⁻² h⁻¹, and the maximum rate of nitrification was 182 ± 28.9 μmol N m⁻² h⁻¹ Uptake of NO₃ ⁻ was quantitatively more important than denitrification throughout the wetland. Rates of N cycling varied spatially within the wetland, with denitrification dominating in the downstream deoxygenated region of the wetland. Studies that use fractionation of N to derive rate estimates must exercise caution when interpreting the net isotope effect. We suggest a sampling procedure for future natural abundance studies that may help improve the accuracy of N cycling rate estimates.