Denitrification in intact sediment cores from a constructed wetland: Examining the isotope pairing technique

Constructed wetlands aid in the removal of NO3- from surface waters due to enhanced rates of denitrification. In this study intact sediment cores from a constructed wetland were used to measure denitrification rates via the isotope pairing technique and to compare with denitrification rates measured via a mass balance of NO3- disappearance from the water column. The traditional application of the isotope pairing technique calls for the use of mechanical mixing to ensure the transfer of tracer 15NO3- into the sediments. However, artificial mixing can disrupt natural redox processes near the sediment-water interface, and thus may yield rates that are not representative of field conditions. The objective of this study was to examine the applicability of the isotope pairing technique for intact sediment cores and to determine the adjustments that may be necessary in such applications. The 15NO3- was added to the overlying water of intact sediment cores and was transported into sediments only by natural diffusion processes. Using Br- as a conservative tracer, it was determined that passive diffusion alone allowed 27% of the added compound to reach the zone of denitrification in the sediments. Using these results, the enrichment factor (ε) used in the isotope pairing technique was adjusted to account for this effect. Also, it was determined that agitation of the cores at the end of the incubations was necessary to release residual 15N-labelled N2 gas entrained in the sediment matrix. Only after accounting for these two factors did the denitrification rate from the isotope pairing technique agree with mass balance calculations. The isotope pairing technique can be used to quantify denitrification in intact sediment cores if diffusion limitations are quantified and entrained N2 is released from the sediment matrix before final isotope analyses are conducted