Temporal and spatial variability in the cycling of nitrogen within a constructed wetland: A whole‐system stable‐isotope‐addition experiment

Abstract

Constructed wetlands attenuate effluent nutrients, are hydrodynamically well defined, and are a useful proxy for the study of nitrogen (N) transformation in eutrophic natural systems. A whole-system stable-isotope addition was undertaken to describe the N cycling within a constructed wetland. Addition of 15 NH z 4 and particulate organic 15 N (PO 15 N) and a conservative tracer (Li + ) revealed that, initially, sedimentation of PO 15 N and assimilatory uptake of 15 NH z 4 near the wetland inlet removed most of the added 15 N. Denitrification of NO { 3 dominated inorganic N dynamics and was higher upstream than downstream owing to the greater availability of NO { 3 upstream. More NH z 4 was mineralized upstream where PON settlement was highest, and settled PON started being mineralized within 14 d. Nitrification was insignificant upstream but was an important process in the downstream region of the wetland in spite of low oxygen concentration. In the medium term (2-8 weeks), the PO 15 N initially removed to the sediments continued to be mineralized, releasing 15 NH z 4 back to the water column. Remineralized 15 N spiraled through sediment and then macrophyte pools. A dry-out period resulted in a minor washout of N during the subsequent inundation. After 157 d, 30.8% 6 7.3% of the added 15 N was still in sediments, 7.4% 6 3.8% was in plants, 40.8% 6 8.3% had been lost most likely as 15 N 2 , and the remainder had been released in the wetland outlet water. Internal recycling retards the flow of N through wetlands, and shortterm retention leads to eventual enhanced removal through denitrification.Sewage treatment plant (STP) discharge can lead to persistent eutrophication and a loss in recreational and commercial value of receiving waterways (Cloern 2001). Constructed wetlands (CWs) are becoming increasingly popular as a tertiary treatment option for STPs. Constructed wetlands are known to remove nutrients from an effluent stream; however, the magnitude of this removal and the processes involved can vary over different timescales (Kadlec and Knight 1996;Vymazal 2007). The type and magnitude of nutrient-cycling processes may also vary spatially within wetlands (Johansson et al. 2004;Scott et al. 2005 Scott et al. , 2008. Furthermore, disturbances to the wetland, including periodic dry out conducted as part of routine management protocols, are suspected of having important effects on N dynamics (Davidsson et al. 1997;Davidsson and Stahl 2000). However, the role of periodic dry out on wetland N cycling remains unclear (Ishida et al. 2006). In order to improve the management of CWs, it is critical to understand: (1) the temporal variability in the cycling of N during treatment, from the very short term (days) to the longer term (up to 6 months), (2) the spatial variability in N cycling, and (3) the ways in which N dynamics change in response to disturbances such as periodic dry out.The fate of N within wetlands has been successfully studied using intact core incubations and mesocosm-scale experiments (Davidsso...

Fate of microphytobenthos nitrogen in subtropical subtidal sediments: A 15 N pulse-chase study

“…Long‐term 15 N labeling studies of freshwater and marine marshes have shown the highest losses during the first month (De Laune et al ; White and Howes ; Gribsholt et al ), although our losses were much higher. For example, after one month 76% of the added 15 N remained in a salt marsh system (White and Howes ), 66% remained in a freshwater constructed wetland (Erler et al ) and 50% remained in a tidal freshwater system (Gribsholt et al ). All these studies with higher retention showed rapid transfer of 15 N into plants, which enhances the short‐term retention.…”

Nitrogen transformations in a through‐flow wetland revealed using whole‐ecosystem pulsed 15N additions

“…Whole ecosystem 15 N isotope addition techniques recently refined for headwater streams (Mulholland et al 2008) could yield new insights into the processing of N in hydrologically complex aquatic ecosystems, but so far there have been only a few applications within wetlands (Tobias et al 2003;Gribsholt et al 2005;Erler et al 2010). Tracer addition experiments using 15 N have demonstrated the importance of in-stream processes (assimilation and denitrification) in reducing N export from watersheds Mulholland et al 2008).…”

Wastewater treatment for land-based aquaculture: improvements and value-adding alternatives in model systems from Australia

“…Blue boxes represent pre-treatment options to treat Group 1 constituents; green boxes represent primary treatment options to treat Group 2 constituents; red boxes represent secondary treatment options to treat Group 3 constituents; and grey boxes represent secondary treatment options to treat Group 4 constituents. Based on Gray (2005) and Hammer & Hammer (2008) fication and anammox pathways occurring in the sludge and NO 3 − and NH 4 + are preferentially assimilated by algae (Erler et al 2010, de Paula Silva et al 2012. Conversion from DON to dissolved inorganic nitrogen (DIN) is therefore beneficial to reducing total nitrogen (Erler et al 2010).…”

“…Based on Gray (2005) and Hammer & Hammer (2008) fication and anammox pathways occurring in the sludge and NO 3 − and NH 4 + are preferentially assimilated by algae (Erler et al 2010, de Paula Silva et al 2012. Conversion from DON to dissolved inorganic nitrogen (DIN) is therefore beneficial to reducing total nitrogen (Erler et al 2010).…”

“…Furthermore, denitrification and anammox rates of (means ± SE) 199.4 ± 18.7 and 965.3 ± 122.8 µmol N m −2 h −1 , respectively, have been recorded in freshwater wetlands treating MWW (Erler et al 2008). The physical design of wetlands and application of wastes should be optimised to enhance oxidation of DON to DIN and improve nitrogen removal (Erler et al 2010)…”