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 15NH4+ and particulate organic 15N (PO15N) and a conservative tracer (Li+) revealed that, initially, sedimentation of PO15N and assimilatory uptake of 15NH4+ near the wetland inlet removed most of the added 15N. Denitrification of NO3‐ dominated inorganic N dynamics and was higher upstream than downstream owing to the greater availability of NO3− upstream. More NHz 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 PO15N initially removed to the sediments continued to be mineralized, releasing 15NH4+ back to the water column. Remineralized 15N 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% ± 7.3% of the added 15N was still in sediments, 7.4% ± 3.8% was in plants, 40.8% ± 8.3% had been lost most likely as 15N2, and the remainder had been released in the wetland outlet water. Internal recycling retards the flow of N through wetlands, and short‐term retention leads to eventual enhanced removal through denitrification.